AS1.13 | Tropical Meteorology and Tropical Cyclones
EDI
Tropical Meteorology and Tropical Cyclones
Convener: Enrico Scoccimarro | Co-conveners: Allison Wing, Alyssa Stansfield, Eric Maloney
Orals
| Tue, 25 Apr, 08:30–12:30 (CEST), 14:00–18:00 (CEST)
 
Room M1
Posters on site
| Attendance Wed, 26 Apr, 10:45–12:30 (CEST)
 
Hall X5
Orals |
Tue, 08:30
Wed, 10:45
The understanding of tropical phenomena and their representation in numerical models still raise important scientific and technical questions, particularly in the coupling between the dynamics and diabatic processes. Among these phenomena, tropical cyclones (TC) are of critical interest because of their societal impacts and because of uncertainties in how their characteristics (cyclogenesis processes, occurrence, intensity, latitudinal extension, translation speed) will change in the framework of global climate change. The monitoring of TCs, their forecasts at short to medium ranges, and the prediction of TC activity at extended range (15-30 days) and seasonal range are also of great societal interest.

The aim of the session is to promote discussions between scientists focusing on the physics and dynamics of tropical phenomena. This session is thus open to contributions on all aspects of tropical meteorology between the convective and planetary scale, such as:

- Tropical cyclones,
- Convective organisation,
- Diurnal variations,
- Local circulations (i.e. island, see-breeze, etc.),
- Monsoon depressions,
- Equatorial waves and other synoptic waves (African easterly waves, etc.),
- The Madden-Julian oscillation,
- etc.

We especially encourage contributions of observational analyses and modelling studies of tropical cyclones and other synoptic-scale tropical disturbances including the physics and dynamics of their formation, structure, and intensity, and mechanisms of variability of these disturbances on intraseasonal to interannual and climate time scales.
Findings from recent field campaigns are also encouraged.

Orals: Tue, 25 Apr | Room M1

Chairpersons: Eric Maloney, Allison Wing
Tropical Convection and Tropical Waves I
08:30–08:40
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EGU23-8949
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AS1.13
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ECS
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On-site presentation
Marlon Maranan, Idene-Flore Mantho T., Andreas H. Fink, Derbetini A. Vondou, Peter Knippertz, and Roderick van der Linden

Tropical waves, particularly convectively coupled equatorial waves (CCEWs), are known to modulate rainfall in tropical Africa on intraseasonal down to convective time scales, the latter of which includes the dynamics of heavy rainfall events. Data scarcity in large parts of Africa, especially in equatorial Africa, has long prevented a clearer picture on the regional variability of extreme rainfall. Thus, making use of globally gridded satellite data and a unique in-situ rainfall dataset for Cameroon, this study aims for a systematic comparison of the role of tropical waves on the occurrence and variability of intense rainfall over western equatorial Africa.

For the study period 2001-2019 in a selected domain over Cameroon, heavy daily rainfall (i.e. the 20% strongest and spatially most extensive) events are identified using both the satellite-based rainfall estimates of the Integrated Multi-satellite Retrievals for Global Precipitation Measurement (IMERG) and largely unique station data from the Karlsruhe African Surface Station-Database (KASS-D). The outgoing longwave radiation (OLR) dataset of the National Oceanic and Atmospheric Administration (NOAA) are then used (a) to support evidence of the occurrence of the intense rainfall events, and (b) to apply a wavenumber-frequency filtering in order to evaluate the co-occurrence of tropical waves around these events. These include the fast modes such as Kelvin waves and tropical disturbances (TD), in the study region commonly represented by African Easterly Waves (AEWs), as well as slow modes represented by equatorial Rossby waves and the Madden-Julian Oscillation (MJO). Finally, to account for regional differences in seasonal rainfall characteristics, the analysis is performed for a southern and northern sub-domain during the bi-modal (March–May/September–November) and unimodal (May–October) rainy seasons, respectively.

Results show that: 1) the passage of Kelvin waves and TDs have the strongest impact on daily rainfall rates in the two sub-regions, whereas the effect of the MJO is the weakest ; 2) the modulation by Kelvin waves is strongest in southern Cameroon whereas that of TDs is strongest in the north; 3) there is a shift between the wet wave phases in OLR and rainfall (IMERG, KASSD); 4) up to 78% of the cases with heavy rain coincide with the passage of a tropical wave; 5) Kelvin and TD are again the most likely to be associated with a heavy rainfall event, featuring an up to five times higher local wave intensity as compared to the other waves.

To further test potential dependencies of results on the applied wave identification method, tropical waves have also been identified with a 2D spatial projection method based on parabolic cylinder functions (PCFs) using horizontal wind fields from ERA5. Here, first results suggest that the projection method overall yields less intense and slower Kelvin waves. Furthermore, the occurrence of a Kelvin wave appears to be related to heavy rainfall to a lesser degree compared to the wavenumber-frequency approach. This potentially stresses the importance of a careful choice of the suitable wave identification method for a given application, the details of which are currently evaluated.

How to cite: Maranan, M., Mantho T., I.-F., Fink, A. H., Vondou, D. A., Knippertz, P., and van der Linden, R.: Impact of tropical waves on rainfall modulation and heavy rainfall event occurrence over western equatorial Africa, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8949, https://doi.org/10.5194/egusphere-egu23-8949, 2023.

08:40–08:50
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EGU23-2007
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AS1.13
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ECS
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On-site presentation
Ashar Aslam, Juliane Schwendike, Simon Peatman, Cathryn Birch, Massimo Bollasina, and Paul Barrett

Patterns in extreme precipitation across the Maritime Continent in Southeast Asia are known to be modulated by many processes, from large-scale modes of variability such as the Madden-Julian Oscillation and planetary waves, to finer-scale processes such as the diurnal cycle. Transient mid-level dry air intrusions are an example of a process not extensively studied over the Maritime Continent, which has the potential to influence rainfall patterns.  

Through Lagrangian trajectory and event composite analyses, we use a humidity metric which identifies mid-level dry air intrusions. These intrusions originate from upper-level disturbances along the subtropical jet. Mid-level cyclonic circulation anomalies northwest of Australia from December-February (DJF) intensify westerlies in the southern Maritime Continent, advecting dry air eastward. In contrast, mid-level anticyclonic circulation anomalies northwest of Australia from June-August (JJA) intensify southern Maritime Continent easterlies, advecting dry air westward. The resultant transport direction of associated air parcels is also dependent on the seasonal low-level monsoon circulation, and potentially convective entrainment.  

Dry air intrusions are found to be important in influencing low-level wind circulations and rainfall patterns in the southern Maritime Continent. Dry air suppresses rainfall over seas near to the southern Maritime Continent in both seasons. Further suppression matches intrusions trajectories, such as over southern Maritime Continent islands in DJF, and the Indian Ocean in JJA. In both seasons, there is enhanced rainfall to the east of the intrusion, where there is moist return flow to the extratropics.  

How to cite: Aslam, A., Schwendike, J., Peatman, S., Birch, C., Bollasina, M., and Barrett, P.: Mid-Level Dry Air Intrusions over the southern Maritime Continent, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2007, https://doi.org/10.5194/egusphere-egu23-2007, 2023.

08:50–09:00
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EGU23-578
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AS1.13
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ECS
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Virtual presentation
Northward Prpogating BSISO over South Asia: connection between moisture advection and vortex tilting
(withdrawn)
Sambrita Ghatak and Jai Sukhatme
09:00–09:10
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EGU23-6549
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AS1.13
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ECS
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On-site presentation
Natasha Senior, Adrian Matthews, Ben Webber, Beata Latos, and Dariusz Baranowski

The Indonesian island of Sulawesi lies at the heart of the Maritime Continent, a region prone to extreme rainfall. On seasonal timescales, rainfall frequency and intensity increases during the monsoon season (Nov-March). On subseasonal scales, rainfall is modulated by the Madden-Julian Oscillation (MJO) which increases moisture and moisture convergence in its active phase. Higher frequency modes include convectively coupled equatorial waves which influence rainfall variability on daily timescales. On 22nd January 2019, these large-scale meteorological drivers coincided resulting in the South Sulawesi region experiencing its largest flood on record. Specifically, the extreme rainfall event was linked to a convectively coupled Kelvin wave (CCKW) and a convectively coupled equatorial Rossby wave (CCERW) embedded within an active MJO envelope, as well as a cross equatorial cold surge (Latos et al, 2021). Interactions between these modes resulted in increased moisture transport and convergence that lead to the development of a mesoscale convective system (MCS) over Java and the Java Sea on 21st January 2019. This MCS traversed towards Sulawesi bringing extreme rainfall to the region in the evening and overnight on the 22nd reaching its peak mid-afternoon. Cases like this present a unique challenge for forecasters, to not only to accurately represent the individual equatorial modes but their interactions. In the present work we study the MCS in reanalysis and satellite data and discuss how the various equatorial modes contributed to its development. Then we examine its representation in different convection permitting Met Office Unified Model (MetUM) configurations. We find that the MetUM performed well in capturing the trajectories of the equatorial modes, however the representation of the MCS itself varies between ensemble members and model configurations. We further examine how well the RAL1T+ configuration represents the equatorial modes through comparing filtered fields of daily model data at fixed lead times to those in observations.

How to cite: Senior, N., Matthews, A., Webber, B., Latos, B., and Baranowski, D.: Extreme precipitation in South Sulawesi triggered by equatorial waves and its representation in MetUM forecasts, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6549, https://doi.org/10.5194/egusphere-egu23-6549, 2023.

09:10–09:20
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EGU23-4550
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AS1.13
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ECS
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Virtual presentation
Ahmed Shaaban and Paul Roundy

Stratospheric Kelvin waves are known to be absorbed by the background flow via mechanical and thermal damping and, to less extent, by the critical layer interaction. Critical layer interaction occurs when the Kelvin waves' phase speed approaches the background flow's speed. This study aims to depict the structure of the Kelvin waves while approaching the critical layer, where the phase speed of the wave matches the speed of the background flow. In the time domain, the wavelet filtering technique filters Kelvin waves at a specific location and phase speeds using ERA-I zonal wind. Linear regression yields the pattern of specific phase speed's Kelvin wave. Yet, the critical layer interaction of the Kelvin waves with the environmental flow could be studied by choosing a background environment in which its flow speed matches the wave's phase speed, which could be implemented using the varying-coefficient regression technique. We found that the in-phase relationship between the zonal wind and height, associated with the structure of the Kelvin waves, relaxes with the decreasing of the Doppler-shifted speed; then, at a further reduction of the Doppler-shifted speed, the Gill pattern appears. Furthermore, Kelvin waves were found to be absent under an environment of westerly shear.

How to cite: Shaaban, A. and Roundy, P.: On the critical layer interaction of the stratospheric Kelvin waves, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4550, https://doi.org/10.5194/egusphere-egu23-4550, 2023.

09:20–09:30
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EGU23-845
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AS1.13
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ECS
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Virtual presentation
Shreya Keshri and Suhas Ettammal

In this study we have conducted a survey of Mixed Rossby-Gravity (MRG) wave events in the upper troposphere and quantified their association with the intrusions of extratropical disturbances for the period 1979-2019. MRG events are identified by projecting the equatorial meridional winds at 200 hPa onto the meridional structure of theoretical MRG waves2390 MRG events are identified and majority (61%) of them occurred during May-October months, and 65% of the total MRG events occurred over the central-east Pacific and Atlantic Ocean domains. Not only the frequency of occurrence but also the amplitude, wavenumber and trapping scale of the MRG events are found to exhibit a clear seasonality. MRG events associated with intrusions of extratropical disturbances are identified as when the potential vorticity on the 350K isentropic surface at 15° latitude exceeded 1 PVU in the vicinity of the MRG events. We find that 37% of the MRG events are intrusion MRG events and a large majority (88%) of such events occurred over the central-east Pacific and Atlantic Ocean domains. It is also noteworthy that nearly 70% of such intrusions occurred in the winter Hemisphere where the westerly wind ducts are well developed. Over the central-east Pacific during Northern Hemispheric (NH) winter, it is observed that the amplitude of intrusion MRG events are larger and have a larger meridional extent compared to non-intrusion MRG events. They also exhibit a similar spatial scale as the extratropical disturbances implying that resonant interactions may be a primary mechanism for the genesis of MRG events. During NH summer, on the other hand, MRG events are primarily triggered by convective processes and the extratropical disturbances may be instrumental in amplifying their amplitude. 

How to cite: Keshri, S. and Ettammal, S.: A survey of Mixed Rossby-Gravity waves and quantification of their association with extratropical disturbances., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-845, https://doi.org/10.5194/egusphere-egu23-845, 2023.

09:30–09:40
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EGU23-5340
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AS1.13
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ECS
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Virtual presentation
Mehak Na, Shreya Keshri, and Suhas Ettammal

Mixed Rossby-Gravity (MRG) Waves are westward propagating synoptic scale equatorial disturbances which play a crucial role in the formation of tropical cyclones and tropical depressions, and they constitute a significant part of various modes of tropical variability such as the Madden-Julian Oscillation (MJO) and Quasi-Biennial Oscillation (QBO). In this study, we have investigated the trends and Inter Annual Variability (IAV) in the occurrence of upper tropospheric MRG events using ERA-I reanalysis data for the period 1979-2018. The MRG events are identified by projecting the upper tropospheric meridional winds onto the theoretical spatial structure of MRG waves. A steady increasing trend is observed in the occurrence of MRG events which is contributed by the MRG events associated with the intrusion of extratropical disturbances. The possible factors that govern the observed trends and IAV in the occurrences of MRG events are El Nino Southern Oscillation (ENSO), MJO and extratropical forcing. The MRG events over the central and Eastern Pacific contribute maximum to the IAV. ENSO explains about 25% of the IAV. It exhibits a positive correlation with non-intrusion MRG events and a negative correlation with intrusion MRG events. These observations have been investigated by exploring the strength and the extent of the westerly duct at 200 hPa and the Outgoing Longwave Radiation (OLR) during El Nino and La Nina years over the central-Eastern Pacific ocean. The convectively active state of the MJO over the Western Pacific explains 20% of the IAV over the central-Eastern Pacific. Besides ENSO, MJO exhibits a diametrically opposite correlation with intrusion and non-intrusion MRG events. The antisymmetric heating with respect to the equator, associated with the MJO, enhances non-intrusion MRG events. The subtropical easterlies forbid the intrusion of extratropical disturbances, thereby lowering the occurrences of intrusion MRG events. The increasing trend in the intrusion of extratropical disturbances explains the observed trend in the upper tropospheric MRG events. Such an increasing trend is not observed in the strength or extent of the upper tropospheric westerly duct over the central-Eastern Pacific.

How to cite: Na, M., Keshri, S., and Ettammal, S.: Major Factors Governing the Trends and Interannual Variability in the Occurrences of Mixed Rossby-Gravity Wave Events, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5340, https://doi.org/10.5194/egusphere-egu23-5340, 2023.

