AS1.9
Orals: Thu, 27 Apr | Room M1
Mesoscale convective systems (MCSs) are important components of the Earth’s weather and climate systems. They produce a large fraction of tropical rainfall and their top-heavy heating profiles can feedback onto atmospheric dynamics. Understanding the large-scale environmental precursor conditions that cause their formation is normally done as case studies or on a regional basis. Here, we take a global view on this problem, linking tracked MCSs to the environmental conditions that lead to their growth and maintenance. We consider common variables associated with deep convection, such as CAPE, total column water vapour and moisture convergence. We take care to distinguish between conditions associated with deep convection, and conditions associated with MCSs specifically. Furthermore, we pose the question in a way that is useful for the development of an MCS parametrization scheme, by asking what environmental conditions lead to MCS occurrence, instead of locating an MCS and then finding the associated conditions.
How to cite: Muetzelfeldt, M., Plant, R., and Christensen, H.: Environmental Precursors to Mesoscale Convective Systems, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-16303, https://doi.org/10.5194/egusphere-egu23-16303, 2023.
Based on operational radar observations and high-resolution analyses from the Variational Doppler Radar Analysis System (VDRAS), a bow echo producing high-winds and heavy rainfall that occurred over South China in the pre-rainy season is studied. Results show that this bow echo developed from a quasi-linear convective system (QLCS) and acquired a well-defined bow shape after merging with a pre-line convective cell (CC). Interestingly, the rear-inflow jet (RIJ), which has been well recognized to play a key role in the formation of a bow echo, was absent in this merger-formation bow echo (MFBE). This is ascribed to the weak cold pool and line-end vortices generated within the QLCS as it developed in the monsoon environment of high humidity and weak low-level vertical wind shear.
A new pathway of bow echo formation was proposed instead, which highlighted the importance of the low-level mesovortex (MV) on the leading edge of the QLCS. The MV originated from a weak vertical vorticity band ahead of the QLCS. Vertical vorticity budget analyses revealed that the enhanced stretching effect during the QLCS-CC merger was the main cause of the growth of the MV. The well-developed MV thereby provided a RIJ-like flow wrapping cyclonically from north of the QLCS, forcing the QLCS to distorted into a bow echo. This MV contributed foremost to the near surface gales as well.
Combined with the well-resolved dynamical processes aforementioned, observations from an S band polarimetric radar are employed, aiming to uncover the microphysical and dynamical structures and their interaction processes accounting for the heavy rainfall. The precipitation was shown to be featured of high concentration of large hydrometeors, with maxima basically limited within the intensified MV. The deep QLCS developing far above the freezing level favored for significant ice-phase processes, further enhancing rain rate through melted graupel and hail. High spatiotemporal correlation between the precipitation extremes and the MV suggests the non-negligible role the MV played to determine the microphysics process of this precipitation, which required more detailed researches next.
How to cite: Liu, Q., Xu, X., Zhao, K., and Zhou, A.: A Merger-Formation Bow Echo Caused by Low-Level Mesovortex in South China, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-4332, https://doi.org/10.5194/egusphere-egu23-4332, 2023.
Spring precipitation over the southeastern Tibetan Plateau (SETP) produces more than 34% of annual precipitation, which is comparable to summer precipitation. This pre-monsoon rainfall phenomenon, influenced synthetically by atmospheric circulations and topography, makes the SETP an exception to its surroundings. Here, fine-scale characteristics and typical synoptic backgrounds of this unique phenomenon have been investigated. The spring precipitation over the SETP is characterized by high frequency at hourly scale, with a single diurnal peak at night. Event-based analysis further demonstrates that the spring precipitation is dominated by long-lasting nocturnal rainfall events. From early to late spring, the dominant synoptic factor evolves from terrain-perpendicular low-level winds to atmospheric moisture, influencing the spatial heterogeneity and fine characteristics of the spring precipitation. The westerly-dominated type, featured by lower geopotential height over the TP and enhanced westerlies along the Himalayas, produces limited-area precipitation at those stations located at topography perpendicular to low-level winds. In contrast, the moisture-dominated type is featured by an anomalous cyclone over the Bay of Bengal and induces widespread precipitation around the SETP, which is the leading contributor to the spring precipitation there. Due to the moist environment and weak instability, the spring precipitation influenced by the moisture-dominated type is characterized by long-lasting nocturnal events, with a large portion of weak precipitation. Findings revealed in this study complete the picture of spring precipitation influenced by different dominant synoptic factors over the SETP, which deepen the current understanding of the joint influence of circulation and topography on the hydrological cycle of complex terrains.
