AS1.5

How to cite: Knippertz, P., Dias, J., Fink, A. H., Gehne, M., Kiladis, G., Kikuchi, K., Methven, J., Rasheeda Satheesh, A., Roundy, P. E., Schlueter, A., Wheeler, M. C., Woolnough, S. J., Yang, G.-Y., and Žagar, N.: The Art of Identifying Equatorial Waves, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-724, https://doi.org/10.5194/egusphere-egu21-724, 2021.

Convectively coupled equatorial Kelvin waves (CCKWs) are tropical weather systems that bring high impact weather and flooding, particularly in the Maritime Continent. They are a key component of the tropical climate system through scale interactions with other phenomena such as the Madden--Julian oscillation (MJO). CCKWs share many key features with theoretical, dry, linear equatorial Kelvin waves, such as a predominantly zonal component of their horizontal wind anomalies, and eastward propagation. Here, a vorticity budget for CCKWs is constructed using reanalysis data, to identify the basic mechanisms of eastward propagation and the observed growth. The budget is closed, with a small residual. Vortex stretching, from the divergence of the Kelvin wave acting on planetary vorticity (the -f D term), is the sole mechanism by which the vorticity structure of a theoretical Kelvin wave propagates eastward. This term is also the key mechanism for the eastward propagation of CCKWs, but its different phasing also leads to growth of the CCKW. However, unlike in the theoretical wave, other vorticity source terms also play a role in the propagation and growth of CCKWs. In particular, vortex stretching from the divergence of the CCKW acting on its own relative vorticity (the -ζ D term) is actually the largest source term, and this contributes mainly to the growth of the CCKW, as well as to eastward propagation. Horizontal vorticity advection (and to a lesser extent, vertical advection), counters the vortex stretching, and acts to retard the growth of the CCKW. The tilting of horizontal vorticity into the vertical also plays a role. However, the meridional advection of planetary vorticity (the -β v term, the main mechanism for westward propagation of Rossby waves), is negligible. The sum of the source terms in this complex vorticity budget leads to eastward propagation and growth of the CCKWs. The implications for numerical weather prediction, forecasting and climate simulations are discussed.

How to cite: Matthews, A.: Propagation and growth mechanisms for convectively coupled equatorial Kelvin waves, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-547, https://doi.org/10.5194/egusphere-egu21-547, 2021.

Recent studies found that the coupling of equatorial waves to convection is key to improving weather forecasts in the tropics on the synoptic to the subseasonal timescale but many models struggle to realistically represent this coupling. To study the underlying mechanisms of convectively coupled equatorial waves, we use aquaplanet simulations with the ICOsahedral Nonhydrostatic (ICON) model in a tropical channel configuration with a horizontal grid spacing of 13 km and with a prescribed zonally symmetric, latitudinally varying sea surface temperature. We compare simulations with parameterized and explicit deep/shallow convection. Using wave identification tools that are based on Fourier filtering in time and space and on projections of dynamical fields on theoretical wave patterns, we observe a predominance of equator-symmetric equatorial waves such as Kelvin waves and slow large-scale variability resembling the Madden-Julian Oscillation.

To diagnose interactions between the equatorial waves and convection, we use a moist static energy (MSE) framework. A budget analysis for column integrated MSE shows that spatial anomalies of the net shortwave and longwave radiation and the surface enthalpy flux increase the spatial variance of the column MSE, while advection dampens variability. For wave-convection coupling we employ a wave composite technique for the terms of the MSE budget. Results from this analysis will be presented at the conference. The same filtering tools and diagnostics are applied to a realistic ICON simulation with a 2.5 km horizontal grid spacing from the DYnamics of the Atmospheric general circulation Modeled On Non-hydrostatic Domains (DYAMOND) project.

How to cite: Jung, H., Knippertz, P., Hoose, C., Ruckstuhl, Y., Redl, R., and Janjic, T.: Disentangling the mechanisms of wave-convection coupling using idealized simulations in a tropical channel: wave composites of the moist static energy budget, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9388, https://doi.org/10.5194/egusphere-egu21-9388, 2021.

Rainfall variability over West Africa remains a major challenge for numerical weather prediction (NWP). Due to the largely stochastic and sub-grid nature of tropical convection, current NWP models still fail to provide reliable precipitation forecasts – even for a 1-day leadtime – and are barely more skillful than climatology-based forecasts. Thus, several recent studies have investigated the presumably more predictable influence of tropical waves on environmental conditions for convection and found distinct and coherent (thermo-)dynamical patterns depending on the type and phase of the wave. Of particular interest in this context is the interaction of the wave with the lifecycle of usually westward propagating mesoscale convective systems (MCSs), which are the major providers of rain in the region and can occasionally even lead to flooding. The exact mechanisms and strength of this interaction are still not entirely known.

This study combines two recent datasets in a novel way in order to systematically investigate the influence of tropical waves on MCS characteristics and lifecycle. First, MCSs are tracked within northern tropical Africa (20°W-30°E / 2°-15°N) over an 11-year period during the West African rainy season (April-October) using infrared brightness temperature fields provided by the Spinning enhanced visible and infrared imager (SEVIRI). Second, tropical waves are isolated by applying a filtering method in the wave-frequency domain to precipitation data of the Tropical Rainfall Measuring Mission (TRMM) within the 5°-15°N latitude band for the same target period. By combining the two datasets in space and time, the magnitude and phase of each wave is known at every timestep of the MCS tracks, which enables a systematic investigation of MCS characteristics as a function of wave properties.