09:40–09:50
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EGU23-2020
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AS1.13
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On-site presentation
Adrian Matthews

Convectively coupled equatorial Rossby waves (CCERWs) are an intrinsic part of the spectrum of tropical weather systems, and can bring extreme precipitation to tropical locations. They are usually interpreted as modified versions of the theoretical dry equatorial Rossby wave solutions of the shallow water equations. However, the structure and dynamics of CCERWs are rather different to their theoretical cousins. Here, a vorticity budget is presented for both theoretical equatorial Rossby waves and for CCERWs (based on reanalysis data). The different strengths of the vorticity budget terms between the theoretical waves and CCERWs gives insights into CCERW propagation and growth mechanisms, and provides a focus and testbed for future model and forecast improvements.

How to cite: Matthews, A.: Vorticity budget of convectively coupled equatorial Rossby waves: propagation and growth mechanisms, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2020, https://doi.org/10.5194/egusphere-egu23-2020, 2023.

09:50–10:00
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EGU23-15063
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AS1.13
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On-site presentation
Simon Peatman, Cathryn Birch, Juliane Schwendike, John Marsham, Chris Dearden, Stuart Webster, Emma Howard, Steven Woolnough, Ryan Neely, and Adrian Matthews

The Maritime Continent, located within the Indo-Pacific warm pool, experiences some of the most intense convective rainfall on Earth, with a pronounced diurnal cycle. The spatio-temporal variability of convection, its organisation and its offshore propagation away from the islands overnight all depend on many factors including the topography of island coastlines and mountains, and large-scale weather phenomena such as the Madden-Julian Oscillation, El Niño–Southern Oscillation and equatorial waves. However, numerical weather prediction and climate models typically suffer from considerable biases in simulating the diurnal convection and its propagation, hence there is a need to improve our understanding of the underlying physical mechanisms of these phenomena.

While the nocturnal offshore propagation of convection is often thought to be forced by gravity waves triggered by land-based diurnal convection, alternative hypothesized mechanisms exist in the literature, related to the propagation of the offshore land breeze and cold pools. Using convection-permitting simulations of selected case studies of convection propagating offshore from Sumatra, we find a squall line propagating overnight due to low-level convergence caused by the land breeze and environmental winds. This is reinforced by cold pools, which we diagnose using model tracers. However, gravity waves also play a role, triggering localized (non-organized) convection which does not itself propagate, but can appear as propagation along wave trajectories when compositing the diurnal cycle over many days.

The investigation is extended to other coastlines in the Maritime Continent, using convection-permitting simulations for 900 days during boreal winters, to demonstrate broader evidence for these physical mechanisms; to understand why the offshore propagation occurs on some days but not others; and to show how the strength, timing and causes of offshore propagation vary for different Maritime Continent islands, due to variations in the large-scale winds, orography and the topography of coastlines.

How to cite: Peatman, S., Birch, C., Schwendike, J., Marsham, J., Dearden, C., Webster, S., Howard, E., Woolnough, S., Neely, R., and Matthews, A.: Physical mechanisms of offshore propagation of convection in the Maritime Continent, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15063, https://doi.org/10.5194/egusphere-egu23-15063, 2023.

10:00–10:10
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EGU23-6722
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AS1.13
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Highlight
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On-site presentation
Chuing-Wen June Chang, Min-Hui Lo, Wan-Ling Tseng, Yu-Cian Tsai, and Jia-Yuh Yu

Deforestation is a major issue affecting both regional and global hydroclimates. This study investigated the effect of deforestation in the Maritime Continent (MC) on tropical intraseasonal climate variability. Using a global climate model with Madden–Julian Oscillation (MJO) simulations, we examined the effect of deforestation over the MC region by replacing the forest canopy with grassland. The results revealed that under constant orographic and land–sea contrast forcing, the modification of the canopy over the MC altered the characteristics of the MJO. We noted the amplification of the MJO and increases in wet–dry fluctuation and the zonal extent. We analyzed more than 100 MJO cases by performing K-means clustering and determined that the continuous propagation of the MJO over the MC increased in 35% and 61% of the total 110 cases in the control and deforestation experiments, respectively. This phenomenon was associated with more substantial vanguard precipitation, increased soil moisture, and a suppressed diurnal cycle in land convection. Furthermore, when the MJO convection was over the Indian Ocean (IO), we observed the enhancement of low-level moisture over the MC region in the deforestation experiment. Grassland surface forcing provides a thermodynamic source for triggering instability in the atmosphere, resulting in low-level moisture convergence. The MJO exhibited a stronger energy recharge–discharge cycle in the deforestation experiment than in the control experiment, and this difference between the experiments enlarged from the IO to MC.

 

How to cite: Chang, C.-W. J., Lo, M.-H., Tseng, W.-L., Tsai, Y.-C., and Yu, J.-Y.: Impact of Deforestation in the Maritime Continent on the Madden–Julian Oscillation, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6722, https://doi.org/10.5194/egusphere-egu23-6722, 2023.

10:10–10:15
Coffee break
Chairpersons: Eric Maloney, Allison Wing
Tropical Convection and Tropical Waves II
10:45–10:55
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EGU23-3655
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AS1.13
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ECS
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On-site presentation
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Masoud Rostami, Bowen Zhao, and Stefan Petri

By means of a new multilayer pseudo-spectral moist-convective thermal rotating shallow-water (mcTRSW) model in a full sphere, we present a possible equatorial adjustment beyond Gill’s mechanism for the genesis and dynamics of the Madden–Julian oscillation (MJO). According to this theory, an eastward-propagating MJO-like structure can be generated in a self-sustained and self-propelled manner due to nonlinear relaxation (adjustment) of a large-scale positive buoyancy anomaly, depressed anomaly, or a combination of these, as soon as this anomaly reaches a critical threshold in the presence of moist convection at the Equator. This MJO-like episode possesses a convectively coupled “hybrid structure” that consists of a “quasi-equatorial modon” with an enhanced vortex pair and a convectively coupled baroclinic Kelvin wave (BKW), with greater phase speed than that of dipolar structure on an intraseasonal time-scale. Interaction of the BKW, after circumnavigating the entire Equator, with a new large-scale buoyancy anomaly may contribute to excitation of a recurrent generation of the next cycle of MJO-like structure. Overall, the generated “hybrid structure” captures a few of the crudest features of the MJO, includingits quadrupolar structure, convective activity, condensation patterns, vorticity field, phase speed, and westerly and easterly inflows in the lower and upper troposphere. Although moisture-fed convection is a necessary condition for the “hybrid structure” to be excited and maintained in the proposed theory in this study, it is fundamentally different from moisture-mode theories, because the barotropic equatorial modon and BKW also exist in “dry” environments, while there are no similar “dry” dynamical basic structures in moisture-mode theories. The proposed theory can therefore be a possible mechanism to explain the genesis and backbone structure of the MJO and to converge some theories that previously seemed divergent (DOI:10.1002/qj.4388). 

How to cite: Rostami, M., Zhao, B., and Petri, S.: A new born theory for the genesis and dynamics of Madden–Julian oscillation-like structure, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3655, https://doi.org/10.5194/egusphere-egu23-3655, 2023.

10:55–11:05
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EGU23-3995
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AS1.13
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ECS
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Virtual presentation
Liset V. Proveyer and Alejandro Jaramillo

This research aims to determine how the Madden-Julian oscillation (MJO) influences extreme summer precipitation events in the Metropolitan Area of the Mexican Valley (MAMV). Using the Real-time Multivariate MJO Index (RMM), we found a higher frequency of days with extreme events during phases 1 and 2 of the MJO (wet phases), with the lowest occurrence during phases 6, 7, and 8 (dry phases). These frequencies are associated with positive (negative) humidity anomalies in the whole atmospheric column of the study region during the wet (dry) phases. The interaction of a humid flow from the Caribbean Sea with the mountain systems of the region plays a fundamental role in the occurrence of deep convection. Also, the formation of mesoscale convective systems in the central region of the Mexican territory contributes to the moisture content in the Mexican Valley. We used the Dynamic Recycling Model to quantify the relative contributions of different source regions to the atmospheric humidity in MAMV. We found that the greatest contributions to the humidity anomalies are from the Caribbean Sea and Central Mexico during the wet phases of the MJO. During the remaining phases, we observe a weakening of the humid flow from the east as the Caribbean low-level jet intensifies. Additionally, during phases 7 and 8, the mountainous systems that limit the MAMV constitute natural barriers to the flow of humidity that tends to be predominantly from the eastern Pacific. The MAMV is highly vulnerable to extreme precipitation events and their effects, such as pluvial floods and landslides. Therefore, studying the phenomena that modulate these extreme events is essential to improve their predictability and perform better risk management.

How to cite: Proveyer, L. V. and Jaramillo, A.: Impact of the MJO on the extreme precipitations in the Mexican Valley, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3995, https://doi.org/10.5194/egusphere-egu23-3995, 2023.

11:05–11:15
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EGU23-4491
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AS1.13
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ECS
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Virtual presentation
Luis Lazcano and Christian Domínguez

The Madden-Julian Oscillation (MJO) is the main mode of intraseasonal variability over the tropics. We aim to explore the MJO modulation on extreme precipitation events by analyzing atmospheric variables from ERA5 and oceanic variables from the NOAA and HYCOM reanalysis over the Eastern Pacific Ocean from May to November during the 1982-2018 period. The Real-time Multivariate MJO Index (RMM) is used to define the MJO phases. Only strong MJO phases are considered for this study. During MJO phases 3-7, the Eastern Pacific Ocean warm pool goes through a large expansion, but rainfall decreases near the Mexican Pacific coast. On the other hand, MJO phases 8, 1, and 2 induce an increase in precipitation over the continental part of the Middle Americas, making extreme precipitation events more frequent, but these phases decrease the warm pool extension near the Pacific coast. Additionally, the MJO compounds are classified according to El Niño Southern Oscillation (ENSO) years. MJO phases under Neutral and El Niño years have similar patterns in the atmospheric variables. However, these patterns drastically change in MJO phases under La Niña years, as the warm pool expansion decreases and the decrease/increase in rainfall is more intense compared to Neutral and El Niño years. We also noticed that the warm pool expansion and extreme precipitation events do not occur simultaneously. There is a lag of 7 days, as previous studies found but for the Indian Ocean. We conclude that these results are important to understand the air-ocean coupling for MJO and its application for sub-seasonal forecasts.

How to cite: Lazcano, L. and Domínguez, C.: Influence of the MJO on extreme precipitation events over the Eastern Pacific Ocean, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4491, https://doi.org/10.5194/egusphere-egu23-4491, 2023.

11:15–11:25
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EGU23-15259
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AS1.13
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ECS
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Virtual presentation
Julia Crook, Fran Morris, Rory Fitzpatrick, Simon Peatman, Juliane Schwendike, Thorwald Stein, Cathryn Birch, and Sam Hardy

Southeast Asia is a region dominated by intense convection and characterised by the high-impact weather associated with synoptic scale tropical depressions, typhoons, or tropical cyclones (TCs). However, more localised convection such as mesoscale convective systems (MCSs) can also produce intense precipitation which can be a major risk for loss of lives and property for the communities in the region. Due to these high-impact weather features, its complex orography, and the significant impact of large-scale weather features on its meteorological variability, predicting weather in Southeast Asia is of great importance and scientific interest, but is a challenge. We aim to characterise the distribution of MCSs in the region and capture how the systems are modulated by the Madden-Julian Oscillation (MJO) and equatorial waves. 

MCSs in Southeast Asia between 2015 and 2020 were tracked using Himawari satellite data, their associated rainfall estimated using IMERG, and classified by lifetime and propagation speed. TC-related rainfall was also deduced using data from IBTrACS to identify certain cloud clusters as associated with TCs. Between 10S and 10N, MCSs account for 45-70% of the precipitation between November and April, and over most of the region, the fractional MCS contribution to rainfall is higher than average on extreme wet days (>55%). Long-lived  (>12 hours) MCSs contribute disproportionately, providing 84% of the rainfall despite comprising only 34% of all MCSs.

The MJO modulates MCS rainfall in a similar way to total rainfall, contributing >50% of the total rainfall anomaly, with the number of MCSs being greater in convectively active phases. However, in the West part of the region there are more fast-moving MCSs in the active MJO phases and more slow-moving MCSs in the inactive phases, resulting in fast-moving MCSs having a greater impact on the MJO-associated variation in MCS rainfall. This variation in MCS rainfall is larger in the West part of the region than the East. Meanwhile, variation in the area-mean rainfall rate within the storms, and sizes of storms were less well correlated with MCS rainfall in different phases; when areas were large, area-mean rainfall rate was generally low, and vice versa, providing compensating effects.

In the low-level convergence phase of an equatorial Kelvin wave, MCS rainfall and non-organized rainfall both increase, accounting for 20-50% of local rainfall anomalies, a pattern which is again enhanced in the West of the region. By contrast, Westward-propagating Mixed Rossby-Gravity waves, and Rossby-1 waves, do not strongly modulate MCS rainfall, and instead their rainfall anomalies are dominated by TC-related rainfall.

These relationships between MCSs and the MJO and Kelvin waves provide useful insight into forecasting MCSs in Southeast Asia by utilising knowledge of the synoptic weather regimes that are or will be affecting the region.

How to cite: Crook, J., Morris, F., Fitzpatrick, R., Peatman, S., Schwendike, J., Stein, T., Birch, C., and Hardy, S.: Impacts of the MJO and Equatorial Waves on Tracked Mesoscale Convective Systems Over Southeast Asia, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15259, https://doi.org/10.5194/egusphere-egu23-15259, 2023.

11:25–11:35
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EGU23-2857
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AS1.13
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On-site presentation
Zeljka Stone, David Raymond, and Stipo Sentic

The OTREC (Organization of Tropical East Pacific Convection) field project took place from August 5 to October 3, 2019. The operational center was in Liberia, Costa Rica. During OTREC, we performed 127 research flight hours in the area of the Eastern Pacific and southwest Caribbean. We deployed 648 dropsondes in a grid to evaluate mesoscale thermodynamic and vorticity budges. We also used the Hiaper Cloud Radar to determine the characteristics of cloud populations. Both of these tools were deployed from the NSF/NCAR Gulfstream V aircraft.

The Eastern Pacific has a strong cross-equatorial gradient in sea surface temperature. The southwest Caribbean exhibits uniform ocean temperatures. The two regions together provide a broad range of atmospheric conditions and a great deal of diversity in convective behavior.

The main goal of the project was to study convection in diverse environments to improve global weather and climate models. In this talk I will present an overview of OTREC, the highlights of the field project and the results that OTREC has yielded that include the thermodynamics of the environment and the vertical mass flux profiles. In particular, column relative humidity, low to mid-tropospheric moist convective instability, and convective inhibition are shown to be useful predictors for moisture convergence, and hence rainfall.

How to cite: Stone, Z., Raymond, D., and Sentic, S.: Organization of Tropical East Pacific Convection Field Project, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2857, https://doi.org/10.5194/egusphere-egu23-2857, 2023.

