AS1.23

EDI
Meso-scale convection and disturbances in high-mountain environments

Mesoscale convective systems (MCSs) and meso-scale storms/disturbances are known to be important precipitation producing/triggering systems in high-mountain environments and on high-altitude plateaus. These meso-scale convective systems and disturbances can lead to severe weather locally and affect lower lying downstream regions.

The aim of this session is to gain an improved understanding of meso-scale systems and the associated processes leading to (extreme) precipitation in mountain regions and/or their downstream areas. We invite contributions on the dynamics of meso-scale storms/disturbances and meso-scale convective systems (including their formation and evolution) as well as smaller-scale convection in connection to atmospheric meso-scale features and how these factors explain spatio-temporal patterns of precipitation and precipitation dynamics. Contributions focussing on individual extreme events or giving climatological perspectives are welcome. Due to the nature of high-mountain environments it is difficult to directly observe their meso-scale atmospheric features and link these to the occurrence and spatio-temporal variability of precipitation. Therefore, contributions integrating remote sensing data, in-situ observations, and high-resolution models, especially those that explicitly resolve convections are particularly welcome.

This session is connected to the recently launched WRCP-CORDEX flagship pilot study “High resolution climate modelling with a focus on mesoscale convective systems and associated precipitation over the Third Pole region”.

Convener: Julia CurioECSECS | Co-conveners: Deliang Chen, Kalli Furtado, Julia KukuliesECSECS
vPICO presentations
| Thu, 29 Apr, 11:45–12:30 (CEST)

vPICO presentations: Thu, 29 Apr

Chairpersons: Julia Curio, Julia Kukulies, Kalli Furtado
11:45–11:50
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EGU21-1313
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solicited
Andreas F. Prein

Mesoscale-Convective Systems (MCSs) are prolific rain-producers and are responsible for most flash flood events in mid-latitudes. Global hotspots of MCS occurrence are downstream of major mountain regions such as the Rocky Mountains, the Andes, and the Himalayas. This is because of the effects of mountain barriers on circulation patterns, moisture transport, and convective initiation. Realistically simulating MCSs in climate models is essential for representing the water and energy cycle and flood and severe convective weather assessments. However, state-of-the-art climate models have substantial biases in simulating MCSs and orographic impacts on downstream environments resulting in large uncertainties and errors in assessing climate change impacts on water availability and extreme events. Here we present that kilometer-scale models, which have an improved representation of orography and can represent deep convective processes explicitly, show a step improvement in simulating organized convective storms compared to coarser-resolution models. We will show examples of these improvements from kilometer-scale simulations over the Tibetan Plateau, North- and South America. We will also show sensitivities to the model setup and feedback processes and end with discussing remaining challenges and future prospects.

How to cite: Prein, A. F.: Modeling of Mesoscale-Convective Systems Downstream of Mountain Regions, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1313, https://doi.org/10.5194/egusphere-egu21-1313, 2021.

11:50–11:52
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EGU21-5531
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ECS
Aditya Jaiswal, Manish Naja, and Samaresh Bhattacharjee

Deep convection is known to be critical for the transport of mass and momentum flux, heat and moisture throughout in the upper troposphere and lower stratosphere region. Hence it modifies the heat budget and general circulation in the atmosphere. Earlier studies have noted very strong instability in the atmosphere over Himalayan foothills, triggering occasional intense convection due to the orographic lifting of the low level moist flow.  However due to the lack of observational network over this complex terrain, a comprehensive analysis of these events and their impacts have not been done.