How to cite: zhao, Y., li, J., ren, L., li, N., and li, P.: Fine-scale characteristics and dominant synoptic factors of spring precipitation over complex terrain of the southeastern Tibetan Plateau, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-1347, https://doi.org/10.5194/egusphere-egu23-1347, 2023.
The topography plays an essential role in initiation and development on precipitating clouds, therefore has a profound effect on the ultimate spatial distributions of precipitation. This study investigates the fine-scale characteristics of synoptic-induced precipitation over Southwest China, a region characterized by a sequence of steep mountains aligned roughly north-south. Based on the convection-permitting simulation for a realistic case, the results show that the model successfully reproduces the observed precipitation, which is induced by a low-level shear line over Southwest China. The spatial distribution of precipitation over three small-scale mountains (named as M1, M2 and M3 from east to west) exhibits distinct inhomogeneity. The precipitation is notably enhanced on the leeward slope of M1, the high-altitude area of M2, as well as the windward slope of M3, which is driven by the steep topography relief, through exerting dominant influences on the local atmospheric circulations. Further results of the high-resolution experiment shows that the thermal instabilities and topographic lifting over the high-altitude ridges are beneficial to the enhanced precipitation. In addition, the small-scale vortex generated on the leeward slope of M1, as well as the convergence zones established over M2 and the windward slope of M3, dynamically contribute to the intensification of precipitation over these three small-scale mountains. In sensitivity simulation with the terrain height of M2 reduced to the comparable height as the other two mountains, the enhanced precipitation decreases significantly over M2. The dynamic blocking effect of M2 on airflow is weakened, leading to the maximum precipitation over M3 moving to its mountaintop.
How to cite: Zhang, M., Li, J., and Li, N.: Spatial inhomogeneity of synoptic-induced precipitation in a region of steep topographic relief, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-10335, https://doi.org/10.5194/egusphere-egu23-10335, 2023.
Mesoscale convective systems (MCSs) downstream of the Tibetan Plateau (TP) exhibit unique precipitation features. These MCSs can have damaging impacts and there is a critical need for improving the representation of MCSs in numerical models. However, most global climate models are typically run at resolutions that are too coarse to reasonably resolve MCSs, and it is still unclear how well higher-resolution global models can reproduce the precipitation characteristics of MCSs. In this study, the sensitivity of MCSs simulated by a global high resolution (~10km), atmosphere-only climate model to different treatments of convection (with and without parametrized convection, and a hybrid representation of convection) have been investigated. The results show that explicit convection (i.e., non-parameterized) can better reproduce the observed pattern of MCS precipitation over the East Asian Summer Monsoon (EASM) region. In general, explicit convection better simulates the diurnal variability of MCSs over the eastern China, and is able to represent the distinctive diurnal variations of MCS precipitation over complex terrain particularly well, such as the eastern TP and the complex terrain of central-northern China. It is shown that explicit convection is better at simulating the timing of initiation and subsequent propagating features of the MCS, resulting in better diurnal variations and further a better spatial pattern of summer mean MCS precipitation. All three experiments simulate MCS rainfall areas which are notably smaller than those in observations, but with much stronger rainfall intensities, implying that these biases in simulated MCS morphological characteristics are not sensitive to the different treatment of convection.
How to cite: Li, P., Muetzelfeldt, M., Schiemann, R., Chen, H., Li, J., Furtado, K., and Zhuang, M.: Sensitivity of simulated mesoscale convective systems over East Asia to the treatment of convection in a high-resolution GCM , EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-5342, https://doi.org/10.5194/egusphere-egu23-5342, 2023.