Tropical Cyclones I
11:35–11:45
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EGU23-75
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AS1.13
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ECS
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On-site presentation
Quinton Lawton and Sharanya Majumdar

In recent years, research has illuminated a distinct relationship between Convectively Coupled Kelvin Waves (CCKWs) and tropical cyclone (TC) formation. In basins that support TCs, there is a pronounced increase in the number of TC genesis events 1-3 days following the passage of a CCKW’s convectively-active phase. It has been hypothesized that this lagged relationship could be the result of the modification of environmental and kinematic factors by the CCKW. However, little work has been done to try to connect these environmental changes to the processes involved in TC genesis. Observational and modeling studies alike have indicated that the development of TCs may be intimately tied to convective-radiative feedbacks and pre-moistening of the atmosphere. How might CCKWs be impacting these processes?

To investigate this, we leverage a 39-year database of African Easterly Waves (AEWs) and associated TC genesis events in the Atlantic Ocean basin from 1981 to 2019. Environmental composites of ERA5 reanalysis and satellite data show an increase in column specific humidity and convective coverage beginning two days prior to TC genesis. This supports previous hypotheses of AEW trough preconditioning. A moist static energy (MSE) variance budget surrounding AEWs is also calculated. This analysis indicates that the dominant source of MSE variance during TC genesis – a proxy for convective aggregation – are longwave-radiative feedbacks, further solidifying the role of convection-related feedbacks in TC development.

Environmental fields around developing AEWs are then composited relative to passing CCKWs. Convectively-active CCKWs temporarily promote an increase in convection, specific humidity, and relative vorticity around AEWs. AEW-CCKW passages are shown to be quite common, with 76% of all developing AEWs passing at least one CCKW in their lifetime. We also compare AEW-CCKW passages that result in TC genesis versus those that do not. The primary discriminator between these two outcomes appears to be convective coverage and diabatic heating at the time of CCKW passage. There is also a pronounced increase in the longwave-radiative feedback term following the CCKW passage for cases that result in TC genesis.

While it is hard to separate the simultaneous effects of a multi-day TC genesis process from that of passing CCKWs, this analysis provides at least circumstantial evidence that CCKW-related modifications to convection and humidity could play an indirect role in preconditioning the AEW and a direct role in strengthening radiative-convective feedbacks. These results also motivate investigation of AEW-CCKW interactions in numerical simulations, which may be more suited to investigate cross-scale interactions and better determine causality.

How to cite: Lawton, Q. and Majumdar, S.: Kelvin Waves and Tropical Cyclogenesis: Connections to Convection and Moisture, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-75, https://doi.org/10.5194/egusphere-egu23-75, 2023.

11:45–11:55
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EGU23-10985
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AS1.13
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ECS
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On-site presentation
Kelly Núñez Ocasio and Chris Davis

In a recent model evaluation of the African Easterly Wave (AEW) that became Helene (pre-Helene; 2006) over the Atlantic, the wave was categorized as a mixed-off-equatorial moisture mode during tropical cyclogenesis as it evolved under weak temperature gradient balance. It was found that the simulated pre-Helene waves were more intense and overall slower than in ERA5, especially the wave that evolved in a more moisture-rich environment. The growth and propagation of the wave were related to the position of the convection with respect to the center of the wave vortex. The influence of environmental moisture on wave propagation before and during genesis remains an open question. Motivated by the recent findings, in this study, moisture sensitivity experiments are performed with a convection-permitting model to further evaluate the moisture dependency of the pre-Helene wave and later tropical cyclogenesis. The Model for Prediction Across Scales (MPAS) regional configuration is used to allow altering initial and lateral boundary conditions of relative humidity (RH) through the entire atmospheric column using ERA5 pressure-level data. Preliminary results reveal that over land the strength of the wave-trough meridional flow is related to mid-to-upper-level diabatic heating tendencies from clouds located in the northerly phase of the wave and to the lack of shallow convection within the vortex. In MOIST (RH x 1.2 experiment), the wave moves slower, yet organized convection propagates out of phase with the wave speed, ultimately weakening the wave and subsequent tropical cyclogenesis. In CONTROL, where the wave propagates faster, the phasing between wave and convection supports a stronger wave prior to genesis and ultimately genesis when compared to MOIST. A moister atmosphere (MOIST) favors a larger fraction of shallow convection (bottom heavy and weaker updrafts) at the center and ahead of the vortex, detraining the mid-troposphere and weakening the mid-tropospheric vorticity. This leads to a wave that weakens prior to genesis compared to CONTROL as well as a more abrupt decrease in speed prior to genesis. The lack of cloud microphysics heating tendencies in DRY (RH x 0.5 experiment) resulted in a weaker mid-to-upper-level circulation but stronger surface-to-low-level winds. The lack of moisture is detrimental to the simulated pre-Helene; however, a moister environment does not necessarily result in a more intense wave or tropical cyclogenesis event. A wave that propagates more slowly (‘moist wave’ versus ‘dry wave’), does not necessarily favor growth. For further growth, convection that is in phase with the vortex should be deep moist convection.

How to cite: Núñez Ocasio, K. and Davis, C.: Wave Propagation and Growth Dependency to Environmental Moisture: A Case of an Atlantic Tropical Cyclogenesis, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10985, https://doi.org/10.5194/egusphere-egu23-10985, 2023.

11:55–12:05
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EGU23-1799
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AS1.13
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ECS
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Virtual presentation
Vineet Singh, Roxy Mathew Koll, and Medha Deshpande

The cyclones during November in the Bay of Bengal follow two distinct tracks. Analysis for the period 1982–2019 shows that some cyclones move
west-northwestward and make landfall at the Odisha, Andhra Pradesh or Tamil Nadu coast of India, or the Sri Lanka coast, while others move north-northeastwards and make landfall at the West Bengal, Bangladesh or Myanmar coast. Our analysis shows there is a significant difference in the steering winds governing these two different cyclone tracks. The north-northeastward moving cyclones are associated with an anomalous upper-level cyclonic circulation over India which is part of a subtropical Rossby wave train triggered by an anomalous upper-level convergence over the Mediterranean region. This wave train propagates along the subtropical westerly jet from the east Atlantic/Mediterranean region and reaches the Indian subcontinent in 4 days. It induces an anomalous cyclonic circulation over the Indian landmass and provides south-to-north and west-to-east steering over the Bay of Bengal, causing the cyclones to move in a north-northeastward direction. On the other hand, for west-northwestward moving cyclones, there is no Rossby wave intrusion over the Indian subcontinent, hence the cyclones move in a west-northwestward direction assisted by the beta effect and climatological winds which are from east to west over the south and central Bay of Bengal. This shows that the track of cyclones in the north Indian Ocean can be modulated by atmospheric changes in the extratropics and can act as a precursor for the prediction of the track of cyclones in this region.

How to cite: Singh, V., Mathew Koll, R., and Deshpande, M.: Role of subtropical Rossby waves in governing the track of cyclones in the Bay of Bengal, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1799, https://doi.org/10.5194/egusphere-egu23-1799, 2023.

12:05–12:15
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EGU23-1258
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AS1.13
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On-site presentation
Chanh Kieu and The-Anh Vu

This study examines the role of tropical dynamics in the formation of global tropical cyclone (TC) clusters. Using theoretical analyses and idealized simulations, it is found that global TC clusters can be produced by the internal dynamics of the tropical atmosphere, even in the absence of all landmass surface and zonal sea surface temperature (SST) anomalies. Theoretical analyses of a two-dimensional InterTropical Convergence Zone (ITCZ) model reveal indeed some large-scale stationary waves whose zonal and meridional structures could support the formation of TC clusters at the global scale. Additional idealized simulations using the Weather Research and Forecasting (WRF) model confirm these results for a range of experiments. Specifically, the examination of two common tropical wave types including the equatorial Rossby (ER) wave and the equatorial Kelvin (EK) wave shows that ER waves could develop a stationary structure for a range of zonal wavenumbers $m\in[5-11]$, while EK waves do not. This modeling result is consistent with the ITCZ analytical model and suggests that large-scale ER waves could support stationary "hot spots" for global TC formation without any zonal SST anomalies. The findings in this study offer different insights into the importance of tropical waves in producing global TC clusters beyond the traditional explanation based on zonal SST variability.   

How to cite: Kieu, C. and Vu, T.-A.: On the Roles of Tropical Waves in the Formation of Global Tropical Cyclone Clusters, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1258, https://doi.org/10.5194/egusphere-egu23-1258, 2023.

12:15–12:25
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EGU23-4688
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AS1.13
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On-site presentation
Jung-Eun Chu, Axel Timmermann, Pavan Harika Raavi, Sun-Seon Lee, Johnny C. L. Chan, and Hung Ming Cheung

Tropical cyclones (TCs), the generic name for typhoons, are among the most destructive natural hazards. It is important to understand how climate change affects TC frequency, to minimize human and economic losses. Some studies have suggested that the number of TCs will decrease, and their formation will shift poleward. However, there is a major lack of fundamental understanding of the origin and development of the TCs from the initial precursory vortex, called a TC seed. The changes in the number of TC seeds and their survival rate (i.e., the proportion that successfully develops into TCs) will eventually control the future TC frequency. However, key challenges are mainly due to a lack of consensus in TC seed definition and a lack of computing resources for TC modeling.

This study aims to meet the above challenges, through the following three tasks: (1) to identify the representation of TC seeds based on three different definitions from early-stage to matured stage; (2) to investigate the future changes in TC seeds and survival rate and their contribution to the poleward shift in TC genesis location; and (3) to unravel the physical mechanisms responsible for the change in response to climate change. We use a high-resolution fully-coupled Community Earth System Model (CESM) with an atmospheric resolution of 0.25° and an ocean resolution of 0.1° with present-day, doubling, and quadrupling CO2 concentrations. Our results show TC seeds defined by early-stage definition show more equatorward distribution with a strong connection to vertical velocity than those defined by matured stage. Interestingly, all three definitions exhibit a statistically significant reduction in the frequency of TC seeds while that in survival rate is not significant. Details of the methods and mechanisms will be further discussed during the presentation. The outcomes of this project will strengthen fundamental scientific knowledge of the TC seeds and their future change mechanisms, as well as provide a scientific basis for future risk assessment and precautionary strategies.

How to cite: Chu, J.-E., Timmermann, A., Raavi, P. H., Lee, S.-S., Chan, J. C. L., and Cheung, H. M.: Contribution of tropical cyclone seeds in the poleward shift of the tropical cyclone formation, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4688, https://doi.org/10.5194/egusphere-egu23-4688, 2023.

12:25–12:30
Lunch break
Chairpersons: Allison Wing, Enrico Scoccimarro
Tropical Cyclones II
14:00–14:10
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EGU23-6086
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AS1.13
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On-site presentation
Ulrike Lohmann, Bernhard Enz, David Neubauer, and Michael Sprenger

Tropical cyclones are among the most devastating natural phenomena that can cause severe damage when hitting land. Some of this damage could be prevented with more reliable short-term and seasonal forecasts. In the wake of the poorly forecast 2013 North Atlantic hurricane season, Rossby wave breaking has been linked to tropical cyclone activity measured by the accumulated cyclone energy. Here, ERA5 reanalysis data and HURDAT2 tropical cyclone data are used to show that the latitude of the 2 potential vorticity unit (PVU) contour on the 360 K isentropic surface in the western North Atlantic is linked to changes in vertical wind shear and relative humidity during the month of September.

A more equatorward position of the 2 PVU contour is linked to an increase in vertical wind shear and a reduction in relative humidity, as manifested in an increased ventilation index, in the tropical western North Atlantic during September. The more equatorward position of the 2 PVU contour is further linked to a reduction in the number of named storms, hurricane days, hurricane lifetime, and number of tropical cyclones making landfall due to changes in genesis location. In summary, the 2 PVU contour latitude in the western North Atlantic can therefore potentially be used as a predictor in seasonal and sub-seasonal forecasting.

How to cite: Lohmann, U., Enz, B., Neubauer, D., and Sprenger, M.: Influence of Potential Vorticity Structure on North Atlantic Tropical Cyclone Activity, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6086, https://doi.org/10.5194/egusphere-egu23-6086, 2023.

14:10–14:20
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EGU23-5825
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AS1.13
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ECS
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Virtual presentation
Amit Kumar, Atul Kumar Srivastava, and Manoj Kumar Srivastava

The precipitation characteristics of tropical cyclones (TCs) formed between 2014-2021 over the Arabian Sea during the onset phase of monsoon and after the monsoon (post-monsoon) seasons have been investigated through the space-borne dual-frequency precipitation radar of the Global Precipitation Measurement (GPM-DPR) satellite level 2, V07 observation. In a cloud that is producing precipitation, the two-dimensional frequency distribution of the liquid water content (LWC; g/m2) and non-liquid water content (IWC; g/m2) exhibits a clear seasonal and cloud-type dependence. For the precipitating cloud of stratiform origin of TCs in the monsoon and post-monsoon seasons, a significant part of rain droplets is present in the LWC limit of 0-800 g/m2 and the IWC limit of 0-350 g/m2. In contrast to the stratiform precipitation associated with the TCs, the LWC quantity is additionally more, and IWC is less for the convective origin precipitating cloud. In the monsoon and post-monsoon season, the mean values of the mass-weighted mean diameter, Dm (mm), are 1.29 (1.47) mm and 1.27 (1.31) mm, respectively, for the stratiform (convective) cyclonic cloud. It is noticed that when the value of Dm increases, the normalised intercept parameters (Nw) decrease, regardless of the season and cloud type related to the TCs. While stratiform precipitation contains a considerably high concentration of smaller-sized rain droplets during both seasons, the number concentration of bigger rain droplets is significantly high during convective precipitation. From the contoured frequency with altitude diagram (CFAD) plots for Dm and Ze for the cyclonic cloud in both seasons, we observe a large concentration of ice and supercooled liquid particles available above the melting layer and a significant concentration of rain droplets in liquid state present below the melting layer. We derived the contribution of the different microphysical processes (break-up, size-sorting, collision-coalescence, and evaporation processes) in the rain droplets formation below the melting layer. It is found that the process of collision-coalescence is predominating microphysical process for convective precipitation. The break-up process is a primary microphysical process in the precipitating cloud of stratiform origin.

 

How to cite: Kumar, A., Srivastava, A. K., and Srivastava, M. K.: GPM-DPR observed microphysical characteristics of the Arabian Sea tropical cyclone, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5825, https://doi.org/10.5194/egusphere-egu23-5825, 2023.