Recently a Stratosphere Troposphere Radar (wind profiler) operating at VHF frequency of 206.5 MHz has been installed at a high altitude site Aryabhatta Research Institute of Observational Sciences (ARIES) (29.4oN, 79.5o E, 1790 m amsl) in Nainital located in Himalayan foothills, a meteorologically sensitive subtropical region. Using the capability of VHF radar of detecting echoes from both clear air and precipitation,  intense deep convection systems were observed on May 5, 2020 and September 2, 2020. Both the events have been studied in details using the temporal and vertical evolution of  radar parameters like total backscattered power and spectral width. Reanalysis data from MERRA-2 and cloud fraction data of IR and Water Vapour channels of INSAT 3D has also been used to investigate underlying synoptic features of the event. Here, it is suggested that deep convection of the pre-monsoon season was induced due to moisture carried by the western disturbance. While the event in monsoon season was due to the easterly moist flow from the Bay of Bengal. The moisture in the mid - troposphere coupled with the orographic lift led to vigorous updrafts and downdrafts of magnitude reaching up to 16 m/s. Updrafts found to be extending well beyond the tropopause into the lower stratosphere region. From the temporal evolution of vertical wind velocity obtained from ST Radar, a clear demarcation between updrafts and downdrafts region was established during the mature phase of the event due to veering of the wind from lower to upper troposphere which also led to the tilting of the updraft cores. During the event the exchange of the vertical flux of horizontal momentum between upper troposphere and lower stratosphere has also been estimated. A significant enhancement (2 – 3 times) in mean zonal (u'w') and meridional component (v'w') of momentum flux has been observed during convection as compared to quiet period. In the upper troposphere and lower stratosphere region mean flux values even reached up to about 33 m2 s-2. We feel that this study will help in providing the crucial insights of the dynamical features of meso-scale convective phenomenon in the central Himalayan region for the first time.

How to cite: Jaiswal, A., Naja, M., and Bhattacharjee, S.: Probing the dynamical features of intense pre-monsoon and summer monsoon deep convective systems using ARIES Stratosphere Troposphere Radar (206.5 MHz) over the Central Himalayan region , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5531, https://doi.org/10.5194/egusphere-egu21-5531, 2021.

11:52–11:54
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EGU21-8223
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ECS
Julia Kukulies, Julia Curio, and Deliang Chen

Meso-scale weather systems have been identified as major precipitation bearing systems in the Tibetan Plateau (TP) region. They can pose a risk for people's life and livelihoods, by causing flooding, extreme winds and heavy rainfall in the populous downstream regions. As local hydroclimatic conditions and large-scale atmospheric circulation patterns change with global warming, it is important to understand the role of such weather systems and the associated precipitation-producing mechanisms for the regional water cycle. Two important systems which are often named in this context are meso-scale convective systems (MCSs) and Tibetan Plateau vortices (TPVs). MCSs are recognized as cloud clusters that produce large areas of heavy rainfall, while TPVs refer to frequently occurring meso-scale cyclonic vortices around 500 hPa that are initiated over the TP. Only few studies have looked at the relationship between the dynamical disturbances like TPVs and observations of MCSs. We present here the key characteristics of MCSs as observed by satellite observations from the past two decades and compare it to the key characteristics of TPVs identified by minima in relative vorticity in reanalysis data. Further, we explore in what way TPVs and MCSs are linked to each other by focusing on the most extreme cases of both systems. Finally, we discuss the role of large-scale circulation for both TPVs and MCSs and suggest that future research about extreme precipitation around the TP region should focus more on the mechanisms that link together both systems.

How to cite: Kukulies, J., Curio, J., and Chen, D.: Meso-scale weather systems and their interaction in the Tibetan Plateau region, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8223, https://doi.org/10.5194/egusphere-egu21-8223, 2021.

