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, 24–28 Apr 2023, EGU23-8949, https://doi.org/10.5194/egusphere-egu23-8949, 2023.

EGU23-8949

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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, 24–28 Apr 2023, EGU23-2007, https://doi.org/10.5194/egusphere-egu23-2007, 2023.

EGU23-2007

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The mechanism behind the northward propagation of the Boreal Summer Intraseasonal Oscillation (BSISO) over South Asia is still elusive, though many theories have been proposed to explain it. In this study, column- integrated moisture budget and vorticity budget analyses are used on NCEP-NCAR1 and ERA5 reanalyses data to understand the dominant propagation mechanism. While traditional theories have focused on the importance of monsoon winds' vertical easterly shear (via vortex tilting) in preconditioning the atmosphere north of the convection centre by inducing a barotropic vorticity anomaly that supports boundary layer convergence, recent work based on the 'moisture mode' framework suggests that moistening ahead of convection by horizontal moisture advection is necessary for the convection to move northward. In this study, we show that both vortex tilting and moisture advection are essential for northward propagation. The moisture budget analysis shows that when the organized convection first appears over the Indian Ocean, easterlies associated with the Rossby wave response to the convection act on the climatological moisture distribution. This moisture gradient is large along the anomalous wind direction over the Arabian Sea and leads to rapid moistening in the southern Arabian Sea region, while over the Bay of Bengal, this process is not as effective as the moisture gradient is not aligned with the anomalous wind direction over the whole Bay. Thus, the characteristic northwest-southeast oriented convection observed in the BSISO over South Asia comes into being. Hence, a tilted belt of strong convection is present behind the dry anomalies associated with subsidence, and as the convection is associated with strong ascent, the vortex tilting term also acquires a tilted structure aligned with the meridional gradient in vertical velocity. The vorticity budget clearly shows tiling is the dominant term that matches the tendency and leads to the vorticity anomalies, and the dominant term in tilting is the term associated with the meridional gradient of anomalous vertical velocity and vertical shear of zonal mean wind. The pronounced northward movement of vorticity anomalies over the Arabian Sea (more than over the Bay) further increases the tilt of the gyre associated with the convection, thus, winds over the northern Indian landmass, particularly above 20N become south-easterly. These anomalous winds tap into the north-westward moisture gradient that is present over much of the northern Indian landmass and helps move the convection further north over the northern part of the subcontinent. Thus, these two processes together pull the convection band northward over the South Asian monsoon region from the Indian Ocean and generate the NW-SE-oriented structure of the BSISO. We also show the vertical structures of the budget terms, which reveals that the moisture advection is primarily important in the middle to lower free troposphere. 

How to cite: Ghatak, S. and Sukhatme, J.: Northward Prpogating BSISO over South Asia: connection between moisture advection and vortex tilting , EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-578, https://doi.org/10.5194/egusphere-egu23-578, 2023.

EGU23-578

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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, 24–28 Apr 2023, EGU23-6549, https://doi.org/10.5194/egusphere-egu23-6549, 2023.

EGU23-6549

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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, 24–28 Apr 2023, EGU23-4550, https://doi.org/10.5194/egusphere-egu23-4550, 2023.

EGU23-4550

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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 waves. 2390 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, 24–28 Apr 2023, EGU23-845, https://doi.org/10.5194/egusphere-egu23-845, 2023.

EGU23-845

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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, 24–28 Apr 2023, EGU23-5340, https://doi.org/10.5194/egusphere-egu23-5340, 2023.

EGU23-5340

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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, 24–28 Apr 2023, EGU23-2020, https://doi.org/10.5194/egusphere-egu23-2020, 2023.

EGU23-2020

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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, 24–28 Apr 2023, EGU23-15063, https://doi.org/10.5194/egusphere-egu23-15063, 2023.

EGU23-15063

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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, 24–28 Apr 2023, EGU23-6722, https://doi.org/10.5194/egusphere-egu23-6722, 2023.

EGU23-6722

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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, 24–28 Apr 2023, EGU23-3655, https://doi.org/10.5194/egusphere-egu23-3655, 2023.

EGU23-3655

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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, 24–28 Apr 2023, EGU23-3995, https://doi.org/10.5194/egusphere-egu23-3995, 2023.

EGU23-3995

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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, 24–28 Apr 2023, EGU23-4491, https://doi.org/10.5194/egusphere-egu23-4491, 2023.