Preliminary results suggest that long-lived MCSs (lifetime ≥ 12h) frequently couple with the “wet” phase of high-frequency tropical waves, in particular Kelvin, eastward inertia-gravity (EIG), and African easterly waves (AEW). Showing an enhanced occurrence frequency of MCS initiation, the wet phase of AEWs appears to have strong modulation capabilities during the genesis stage and further accompanies these long-lived MCSs during their entire lifetime. In the case of Kelvin waves and EIGs, the wet phase overlaps only with the intensification and maturity stage of these MCSs as a consequence of opposite directions of movement. Similar coupling patterns also exist for mixed Rossby gravity waves (MRGs), although to a weaker extent. Furthermore, no consistent coupling tendencies with long-lived MCSs are evident for low-frequency waves (Madden-Julian Oscillation (MJO), equatorial Rossby wave (ER)), arguably since they act on larger spatio-temporal scales. For short-lived MCSs (lifetime < 6h), the coupling with high-frequency waves is substantially weaker.

In the future we will also address potential influences of wave-wave interactions on MCSs as well as potential differences in coupling mechanisms between the Guinea Coast region and the Sahel farther north. With increasing efforts in the prediction of tropical waves, this study has the potential to aid the short-term forecasting of MCS development and its lifecycle. This can be of particular importance for the anticipation of extreme rainfall events and subsequent risk assessment in West Africa.

How to cite: Maranan, M., Schlueter, A., Fink, A. H., and Knippertz, P.: Influence of Tropical Waves on the Lifecycle of Mesoscale Convective Systems over West Africa, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10716, https://doi.org/10.5194/egusphere-egu21-10716, 2021.

Indian monsoon low-pressure systems (LPSs) are synoptic-scale cyclonic vortices that produce around half of the summer monsoon rainfall over India, and often cause catastrophic floods. Thus, accurate predictions of LPSs are crucial for disaster management and long-term planning. To improve the skill of LPS forecasts, it is important to understand how seasonal forecast models simulate the structure and behaviour of these weather systems. Here we examine in detail the simulation of the structure of LPSs by eleven models of the Subseasonal-to-Seasonal (S2S) prediction project. We use a feature-tracking algorithm to identify LPSs in all S2S models during a common re-forecast period of June–September 1999­­–2010. We then generate composite horizontal and vertical structures and compare them with those of LPSs in ERA-Interim reanalysis. 

The results suggest that LPSs have the weakest intensity as well as precipitation in the Bureau of Meteorology (BoM), Australia, Hydrometeorological Centre of Russia (HMCR) and Japan Meteorological Agency models. Most S2S models simulate the warm-over-cold core structure that is commonly observed in LPSs, except for the BoM and HMCR models, which simulate weak positive temperature anomalies near the LPS centre in the lower troposphere. The vertical structure of relative vorticity is shallower and weaker in all S2S models than in ERA-Interim. In most S2S models, LPS composites feature a drier middle and upper troposphere than in ERA-Interim. There is a strong positive correlation between precipitation and the 925 hPa temperature anomaly in most S2S models and ERA-Interim supporting the hypothesis that evaporative cooling from precipitation and reduced insolation due to significant cloud cover are responsible for the lower-tropospheric cold core.

How to cite: Deoras, A., Hunt, K. M. R., and Turner, A. G.: The structure of Indian monsoon low-pressure systems in the subseasonal-to-seasonal prediction models, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3300, https://doi.org/10.5194/egusphere-egu21-3300, 2021.

In this paper, the processes behind severe convective events over the Arabian Peninsula during spring and autumn seasons and their local-scale impacts are investigated using reanalysis data, satellite-derived and observational products. The focus on the transition seasons is justified as Mesoscale Convective Systems (MCSs) are more common at that time of the year, in particular in the months of March and April. The analysis of 48 events from 2000 to 2019 revealed that they are triggered by low-level wind convergence and moisture advection from the Arabian Sea, Arabian Gulf and/or Red Sea. An equatorward displacement and strengthening of the subtropical jet also precondition the environment, as does the presence of a mid-level trough. The latter is generally part of a large-scale pattern of anomalies that are equivalent barotropic in nature, and therefore likely a response to tropical or subtropical forcing. At more local-scales, a drying of the mid-troposphere between 850 and 250 hPa typically by 50%, a reduction of the upper-level winds by about 5 m s^-1, and an increase in the upper-tropospheric and lower-stratospheric temperature on averaged by 2-3 K, are typically observed during a MCS event. Over the 20-year period, a statistically significant increase in the MCSs’ spatial extent, intensity and duration over the UAE and surrounding region has been found, suggesting that such extreme events may be even more impactful in a hypothetical warming world. The rainfall they generate, on the other hand, shows an increase that is not statistically significant.

How to cite: Nelli, N. R., Francis, D., Fonseca, R., Abida, R., Weston, M., Wehbe, Y., and Al Hosary, T.: The Atmospheric Controls of Extreme Convective Events over the Southern Arabian Peninsula during the Transition Seasons, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3234, https://doi.org/10.5194/egusphere-egu21-3234, 2021.

The climatological state and the seasonal variability of the Arabian Heat Low (AHL) and the Intertropical Discontinuity (ITD) are investigated over the Arabian Peninsula using the 1979-2019 ERA-5 reanalysis data. The AHL is a summertime feature, mostly at 15º-35ºN and 40º-60ºE, exhibiting a clear strengthening over the last four decades in line with the observed increase in surface temperature. However, no clear shift in its position is detected. The AHL has a center over the Arabian Gulf and eastern Arabian Peninsula, co-located with the highest surface temperatures, and another over central Saudi Arabia, driven by low-level wind convergence and subsequent increase in atmospheric thickness. The ITD is the boundary between the hot and dry desert air and the cooler and more moist air from the Arabian Sea. It lies along the Arabian Peninsula’s southern coastline in the cold season but reaches up to 28º N between 50º - 60º E in the summer months. While the former has a rather small diurnal variability, the latter shows daily fluctuations of up to 10º. The ITD exhibited a weak northward migration in the 41-year ERA-5 period, likely driven by the increased sea surface temperatures in the Arabian Sea. On interannual timescales, the El Niño-Southern Oscillation, the Indian Ocean Dipole, and solar-geomagnetic effects play an important role in the AHL’s and ITD’s variability.