11:54–11:56
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EGU21-8470
Qing Bao, Lei Wang, Yimin Liu, Guoxiong Wu, Jinxiao Li, Bian He, and xiaofei Wu

Extreme precipitation events, represented by the extreme hourly precipitation (EHP), often occur in the Tibetan Plateau and surrounding areas (TPS) as a result of the complex topography and unique geographical location of this region and can lead to large losses of human life. Previous studies have shown that the performance of extreme precipitation simulations can be improved by increasing the resolution of the model, although the mechanisms are not yet not clear. In this study, we firstly compared the most recent high-quality satellite precipitation product  with station data from Nepal, which is located on the southern edge of the Tibetan Plateau. The results showed that the GPM dataset can reproduce extreme precipitation well and we therefore used these data as a benchmark for climate models of the TPS. We then evaluated the fidelity of global climate models in the representation of the boreal summer EHP in the TPS using datasets from the CMIP6 High-Resolution Model Intercomparison Project (HighResMIP). We used four global climate models with standard (about 100 km) and enhanced (up to 25 km) resolution configurations to simulate the EHP. The models with a standard resolution largely underestimated the intensity of EHP, especially over the southern edge of the Tibetan Plateau. The EHP can reach up to 50 mm h−1in the TPS, whereas the maximum simulated EHP was <35 mm h−1 for all the standard resolution models. The mean intensity of EHP is about 5.06 mm h−1 in the GPM satellite products, whereas it was <3 mm h−1 in standard resolution models. The skill of the simulation of EHP is significantly improved at increased horizontal resolutions. The high-resolution models with a horizontal resolution of 25 km can reproduce the geographical distribution of the intensity of EHP in the TPS. The intensity–frequency distribution of EHP also resembles that from GPM products, showing the same features up to 50 mm h−1, although it slightly overestimates heavy precipitation events. Finally, we propose possible physical linkages between the simulation of EHP and the impacts of the resolution of the model and physical processes. Phenomena over the Indian Ocean at different timescales and the diurnal variation of precipitation in the TPS are used to propose possible physical linkages as they may play an important part in the simulation of EHP in the TPS. Further analysis shows that an increase in the horizontal resolution helps to accurately reproduce the features of water vapor transport on days with extreme precipitation, the northward-propagating intraseasonal oscillation over the Indian and western Pacific Ocean monsoon regions in the boreal summer, the intensity and number of tropical cyclones over the southern Asian monsoon regions, and the peak time and amplitude of the diurnal cycle of precipitation. This increase in accuracy contributes to the improvements in the simulation of EHP in the TPS. This study suggests improvements to increase the horizontal resolution of the GCMs and lay a solid foundation for the accurate reproduction and prediction of EHP in the TPS.

How to cite: Bao, Q., Wang, L., Liu, Y., Wu, G., Li, J., He, B., and Wu, X.: Effects of the horizontal resolution of climate models on the simulation of extreme hourly precipitation in the Tibetan Plateau and surrounding areas, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8470, https://doi.org/10.5194/egusphere-egu21-8470, 2021.

11:56–11:58
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EGU21-15444
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ECS
Jayesh Phadtare, Jennifer Fletcher, Andrew Ross, Andy Turner, Thorwald Stein, and Reinhard Schiemann

Precipitation distribution around an orographic barrier is controlled by the Froude Number (Fr) of the impinging flow. Fr is essentially a ratio of kinetic energy and stratification of winds around the orography. For Fr > 1 (Fr <1), the flow is unblocked (blocked) and precipitation occurs over the mountain peaks and the lee region (upwind region). While idealized modelling studies have robustly established this relationship, its widespread real-world application is hampered by the dearth of relevant observations. Nevertheless, the data collected in the field campaigns give us an opportunity to explore this relationship and provide a testbed for numerical models. A realistic distribution of precipitation over a mountainous region in these models is necessary for flash-flood and landslide forecasting. The Western Ghats region is a classic example where the orographically induced precipitation leads to floods and landslides during the summer monsoon season. In the recent INCOMPASS field campaign, it was shown that the precipitation over the west coast of India occurred in alternate offshore and onshore phases. The Western Ghats received precipitation predominantly during the onshore phase which was characterized by a stronger westerly flow. Here, using the radiosonde data from a station over the Indian west coast and IMERG precipitation product, we show that climatologically, these phases can be mapped over an Fr-based classification of the monsoonal westerly flow. Classifying the flow as 'High Fr' (Fr >1), 'Moderate Fr' ( 0.5 < Fr ≤ 1) and 'Low Fr' ( Fr ≤ 0.5 ) gives three topographical modes of precipitation -- 'Orographic', 'Coastal' and 'Offshore', respectively.  Moreover, these modes are not sensitive to the choice of radiosonde station over the west coast.