The largest type of deep convection, mesoscale convective systems (MCSs), regulate changes in the hydrological cycle and large-scale tropical circulation. During the Indian summer monsoon (June-September), synoptic-scale systems move across the monsoon zone, causing MCSs to form frequently. MCSs cause widespread and heavy rain throughout the monsoon zone. Past MCS studies in India used either observations or simulation in a short period or with case studies approach. Studies on structure and evolution of MCSs highlighting the organization of convection over the monsoon zone are lacking.
In this study, a 4-month, convection-permitting simulation is conducted over the Indian monsoon zone using the Weather Research Forecast (WRF) model with 4-km grid spacing and two microphysics parameterizations and is compared with observations to evaluate composite MCS characteristics and microphysics sensitivities. We first apply a cloud-tracking algorithm to two high-resolution observation data sets, NASA global merged infrared brightness temperature (IR Tb) and GPM IMERG surface precipitation to identify and track individual MCS events during monsoon. Ground-based S-band radar observations are used to examine the 3-D structures of storms embedded within the tracked MCSs and analyze evolution of convective, stratiform and anvil components of the MCSs. A similar cloud-tracking algorithm is then applied to WRF simulated data (radar reflectivity, IR Tb and precipitation) to identify and track MCS in model simulation. As a result, the observed and simulated MCSs are consistently identified and tracked, making it possible to compare WRF MCS population statistics with observed MCSs.
Results show that the properties of MCS including composite evolution, and frequency distribution are reasonably captured by the two simulations with some noticeable differences. In general, the Thompson simulation produces better agreement with observations for convective area and precipitation amount, MCS propagation speed but exhibits underestimation of stratiform area. The composite evolution of simulated MCS cloud and precipitation structures showed a gradual increase from convective initiation to around the first half of the MCSs lifetime, which was consistent with observations. The MCS eccentricity reaches to minimum value at maximum horizontal extent, indicating a quasi-circular shape of MCS. We observed that PDF of MCS precipitation intensity largely agrees well with observations. The highest altitude reached by intense convective cores (30-dBZ echo-tops) is 8 km, but the model significantly underestimates it. The detailed comparison of multiple aspects of MCSs (e.g., initiation, size, intensity, lifetime, propagation) and embedded storms (e.g., convective-stratiform areas) and associated precipitation between the simulation and observations for one monsoon season will be discussed.
How to cite: Tupsoundare, M., Deshpande, S., Feng, Z., Deshpande, M., Das, S. K., and Hanmante, H.: Comparison of Mesoscale Convective Systems in a Seasonal Convection-Permitting Simulation With Observations Over the Indian Monsoon Zone , EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-5258, https://doi.org/10.5194/egusphere-egu23-5258, 2023.
The seasonal migration of the Intertropical Discontinuity (ITD) is critical for monitoring seasonal moist convective processes and associated rainfall over West Africa. This study constitutes a new analysis of the seasonality of moist convection over West Africa, relative to the ITD, based on NASA's Atmospheric Infrared Sounder (AIRS) measurements from 2003-2018. Results show that AIRS resolves the seasonal march of the ITD, including its inherent diurnal-scale variations. AIRS captures the north - south daytime skin temperature dipole around the ITD, with greater relative temperatures to the north, especially during March - August. In the vicinity of the nighttime ITD, AIRS profiles indicate increased instability that is characteristic of nocturnal thunderstorm propagation. On thunderstorm days, the mean latitude of the AIRS-derived ITD is displaced 3o , 0.2o, and 2o north of its DJF, MAM, and SON climatological positions, respectively, and 1.2o south in JJA. The findings of this study are critical to building local tropical weather forecasting capacity and capabilities in West Africa.
How to cite: Osei, M.: Observation of the moist convective environment of West Africa by the Atmospheric InfraRed Sounder, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-1683, https://doi.org/10.5194/egusphere-egu23-1683, 2023.