How to cite: Fonseca, R., Francis, D., Nelli, N., and Thota, M.: Climatology of the Heat Low and the Intertropical Discontinuity in the Arabian Peninsula, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3670, https://doi.org/10.5194/egusphere-egu21-3670, 2021.

The impact of diurnal precipitation over Sumatra Island, the Indonesian Maritime Continent (MC), on synoptic disturbances over the eastern Indian Ocean is examined using high-resolution rainfall data from the Global Satellite Mapping of Precipitation project and the Japanese 55-year Reanalysis data during the rainy season from September to April for the period 2000–2014. When the diurnal cycle is strong, the high precipitation area observed over Sumatra in the afternoon migrates offshore during nighttime and reaches 500 km off the coast on average. The strong diurnal events are followed by the development of synoptic disturbances over the eastern Indian Ocean for several days, and apparent twin synoptic disturbances straddling the equator develop only when the convective center of the Madden–Julian Oscillation (MJO) lies over the Indian Ocean (MJO-IO). Without the MJO, the synoptic disturbances develop mainly south of the equator. The differences in the locations and behaviors of active synoptic disturbances are related to the strength of mean horizontal winds in the lower troposphere. During the MJO-IO, the intensification of mean northeasterly winds in the northern hemisphere blowing into the organized MJO convection in addition to mean southeasterly winds in the southern hemisphere facilitate the formation of the twin disturbances. These results suggest that seed disturbances arising from the diurnal offshore migration of precipitation from Sumatra develop differently depending on the mean states over the eastern Indian Ocean. Furthermore, it is shown that the MJO events with the strong diurnal cycle tend to have longer duration and continuing eastward propagation of active convection across the MC, whereas the convective activities of the other MJO events weaken considerably over the MC and develop again over the western Pacific. These results suggest that the strong diurnal cycle over Sumatra facilitates the smooth eastward propagation of the intraseasonal convection across the MC.

How to cite: Seiki, A., Yokoi, S., and Katsumata, M.: The impact of diurnal precipitation over Sumatra Island, Indonesia, on synoptic disturbances and its relation to the Madden-Julian Oscillation, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3677, https://doi.org/10.5194/egusphere-egu21-3677, 2021.

This study analyzes wind speed and surface latent heat flux anomalies from the Cyclone Global Navigation Satellite System (CYGNSS), aiming to understand the physical mechanisms regulating intraseasonal convection, particularly associated with the Madden-Julian oscillation (MJO). The importance of wind-driven surface flux variability for supporting east Pacific diurnal convective disturbances during boreal summer is also examined. An advantage of CYGNSS compared to other space-based datasets is that its surface wind speed retrievals have reduced attenuation by precipitation, thus providing improved information about the importance of wind-induced surface fluxes for the maintenance of convection. Consistent with previous studies from buoys, CYGNSS shows that enhanced MJO precipitation is associated with enhanced wind speeds, and that associated surface heat fluxes anomalies have a magnitude about 7%-12% of precipitation anomalies. Thus, latent heat flux anomalies are an important maintenance mechanism for MJO convection through the column moist static energy budget. A composite analysis during boreal summer over the eastern north Pacific also supports the idea that wind-induced surface flux is important for MJO maintenance there. We also show the surface fluxes help moisten the atmosphere in advance of diurnal convective disturbances that propagate offshore from the Colombian Coast during boreal summer, helping to sustain such convection.  

How to cite: Maloney, E., Bui, H., Riley Dellaripa, E., and Singh, B.: Wind Speed, Surface Flux, and Convection Coupling from CYGNSS Data, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12855, https://doi.org/10.5194/egusphere-egu21-12855, 2021.

This study investigates the impact of the diurnal cycle of incoming solar radiation on the spontaneous organization of convective clouds, hereafter self-aggregation. We run 3D cloud-resolving simulations in the RCE framework with interactive sea surface temperature (SST). SST is allowed to interact with the atmosphere using a slab ocean ( H = 1 - 200 meters) with a fixed mean but locally varying temperature. The self-aggregation of deep clouds starts with the appearance of dry patches that grow in size while getting drier, and confine the moist convention into a small fraction of the domain, consistent with previous studies of self-aggregation.

Interactive SST has been confirmed to decelerate or prevent self-aggregation. However, our finding shows that including the diurnal cycle reduces the impact of slab depth on the self-aggregation so that the aggregation proceeds much faster for shallower slabs (1,2 or 5 meters). For deeper slabs (50 and 200 meters) the self-aggregation progress is negligibly affected by the diurnal cycle. The accelerated self-aggregation with shallow slabs is found to be related to the mechanism by which the dry patches are triggered.

The triggering of dry patches is typically assumed to be a random process; however, we find that, especially with shallow ocean slabs, the dry patches form in places of cold pools. In other words, the lower tropospheric and boundary layer dryness induced by cold pools as well as surface temperature cooling by cloud shading can persist long enough to ensure a divergent flow, which was found to be important for self-aggregation. With shallow slabs, the negative SST anomaly under the cold pools thermally enhances the radiatively driven night time divergent flow and dries the boundary layer rapidly. The negative moisture anomaly persists even during daytime when the surface warms in dry regions and ensures a divergent flow, however weak, that then leads to the formation of dry patches in the following days. This process significantly accelerates the appearance of first dry patches. Moreover, this mechanism results in the occurrence of self-aggregation for shallow slabs (H=1 or 2 meters ) for which the self-aggregation does not proceed with constant solar radiation in our simulations. The enhanced divergent flow does not play a role for deep slabs as the SST anomalies are very small. Once the self-aggregation is triggered, its progress becomes negligibly affected by the diurnal cycle.

How to cite: Shamekh, S., Muller, C., Duvel, J.-P., Hohenegger, C., and D'Andrea, F.: How does the diurnal cycle of incoming solar radiation affect self-aggregation of convective clouds?, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8205, https://doi.org/10.5194/egusphere-egu21-8205, 2021.