14:20–14:30
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EGU23-7870
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AS1.13
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Highlight
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On-site presentation
Gregory Foltz and the 2022 NOAA Saildrone Hurricane Observations Team

During the 2022 Atlantic hurricane season, uncrewed systems were used in an innovative and coordinated effort to measure the upper ocean and air-sea interface inside and outside of tropical cyclones. The main objectives were to advance understanding of air-sea interactions in and around tropical cyclones and aid forecaster situational awareness, with the ultimate goal of improving tropical cyclone intensity forecasts. The uncrewed systems included seven saildrones and five underwater gliders that operated in the western Atlantic Ocean, Caribbean Sea, and Gulf of Mexico. Nearly collocated and simultaneous measurements were acquired by an underwater glider and saildrone through the eye of Hurricane Fiona south of Puerto Rico in September. Another saildrone was directed through Fiona after it had intensified to a Category 4 Hurricane in the North Atlantic, measuring sustained winds of 35 m/s and significant wave height of 15 m. Two other saildrones obtained measurements in Fiona when it was a tropical storm east of the Caribbean and as a Category 1 hurricane north of Puerto Rico. Later in September, after Hurricane Ian made landfall in southwestern Florida and then re-intensified to a hurricane east of Florida, a saildrone was directed through its center, measuring winds of 29 m/s and an air-sea temperature difference of 8 deg. C near the Gulf Stream. This presentation gives an overview of the 2022 effort and the data acquired, discusses challenges and lessons learned, and looks toward the future of uncrewed systems observations in tropical cyclones.

How to cite: Foltz, G. and the 2022 NOAA Saildrone Hurricane Observations Team: Ocean-Atmosphere Observations from Uncrewed Saildrones and Gliders during the 2022 Atlantic Hurricane Season, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7870, https://doi.org/10.5194/egusphere-egu23-7870, 2023.

14:30–14:40
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EGU23-16737
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AS1.13
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On-site presentation
Zorana Jelenak, Joe Sapp, Paul Chang, Clayton Bjorland, James Carslwell, and Stephen Guimond

The three dimensional observations of wind and rain within the inner core of major hurricanes in the Atlantic basin were collected utilizing the Imaging Wind and Rain Profiler (IWRAP) on board a National Oceanographic Atmospheric Administration (NOAA) P-3 aircraft during the 2020-2022 hurricane seasons. With 30 m vertical and 150 m horizontal resolution, these measurements represent the highest resolution hurricane boundary layer (HBL) observations collected to date with a remote sensing instrument. State of the art IWRAP radar control and data acquisition system collects both in-phase (I) and quadrature (Q) signals for the entire observational profile. This allows for the full spectrum to be derived by utilizing a series of Fast Fourier Transforms (FFTs) on every single range gate resulting in the availability of the observations within lowest 500 m of the HBL. Previously, these observations were only possible from drop sondes. High resolution reflectivity profiles are processed into both three dimensional wind and rain products utilizing Ku- and C-band microwave observations. Over the course of three hurricane seasons, observations of 9 major hurricanes were collected and processed.

            With its unprecidented resolution these measurements are providing insights into turbulant processes within HBL of the major hurricanes and can possibly lead into new HBL parametarization of the hurricane models. Validation of measurements were carried out with flight level aircraft measurements, Step Frequency Microwave Radiometer surface wind measurements and Tail Doppler Radar 3d Wind and Reflectivity data.

            The measurement technique, collected data, validation results and data availability will be discussed and presented.

How to cite: Jelenak, Z., Sapp, J., Chang, P., Bjorland, C., Carslwell, J., and Guimond, S.: Three Dimensional Wind and Rain Aircraft-Based Observations within the Hurricane Inner Core, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16737, https://doi.org/10.5194/egusphere-egu23-16737, 2023.

14:40–14:50
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EGU23-15922
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AS1.13
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On-site presentation
Frederic Tridon, Alessandro Battaglia, and Anthony Illingworth

The WIVERN (WInd VElocity Radar Nephoscope) mission, currently under the Phase-0 of the ESA Earth Explorer program, promises to complement AEOLUS Doppler wind lidar by globally observing, for the first time, vertical profiles of winds in cloudy areas. The objective of this work is to assess the potential of WIVERN for sampling tropical cyclones from the long-term CloudSat dataset. Realistic WIVERN synthetic observations are produced thanks to the recently developed end to end simulator of the WIVERN dual-polarization Doppler conically scanning 94 GHz radar based on CloudSat reflectivity observations and ECMWF co-located winds. The resulting multi-year dataset provides statistics on how well the WIVERN mission can sample the cloud systems associated to tropical cyclones and monitor their genesis and lifecycle. The analysis of the results provides statistics for addressing the following questions for tropical systems: What is the frequency of reliable wind estimates as a function of height? What is the effect of ghost echoes produced by cross-polarization? What is the impact of noise error and how often will the 94 GHz radar signal be fully attenuated by rain?

How to cite: Tridon, F., Battaglia, A., and Illingworth, A.: The Potential of the W-band polarization diversity Doppler radar envisaged for the WIVERN mission for sampling tropical cyclones, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15922, https://doi.org/10.5194/egusphere-egu23-15922, 2023.

14:50–15:00
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EGU23-5056
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AS1.13
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ECS
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On-site presentation
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Xinyan Zhang and Weixin Xu

The radially-outward propagating, cloud-top cooling, diurnal pulse (DP) is a prominent feature in tropical cyclones (TCs) that has important implications for changes in TC structure and intensity. By using an objective diurnal-pulse identification algorithm, this study characterizes DPs both globally and regionally over various ocean basins and examines their relationships to TC structure and intensity. Active DPs (ACTDPs) occur on 52% of TC days globally. They are the most frequent over the Northwest Pacific (NWP, 60.4%). The median duration and propagation distance of ACTDPs are 12–15 h and 500–600 km, respectively. Some ACTDPs (20–25%) last longer than 18 h and propagate as far as 700–800 km. Although the mean propagation speed of ACTDPs is 11–13 m s-1, persistent ACTDPs (lasting >15 h) mostly propagate at speeds similar to internal inertial gravity waves (5–10 m s-1). Most ACTDPs initiate in the inner core overnight, in phase with inner-core deep convection. Nearly half of the ACTDPs are coupled with the outward propagation of precipitation within TCs. The TC inner-core deep convection is significantly enhanced on ACTDP days. Specifically, the 20 dBZ echo top in the upshear quadrant of TCs rises the most evidently with the occurrence of the ACTDP, leading to a more symmetric structure of the inner-core convection. The occurrence ACTDPs may promote the rapid intensification (RI) of TCs. The frequency and duration of ACTDPs are strongly correlated with the TC intensification rate. RI TCs have a markedly higher frequency of the very long-duration ACTDPs (≥15h) and longer mean pulse duration than steady-state and gradually intensifying TCs. Overall, the DP is a potentially useful signal for the RI of TCs.

How to cite: Zhang, X. and Xu, W.: Relationships of the Diurnal Pulse to Structure and Intensity of Tropical Cyclones, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5056, https://doi.org/10.5194/egusphere-egu23-5056, 2023.

15:00–15:10
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EGU23-923
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AS1.13
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ECS
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On-site presentation
Jie Sun, Ming Cai, Guosheng Liu, Ruikai Yan, and Da-Lin Zhang

The central theme of this study is to explore if and how the intensity of a tropical cyclone (TC) is related to its size. This subject has puzzled atmospheric scientists since the work of Depperman (1947) but the existence of this relationship still remains elusive. The improved understanding of the intensity-size relationship of TCs will help coastal communities to prepare for the maximum potential damage as both the intensity and size have important impacts on wind damages, storm surges, and flooding. This study considers 33 years (1988–2020) of TC records of maximum surface winds and radii of maximum and gale-force winds over the North Atlantic Basin derived from the Extended Best Track Dataset. Analysis of these TC records reveals a robust positive correlation between loss of earth and relative angular momentum. This finding together with the inspiration from the seminal work of Emanuel and his collaborators leads us to combine absolute angular momentum and its frictional loss as a radially invariant quantity, referred to as “effective absolute angular momentum” (eAAM), for radial profiles of TC surface winds. It is demonstrated that the eAAM model can reproduce the observed complex intensity-size relationship of TCs, which can be further reduced to a quasi-linear one after factoring out the angular momentum loss and the radius of maximum surface winds. The findings of this study would not only advance our understanding of the complex TC intensity-size relation, but also allow for operational assessments of TC severity and potential damage just using its outer wind information.

How to cite: Sun, J., Cai, M., Liu, G., Yan, R., and Zhang, D.-L.: Uncovering the Intrinsic Intensity-Size Relationship of Tropical Cyclones, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-923, https://doi.org/10.5194/egusphere-egu23-923, 2023.

15:10–15:20
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EGU23-6628
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AS1.13
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ECS
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On-site presentation
Evan Jones, Allison Wing, and Rhys Parfitt

As tropical cyclones (TCs) undergo extratropical transition (ET), they develop distinct frontal boundaries across the resulting extratropical cyclone. In the North Atlantic Ocean (NATL), this can often happen near the Gulf Stream (GS). Previous work has demonstrated that the GS can influence the development of fronts in midlatitude winter cyclones. The mechanisms of air-sea interactions associated with WBCs occur at multiple spatiotemporal scales, with the extent and exact nature of those interactions debated within the literature. Could the influence of the GS on frontal development in midlatitude winter storms also apply to storms undergoing ET? Here, we present both an observational-based statistical analysis, as well as results from case-study simulations, of a possible pathway for the GS to influence TCs undergoing ET via local small-scale SST gradient changes.

Composites of NATL TCs indicate that the magnitude of the GS sea surface temperature (SST) gradient in the time prior to the TC passing is significantly weaker for TCs that begin the ET process but ultimately do not complete it, compared with TCs that do complete ET. Using a simple index of the GS SST gradient strength, both the sensible heat flux gradient and, to a lesser degree, lower-tropospheric diabatic frontogenesis are shown to scale with the local SST gradient used in this index. Our results suggest that there is some support for a mechanism in which the GS SST gradient influences the sensible heat flux gradient and subsequent surface diabatic frontogenesis in the region, impacting the favorability of the environment for a passing TC to complete ET.

To investigate this possible mechanism more closely and establish causality, we use the Weather Research and Forecasting (WRF) model to test case study simulations of Hurricane Teddy as it undergoes ET near the GS. We analyze this by modifying the magnitude and strength of the local grid point-scale SST gradient strength associated with the GS in the North Atlantic in the days prior to Teddy passing over the GS. These different simulations are then compared to determine impacts in terms of the track, intensity, frontal development, strength of both the adiabatic and diabatic frontogenesis, during Teddy’s ET. These results provide insight into the dynamical mechanisms by which surface forcing could exert an influence on ET.

How to cite: Jones, E., Wing, A., and Parfitt, R.: Investigating A Potential Pathway for Gulf Stream Influence on the Extratropical Transition of North Atlantic Tropical Cyclones, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6628, https://doi.org/10.5194/egusphere-egu23-6628, 2023.

15:20–15:30
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EGU23-6747
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AS1.13
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ECS
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Virtual presentation
Zi-Qi Liu and Zhe-Min Tan

The variation of the thermodynamic cycle and energy of tropical cyclones (TCs) under vertical wind shear (VWS) is analyzed, and its associated TC thermal and dynamical structure evolutions are explored. The thermodynamic cycles extracted using the Mean Airflow as Lagrangian Dynamics Approximation (MAFALDA) method show that the maximum energy obtained by the TC decreases with the reduction of storm intensity in VWS. The thermodynamic cycles of sheared TC experience a two-stage evolution. During the early stage, the ascending branch of the MAFALDA cycle shifts toward lower entropy, which is attributed to the reduction of the entropy in the eyewall and the increase of the upward motion and entropy outside the eyewall. In the latter stage, the entropy increases, and the downward motion weakens in the ambient and upper troposphere, allowing the descending legs shifts toward high values of entropy. A backward Lagrangian diagnostic of air parcels associated with variations in thermodynamic cycles is employed to analyze the relative importance of distinct pathways. In addition to the low-, mid-, and upper-level ventilation pathways, the enhanced inner and outer rainbands, outward advection of high entropy air in mid- and upper-troposphere eyewall, the outflow layer with reduced height, and the inflow below the outflow layer are also important for the reduction of the energy gained by TC.

How to cite: Liu, Z.-Q. and Tan, Z.-M.: How Vertical Wind Shear Impacts Tropical Cyclone by Different Thermodynamic Pathways: Energetics and Lagrangian Analysis, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6747, https://doi.org/10.5194/egusphere-egu23-6747, 2023.

15:30–15:40
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EGU23-873
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AS1.13
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ECS
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On-site presentation
Ashish Navale and Karthikeyan Lanka

Cyclones lead to heavy precipitation in a very short period causing severe damage to life and socio-economy along its track. Globally, it is projected that there will be an increase in extreme weather events, which will lead to flooding in places like the Indian subcontinent because of irregular monsoon patterns and cyclonic storms. Extremely rare climatic events occasionally display unexpected phenomena, and cyclone Gulab and Shaheen's formation was one such extraordinary occurrence. Cyclone Gulab developed over the Bay of Bengal on 25th September 2021. The cyclone moved westward and made landfall on the east coast of India in the state of Andhra Pradesh on 26th September. Cyclone Shaheen formed in the North East Arabian sea from the remnants of cyclone Gulab. Although these cyclones were not particularly powerful compared to others in this region, it followed a very unusual track. As the cyclone entered the land, it started losing energy but continued to move across the Indian peninsula as a low-pressure system before emerging into the North Eastern Arabian Sea. Favorable atmospheric and oceanic factors for cyclogenesis in this region caused the system to reintensify on 1st October 2021. The system continued to move westward steadily for two days and intensified into a severe cyclonic storm, Shaheen. On 3rd October, cyclone Shaheen made landfall on the Northeastern coast of Oman and made history as the first severe cyclone to strike the Northern coast of Oman for one and a half-century.

After the landfall of cyclone Gulab, the low-pressure system sustained over land and eventually developed into cyclone Shaheen, suggesting that land was a significant source of moisture. Thus, in this study, we quantified the moisture contributed by land in the form of evapotranspiration to the cyclones Gulab and Shaheen. We used an Eulerian water tracking technique incorporated in the state-of-the-art Weather Research and Forecasting (WRF) model to track moisture. The model allows us to specify a source region of moisture originating as evapotranspiration, which can be tracked throughout the atmosphere. This moisture is tracked till it results in precipitation or advects out of the domain. The precipitation associated with this tracked moisture is termed recycled precipitation. ERA5, a fifth-generation ECMWF atmospheric reanalysis data, is used to set up the model's initial and boundary conditions. The microphysical, cumulus, and planetary boundary layer schemes used are WSM6, Kain-Fritsch, and YSU, respectively. Eulerian water tracking being one of the most accurate tracking techniques, will enable us to get accurate contributions of different regions and land use to the cyclonic system. In this study, we mainly focus on the contributions of moisture from the forested areas and understanding the role of antecedent soil moisture in sustaining the low-pressure system across the Indian landmass. Our results showed that Northeast India and Myanmar's dense vegetated regions contributed copious amounts of moisture to the cyclonic systems in the Bay of Bengal.

How to cite: Navale, A. and Lanka, K.: Role of land in the unusual track of cyclones Gulab and Shaheen, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-873, https://doi.org/10.5194/egusphere-egu23-873, 2023.