Hail is by far the greatest contributor to insured losses from severe convective storms on an annual basis. Individual severe convective storm outbreaks can cause hail losses well above EUR 1 bn. In May and June 2022 a series of such events impacted France, Germany and Belgium. Of these, those occurring on 19-22 June 2022 were particularly damaging as they hit the large metropolitan region of Ile-de-France. There were many reports of large hailstones, causing significant damage to property and motor vehicle. Total insured hail loss estimates in France alone exceeded EUR 2.4 bn, of which EUR 1.34 bn of property loss and EUR 1.08 bn of motor vehicle loss. These were the largest hail events in France in terms of losses since Storm Ela’s, which on 9-10 June 2014 resulted in insured hail losses in excess of 900 mn in 2021 EUR.
Common denominator to these two impactful events were persistent meteorological situations conducive to large-scale severe convective storms for several consecutive days. These compounded with local conditions favorable for the development of severe hail. Maximum hailstone sizes of 12 cm in diameter were observed in the administrative regions of Centre-Val-De-Loire (Ela) and Occitanie (June 2022). In this study we present a reconstruction of these events based on eye-witness reports cross-referenced with weather radar data. We analyze the synoptic configurations and pre-convective environments that characterized them, with particular focus on those properties and features that are peculiar to severe hail-forming thunderstorms. These event reconstructions are part of our effort to construct a Realistic Disaster Scenario (RDS) model for France and Belgium to stress test both individual client portfolios and the market as a whole.
How to cite: Panosetti, D., Allen, C., Yaqubi, O., and Thollon, O.: Severe hail in France: reconstruction of Storm Ela’s and late June 2022 hailstorms, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-10212, https://doi.org/10.5194/egusphere-egu23-10212, 2023.
It has been known that potential vorticity (PV) diagnostics can be employed to (a) evaluate large-scale dynamics of hurricane movement, and (b) assess the storm’s influence on its own track. Moreover, PV variations and temperature adjustments at the tropospheric interior and the associated general circulation theory are closely related to the surface PV. Diagnosis of the surface PV is, however, complicated due to data availability/coarse-resolution configurations across the wide terrain in topographic regions. In this work we develop a high-resolution configuration for the Gulf of Mexico (GoM) region for employing in the Weather Research and Forecasting (WRF) model to study Hurricanes Harvey and Ida. The new configuration includes horizontal resolutions of 5 km and 1.67 km in the main and nest domains, respectively; 55 vertical heights (potential pressure levels) are also considered. Forecasts of Hurricane Harvey for 132-hours (5days + 12 hrs), and Hurricane Ida for 78-hours (3days + 6 hrs) are performed, and outputs are stored at every 15 minutes. It is shown that surface PV changes its sign when the Hurricane Harvey/Ida arrives over the land while PV at high altitudes are conserved. We show that the surface PV change is due to the change of vertical temperature gradient at the surface (i.e. change of surface layer stability). These dynamical evolutions are coincident with an increase in precipitation rate and accumulated precipitation of hurricane aftermath. We discuss how these meteorological processes can possibly influence hurricane movements.
How to cite: Khani, S. and Dawson, C. N.: Potential vorticity and surface layer stability in hurricane movement: case studies of Hurricanes Harvey and Ida, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-11156, https://doi.org/10.5194/egusphere-egu23-11156, 2023.
Posters on site: Thu, 27 Apr, 14:00–15:45 | Hall X5
The Amazon rainforest holds more than 40% of all remaining tropical rainforest and is a key component of the climate system. The scale of deforestation in the Amazon significantly impacts both local and global climates. Under a business-as-usual scenario as much as 40% of the Brazilian Amazon rainforest will be lost by 2050. Despite the magnitude of these changes and its importance, the overall effects of deforestation on rainfall remain uncertain. Land-use change influences rainfall through a variety of mechanisms acting at local to continental scales. As such, previous research indicates conflicting responses to rainfall depending on the scales studied. In reality, rainfall processes interact across these scales, but until recently have been impossible to capture within a single model due to computational expense. Consequently we are unable to rely on these simulations as future estimations of rainfall for such a sensitive and anthropogenically impacted region as the Amazon.