We investigate the impact of air-sea coupling on the simulation of the intraseasonal variability of rainfall over the South Pacific using the ECHAM5 atmospheric general circulation model coupled with Snow-Ice-Thermocline (SIT) ocean model. We compare the fully coupled simulation with two uncoupled simulations forced with sea surface temperature (SST) climatology and daily SST from the coupled model. The intraseasonal rainfall variability over the South Pacific Convergence Zone (SPCZ) is reduced by 17% in the uncoupled model forced with SST climatology and increased by 8% in the uncoupled simulation forced with daily SST. The coupled model best simulates the key characteristics of the two intraseasonal rainfall modes of variability in the South Pacific, as identified by an Empirical Orthogonal Function (EOF) analysis. The spatial structure of the two EOF modes in all three simulations is very similar, suggesting these modes are independent of air-sea coupling and primarily generated by the dynamics of the atmosphere. The southeastward propagation of rainfall anomalies associated with two leading rainfall modes in the South Pacific depends upon the eastward propagating Madden-Julian Oscillation (MJO) signals over the Indian Ocean and western Pacific. Air-sea interaction seems crucial for such propagation as both eastward and southeastward propagations substantially reduced in the uncoupled model forced with SST climatology. Prescribing daily SST from the coupled model improves the simulation of both eastward and southeastward propagations in the uncoupled model forced with daily SST, showing the role of SST variability on the propagation of the intraseasonal variability, but the periodicity differs from the coupled model. The change in the periodicity is attributed to a weaker SST-rainfall relationship that shifts from SST leading rainfall to a nearly in-phase relationship in the uncoupled model forced with daily SST.

How to cite: Pariyar, S. K., Keenlyside, N., and Tseng, W.-L.: The role of air-sea coupling on the simulation of intraseasonal rainfall variability over the South Pacific in ECHAM5-SIT, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12770, https://doi.org/10.5194/egusphere-egu21-12770, 2021.

Low-level cloud cover (LCC) in western Central Africa is an important factor for the persistence of the dense evergreen tropical forest, as it keeps conditions cool, humid, and light-deficient. A quantitative understanding of the mechanisms controlling LCC is an important prerequisite to anticipate future changes, particularly as climate and weather models have been shown to struggle with a realistic representation of low clouds. This is a major goal of the French-German project Dynamics, Variability, and Bioclimatic Effects of Low Clouds in Western Central Africa (DYVALOCCA, ) launched in 2020.

Here we present an analysis of historical station data from the database ISD (Integrated Surface Database) and MIDAS (Met Office Data Archive System), ERA-5 reanalysis, and satellite data from the Meteosat Second Generation (MSG) focusing on the country of Gabon and surroundings. Station data (ISD and MIDAS) show a higher LCC during the major dry season months of July, August, and September (JAS) compared to the two rainy seasons and the other shorter dry season in boreal winter. During typical days in JAS, the LCC that thickens at night tends to break up at daytime near the coast and over the interior plateau, while it remains overcast at the windward site and over the crests of the Crystal and Chaillu low mountain ranges. Thus, stations at the coast have a different diurnal LCC cycles compared to stations in the interior of Gabon. The diurnal amplitudes of LCCs in the interior are lower and the maximum and minimum LCC occurs later in the day compared to coastal stations.

A comparison to the station data shows that LCC is generally underestimated in ERA-5. At the diurnal scale, LCC over the plateaus of eastern Gabon often does not dissolve as fast as in the ERA-5 reanalysis. Data from the Spinning Enhanced Visible and Infrared Imager (SEVIRI) satellite corroborates the underestimation by ERA-5. The RGB Night Microphysical Scheme (NMS) with SEVIRI data to determine LCC in the region shows an acceptable fit to the station data. Other satellite products such as CLAAS-2 do not deliver as good an estimate of the LCC in western Central Africa as the NMS.

Future work will employ Cloudsat-Calipso data to enhance our understanding of the vertical distribution of clouds. The best suited data sets will then be used as validation data set for convection-permitting modeling studies of example nights and days.

How to cite: Aellig, R., Gerighausen, J., Fink, A., Knippertz, P., and Philippon, N.: Satellite- and station-based climatology of low-level cloud cover during the long dry season in western Central Africa, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9552, https://doi.org/10.5194/egusphere-egu21-9552, 2021.

This study investigates the synoptic-scale flows associated with extreme rainfall systems over the Asian–Australian monsoon region (90°E–160°E and 12°S–27°N). On the basis of the statistics of the 17-year Precipitation Radar observations from Tropical Rainfall Measurement Mission, a total of 916 extreme systems, with both the horizontal size and maximum rainfall intensity exceeding the 99.9^th percentiles of the tropical rainfall systems, are identified over this region. The synoptic wind pattern and rainfall distribution surrounding each system are classified into four major types: vortex, coastal, coastal with vortex, and none of above, with each accounting for 44%, 29%, 7%, and 20%, respectively. The vortex type occurs mainly over the off-equatorial areas in boreal summer. The coast-related types show significant seasonal variations in their occurrence, with high frequency in the Bay of Bengal in boreal summer and on the west side of Borneo and Sumatra in boreal winter. The none-of-the-above type occurs mostly over the open ocean, and in boreal winter, these events are mainly associated with the cold surge events. The environment analysis shows that coast-related extremes in the warm season are found within the areas where high total water vapor and low-level vertical wind shear occur frequently. Despite the different synoptic environments, these extremes show a similar internal structure, with broad stratiform and wide convective core (WCC) rain. Furthermore, the maximum rain rate is located mostly over the convective area, near the convective–stratiform boundary in the system. Our results highlight the critical role of the strength and direction of synoptic flows in the generation of extreme rainfall systems near coastal areas. With the enhancement of the low-level vertical wind shear and moisture by the synoptic flow, the coastal convection triggered diurnally has a higher chance to organize into mesoscale convective systems and hence a higher probability to produce extreme rainfall.

How to cite: Chen, W.-T., Jian, H.-W., Chen, P.-J. C., Wu, C.-M., and Rasmussen, K. L.: The Synoptically-Influenced Extreme Precipitation Systems over Asian-Australian Monsoon Region observed by TRMM Precipitation Radar, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8204, https://doi.org/10.5194/egusphere-egu21-8204, 2021.