15:40–15:45
Coffee break
Chairpersons: Enrico Scoccimarro, Allison Wing
Tropical Cyclones III
16:15–16:25
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EGU23-16538
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AS1.13
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ECS
|
On-site presentation
Dependence of the rapid intensification on the initial size of tropical cyclones over the North Indian Ocean
(withdrawn)
Kasturi Singh
16:25–16:35
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EGU23-15866
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AS1.13
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ECS
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Virtual presentation
Yerni Srinivas Nekkali, Krishna Kishore Osuri, Ananda Kumar Das, and Dev Niyogi

Tropical cyclones (TCs) are one of the natural destructive weather phenomena. The accurate prediction of TC intensity is dependent on the understanding of the physical processes behind that. This study exposes the importance of microphysical (MP) processes in the rapid intensity changes of cyclones. For this, tropical cyclone simulations were made from the WRF model with a double nested (9 km-Static and 3 km-moving nests) configuration. This study shows that the heating generated by the MP processes in the TC’s inner-core region is highly (moderately) correlated with precipitated (non-precipitated) hydrometeors. During the rapid intensification (RI) period, heat-released microphysical processes such as condensation, freezing due to the accretion of liquid hydrometeors with ice particles, and deposition, etc., are dominant as compared to cooling-induced processes. In addition, the saturated envelope in the TC Phailin (2013) is responsible for more convection, heating, and hence consecutive RI episodes. While dry air intrusion hampers the prolonged RI episodes in TC Fani (2019). However, rapid weakening (RW) in TC Lehar (2013) is promoted by asymmetric, limited convection, and hence, lesser heating. During this RW period, the warm rain (ice) microphysical processes mainly produce heating (cooling).

How to cite: Nekkali, Y. S., Osuri, K. K., Das, A. K., and Niyogi, D.: Investigation of the microphysical processes during the rapid intensity changes of tropical cyclones over the Bay of Bengal: A modelling approach, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15866, https://doi.org/10.5194/egusphere-egu23-15866, 2023.

16:35–16:45
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EGU23-8613
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AS1.13
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ECS
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Virtual presentation
Sara Müller, Xiaoli Guo Larsén, and David Verelst

Tropical cyclones are associated with extreme wind speeds, enhanced turbulence, vertical wind shear, and veer. All these elements increase loads acting on structures such as wind turbines, bridges, and high-rise buildings. While most studies focus on maximal wind speeds in tropical cyclones, we analyze wind shear and veer in the lowest 300 m of the atmosphere, which is relevant for wind energy applications. We use the Weather Research and Forecasting model to model and analyze the distribution and spatial structure of wind shear and veer in Typhoon Megi (2016) at different radii. We found maximal mean shear and veer in the eyewall region. Shear and veer are on average smaller in the rainbands, but their respective distribution is positively skewed due to spatially organized outliers. These outliers are associated with convective cells and downdrafts, that propagate over structures with speeds of around 30 ms⁻¹. Consequently, structures experience rapid changes in shear and veer. We further analyze vertical cross-sections through convective cells and their propagation velocity. The study highlights differences in characteristics of the low-level wind field between the eyewall region and rainbands, which suggest distinct forces acting on structures.

How to cite: Müller, S., Guo Larsén, X., and Verelst, D.: Veer and shear in the tropical cyclone lower boundary-layer, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8613, https://doi.org/10.5194/egusphere-egu23-8613, 2023.

16:45–16:55
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EGU23-3718
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AS1.13
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On-site presentation
Hsiao-Chung Tsai, Tzu-Ting Lo, Meng-Shih Chen, Yun-Jing Chen, Jui-Ling Kuo, and Han-Yu Hsu

In this study, week-1 to week-4 forecasts of tropical cyclones (TCs) in the western North Pacific are evaluated. The CWB TC Tracking System 2.0 (Lo et al. 2021) is used to objectively detect TCs in the 46-day ECMWF ensemble (ENS) forecasts in the 2021 season and also the reforecasts during 2001-2020. Preliminary evaluations of the probabilistic TC activity forecasts in the 20-year reforecasts show promising forecast skills. The reliability diagrams indicate slight over-forecasting bias in the weeks 1-4 forecasts, and the AUCs (Area Under Curves) are ranging from 0.91 (week-1) to 0.80 (week-4). The relationship between the TC activity forecast skill and the western North Pacific summer monsoon (WNPSM) is also investigated. The WNP monsoon index (WNPMI) proposed by Wang et al. (2001) is computed to provide a measure for the summer monsoon, and the TC forecast skills are evaluated under different levels of the WNPMI. To identify the potential false alarms, a spatial-temporal track clustering technique (Tsai et al. 2019) is implemented to objectively group similar vortex tracks in the 51-member forecasts. The corresponding ensemble mean track for each cluster is then used for performing the event-based verifications after the end of season. More details about the TC forecast verifications in weeks 1-4 using the ECMWF monthly ensemble will be presented in the meeting.

How to cite: Tsai, H.-C., Lo, T.-T., Chen, M.-S., Chen, Y.-J., Kuo, J.-L., and Hsu, H.-Y.: Relationship between the Tropical Cyclone Forecast Skill and the Western North Pacific Summer Monsoon in the ECMWF Monthly Ensemble, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3718, https://doi.org/10.5194/egusphere-egu23-3718, 2023.

16:55–17:05
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EGU23-10481
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AS1.13
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ECS
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On-site presentation
Robert Nystrom and Falko Judt

The very active month of September 2020 included the formation of 10 named storms, the most on record for the month of September, and 5 concurrent tropical cyclones (TCs) in the North Atlantic basin on September 14th. The Model for Prediction Across Scales (MPAS) is used to explore potential opportunity to predict TC activity out to 4 weeks. First, the MPAS model climatology for September TC activity is established. Next, the predictability of an active September is explored using MPAS simulations with initial atmospheric and oceanic conditions from the global forecast system (GFS) and compared with MPAS climatology. MPAS simulations for 2020 are initialized over the last two weeks of August and run freely through September. The total number of TCs, TC days, accumulated cyclone energy (ACE), and the track density are each evaluated relative to observations. In addition, the simulations resulting in the most and least active month are analyzed in further detail to understand why those model simulations predicted an active or inactive September. Lastly, differences with and without a regionally refined 3 km mesh are explored.

How to cite: Nystrom, R. and Judt, F.: Predictions of North Atlantic tropical cyclone activity out to 4 weeks with global MPAS simulations, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10481, https://doi.org/10.5194/egusphere-egu23-10481, 2023.

17:05–17:15
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EGU23-3335
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AS1.13
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ECS
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On-site presentation
Stella Bourdin, Sébastien Fromang, Arnaud Caubel, Josefine Ghattas, Yann Meurdesoif, and Thomas Dubos

The availability of a new icosahedral dynamical core (DYNAMICO) for the IPSL model was the opportunity to participate in the HighResMIP protocol. We present the results of four historical 1950-2015 atmosphere-only (forced SST) simulations at horizontal resolutions equal to 200, 100, 50, and 25 km. We compare them with two simulations that use the same configuration but were performed with the previous longitude-latitude dynamical core at 250 and 75km horizontal resolutions.

We use these simulations to perform the first assessment of Tropical Cyclones (TC) in the IPSL model. This evaluation is done across four resolutions, gathering methodologies from recent literature (Roberts et al., 2020 a&b; Moon et al., 2020; Chavas et al., 2017; Camargo et al., 2020; Bourdin et al., 2022).
We first show that the results obtained with DYNAMICO compare favorably with the previous dynamical core of the IPSL model.
Then, we analyze how increasing horizontal resolution from 200km to 50km improves the TC climatology. Our results align with the current expectation that frequency and geographical distribution get closer to the observation but that the intensity is still significantly under-resolved.
In the highest-resolution simulation TC activity in the North Atlantic basin is well represented in terms of geographical distribution and inter-annual variability. However, regional biases remain, especially in the Western North Pacific, where there is a significant deficit in TC number and a shift of activity towards the east of the basin. These regional biases are robust with resolution but are not associated with any obvious climatological bias in the simulations.
Finally, we study composites, TC size, and life cycles to document the physics of the model's TCs. They show that the model simulates realistic TC structures with primary and secondary circulations, an eyewall, and a warm core. TC size diminishes with resolution and less so with intensity.

We conclude that the IPSL model is able to simulate a realistic climatology of Tropical Cyclones at 25 km horizontal resolution, with maximum intensities limited by the current maximum resolution.

How to cite: Bourdin, S., Fromang, S., Caubel, A., Ghattas, J., Meurdesoif, Y., and Dubos, T.: Tropical Cyclones in High-Resolution Global Climate Simulations with the IPSL Model, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3335, https://doi.org/10.5194/egusphere-egu23-3335, 2023.

17:15–17:25
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EGU23-3704
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AS1.13
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Virtual presentation
Difei Deng and Elizabeth Ritchie

Tropical Cyclone Debbie (2017) made landfall near Airlie Beach on 28 March 2017 causing 14 fatalities and an estimated US$2.67B economic loss and was ranked as the most dangerous cyclone to hit Australia since TC Tracy in 1974. In addition to the extreme flooding as TC Debbie moved onshore and down the east coast of Australia, it intensified rapidly just offshore from Category 2 to Category 4 in approximately 18 hours and finally made landfall as a Category 4 TC, causing widespread and disastrous damage.

 

A high-resolution WRF simulation (1-km horizontal, and 10-min temporal resolution) is used to analyze the inner-core structure and evolution during the offshore rapid intensification period in the current conditions and potential future change. In current condition, Debbie’s a rapid intensification (RI) stage is characterized by three rounds of eyewall breakdown into mesovortices and re-development events. Each round of breakdown and re-establishment brings high potential vorticity and equivalent potential temperature air back into the eyewall, re-invigorating eyewall convection activity and driving intensification. The potential future changes in the inner-core structure and eyewall evolution will also be discussed using WRF with the Coupled Model Intercomparison Project Phase 6 (CMIP6) perturbed conditions to better assess the possible TC intensity change under different climate change scenarios.

How to cite: Deng, D. and Ritchie, E.: High-resolution simulation of Tropical Cyclone Debbie (2017):The current and future changes in the inner-core structure and evolution during offshore intensification., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3704, https://doi.org/10.5194/egusphere-egu23-3704, 2023.

17:25–17:35
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EGU23-8710
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AS1.13
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ECS
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Virtual presentation
Tsz-Kin Lai and Ralf Toumi

Many aspects of tropical cyclone (TC) properties at the surface have been changing but any systematic vertical changes are unknown. Here we document a recent trend of high thick clouds of TCs. The global inner-core high thick cloud fraction measured by satellite has decreased from 2002 to 2021 by about 10% per decade. The TC inner-core surface rain rate is also found to have decreased during the same period by a similar percentage. This suppression of high thick clouds and rain has been largest during the intensification phase of the strongest TCs. Hence, these two independent and consistent observations suggest that the TC inner-core convection has weakened and that TCs have become shallower recently at least. For this period the lifetime maximum intensity of major TCs has not changed and this suggests an increased efficiency of the spin-up of TCs.

How to cite: Lai, T.-K. and Toumi, R.: Has there been a recent shallowing of tropical cyclones?, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8710, https://doi.org/10.5194/egusphere-egu23-8710, 2023.

17:35–17:45
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EGU23-1692
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AS1.13
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Highlight
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Virtual presentation
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Yuan Sun, Wei Zhong, Hongrang He, and Yao Yao

Understanding the impact of climate change on tropical cyclones (TCs) has become a hot topic. The slowdown of TC translation speed contributes greatly to the locally accumulated TC damage. While the recent observational evidence shows that TC translation speed has decreased globally by 10% since the mid-twentieth century, the robustness of the trend is questioned by other studies as effects of changes in observational capability can strongly affect the global trend. Moreover, none of the published studies considered dependence of TC slowdown on TC intensity. This is the caveat of these analyses as the effect of TC slowdown is closely related to TC intensity. Here, we investigate the relationship between TC translation speed trend and TC intensity, and reveal possible reasons for the trend. We show that the global slowing trend without weak TC moments (≤ 17 m s-1) is about double of that with weak TC moments in a recent study. This is because the slowing trend is dominated by strong TCs’ trend. Stronger (weaker) TCs tend to be controlled more by upper-level (lower-level) steering flow, and the calculated trend of upper-level steering flow is much larger than that of lower-level steering flow. This may be an important reason for the large difference between the slowing trend without weak TC moments and that with weak TC moments. Furthermore, the changes of TC tracks (including inter-basin trend and latitudinal shift), which are partly attributed to data inhomogeneity, make a much larger contribution to the slowing trend, compared with the weakening of tropical circulation, which is related to anthropogenic warming.

How to cite: Sun, Y., Zhong, W., He, H., and Yao, Y.: The slowdown tends to be greater for stronger tropical cyclones, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1692, https://doi.org/10.5194/egusphere-egu23-1692, 2023.

17:45–17:55
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EGU23-3628
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AS1.13
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Highlight
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On-site presentation
Chia-Ying Lee, Suzana Camargo, Adam Sobel, Richard Seager, Boniface Fosu, and Kevin Reed

The response of tropical cyclone activity to anthropogenic radiative forcing remains uncertain, with even the direction of the change uncertain in some respects (e.g., TC frequency), both globally and regionally. One important source of uncertainties is the Pacific zonal SST gradient. At the interannual time scale, this SST gradient, through the El Niño Southern Oscillation, is known to strongly influence global TC activity. Global climate models in CMIP5/6 generations project this SST gradient to weaken and lead to a more El Niño-like mean state in the future. Observations over the past several decades, however, show a strengthening of the SST gradient and thus a more La Niña-like mean state. While the observed strengthening of the SST gradient may be due to natural variability or merely an observational issue, some recent studies have marshalled evidence, backed up with modeling, to argue that the projected weakening is erroneous and a consequence of a common climatological cold tongue bias that has persisted in a few generations of global climate models. If the above argument is correct, at the transient forced response of Pacific SST over the upcoming decades will be towards a La Niña-like mean state, in contrast to the climate models. This means that the projected trends in TC activity from current state-of-the-art global climate models may be incorrect in some basins. In this presentation, we will report an initial investigation of the above problem using synthetic TCs from the Columbia tropical cyclone HAZard model (CHAZ) downscaled from CMIP6 models. Although all show El Niño-like forced responses, we will group the CMIP6 models/members based on the magnitudes of their climatological cold tongue biases, their historical trends of the zonal and meridional SST gradients, and the correlation between their trends and the observed one. For each stratified group, we will then evaluate SST gradient projections and how these projections affect the large-scale atmospheric and oceanic environment conditions that are important to TC activity and thus influence the forced trends in the CHAZ-CMIP6 downscaled TCs.  This work will inform on how much a potential model bias towards the wrong sign of the tropical Pacific zonal SST gradient change matters for projections of global TCs.

How to cite: Lee, C.-Y., Camargo, S., Sobel, A., Seager, R., Fosu, B., and Reed, K.: Forced trends in the tropical Pacific and global tropical cyclones: An investigation using a statistical-dynamical downscaling model, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3628, https://doi.org/10.5194/egusphere-egu23-3628, 2023.