How to cite: Phadtare, J., Fletcher, J., Ross, A., Turner, A., Stein, T., and Schiemann, R.: Precipitation modes over the Western Ghats orography during the summer monsoon season, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15444, https://doi.org/10.5194/egusphere-egu21-15444, 2021.

11:58–12:00
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EGU21-5166
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ECS
Monika Feldmann, Marco Gabella, and Alexis Berne

Persistent rotation is a strong indicator of severe weather hazards in convective storms. This work presents a multi-year record of mesocyclonic rotation tracks derived from archived operational radar data in Switzerland. In addition to the general occurrence, underlying seasonal and daily trends, as well as the influence of synoptic weather situation and terrain are explored.
The applied mesocyclone detection presents a combination of thunderstorm cell detection and tracking and rotation identification. The thunderstorm cell detection hereby isolates areas of interest, that are then feed into the rotation detection. The complex terrain of the Swiss Alps and the different environmental conditions leading to persistent rotation in convection required some adaptations to the typical definition of mesocyclonic rotation. A combination of rotational velocity, vorticity, vertical extent and temporal continuity are used to detect mesocyclonic rotation and identify their tracks.
The multi-year record shows considerable variability between the years. A large number of rotation tracks however does not necessarily correspond to a large number in thunderstorms. There is no strict preference on rotation direction, with a slightly higher fraction of cyclonic detections over anticyclonic detections. A spatial overview of the identified events clearly shows the influence of terrain. Pre-Alpine valleys, particularly with lakes, seem to provide favorable conditions for rotation in convection. The largest incidence is located to the South of the Alps in the valleys of the lakes Maggiore, Lugano and Como.

How to cite: Feldmann, M., Gabella, M., and Berne, A.: Characterization of mesocyclonic rotation in severe convection over the Swiss Alps, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5166, https://doi.org/10.5194/egusphere-egu21-5166, 2021.

12:00–12:02
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EGU21-9715
Oliver Branch, Andreas Behrendt, Osama Alnayef, Florian Späth, Thomas Schwitalla, Marouane Temimi, Michael Weston, Sufian Farrah, Omar Al Yazeedi, Siddharth Tampi, Karel de Waal, and Volker Wulfmeyer

We present exciting Doppler lidar and cloud radar measurements from a high-vantage mountain observatory in the hyper-arid United Arab Emirates (UAE) - initiated as part of the UAE Research Program for Rain Enhancement Science (UAEREP). The observatory was designed to study the clear-air pre-convective environment and subsequent convective events in the arid Al Hajar Mountains, with the overarching goal of improving understanding and nowcasting of seedable orographic clouds. During summer in the Al Hajar Mountains (June to September), weather processes are often complex, with summer convection being initiated by several phenomena acting in concert, e.g., interaction between sea breeze and horizontal convective rolls. These interactions can combine to initiate sporadic convective storms and these can be intense enough to cause flash floods and erosion. Such events here are influenced by mesoscale phenomena like the low-level jet and local sea breeze, and are constrained by larger-scale synoptic conditions.