In order to predict the intensity and location of extreme weathers, such as torrential rainfall by individual thunderstorm or typhoon, we are developing the new methodology of weather monitoring using a ground AWS network with lightning sensors and micro-satellites weighting about 50kg, which will realize quasi-real-time thunderstorm monitoring with broad coverage. Based on the AWS network data, we plan to operate micro-satellites in nearly real-time, manipulating the attitude of satellite for capturing the most dangerous or important cloud images for 3D reconstruction. We have developed and launched several micro-satellites and been improving the target pointing operation for this decade. We succeeded in obtaining the images of the typhoon center at a resolution of 60-100 m for Typhoon Trami in 2018 and Typhoon Maysak in 2020. Using 4 or a few 10s images captured from different angles by one micro-satellite when it passed over the typhoon area, 3D models of typhoon eye were reconstructed, which have a ground resolution of ~100 m. Due to the unusual temperature profile around typhoon eye, it’s very difficult to estimate the heigh distribution of cloud top only with a thermal infrared image at a resolution of 2 km taken by geostationary meteorological satellite. This is one of the biggest limitations in estimating the precise intensity of typhoons, namely, the center pressure or the maximum wind velocity. The on-demand flexible operation of micro-satellite will achieve the high accuracy estimation of typhoon intensity as well as the speed estimation of individual thunderstorm development, which can be applied to disaster management. This research was conducted by a mixed team of Japan and the Philippines, supported by Science and Technology Research Partnership for Sustainable Development (SATREPS), which is funded by Japan Science and Technology Agency (JST) / Japan International Cooperation Agency (JICA).

How to cite: Takahashi, Y., Sato, M., Kubota, H., Ishida, T., Castro, E., Algodon, M., Perez, G., and Marciano, J.: Coordinated observation system for extreme weathers consisting of AWS network with lightning sensor and micro-satellites, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14649, https://doi.org/10.5194/egusphere-egu21-14649, 2021.

An approach is proposed [1] for determining the precise time of the start of tropical cyclogenesis, which includes a combined analysis of data from cloud-resolving numerical modeling and GOES Imagery. The approach is based on the similarity of patterns in the fields of vertical helicity (numerical simulation) and temperature (satellite data), allowing for the localization of intense rotating convective clouds known as the Vortical Hot Towers. As a theoretical ground, we applied a hypothesized (to date) interpretation of tropical cyclogenesis as a large-scale instability caused by the mechanism of the turbulent vortex dynamo in the atmosphere [1,2], and with bearing in mind the crucial role of Vortical Hot Towers in providing the dynamo-effect [2]. In this context, birth of a hurricane is considered as an extreme threshold event in the helical atmospheric turbulence of a vorticity-rich environment of a pre-depression cyclonic recirculation zone. Helical turbulence is characterized by the broken mirror symmetry and permits an existence of inverse energy cascade in three-dimensional cases. In order to trace and analyze processes of self-organization in the tropical atmosphere, that span scales from convective clouds with horizontal dimensions of 1-5 km to mesoscale vortices of hundreds of kilometers, we used the post-processing [1-3] of data from cloud-resolving numerical simulations [4].  Implementation of the proposed approach revealed that large-scale vortex instability can begin a few hours, or even dozens of hours, before the formation of the Tropical Depression. This work was supported by the research project “Monitoring” No. 01200200164.

References

[1] Levina, G. V., 2020. Birth of a hurricane: early detection of large-scale vortex instability. J. Phys.: Conf. Ser., 1640  012023,  doi:10.1088/1742-6596/1640/1/012023

[2] Levina, G. V., 2018. On the path from the turbulent vortex dynamo theory to diagnosis of tropical cyclogenesis. Open J. Fluid Dyn., 8, 86–114,  https://doi.org/10.4236/ojfd.2018.81008

[3] Levina, G. V. and M. T. Montgomery, 2015. When will Cyclogenesis Commence Given a Favorable Tropical Environment?  Procedia IUTAM, 17, 59–68, https://doi.org/10.1016/j.piutam.2015.06.010

[4] Montgomery, M. T., M. E.  Nicholls, T. A. Cram, and A. B. Saunders, 2006: A vortical hot tower route to tropical cyclogenesis. J. Atmos. Sci., 63, 355–386,  https://doi.org/10.1175/JAS3604.1

How to cite: Levina, G.: Diagnosis of Pre-Depression Large-Scale Vortex Instability in the Tropical Atmosphere, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6966, https://doi.org/10.5194/egusphere-egu21-6966, 2021.

We evaluate the impact of temperature at the upper troposphere and lower stratosphere (UTLS) on the tropical cyclone (TC) generation and its development by using the nonhydrostatic atmosphere-ocean coupling axisymmetric numerical model [Rotunno and Emanuel, 1987; Ito et al., 2010]. In the case of cold simulation at UTLS, the maximum wind and the minimum sea level pressure are increased and decreased than the control run, respectively. The magnitude of intensity change is the approximately 4 times larger than the change estimated from the MPIs (Maximum Potential Intensity [Bister and Emanuel,1998; Holland, 1997]). Further, during the development phase, the cold air mass intrudes to the middle troposphere from the upper troposphere at the center of TC, which is not seen in the warm case, leading the atmosphere unstable and enhanced the upward motion and then the TC got stronger.

How to cite: Eguchi, N., Kobayashi, K., Ito, K., and Nasuno, T.: The impact of upper tropospheric temperature change on tropical cyclone, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13735, https://doi.org/10.5194/egusphere-egu21-13735, 2021.