17:55–18:00

Posters on site: Wed, 26 Apr, 10:45–12:30 | Hall X5

Chairpersons: Eric Maloney, Enrico Scoccimarro
Tropical Convection and Tropical Waves
X5.31
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EGU23-1541
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AS1.13
Enrico Scoccimarro and Daniele Peano

Rapid intensification/weakening (RI/RW) refers to a significant increase/decrease in tropical cyclone (TC) intensity over a short period of time. A TC can also undergo multiple RI/RW events during its lifetime, and these events pose a significant challenge for forecasting TC activity. In fact, RW is one major source of large intensity forecasting errors as well as RI. These processes can be associated to particular large-scale conditions, both in terms of atmospheric drivers - such as vertical wind shear or dry air intrusion - and oceanic drivers - such as sea surface temperature (SST) gradient.

In this work we aim to verify the ability of the new CMCC-CM3 model (a preliminary version of the General Circulation Model that will take part to the 7th Coupled Model Intercomparison Project - CMIP7 effort) in representing Tropical Cyclone activity with a particular focus on RI and RW. The simulations used in this work have been provided within the EU project BlueAdapt at a 25km horizontal resolution in atmosphere and ocean components, ensuring the representation of realistic TCs both in terms of spatial variability and intensity. Less agreement is found in representing RI/RW timing and duration, but better results are obtained, compared to the previous version of the model CMCC-CM2. The role of the ocean in determining RI and RW is also investigated.

How to cite: Scoccimarro, E. and Peano, D.: Rapid Intensification and Rapid Weakening of Tropical Cyclones, as represented by the CMCC-CM3 Climate Model, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1541, https://doi.org/10.5194/egusphere-egu23-1541, 2023.

X5.32
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EGU23-4002
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AS1.13
Eric Maloney, Michael Natoli, Emily Riley Dellaripa, Hien Bui, Charlotte DeMott, and Ewan Short

Environmental conditions supporting offshore propagation of diurnal precipitation near tropical coastlines are examined. In particular, the effect of the near-coastal background wind, moisture, and surface wind speed and fluxes on offshore precipitation propagation is assessed for the Philippines, northern Australia, and Panama Bight region near Colombia. Reanalysis fields, satellite precipitation, surface wind speed (from the CYGNSS satellite), and flux observations, and the Cloud Model 1 (CM1) are used in this work. In general, a moist offshore environment and enhanced wind-driven surface fluxes support offshore propagation of strong diurnal convective disturbances. Near the west coast of Luzon, a weak offshore wind in the lower free troposphere also supports offshore propagation, as often occurs in the transition phases of the boreal summer intraseasonal oscillation from suppressed to enhanced daily mean convection. Vertically-integrated moist static energy budget analysis is used to support these results. Sensitivity tests with the CM1 verify the importance of weak offshore flow and a moist offshore environment for supporting offshore propagation of diurnal precipitation.

How to cite: Maloney, E., Natoli, M., Riley Dellaripa, E., Bui, H., DeMott, C., and Short, E.: Effect of the Coastal Large-Scale Environment on the Tropical Diurnal Cycle, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4002, https://doi.org/10.5194/egusphere-egu23-4002, 2023.

X5.33
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EGU23-4126
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AS1.13
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ECS
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K. S. S. Sai Srujan, Sukumaran Sandeep, and Hariprasad Kodamana

About 60% of the rainfall during the Indian Summer Monsoon (ISM) is manifested by the synoptic-scale storms form over North Bay of Bengal (BoB) and the adjacent land area known as “Low-Pressure Systems” (LPS). Unlike tropical cyclones, the storms during this season (LPSs) are embedded in the background monsoon flow, which makes them difficult to predict, considering the chaotic nature of the monsoon. Nearly one-third of these synoptic-scale storms are formed due to the amplification of disturbance which is propagating from the Western North Pacific (WNP) (categorized as “downstream LPS”). We observed an association of tropical cyclones (TCs) originating over WNP with the genesis mechanisms of downstream LPS over the BoB. The TCs over the WNP are classified into different clusters based on different features like length, genesis location, landfall, etc., using the gaussian mixture models. We found that four major clusters of WNP TCs are responsible for triggering 83% of the downstream LPS genesis. We established a causality using the transfer entropy analysis between the fluctuations in mean sea-level pressure over BoB and the Rossby wave activity over the WNP prior to the initiation of an LPS.

Our results suggest a plausible prediction of downstream LPS at least a week ahead. The current generation of climate models has low skill in simulating the LPS; understanding the dynamics behind the genesis of LPS is the way to improve the LPS-related precipitation in climate models. The recent advancement in using AI/ML in predicting various weather and climate phenomena, including our recent study in predicting the synoptic-scale sea-level pressure using the ConvLSTM model explains the importance of dynamics-based data-driven ML models to predict complex weather patterns. Understanding the dynamics of such physical phenomena will help in identifying the appropriate predictors for the data-driven ML models.

How to cite: Srujan, K. S. S. S., Sandeep, S., and Kodamana, H.: A Dynamical Framework to Understand and Predict the Indian Summer Monsoon Low Pressure Systems, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4126, https://doi.org/10.5194/egusphere-egu23-4126, 2023.

X5.34
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EGU23-8675
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AS1.13
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ECS
Akshay Deoras, Andrew G. Turner, and Kieran M. R. Hunt

Sri Lanka is affected by extreme precipitation events every year that cause floods, landslides, and tremendous economic losses. Unlike for other countries in South Asia such as India, there has been a limited investigation of weather patterns associated with extreme precipitation events in Sri Lanka. In this study, we use the ERA5 reanalysis dataset to understand the association between extreme precipitation events and 30 weather patterns, which were originally derived to represent the variability of the Indian climate during January–December 1979–2016. Furthermore, we analyse the modulation of extreme precipitation events by the Madden-Julian Oscillation (MJO). We also use the daily rainfall data from 51 meteorological stations in Sri Lanka to take some account of the observational uncertainty.

We find that weather patterns that are most common during the northeast monsoon (December–February) and second intermonsoon (October–November) seasons produce the highest number of extreme precipitation events. Moreover, extreme precipitation events occurring during these two seasons are more persistent than those during the southwest monsoon (May–September) and first intermonsoon (March–April) seasons. The frequency of extreme precipitation events is enhanced (suppressed) in MJO phases 1–4 (5–8) for most weather patterns. The results of this study could benefit meteorologists, hydrologists, and researchers in developing forecasting products based on the identification of these weather patterns and MJO phases in numerical weather prediction and the subseasonal-to-seasonal prediction models, envisaging improved disaster preparedness in Sri Lanka.

How to cite: Deoras, A., Turner, A. G., and Hunt, K. M. R.: The influence of weather patterns and the Madden-Julian Oscillation on extreme precipitation over Sri Lanka, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8675, https://doi.org/10.5194/egusphere-egu23-8675, 2023.

X5.35
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EGU23-10920
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AS1.13
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Ma. Cathrene Lagare, Takeshi Yamazaki, and Junshi Ito

Mesoscale convective systems (MCSs) are organized clusters of convection that often bring in heavy to extreme rainfall, which can cause devastating effects such as flooding, landslides, and significant crop and infrastructural damages. Studies on severe weather in the Philippines, a part of the Maritime Continent where frequent and intense convective activities occur, focus predominantly on synoptic-scale systems (e.g., tropical cyclones). The characteristics of MCSs in the Philippines remain understudied. 

Motivated by this research gap, a long-term MCS climatology over the Philippines was constructed using the global MCS tracking database of Feng et al. (2021), and its large-scale environments are investigated to understand the formation of MCSs. Preliminary results show that large-scale flows largely affect MCS formation. MCSs occur more frequently during the peak of the Asian summer monsoon (JJA), producing large rainfall amounts over the west of the Philippines. Meanwhile, the Asian winter monsoon during DJF has a different effect on MCS formation in the Philippines as it does not directly correspond to high occurrences of MCSs. However, the convective systems during DJF still produce high rainfall amounts over the east of the Philippines. Based on these results, additional analyses for the MCSs during the boreal winter are conducted. 

 

Reference:

Feng, Z., Leung, L. R., Liu, N., Wang, J., Houze Jr, R. A., Li, J., ... & Guo, J. (2021). A global high-resolution mesoscale convective system database using satellite-derived cloud tops, surface precipitation, and tracking. Journal of Geophysical Research: Atmospheres, 126(8), e2020JD034202.

How to cite: Lagare, Ma. C., Yamazaki, T., and Ito, J.: Characteristics of Mesoscale Convective Systems in the Philippines, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10920, https://doi.org/10.5194/egusphere-egu23-10920, 2023.

X5.36
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EGU23-12516
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AS1.13
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ECS
Julia Windmiller and Bjorn Stevens

The intertropical convergence zone (ITCZ) is a central component of the global circulation system, but remarkably little is known about the dynamical and thermodynamical structure of the convergence zone itself. This is true even for the structure of the low-level convergence that gives the ITCZ its name. Following on from the major international field campaigns in the 1960s and 70s, we performed extensive atmospheric profiling of the Atlantic ITCZ during a ship-based measurement campaign aboard the research vessel SONNE in summer 2021. Combining the data we collected during our north-south crossing of the ITCZ with reanalysis data shows that there are generally two low-level convergence lines that roughly mark the southern and northern edges of the region of intense precipitation. Based on the location of these two edges, we construct a composite view of the structure of the Atlantic ITCZ. The ITCZ, far from being simply a region of enhanced deep convection, has a rich inner life, i.e., a rich dynamical and thermodynamic structure that changes throughout the course of the year and has a northern edge that differs systematically from the southern edge. 

How to cite: Windmiller, J. and Stevens, B.: The inner life of the Atlantic ITCZ, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12516, https://doi.org/10.5194/egusphere-egu23-12516, 2023.

X5.37
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EGU23-127
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AS1.13
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ECS
samira El Gdachi, Pierre Tulet, and Anne Réchou

Numerical Weather Prediction models still have difficulties to predict local-scale phenomena, such as thermal breezes circulation.  They are local driven wind systems that form over coastal zones (sea/land breeze) or mountainous terrain (slope/valley breeze), produced by the buoyancy effects associated with the diurnal cycle of heating and cooling of the lower atmospheric layers (Zardi et Whiteman, 2013). These circulations can drive abrupt changes that generate localized wind gusts, extreme precipitation, air pollution episodes in the lower layers, or sea state perturbations. 

The characteristics of the volcanic and tropical island of Reunion Island  (Indian Ocean, 21°07’S, 55°32’E) offer an exceptional natural field of investigation for these process studies. The meteorological circulations on Reunion Island have been extensively studied by Lesouëf et al. (2010), Durand et al. (2014), Tulet et al. (2017), Foucart et al. (2018), and Réchou et al. (2019). These works show that the island is affected by a regime of southeast trade winds, which is intense in winter (June-August) and moderate to weak in summer (December to February). This weather regime is the cause of intense winds on the southwest and northeast edges of the island and a branch of northwesterly leeward circulation forcing in the northwest of the island (Maïdo area). In this region, thermal circulations are added to this regional circulation. This return loop occurs almost daily in this part of the island in the boundary layer. The oceanic air masses are advected on the slopes of the Maïdo area by the sea and valley breezes. This convection on the mountain slopes causes an almost daily formation of clouds, which are generally weakly developed vertically and generally with low water content. 

An intensive measurement campaign BIOMAÏDO (Bio-physicochemistry of tropical clouds at Maïdo) took place from 11 March to April 7, 2019, at Réunion Island, in order to study the chemical and biological composition of the air mass, the formation processes of secondary organic matter in heterogeneous environments, the dynamics and the evolution of the boundary layer, and the macro and micro-physical properties of clouds.  

In this study, we detail and analyze the dynamics circulations using the observations of the campaign and compare them to a high-resolution (100m horizontal resolution) numerical simulation with the Meso-NH model. Such a model turned during the selected days in which a dynamical connection between the sites was found (Rocco et al., 2022).

The preliminary results have shown that a vertical resolution smaller than a few meters (~1m)  is needed to capture the katabatic flows and the structure of the valley boundary layer, these circulations have an abrupt variation (~1 hour) and the anabatic flow takes nearly 1 h to arrive to the top of the mountain. 

The temporal and spatial structure of this breezes regimes is analyzed with the use of the wet bulb potential temperature (Davies-Jones., 2007), and the turbulence kinetic energy budgets determined by the numerical model;  this study aims to quantify which processes have the most important role during the diurnal breeze evolution. 

How to cite: El Gdachi, S., Tulet, P., and Réchou, A.: Dynamic circulations and Windward Flow over Reunion Island, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-127, https://doi.org/10.5194/egusphere-egu23-127, 2023.

X5.38
|
EGU23-7841
|
AS1.13
|
ECS
|
Jack Mustafa, Adrian Matthews, Rob Hall, Karen Heywood, and Marina Azaneu

The meteorological diurnal cycle over the Maritime Continent is a major component of observed variability and features see-sawing of intense precipitation from over land through the afternoon and evening to over surrounding seas and oceans through the night into the morning. This high-frequency land-locked mode of variability interacts with lower-frequency propagating modes of tropical variability, such as the Madden-Julian Oscillation, therefore accurate forecasting of downstream impacts of these intraseasonal modes of variability depends on accurate understanding and model representation of the diurnal cycle.

In this presentation we compare the observed diurnal cycle of precipitation with the diurnal cycle generated by regional hindcast runs of the UK Met Office Unified Model with parameterised and with explicitly-resolved convection. A novel characterisation framework is used to quantify the cycle at each location in order to optimise the intuitive simplicity and the completeness of the characterisation.

How to cite: Mustafa, J., Matthews, A., Hall, R., Heywood, K., and Azaneu, M.: The diurnal cycle of precipitation over the Maritime Continent: characterisation in observations and models, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7841, https://doi.org/10.5194/egusphere-egu23-7841, 2023.

X5.39
|
EGU23-16512
|
AS1.13
|
ECS
|
Beata Latos, Dariusz Baranowski, Maria Flatau, Jens de Bruijn, Katarzyna Barabasz, Michał Łabuz, Donaldi Permana, and Jaka Paski

Indonesia, with its tropical and monsoonal climate, is exposed to heavy precipitation and enormous rainfall accumulation which results in weather-driven hazards, including extreme rainfall events and floods. There are several conventional sources of data to estimate potential of anomalously high precipitation in Indonesia, including rain gauge data, satellite data and meteorological reanalysis. Even though they allow assessment of precipitation variability, their usefulness is limited by biases and data gaps. Furthermore, assessment of a variability in precipitation patterns is not the same as identification of their adverse societal effects, such as floods.  