EGU23-4491

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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, 24–28 Apr 2023, EGU23-15259, https://doi.org/10.5194/egusphere-egu23-15259, 2023.

EGU23-15259

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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, 24–28 Apr 2023, EGU23-2857, https://doi.org/10.5194/egusphere-egu23-2857, 2023.

EGU23-2857

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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, 24–28 Apr 2023, EGU23-75, https://doi.org/10.5194/egusphere-egu23-75, 2023.

EGU23-75

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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, 24–28 Apr 2023, EGU23-10985, https://doi.org/10.5194/egusphere-egu23-10985, 2023.

EGU23-10985

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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, 24–28 Apr 2023, EGU23-1799, https://doi.org/10.5194/egusphere-egu23-1799, 2023.

EGU23-1799

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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, 24–28 Apr 2023, EGU23-1258, https://doi.org/10.5194/egusphere-egu23-1258, 2023.

EGU23-1258

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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, 24–28 Apr 2023, EGU23-4688, https://doi.org/10.5194/egusphere-egu23-4688, 2023.

EGU23-4688

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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, 24–28 Apr 2023, EGU23-6086, https://doi.org/10.5194/egusphere-egu23-6086, 2023.

EGU23-6086

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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, D_m (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 D_m 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 D_m and Z_e 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, 24–28 Apr 2023, EGU23-5825, https://doi.org/10.5194/egusphere-egu23-5825, 2023.

EGU23-5825

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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, 24–28 Apr 2023, EGU23-7870, https://doi.org/10.5194/egusphere-egu23-7870, 2023.

EGU23-7870

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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, 24–28 Apr 2023, EGU23-16737, https://doi.org/10.5194/egusphere-egu23-16737, 2023.

EGU23-16737

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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, 24–28 Apr 2023, EGU23-15922, https://doi.org/10.5194/egusphere-egu23-15922, 2023.

EGU23-15922

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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, 24–28 Apr 2023, EGU23-5056, https://doi.org/10.5194/egusphere-egu23-5056, 2023.

EGU23-5056

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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, 24–28 Apr 2023, EGU23-923, https://doi.org/10.5194/egusphere-egu23-923, 2023.

EGU23-923

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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, 24–28 Apr 2023, EGU23-6628, https://doi.org/10.5194/egusphere-egu23-6628, 2023.

EGU23-6628

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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, 24–28 Apr 2023, EGU23-6747, https://doi.org/10.5194/egusphere-egu23-6747, 2023.

EGU23-6747

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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 25^th September 2021. The cyclone moved westward and made landfall on the east coast of India in the state of Andhra Pradesh on 26^th 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 1^st October 2021. The system continued to move westward steadily for two days and intensified into a severe cyclonic storm, Shaheen. On 3^rd 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, 24–28 Apr 2023, EGU23-873, https://doi.org/10.5194/egusphere-egu23-873, 2023.

EGU23-873

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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, 24–28 Apr 2023, EGU23-15866, https://doi.org/10.5194/egusphere-egu23-15866, 2023.

EGU23-15866

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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, 24–28 Apr 2023, EGU23-8613, https://doi.org/10.5194/egusphere-egu23-8613, 2023.

EGU23-8613

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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, 24–28 Apr 2023, EGU23-3718, https://doi.org/10.5194/egusphere-egu23-3718, 2023.

EGU23-3718

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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 14^th. 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, 24–28 Apr 2023, EGU23-10481, https://doi.org/10.5194/egusphere-egu23-10481, 2023.

EGU23-10481

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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, 24–28 Apr 2023, EGU23-3335, https://doi.org/10.5194/egusphere-egu23-3335, 2023.

EGU23-3335

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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, 24–28 Apr 2023, EGU23-3704, https://doi.org/10.5194/egusphere-egu23-3704, 2023.

EGU23-3704

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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, 24–28 Apr 2023, EGU23-8710, https://doi.org/10.5194/egusphere-egu23-8710, 2023.

EGU23-8710

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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, 24–28 Apr 2023, EGU23-1692, https://doi.org/10.5194/egusphere-egu23-1692, 2023.

EGU23-1692

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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, 24–28 Apr 2023, EGU23-3628, https://doi.org/10.5194/egusphere-egu23-3628, 2023.

EGU23-3628

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