Tropical cyclones are a weather phenomenon that can devastate coastlines and cause substantial harm to human life and infrastructure every year. Their seasonal prediction is an effort that has been undertaken for several decades. These predictions are generally useful and have skill. The 2013 season was predicted as above average in activity by all forecasting agencies, but was one of the least active on record. A previously proposed reason for this is the abundance of Rossby wave breaking in the north Atlantic, which dries and cools the tropics by mixing in extratropical air. While the existence of this mechanism is not disputed, other pathways linked to the interactions between tropical and extratropical air masses are suggested and evaluated in this study

The numerical model ICON is used in Limited Area Mode (~13 km horizontal resolution) to simulate the north Atlantic, using ERA5 data for the hurricane season of 2013 to prescribe initial and boundary conditions. To influence Rossby wave breaking, a set of simulations uses 30 day running mean boundary conditions in the northern part of the domain, while a reference set uses regular boundary conditions everywhere along the boundary. Though the results do not falsify the aforementioned hypothesis of the abundance of Rossby wave breaking influencing tropical cyclone activity, they suggest that other mechanisms, such as changes in steering flow, tropopause temperature and wind shear, could also be responsible for changes in tropical cyclone activity. Furthermore, the accumulated cyclone energy seems to be rather closely related to the mean latitude of the 2 potential vorticity unit contour on the 350 K isentropic surface within a small longitudinal window in the western Atlantic.

How to cite: Enz, B., Neubauer, D., Sprenger, M., and Lohmann, U.: Tropical Cyclones in Reduced Rossby Wave Breaking Environments, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6015, https://doi.org/10.5194/egusphere-egu21-6015, 2021.

Global warming influences tropical cyclones (TC) and their impacts in different ways. Warmer sea surface temperatures (SST) are expected to lead to stronger intensification, the increased water holding capacity of warmer air increases the precipitation brought by TCs. These are thermodynamic changes that are rather well understood.

When it comes to the influence of circulation changes on tropical cyclone activity open questions remain: Will there be more or less TCs in a warmer world? And what would be the physical mechanism for a change in TC frequencies?

TC formation and intensification not only depends on the available energy but also on the large-scale atmospheric circulation. For instance, TC development is strongly hampered when the vertical wind shear (difference between upper and lower level wind speeds) is high.

Here we present a tropical cyclone season emulator for the Atlantic basin that produces TCs based on SSTs averaged over the Atlantic main development region and daily time series of weather patterns obtained from a self-organizing map clustering. The emulator is based on probabilities for storm genesis, storm length and intensity changes that were empirically assessed using the ERA5 reanalysis and IBTrACS TC observations.

We see different applications for this emulator:
1) While most global circulation models (GCM) fail to adequately simulate TCs, their projections for SSTs and large-scale weather patterns contains valuable information. Using our emulator, we could indirectly analyse TC activity projections for all available GCMs.
2) In the emulator thermodynamic (SSTs) and dynamic influences (weather patterns) are distinct inputs. This allows us to construct different counterfactuals to attribute changes in TC activity to thermodynamic or dynamic changes. For example, the emulator could be used to simulate TC seasons with large scale circulation as observed in 2017 but with preindustrial SSTs, so as to analyse the extent to which warming of the ocean surface had contributed to the extreme hurricane season of 2017.

How to cite: Pfleiderer, P., Nath, S., Jaye, A., and Schleussner, C.-F.: Studying dynamic and thermodynamic influences on hurricane activity with a tropical cyclone emulator, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8399, https://doi.org/10.5194/egusphere-egu21-8399, 2021.

Tropical cyclones (TCs) and easterly waves (EWs) produce significant seasonal rainfall over the tropical and subtropical North America. When TC activity over the tropical eastern Pacific (TEP) or the Intra Americas Seas (IAS) is below-normal (above-normal), regional precipitation may be below (above-normal). However, it is not only the number of TCs what may change seasonal precipitation, but the trajectory of the systems. TCs induce intense precipitation over continental regions if they are close enough to shorelines, for instance, if the TC center is located less than 500 km-distant from the coast. However, if TCs are more remote than this threshold distance, the chances of rain over continental regions decrease, particularly in arid and semi-arid regions. In addition, a distant TC may induce subsidence or produce moisture divergence that inhibits, at least for a few days, convective activity farther away than the threshold distance.

EWs can produce up to 50% of seasonal rainfall and contribute substantially to interannual regional rainfall variability. An observational analysis shows that the El Niño Southern Oscillation (ENSO) affects EW frequency and therefore, their contribution to seasonal rainfall. In recent years, TC activity over the Main Development Region (MDR) of the tropical North Atlantic has a negative impact on regional seasonal precipitation over northern South America. High TC activity over MDR corresponds to below-normal precipitation because it reduces the EW activity reaching northern South America through the recurving of TC tracks. Recurving TC tracks redirect moisture away from the tropical belt and into the mid-latitudes. However, this relationship only holds under neutral ENSO conditions and the positive phase of the Atlantic Multidecadal Oscillation. A 10-member regional model multi-physics ensemble simulation for the period 1990–2000 was analyzed to show the relationships are robust to different representations of physical processes. This new understanding of seasonal rainfall over the tropical Americas may support improved regional seasonal and climate outlooks.

How to cite: Dominguez, C.: Tropical Cyclone and Easterly Wave Relationship in Regional Precipitation over the Tropical and Subtropical North America, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-307, https://doi.org/10.5194/egusphere-egu21-307, 2021.

The Philippines is exposed to Tropical Cyclones (TCs) throughout the year due to its location in the western North Pacific. While these TCs provide much-needed precipitation for the country’s hydrological cycle, extreme precipitation from TCs may also cause damaging hazards such as floods and landslides. This study examines the relationship between TC extreme precipitation and TC characteristics, including movement speed, intensity, and season, for westward-moving TCs crossing Luzon, northern Philippines. We measure extreme precipitation by the Weighted Precipitation Exceedance (WPE), calculated against a 95^th percentile threshold, which considers both the magnitude and spatial extent of TC-related extreme precipitation.