The Doppler lidar and cloud radar were employed for approximately two years at a high vantage-point to capture valley wind flows and observe convective cells. The instruments were configured to run synchronized polar (PPI) scans at 0°, 5°, and 45° elevation angles and vertical cross-section (RHI) scans at 0°, 30°, 60, 90°, 120°, and 150° azimuth angles. Using this imagery, along with local C-band radar and satellite data, we were able to identify and analyze several convective cases. To illustrate our results, we have selected two cases under unstable conditions - the 5 and 6 September 2018. In both cases, we observed areas of low-level convergence/divergence, particularly associated with wind flow around a peak 2 km to the south-west of the observatory. The extension of these deformations are visible in the atmosphere to a height of 3 km above sea level. Subsequently, we observed convective cells developing at those approximate locations – apparently initiated because of these phenomena. The cloud radar images provided detailed observations of cloud structure, evolution, and precipitation. In both convective cases, pre-convective signatures were apparent before CI, in the form of convergence, wind shear structures, and updrafts.

These results have demonstrated the value of synergetic observations for understanding orographic convection initiation, improvement of forecast models, and cloud seeding guidance. The manuscript based on these results is now the subject of a peer review (Branch et al., 2021).

 

Branch, O., Behrendt, Andreas Alnayef, O., Späth, F., Schwitalla, Thomas, Temimi, M., Weston, M., Farrah, S., Al Yazeedi, O., Tampi, S., Waal, K. de and Wulfmeyer, V.: The new Mountain Observatory of the Project “Optimizing Cloud Seeding by Advanced Remote Sensing and Land Cover Modification (OCAL)” in the United Arab Emirates: First results on Convection Initiation, J. Geophys. Res.  Atmos., 2021. In review (submitted 23.11.2020).

How to cite: Branch, O., Behrendt, A., Alnayef, O., Späth, F., Schwitalla, T., Temimi, M., Weston, M., Farrah, S., Al Yazeedi, O., Tampi, S., de Waal, K., and Wulfmeyer, V.: First Doppler lidar and cloud radar measurements of orographic convection initiation from a mountain observatory in the Al Hajar Mountains of the United Arab Emirates., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9715, https://doi.org/10.5194/egusphere-egu21-9715, 2021.

12:02–12:04
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EGU21-10800
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ECS
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Highlight
Jonas Van de Walle, Wim Thiery, and Nicole P.M. van Lipzig

Severe thunderstorms pose a constant threat to more than 30 million people living along the shores of Lake Victoria (East Africa). Thousands of fishermen lose their lives on the lake every year, and capsizing accidents with passenger ferries and transport boats are frequently reported. Moreover, hazardous thunderstorms affect people living inland, continuously facing flood risks.

In this data scarce region, atmospheric models are particularly useful tools to better understand the region’s complex climate, especially when simulated at convection-permitting resolution. For example, such models already demonstrated the importance of the lake in determining the diurnal precipitation cycle, and highlighted the role that mountain blocking of easterly trade winds plays in explaining the regional rainfall pattern.

Such models also allow us to generate high-resolution future projections for this region. In this study, a surrogate global warming approach has been applied. In a first simulation, the ensemble mean of the recent global climate projections from the CMIP6 data set was used to perturb the lateral boundary conditions from the ERA 5 reanalysis. In this ensemble mean, variations in (large scale) atmospheric dynamics are negligible and the climate change signal is mainly determined by the increased water vapour related to the warming and the response of the mesoscale circulation to differential lake/land heating. Specifically, while increased water vapour tends to increase total precipitation, weakened mesoscale circulation makes the over-lake rainfall to reduce instead. In a second simulation, a CMIP6 member with larger large-scale dynamical changes in the region was chosen to perturb the ERA5 lateral boundary data, thereby changing both the thermodynamics and the dynamical fields. Combining both simulations enables us to study the effects of changed large-scale dynamics and its interaction with the mountain peaks on mean and extreme precipitation in the region, thereby gaining insight in expected future changes of the region’s hazardous thunderstorms.

How to cite: Van de Walle, J., Thiery, W., and P.M. van Lipzig, N.: Large-scale versus regional drivers of climate change in the Lake Victoria basin, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10800, https://doi.org/10.5194/egusphere-egu21-10800, 2021.

12:04–12:30