Due to the proliferation of social media, these conventional data sets can be supplemented with crowd-sourced information that can potentially provide longer-term, accurate records and cover a larger area. In this study, we demonstrated that Twitter is a useful source for flood detection and created a flood database. Twitter-based flood database is derived for subregions of major islands within Indonesia: Java, Sumatra, Borneo and Sulawesi, and validated against data from governmental reports and local paper articles. Results show that Twitter-based retrieval performs well in comparison with other sources, but only in regions characterized by sufficiently large pool of active users. 

Flood events and extreme rainfall events (defined using in-situ and satellite data) were compared in terms of their spatial and temporal distribution, as well as their meteorological drivers. In general, on each of the island, there is a seasonal cycle: a wet season during boreal winter, when the Southeast Asian monsoon provides an environment supportive of rain events, and a dry season during boreal summer. On intraseasonal scale, Madden-Julian Oscillation (MJO) creates the conditions favorable for weather extremes. MJO activity causes an increase in the local rainfall rate, with a significant increase in a chance of observing extreme precipitation during favorable MJO phase.  

How to cite: Latos, B., Baranowski, D., Flatau, M., de Bruijn, J., Barabasz, K., Łabuz, M., Permana, D., and Paski, J.: Spatial and temporal variability of floods in Indonesia based on governmental data, Twitter messages and paper reports, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16512, https://doi.org/10.5194/egusphere-egu23-16512, 2023.

X5.40
|
EGU23-16746
|
AS1.13
Srinivasa Ramanujam Kannan

Kal Baishaki, a heavy thunderstorm event, recurs yearly during the premonsoon period (March-May) over the Indo-Gangetic plain and northeastern part of India. The event is highlighted by vigorous thunderstorm activity often associated with lightning and heavy to very heavy precipitation. Though the event has significant health and economic impact, the precipitation characteristics are not clearly understood. This is mainly due to a lack of continuous observation across a vast area covering West Bengal, Jharkhand, Orissa, Bihar, Assam, and other northeastern states of India. Precipitation measuring instruments onboard the Tropical Rainfall Measuring Mission satellite, followed by the Global Precipitation Measurement Mission, have provided an unprecedented data set that provides rainfall estimates at a high spatial, but sparsely temporal scale. The accuracy of data products has been refined over the years by comparing them with measurements from ground stations worldwide. The present work aims to consider rainfall data measured between 2001 and 2020 using remotely sensed instruments to analyse the precipitation characteristics of the significant rain event using a time series analysis approach.

How to cite: Kannan, S. R.: A study on precipitation characteristics of Kal Baishakhi: a premonsoon thunderstorm event, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16746, https://doi.org/10.5194/egusphere-egu23-16746, 2023.

Tropical Cyclones
X5.41
|
EGU23-44
|
AS1.13
|
ECS
|
Rahul Raghudhas, Jayanarayanan Kuttippurath, Arun Chakraborty, and Akhila Rajeev

We investigate the changes in cyclogenesis and tropical cyclone (TC) activity by the warped life cycle of MJO triggered by the two-fold expansion of the warm pool that occurred during the period of the past 40 years (1979-2019). To study the impact of MJO on TC genesis and activity, we have used the genesis potential index (GPI), accumulated cyclonic energy (ACE) and frequency of cyclones in the active, moderately active and non-active periods of MJO in the North Indian Ocean (NIO) and Western North Pacific (WNP). We find an inverse characteristic of anomalies of relative humidity, vertical wind shear, absolute vorticity, potential intensity, GPI and sea surface temperature over the tropical region between active and non-active years of MJO (1979-2019). High TC activity is experienced during the moderately active years of MJO over the Bay of Bengal (BoB) and WNP. The impact of MJO on TC activity over WNP from October to December (OND) is not particularly dominant during the active years. The genesis of TCs over the Arabian Sea (AS) have also increased during the active years of MJO; indicating that the impact of MJO is increasing over AS. In addition, stalling of eastward propagation of MJO is noticed over the Maritime Continent (MC) during the active and moderately active MJO years. After phase 5, a strong decline in the trend of the phase duration over WNP is noticed, which can be attributed to the reduced TC genesis and activity over WNP during the MJO active years. Reduced MJO activity during OND over WNP, along with lower absolute vorticity and vertical velocity, resulted in lower TC activity and genesis. Our analysis reveals the basin dependency of TC activity and genesis over AS, BoB and WNP due to the stalled propagation of MJO over MC by the extended Indo-Pacific warm pool driven by anthropogenic activities.

How to cite: Raghudhas, R., Kuttippurath, J., Chakraborty, A., and Rajeev, A.: Influence of MJO on cyclone activity in the north Indian Ocean and Western North Pacific, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-44, https://doi.org/10.5194/egusphere-egu23-44, 2023.

X5.42
|
EGU23-16246
|
AS1.13
Jie Ming

Characterizing inflow structure is important to better represent tropical cyclone impacts in numerical models. While much research has considered the impact of storm translation on the distribution of inflow angle, comparatively less research has examined its distribution relative to the environmental wind shear. This study analyzes data from 3,655 dropsondes in 44 storms to investigate the radial and shear-relative distribution of surface inflow angle. Emphasis is placed on its relationship with intensity change. The results show that the radial variation in the inflow angle is small and not significantly dependent on the shear magnitude or intensity change rate. In contrast, the azimuthal distribution of the inflow angle shows a significant asymmetry, with the amplitude of the asymmetry increasing with shear magnitude. The maximum inflow angle is located in the downshear side. The degree of asymmetry is larger in the outer core than in the eyewall. Intensifying storms have a smaller degree of asymmetry than steady-state storms under moderate shear.

How to cite: Ming, J.: The Shear-Relative Variation of Inflow Angle and Its Relationship to Tropical Cyclone Intensification, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16246, https://doi.org/10.5194/egusphere-egu23-16246, 2023.

X5.43
|
EGU23-4734
|
AS1.13
Jinyeon Kim, Dongjin Kim, Daejoon Kim, Joohyung Son, and Dong-Ju Ham

The official tropical cyclone information in Korea includes a deterministic forecast position of TC center and its uncertainty with the 70% probability circle, which is statistically determined by previous 3 year’s operational track errors. Therefore, the current probability circle does not represent situational uncertainty. In this study, it is investigated for using an ensemble prediction system (EPS) to represent the TC position uncertainty with three different methods: circle (CIR), ellipse with an along-track and a cross-track axes (EAC), ellipse with eigenvector axes (EEV). Five single EPSs, ECMWF, NCEP, UKMO-UM, JMA and KMA-UM, and two multiple ensembles, a simple one (SME) and a calibrated one (CME) which coincides the ensemble means, were evaluated. The methods and the ensembles were verified for 5 days with the hit rate which is defined as the percentage of the observed TC central positions within circles or ellipses.
In order to verify the new probability areas as well as the operation, the hit rate which is defined as the percentage of the observed TC central positions within 70% probability circle or ellipses were used. The operational radii have over 70% hit rate, around 0.8 for all forecast times. It means that the official forecast skill is getting better year by year and the current circle is overestimated. EPS based circle or ellipse showed better performance apart from the EAC. In more detail, the CME for both circle and ellipse method outperformed the operational method until 48 forecast hours. Since the five single EPSs were under-spread at this time, the multiple ensembles could overcome this shortage. After 72 forecast hours, SME and CME are too overspread, so that a single EPS is more likely to be consistent with the 70% probability area.
Although it is definitely sure that the EPS based one is better, there are still limitations to use them. It is difficult to say which method is the best because performance of methods is different according to the forecast time and to get other organizations’ EPS data in real time. Nevertheless, utilizing ensemble for TC track is valuable information since EPS can provide the best method for estimating uncertainty. 

How to cite: Kim, J., Kim, D., Kim, D., Son, J., and Ham, D.-J.: Investigation of TC track uncertainty using multiple ensembles for the official TC forecast, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4734, https://doi.org/10.5194/egusphere-egu23-4734, 2023.

X5.44
|
EGU23-4792
|
AS1.13
Lei Yang, Xi luo, Fenghua Zhou, Dongxiao Wang, and Weiqiang Wang

The differences in the characteristics of the rapid intensification (RI) during the TCs that form in the SCS (referred as local TCs) and that
enter the SCS from the western North Pacific (WNP; referred as entering TCs) have not been well studied, which could contribute the inaccuracy
of current TC intensity forecast in the SCS. In this study, we used TC observations, reanalysis data and model experiments to analyze the RI
occurrences during local TCs and entering TCs in 1980e2016. We found that the significant interannual and interdecadal variations in RI
occurrences during local eastward-moving TCs were related to the strong intraseasonal oscillation (ISO) over the SCS and the WNP under La
Nina conditions. RI during local westward-moving TCs showed insignificant variations as a result of the complex interactions among the
monsoon trough, ISO and the large-scale circulation. RI during entering TCs showed strong interdecadal variations, with increased RI after
1997, even though the total number of entering TCs has decreased since 1997, which is a result of a higher number of entering TCs in the
northwestern quadrant of the WNP, a stronger ISO and weak vertical windshear over the SCS and east of the Philippines under negative phase of
Pacific Decadal Oscillation. The different variations and related mechanisms of RI indicates that distinct forecasting factors should be considered
for intensity prediction during local eastward- and westward-moving TCs and entering TCs.

How to cite: Yang, L., luo, X., Zhou, F., Wang, D., and Wang, W.: Characteristics of rapidly intensifying tropical cyclones in the South China Sea, 1980-2016, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4792, https://doi.org/10.5194/egusphere-egu23-4792, 2023.

X5.45
|
EGU23-5434
|
AS1.13
|
ECS
JaeDeok Lee, Eun-Chul Chang, Kosuke Ito, and Chun-Chieh Wu

This study investigated rapid intensification (RI, +30 kt in 24 h) and rapid weakening (RW, -20 kt in 24 h) for Typhoon Trami (2018) using the Weather Research and Forecasting V4.2 with the three-dimensional Price-Weller- Pinkel ocean model. As with the previous Typhoon Lan (2017) case study, the three-dimensional relative humidity field reproduced from Tropical Cyclones-Pacific Asian Research Campaign for the Improvement of Intensity Estimations/Forecasts dropsonde data and Himawari-8 satellite imagery was assimilated during every tropical cyclone dynamical initialization process. Specifically, dropsonde data obtained from two aircraft campaigns for Lan and Trami is used. Numerical results showed that compared to without this special data assimilation, Trami’s RI and RW simulations were better improved with this special data assimilation with respect to track and intensity forecasts. Around the RI period, vertical wind shear noticeably decreased and convective bursts (vertical velocity ≥ 3 m s-1 with 30 dBZ at 2 km height) significantly increased during the RI period. With these favorable ambient and storm inner-core environments, Trami quickly formed an eye structure. After RI and slow intensification periods, Trami eventually reached the Category 5 Saffir-Simpson hurricane scale. This maximum intensity was almost maintained until it had turned northwards. After that, as its translation speed significantly decreased, RW occurred with substantial upwelling. This upwelling caused a stable boundary layer and made significant asymmetry of surface heat fluxes and convective clouds. During this significant sea surface cooling period, deep convective cells were significantly suppressed in the eyewall area. As a result, Trami underwent RW during this period. To sum up, Trami’s RI may be associated with the reduction of negative dynamic forcing around the RI period, whereas Trami’s RW may be related to negative thermodynamic forcing by ocean cooling with a very slow translation speed during the RW period. More numerical results and detailed analyses of Trami’s RI and RW will be shown in the 2023 EGU General Assembly.

 

Keywords: dropsonde data assimilation, tropical cyclone dynamical initialization, rapid intensification, rapid weakening, WRF atmosphere and ocean coupled model

 

Acknowledgment

This work was funded by the Korea Meteorological Administration Research and Development Program under Grant KMI2022-00410.

 

How to cite: Lee, J., Chang, E.-C., Ito, K., and Wu, C.-C.: Effects of the Assimilation of Relative Humidity Reproduced From T-PARCII and Himawari-8 Satellite Imagery Using Dynamical Initialization and Ocean Coupled Model: A Case Study of Typhoon Trami (2018), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5434, https://doi.org/10.5194/egusphere-egu23-5434, 2023.

X5.46
|
EGU23-6157
|
AS1.13
|
ECS
|
Andrea Polesello, Caroline J. Muller, Claudia Pasquero, and Agostino N. Meroni

Wind Induced Surface Heat Exchange (WISHE) mechanism is considered very important for tropical cyclone intensification in a large part of the scientific literature([1], [2], [3] ): heat flux from the ocean increase with increasing wind speed, building up a positive feedback on the intensification.
Simple WISHE-based models of tropical intensification predict that tropical cyclones intensify up to a steady state at the Potential Intensity (PI), obtained from the balance of heat supply rate from the ocean and dissipation rate in the boundary layer and dependent on boundary conditions only ([1]). The main problem of such models is the fact that they typically drastically simplify the convective motion within the cyclone, assuming a troposphere neutral to moist convection. ([4]).
In our work we tested these predictions in idealized numerical experiments performed using the non-hydrostatic, high-resolution model System for Atmospheric Modelling (SAM). The results showed a significantly different intensity evolution, with the cyclone undergoing a oscillation in surface wind speed with peak intensity significantly lower than the PI.
This intensity evolution was related to that of the environmental conditions along the whole air column: convective heating exports latent and sensible heat in the middle-upper troposphere, increasing environmental air buoyancy and so reducing CAPE. Radiative heating from the clouds further stabilizes the upper troposphere, weakening convection and thus cyclone intensity. After the intensity decay phase the upper level air surrounding the cyclone cools down through radiation emission: entrainment of cold air by the cyclone itself rebuilts CAPE and triggers a new intensification. Despite this work showed some limits in the predictivity of WISHE theory, WISHE feedback itself was proved to be fundamental for tropical cyclone intensification with a sensitivity numerical experiment.

 

[1]  K. Emanuel et al., “Tropical cyclones,” Annual review of earth and planetary sciences, vol. 31,
no. 1, pp. 75–104, 2003

[2]  K. A. Emanuel, “An Air-Sea Interaction Theory for Tropical Cyclones. Part I: Steady-State
Maintenance.,” Journal of Atmospheric Sciences, vol. 43, pp. 585–605, Mar. 1986.

[3]  C. J. Muller and D. M. Romps, “Acceleration of tropical cyclogenesis by self-aggregation
feedbacks,” Proceedings of the National Academy of Sciences, vol. 115, no. 12, pp. 2930–
2935, 2018.

[4]  K. A. Emanuel, “The behavior of a simple hurricane model using a convective scheme based
on subcloud-layer entropy equilibrium,” Journal of Atmospheric Sciences, vol. 52, no. 22,
pp. 3960 – 3968, 1995.

How to cite: Polesello, A., Muller, C. J., Pasquero, C., and Meroni, A. N.: Intensification mechanisms of tropical cyclones, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6157, https://doi.org/10.5194/egusphere-egu23-6157, 2023.