WPE has a significant, moderate positive relationship with TC intensity and a significant, weak negative relationship with TC movement speed. When TCs are classified by pre-landfall intensity, Typhoons (1-minute maximum sustained wind speed > 64 knots) tend to yield higher WPE than non-Typhoons (< 64 knots). On the other hand, when TCs are classified by pre-landfall speed, Slow TCs (movement speed < 11.38 knots) tend to yield higher WPE than Fast TCs (movement speed > 11.38 knots). However, while distributions of WPE are similar between the Southwest Monsoon (June-September) and Northeast Monsoon (October-December) seasons, the relationship between pre-landfall TC intensity and WPE is more pronounced during June-September. These results suggest that it is important to consider the pre-landfall cyclone movement speed, intensity, and season to anticipate extreme precipitation of incoming TCs. A decision table considering these factors is devised to aid in TC extreme precipitation forecasting.

How to cite: Racoma, B. A., Klingaman, N., Holloway, C., Schiemann, R., and Bagtasa, G.: Tropical Cyclone Characteristics Associated with Extreme Precipitation in the Northern Philippines, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15486, https://doi.org/10.5194/egusphere-egu21-15486, 2021.

Typhoons are extreme weather phenomena that inflict damages and casualties around globe. These phenomena are difficult to study because of their chaotic behaviour but the capacity to measure their intensity can help mitigate the hazards that they bring. In the past, several attempts have been done to relate typhoon's intensity with the structural evolution of its eye. This suggests the possible relation between the typhoon intensity with typhoon eye altitude. In this research, we visualize Typhoon Trami’s structure by reconstructing the three-dimensional model inside its eye and analyze the information of its cloud top altitude. An experiment was conducted under the SATREPS/ULAT project (SATREPS: Science and Technology Research Partnership for Sustainable Development, ULAT: Understanding Lightning and Thunderstorm) where images of Typhoon Trami were taken from an aircraft last September 26, 2018. Aircraft images were used to reconstruct the 3D model inside the typhoon eye because they provide closer views of the typhoon than that of geostationary satellite images, making it easier to reconstruct a 3D model. The 3D reconstruction generated covers 43 km region of the typhoon eye at 20.2 m/pixel spatial resolution. Three cross-sections of the 3D model were analyzed, and the resulting altitude distribution was compared with the cloud-top altitude estimated by mapping the brightness temperature of the Himawari Thermal Infrared Band 13 with cloud-top height as measured by NOAA sonde data. From the 3D model, the altitude distribution ranges from 5.3 km to 14.3 km which corresponds with the altitude estimated from the brightness temperature of 6.5 km to 14.3 km. However, regions of altitude difference can also be observed between the two methods. This study shows that a three-dimensional model could be a good mode of typhoon visualization as it shows a more detailed typhoon structure such as the stairstep structures that was detected at some regions within the typhoon eye. This research was supported by SATREPS, funded by Japan Science and Technology Agency (JST) / Japan International Cooperation Agency (JICA).

How to cite: Algodon, M., Takahashi, Y., Sato, M., Kubota, H., Ishida, T., Yamashita, K., Castro, E., Perez, G. J., Marciano, J. J., Matsumoto, J., Hamada, J., Tsuboki, K., and Yamada, H.: 3D Reconstruction of Typhoon and Thunderstorm Cloud Top Using Airborne Camera, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14160, https://doi.org/10.5194/egusphere-egu21-14160, 2021.

The deep convective clouds of developing tropical cyclones (TCs) are highly effective at trapping the infrared (or longwave) radiation welling up from the surface. This “cloud greenhouse effect” locally warms the lower–mid-troposphere relative to the TC’s surroundings – an effect that manifests in all stages of the TC lifecycle. While idealized studies suggest the importance of this feedback for TC formation, this issue has remained unexplored for TCs in nature, where non-zero background flow, wind shear, and synoptic-scale variability are known to greatly constrain TC development.

To address this gap, we examine the potential role of this cloud–infrared (or longwave) radiation feedback in the context of two archetypal storms: Super Typhoon Haiyan (2013) and Hurricane Maria (2017). We conduct a set of numerical model experiments for both storms with a convection-resolving model (WRF-ARW) from the very early stages of TC development. We examine sensitivity experiments wherein this cloud–radiation feedback is removed at various lead-times prior to TC genesis and the onset of rapid intensification (RI). In both storms, removing this cloud–radiation feedback at a lead-time of ~1 day or less leads to delayed and/or weaker intensification than in the control case. When this feedback is removed with a lead-time of two days or longer, however, the storms altogether fail to development and intensify. This local cloud greenhouse effect strengthens the thermally direct transverse circulation of the incipient storm, in turn both promoting saturation within its core and accelerating the spin-up of its surface tangential circulation via angular momentum convergence. These findings indicate that the cloud greenhouse effect plays a critical role in accelerating and promoting TC development in nature. Progress in the prediction of TC formation and intensification has been very limited in recent decades. Cloud–radiation feedback represents a large source of uncertainty in models, which hence manifests as uncertainty in the prediction of TC development. Our findings highlight the pressing need to better constrain this feedback in models. Doing so holds promise for advancing our ability to forecast TCs.

How to cite: Ruppert, J., Wing, A., Tang, X., and Duran, E.: The Critical Role of Cloud–Infrared Radiation Feedback in Tropical Cyclone Development, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-85, https://doi.org/10.5194/egusphere-egu21-85, 2021.

Interactions between clouds, radiation, and circulations are fundamental to tropical climate, but until recently, the impact of these interactions on tropical cyclones (TCs) has been relatively unexplored. Simulations of rotating radiative-convective equilibrium confirm that radiative feedbacks are important for spontaneous TC genesis (in which a TC is allowed to form from random noise). While not strictly necessary, radiative feedbacks significantly accelerate TC genesis and especially contribute in the early stages of genesis. These radiative feedbacks arise from interactions between spatially and temporally varying radiative cooling (driven by the dependence of radiative cooling rate on clouds and water vapor) and the developing tropical cyclone (the circulation of which shapes the structure of clouds and water vapor).  However, TCs in nature are generally observed to form from pre-existing disturbances, calling into question whether radiative feedbacks play a significant role.