In this study, we overcome these limitations by running convection permitting simulations (horizontal resolution 4.5km) over a large domain (6000km covering the majority of South America) using a Tropical configuration of the UK Met Office Unified Model. The high-resolution and continental-scale of these simulations present an opportunity to reduce the uncertainty in Amazonian rainfall estimates within a single model and ensures rainfall processes and interactions across scales are captured. To investigate the impacts of deforestation we will include a series of land-use sensitivity runs making use of a range of socioeconomic scenarios to 2050. Here we present initial results from our simulations, indicating how localised storms, mesoscale convective systems and large-scale circulations respond to land-use change.
How to cite: Bassett, R., Garcia-Carreras, L., Lowe, D., Alves, L., Fisch, G., Halladay, K., Kahana, R., and Veiga, J.: Deforestation and changes in rainfall across the Amazon – reducing uncertainty using a continental scale convection permitting domain , EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-6107, https://doi.org/10.5194/egusphere-egu23-6107, 2023.
Weather models which allow explicit convection can add value to weather forecasting by improving the intensity and timing of precipitating systems and their dynamics, which is particularly valuable in the tropics where moist diurnal convection dominates. In West Africa, convection can become organised to form mesoscale convective systems which are crucial for supplying water but may have devastating impacts, and while convection-permitting models improve forecasts, issues remain in the implementation of operational convection-permitting models. A major problem is initialising weather models in the tropics, where measurements are sparse and weather systems are dominated by nonlinear diabatic processes, which makes data assimilation challenging. Currently, the UK Met Office runs a regional operational convection-permitting model in Africa, the Tropical Africa Model, which is initialised using lower-resolution global models. However, the global models have extremely limited convective-scale structures and as a result, there is a spin-up time of around 12-18 hours before the model begins to accurately reflect precipitation.
To counteract this problem, a “warm-starting” method has been trialled. The warm-starting technique blends large-scale features from the global model and fine-scale fields below a certain length scale from previous runs of the high-resolution model to use as an initial state in a new model run. It combines the more realistic convective structures of the regional model and the more accurate synoptic conditions in the global model. Not only is the approach cheaper and quicker than traditional data assimilation, both in terms of development and computational cost, but it also shows demonstrable improvements in the representation of precipitation for the first 12 hours of the model and beyond relative to simulations where the model has simply been initialised with the global model (a “cold-start”). The cold-start simulations appear to consistently predict rainfall that is too intense even beyond the first 12 hours.
We investigate why warm-starting models produces more realistic rainfall distributions by examining the dynamical structures: producing statistics of rainfall objects as forecasts evolve and examining their connection to the dynamics. We examine the energetics of convection in the convection-permitting models, aiming to provide a justification for the scale length at which we include structure from previous model runs using this technique.
How to cite: Morris, F., Warner, J., Bain, C., Schwendike, J., Parker, D., and Petch, J.: Dynamical Impacts of Warm-Starting Operational Weather Models over Africa, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-5208, https://doi.org/10.5194/egusphere-egu23-5208, 2023.
Convection-permitting (CP) climate simulations represent a major advance in capturing land surface effects on convection. From observational analyses in West Africa, we know that land surface conditions are a major driver of storm initiation as well as intensification during later stages of the storm life cycle. Dry soils of 10 km to several 100s of km scale can cause anomalous warming of the planetary boundary layer and affect horizontal circulations, regional moisture convergence as well as instability. However, to date it remains unclear whether, in a warming climate, larger and more intense storms may change the scale and frequency of surface patterns, feeding back on these identified processes. Here, we evaluate the ability of a pioneering convection-permitting (4.4km) pan-African climate simulation to capture the observed land effects on the pre-convective environment in West Africa and subsequent storm characteristics. This is compared to a CP climate projection representing a decade under a very high emission scenario around 2100 in order to reveal potential changes in process interactions and consequences for organised convection in the future.