X5.47
|
EGU23-6758
|
AS1.13
|
ECS
Xi luo, Lei Yang, Sheng Chen, Dong Liang, Johnny C. L. Chan, and Dongxiao Wang

The track of tropical cyclones (TCs) formed in the South China Sea (SCS) can be divided into eastward and westward directions. Significant decadal variation during 1980–2020 only exists in the number of eastward-moving TCs, especially during July–September, with 47% TCs moving eastward during 1994–2004 (Period II), 22% during 1980–1993 (Period I) and only 15% during 2005–2020 (Period III). This decadal change is related to the zonal shift of Western Pacific Subtropical High (WPSH). An eastward-retreated WPSH during 1994–2004 leads to upward motion and westerly flow anomaly over the northern SCS, and therefore favors TC genesis and eastward motion. The eastward-retreated WPSH is associated with a warm sea surface temperature anomaly over the tropical western-central Pacific which induces a cyclonic flow and weakens the WPSH. With the weaker modulation of WPSH, stronger intraseasonal oscillation (ISO) in the SCS during Period II favors eastward-moving TCs due to the westerly flow associated with the ISO.

How to cite: luo, X., Yang, L., Chen, S., Liang, D., Chan, J. C. L., and Wang, D.: The Decadal Variation of Eastward-Moving Tropical Cyclones in the South China Sea During 1980–2020, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6758, https://doi.org/10.5194/egusphere-egu23-6758, 2023.

X5.48
|
EGU23-15194
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AS1.13
|
ECS
Xueqi Fan

The organization and propagation of inner rainbands of landfalling Typhoon Cempaka (2021) during rapid intensification (RI) are investigated from two ground-based Doppler radars. Dual-Doppler analysis based on ground-based radars provide long-lasting high temporal and spatial three dimensional wind fields to examine the possible mechanisms for the organization of inner rainbands. In the early period when the convections were preferentially located inside the RMW, deformation plays an important role in the formation of inner rainbands. Convective cells were advected by the cyclonically rotating tropical cyclone swirling flow while being deformed into spiral shapes. In the later period when the convections were preferentially located outside the RMW, positive part of wavenumber-2 reflectivity associated with the rainband is collocated with the positive component of wavenumber-2 vorticity. The wavenumber-2 reflectivity moved at an azimuthal phase speed of 64.5% of the local tangential wind and very close to the theoretically predicted speed. It is evident that vortex Rossby wave is associated with the organization of rainband in the later stage.

How to cite: Fan, X.: Differences in the Formation and Evolution of the Inner Rainbands during the Rapid Intensification of Typhoon Cempaka (2021), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15194, https://doi.org/10.5194/egusphere-egu23-15194, 2023.

X5.49
|
EGU23-12007
|
AS1.13
Jeong-Ho Bae, Jung-Rim Lee, Seong-Hee Won, and Dong-Ju Ham

The accuracy of tropical cyclone (TC) forecasts from NWP models have been improved especially for the track. Relatively, TC intensity forecasts still include huge uncertainties though the dynamics, physics processes, and resolutions of NWP systems become higher in both horizontal and vertical. For this reason, many operational centers and academia for TC forecasts implemented statistical prediction systems and Artificial Intelligence (AI) algorithms based on long-term dynamic model forecasts for better predictions of typhoon intensity.
The National Hurricane Center (NHC) developed the Statistical Hurricane Intensity Prediction Scheme (SHIPS) which is a statistical model based on NWP forecasts (parameters from atmosphere and ocean). Also, infrared imagery from geostationary satellite is used as predictors for the regression. SHIPS is implemented for the North Atlantic and East Pacific regions. Otherwise, the Joint Typhoon Warning Center (JTWC) implemented this model for the Northwest Pacific region. Also, Korea Meteorological Administration (KMA) and Japan Meteorological Administration (JMA) developed the statistical based typhoon prediction systems (called STIPS and TIFS, respectively). However, the accuracy of these systems is not stable because it is not easy to define the tendency of NWP forecasts for TC intensity. 
The National Typhoon Center of KMA developed a new statistical model (Statistical Prediction Intensity of Korea mEteorological administrator, SPIKE) for typhoon intensity prediction based on ECMWF forecast. While the ECMWF Integrated Forecast System (IFS) has an excellent performance in forecasting track of typhoons, the intensity tends to be underestimated compared to typhoons analysis information. 
SPIKE is basically developed as a multi-linear regression model, and its predictors are extracted from the IFS forecast. The average prediction error of typhoon intensity of SPIKE in 2022 decreased by about 30% compared to the ECMWF forecasts. However, there was still a limitation, especially for cases of rapid intensification (RI). More studies to reflect real-time intensity, cloud development, center location, and prediction errors of the model are conducted. Then, the second multi-linear regression model to account for these parameters is developed. Finally, an additional improvement of about 30% was achieved. Also, the performance for RI cases developing more than 35 knots within 24 hours was greatly improved. 

How to cite: Bae, J.-H., Lee, J.-R., Won, S.-H., and Ham, D.-J.: Statical prediction system of the typhoon intensity using Numerical Weather Prediction model for correction, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12007, https://doi.org/10.5194/egusphere-egu23-12007, 2023.

X5.50
|
EGU23-3390
|
AS1.13
|
ECS
Xiaoqi Zhang, Gregor C Leckebusch, and Kelvin S Ng

Tropical cyclone activity usually negatively affects people’s lives in coastal countries, especially in East-Asia including China. In order to reduce the effect, various approaches are utilized to study tropical cyclones. The Murray and Simmonds Cyclone Tracking algorithm, which has been mainly used in tracking extratropical cyclones, is, for the first time, applied to detect and track tropical cyclones in the West-Pacific based on mean sea level pressure data. Since this algorithm only requires one variable field as input, if it would achieve similar performance as other more complex tracking algorithms, this could be a good algorithm to use for construction of large physically consistent tropical cyclone event sets.

In the presentation, a preliminary evaluation on the performance of the Murray and Simmonds Cyclone Tracking algorithm on tracking tropical cyclones in West-Pacific, using ERA5 and IBTrACS, will be presented. The sensitivity of the performance of the algorithm on different parameter settings will also be discussed. Furthermore, the added value of combining the Murray and Simmonds Cyclone Tracking algorithm and an impact-based storm tracking algorithm, WiTRACK, from the disaster risk reduction and mitigation perspective will also be demonstrated.

How to cite: Zhang, X., Leckebusch, G. C., and Ng, K. S.: Objective Tracking of Tropical Cyclones and their Impact on Relevant Wind Fields in the West-Pacific for Construction of Physically Consistent Event Sets, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3390, https://doi.org/10.5194/egusphere-egu23-3390, 2023.

X5.51
|
EGU23-14848
|
AS1.13
|
ECS
|
Yegor Hudozhnik and Andreas Windisch

Tropical Cyclones (TCs) are extremely dangerous and destructive events which pose a danger to human lives every year. Conventional TC forecasting methods are computationally intensive and require a relatively large amount of energy and time.

In the light of climate change due to the process of global warming, the behavior of TCs may change, and therefore require the use of modern, more flexible learning methods for estimation and forecasting.

In recent years, the study of the application of Deep Learning (DL) in this area proved to be highly effective. These methods are designed to facilitate the prediction process, as well as automatically detect possible trends that may occur over time.

In this work, an application of neural networks such as LSTMs and GRUs is investigated to forecast tracks and classify the evolution of TC systems using satellite image data series as an input, where historical track data and the satellite image data are used to train the network. Particular attention is paid to adaptivity of DL approaches to recent trends and edge cases.

How to cite: Hudozhnik, Y. and Windisch, A.: Multivariate forecasting of tropical cyclones using combined neural networks., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14848, https://doi.org/10.5194/egusphere-egu23-14848, 2023.

X5.52
|
EGU23-14195
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AS1.13
|
ECS
Kyoungmin Kim, Donghyuck Yoon, Dong-Hyun Cha, and Jungho Im

The tropical cyclone (TC) tracks are usually simulated with the numerical models, which have an intrinsic error, although the performance of numerical models is continuously improving. Recently, machine learning has been suggested as a good tool to correct the intrinsic error of the model outputs. This study used an artificial neural network (ANN) to correct the error of TC tracks hindcasted by the Weather Research and Forecasting (WRF) model over the western North Pacific (WNP). TCs whose intensity was higher than tropical depression (i.e., tropical storm, severe tropical storm, and typhoon) from June to November were hindcasted, and TC positions at 72 h were set as the target of bias correction. WRF model output, best track data, and wind field of reanalysis were used as input variables of ANN. The structure of ANN was optimized for TCs during 2006-2015, and the optimized ANN was verified for TCs from 2016-2018. In the verification of ANN, TCs were classified using k-mean clustering to analyze the results of bias correction because the performance of the numerical model for the TC track varied depending on the region of WNP. The ANN corrected the error of WRF by 8.81% for four clusters where ANN was most effective. Moreover, the post-processing was applied to other clusters with less effect of ANN. Consequently, ANN with post-processing improved the accuracy of WRF by 4.34%.

How to cite: Kim, K., Yoon, D., Cha, D.-H., and Im, J.: Machine learning-based bias correction for tropical cyclone track simulation of the WRF model over the western North Pacific, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14195, https://doi.org/10.5194/egusphere-egu23-14195, 2023.

X5.53
|
EGU23-10
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AS1.13
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ECS
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Pubali Mukherjee and Balaji Ramakrishnan

 

The Northern Indian Ocean has witnessed the genesis of several devastating cyclones over the years due to the typical warm climate. The effect of climate change on these cyclones is an essential topic of research owing to the socio-economic impacts of these cyclones on the coastlines. Climate change is expected to influence the various synoptic parameters of these storms, like translational speed, intensification, frequency, etc. Most of the studies about the impact of climate change on cyclones have been done related to the Atlantic and Pacific Oceans; very few have explored the storms of the Indian Ocean in this context. Considering this context, the present study attempts to understand the track, intensity, and synoptic parameters of Tropical cyclone Vayu-June 2019 under the climate change scenario of RCP 8.5 with the Community Earth Systems Model, CESM data simulated with GPU-based WRF-ARW model. The model is simulated at a 9km single domain with a selected set of physical settings based on the previous studies on the cyclones of the Northern Indian Ocean. The track and intensity of the simulated storm are compared with the present-day hurricane Vayu from the IMD best track estimates.

Interestingly under RCP 8.5, unlike the present-day cyclone Vayu, under RCP 8.5, Vayu would have made landfall along the west coast of India with a sustained wind speed of ~ 15 m/s w. At the same time, he presents a scenario in Vayu weakened over the ocean due to several interactions with the mid-latitude westerlies. The results indicate a considerable change in the future thermodynamics under which Vayu sustained the intensity till landfall. Under RCP 8.5 simulations, the initial posting error is high; other than that, the coming cyclone Vayu seemed to follow a similar track as the present-day storm except for the landfall.

Regarding wind speed intensity, Vayu under RCP 8.5 shows equal wind intensity as that of the present day, with similar underestimation at the mature stage of the storm. The initial results of this study indicate that changes in large-scale thermodynamics in future warming scenarios can influence the modulations in track and intensity of a very severe cyclonic storm like Vayu. Such results highlight the importance of closely monitoring Arabian Sea cyclones to understand the impending disaster mitigations under probable warming scenarios.

 

 

How to cite: Mukherjee, P. and Ramakrishnan, B.: Tropical cyclone Vayu under climate change scenario RCP 8.5, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10, https://doi.org/10.5194/egusphere-egu23-10, 2023.

X5.54
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EGU23-11096
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AS1.13
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ECS
Ho Thi Ngoc Huyen and Jin-Ho Yoon

This study investigated long-term changes of tropical cyclones (TCs) activity and rainfall produced by TCs, hereafter TC-rainfall, over Vietnam in the period of 1979-2021. Furthermore, it is investigated how rainfall changes are influenced by the North Pacific (NP) pattern.

First, it was found that TC activity has not changed significantly in its frequency, and its related rainfall including TC and tropical depression (TDs) produced rainfall, hereafter TCTD-rainfall over entire Vietnam. On the other hand, significant increasing trend of TC activity (XX per 10 year), TC-rainfall (65 mm per 10 year), TCTD-rainfall (75 mm per 10 year), and total rainfall (440 mm per 10 year) is found in North Vietnam in the period of 1979-2021. However, TC, TD activity, TC-rainfall and TCTD-rainfall did not show any significant trends in Central and South Vietnam. Second, decadal and inter-decadal variation of rainfall in North Vietnam are significantly correlated with the North Pacific (NP) pattern during autumn season (October-December). Negative (positive) phases of the NP is characterized by a low (high) sea level pressure (SLP) located over the northern North Pacific Ocean and anomalously warm (cold) sea surface temperature (SST) over central and eastern tropical Pacific, resulting in to less (more) TCs activity and rainfall events over the South China Sea and Vietnam. In summary, it is found that rainfall produced by TCTD exhibits significantly increasing trend in North Vietnam, as well as total rainfall and the NP pattern plays an active role in altering rainfall anomalies in North Vietnam in the decadal and inter-decadal timescales.

How to cite: Thi Ngoc Huyen, H. and Yoon, J.-H.: Tropical Cyclone produced rainfall trends in Vietnam and their relationship with the North Pacific (NP) pattern during 1979 – 2021, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11096, https://doi.org/10.5194/egusphere-egu23-11096, 2023.

X5.55
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EGU23-4664
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AS1.13
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ECS
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Xin Huang, Johnny C. L. Chan, Ruifen Zhan, Zifeng Yu, and Rijin Wan

Persistent heavy rainfall produced by western North Pacific (WNP) tropical cyclones (TCs) can lead to widespread flooding and landslides in Asian countries. On July 2021, unprecedent rainfall amount occurred when Typhoon In-fa passed through the highly populated eastern China. While the associated synoptic features have been analyzed, the extreme characteristics and return periods of rainfall induced by In-fa remain unexplored. Analyses of rainfall data from a WNP TC database of the China Meteorological Administration (CMA) show that Typhoon In-fa not only produces record-breaking rainfall accumulations at individual surface stations, but generates unprecedent rainfall amounts for the whole area of eastern China. Quantitatively, 2, 4, 11, 24 and 55 stations are exposed to once in 200-, 100-, 50-, 20- and 10-year extreme TC rainfall accumulations, respectively, and total rainfall at 75 stations reaches a record high since 1980. Overall, the return period is up to ~481 years for the total rainfall amount accumulated in eastern China during the 1980-2019 baseline. The extremely long rainfall duration is identified as key to the torrential rains in the Yangtze River Delta before In-fa changes its direction of movement from northwestward to northeastward, while the extreme rain rate plays a dominant role in the northern areas afterwards. Probabilities of occurrence of such an unprecedented TC rainfall event have increased in most (~75%) of the eastern China during the period of 2000-2019 compared with those during 1980-1999. Our study highlights the likely increase in risk of extreme TC-induced rainfall accumulations which should be considered in disaster risk mitigation.

How to cite: Huang, X., Chan, J. C. L., Zhan, R., Yu, Z., and Wan, R.: Record-breaking rainfall accumulations in eastern China produced by Typhoon In-fa (2021), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4664, https://doi.org/10.5194/egusphere-egu23-4664, 2023.