Here, I investigate the importance of radiative feedbacks in TC genesis and the mechanisms underlying their influence in a set of idealized cloud-resolving simulations in which a TC is allowed to develop after initialization from a mesoscale warm, saturated bubble on an f-plane, in an otherwise quiescent and moist neutral environment. TC genesis is delayed by a factor of two or three when radiative feedbacks are removed by prescribing a fixed cooling profile or spatially homogenizing the model-calculated cooling profiles. Further analysis and additional mechanism denial experiments pinpoint the longwave radiative feedback contributed by ice clouds as the strongest influence. These results are consistent with recently published case study simulations in which cloud-radiative effects accelerate TC formation and intensification in realistic scenarios. The important takeaway from the results presented here is that that cloud-longwave radiative feedbacks have a profound impact on TC genesis in a hierarchy of model simulations. Improving the representation of cloud-radiative feedbacks in forecast models therefore has the potential to yield critical advancements in TC prediction.

How to cite: Wing, A.: The Importance of Radiative Feedbacks in Tropical Cyclogenesis in Idealized Simulations, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6192, https://doi.org/10.5194/egusphere-egu21-6192, 2021.

Tropical cyclones (TCs) forming over the Bay of Bengal can cause devastation when they make landfall in India and Bangladesh; accurate prediction of their track and intensity is essential for disaster management. TC intensity is moderated by heat, momentum and moisture exchanges between the atmosphere and ocean. In recent years there have been significant improvements in the skill of TC forecasts due to the implementation of coupled atmosphere-ocean models and high-resolution models capable of explicitly resolving small-scale physical processes influencing storm development.

This study evaluates the representation of six TCs in the Bay of Bengal from 2016 to 2019, using both a Met Office Unified Model atmosphere-only configuration (ATM) with 4.4 km grid spacing, and coupled to a 2.2 km resolution NEMO (Nucleus for European Models of the Ocean) ocean model (CPL). To determine the impact of coupling on wind-driven mixing and ocean-atmosphere heat exchange, forecast sea surface temperature (SST) is compared to observations. The impact of coupling on track position and storm intensity is evaluated using predictions of minimum sea level pressure (MSLP) and 10 m maximum sustained winds (MSW). Representation of TC dynamics is assessed by analysing storm structure, using radius of maximum winds and rain rate asymmetry.

Results from the three most intense TC case studies (Fani, Titli, and Vardah) show that SSTs in ATM are too high, while SSTs in CPL are slightly too low, with an overestimation of the cooling response in TC wakes. TC track position errors are small, but intensity error metrics for MSLP and MSW show biases relative to observations. Peak intensity is overestimated for Titli and Vardah in the ATM model configuration; the CPL model configuration generally produces weaker storms than the ATM model configuration. Wind speeds outside the storm centre are high compared to observations, with a greater bias in the ATM model configuration.  Both model configurations produce accurate predictions of radius of maximum winds and rain rate asymmetry, suggesting a good representation of TC dynamics. Much of the variation in rain rate asymmetry in the forecasts can be explained by variations in wind shear.

How to cite: Saxby, J., Crook, J., Birch, C., Holloway, C., Lewis, H., Peatman, S., and Schwendike, J.: Forecasts of tropical cyclones in the Bay of Bengal in a regional convection-permitting atmosphere-ocean coupled model, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8213, https://doi.org/10.5194/egusphere-egu21-8213, 2021.

Tropical cyclones (TCs) in the North Pacific Ocean claim a major socio-economic toll on a yearly basis, and their impacts are projected to be exacerbated due to climate change and increased exposure and vulnerability. Recent examples of Typhoons Mangkhut (2018) and Hagibis (2019) are a reminder of the devastating impacts these storms can have. While the TC activity in the West North Pacific (WNP) and East North Pacific (ENP)  has been the subject of intense investigation, these basins are generally treated separately, rather than considering the storm activity in the North Pacific as a single basin. The influence of climate processes, such as the Pacific Decadal Oscillation (PDO) ,  that operate across the entire North Pacific may not have been considered by focusing on the sub-basins, especially if we are interested in multi-annual and decadal changes. It is reasonable to hypothesize that a climate mode like the PDO could play an important role in terms of TC activity in this basin. However, there is limited evidence that connects these storms and the PDO. Our expectation is that the number of TC days is related to the PDO through the modulation of this climate mode of the SST in the regions where these storms develop. In particular, during the positive phase of the PDO, warm waters close to the equator would lead to conditions favorable to the development of longer-lasting storms compared to the negative PDO phase, which is characterized by lower SST values. We believe that this connection has not been sufficiently considered in the literature because the North Pacific Ocean was not considered as a single basin but broken up into WNP and ENP, confounding the detection of a potential PDO signal. Therefore, in this work we focus on the potential role of the PDO in modulating TC activity, with emphasis on the number of TC active days in the entire North Pacific Ocean. We have selected this metric because the number of TC days provides an integrated information about TC genesis, lifespan, and tracks, and because it exhibits substantial decadal-scale oscillations in TC activity compared to other metrics used to highlight TC activity. We aim to verify the effects of different SST patterns on the spatial distribution of TC genesis in the North Pacific leading to conditions that are more/less favorable for long-lasting TCs under positive/negative PDO phases. A larger number of TC days for storms that tend to develop along the tropics during the positive PDO phase is found. When we stratify the years according to the sign of the PDO phase, the years associated with the positive phase tend to have storms that form at a lower latitude and that last longer  compared with the negative phase. On average, these storms tend to form around 14°N and to result in 240 TC days; during the negative PDO phase, TCs tend to form around 16°N, for a total of 160 TC days.

How to cite: Scoccimarro, E., Villarini, G., Gualdi, S., and Navarra, A.: The Pacific Decadal Oscillation Modulates Tropical Cyclone days in the North Pacific Ocean, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11117, https://doi.org/10.5194/egusphere-egu21-11117, 2021.