How to cite: Klein, C., Barton, E., and Taylor, C.: Changes in land surface effects on organised convection in a convection-permitting climate projection , EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-14475, https://doi.org/10.5194/egusphere-egu23-14475, 2023.
Floods related to heavy precipitation are common over Europe during both the warm and the cold seasons. In a way to better understand these heavy precipitation systems and their potential evolution in a warming climate, several studies investigated the dependency of precipitation extremes to temperature over Europe (e.g. Lenderink et al., 2008; Berg et al., 2013). It was found that the scaling of precipitation extremes can exceed the scaling expected from the Clausius-Clapeyron (CC) relationship, relating temperature to the water holding capacity of the atmosphere. While several potential explanations were proposed, a recent study (Lochbihler et al., 2017) noted the important role of large systems in determining this “super-CC” scaling over the Netherlands.
Building on this study, we further investigate the role of Mesoscale Convective Systems (MCS) in determining the temperature precipitation relationship over Europe. The detection and tracking of MCSs is based on the recent Integrated Multi-satellitE Retrievals for Global Precipitation Measurement (IMERG; Huffman et al., 2019) satellite precipitation climatology. We use the EUropean Cooperation for LIghtning Detection (EUCLID) lightning dataset to distinguish between stratiform (or shallow convective) and deep convective rain patches without introducing bias in precipitation intensity. We select the temperature upstream of the MCS tracks, as a proxy of the moisture source involved in the formation of MCS precipitation.
MCS can display strong dynamical features such as the rear inflow jet or cold pools, of which the effects on precipitation as well as their changes with temperature are still unclear. It suggests that MCS precipitation may deviate significantly from the CC scaling. Additionally, the processes involved in MCS precipitation may differ depending on the stage of the MCS life cycle. We thus characterize the temperature dependency of MCS precipitation and their 2-D structure at different stages of their life cycle. This work contributes to better understanding the drivers of MCS precipitation and how these may evolve in a warming climate.
References
Berg, P., Moseley, C., & Haerter, J. O. (2013). Strong increase in convective precipitation in response to higher temperatures. Nature Geoscience, 6(3), 181-185.
Huffman GJ, Stocker EF, Bolvin DT, Nelkin EJ, Tan J. 2019. GPM IMERG final precipitation L3 half hourly 0.1 degree x 0.1 degree V06, Greenbelt, MD, Goddard Earth Sciences Data and Information Services Center. doi: 10.5067/GPM/IMERG/3B-HH/06
Lenderink, G., & Van Meijgaard, E. (2008). Increase in hourly precipitation extremes beyond expectations from temperature changes. Nature Geoscience, 1(8), 511-514.
Lochbihler, K., Lenderink, G., & Siebesma, A. P. (2017). The spatial extent of rainfall events and its relation to precipitation scaling. Geophysical Research Letters, 44(16), 8629-8636.
How to cite: Da Silva, N. and Hearter, J.: Do Mesoscale Convective Systems precipitation follows the Clausius-Clapeyron relationship?, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-15734, https://doi.org/10.5194/egusphere-egu23-15734, 2023.
In this study we investigate the impact of several selected sources of uncertainty on convective precipitation prediction. For this purpose, we conduct numerical simulations with the ICOsahedral Non-hydrostatic (ICON) model for two consecutive days in June, 2021, on which deep moist convection triggered by different synoptic forcing occurred over southwestern Germany. We use single- and double-moment microphysics schemes and vary the initial soil moisture, grid spacing, and cloud condensation nuclei (CCN) concentration. We compare the results with measurements conducted on the same two days during the Swabian MOSES (Modular Observation Solutions for Earth Systems) field campaign. We find that the applied dry bias (initial soil moisture in the model reduced by 25%) much better represents the actual soil moisture conditions and leads to an improved quantitative precipitation forecast when compared to radar-derived precipitation amounts. Furthermore, the model resolution impacts the precipitation amount, intensity, and the timing of convection initiation: while 1-km runs show the least root mean square error for 24-hour precipitation sums, the onset of convective precipitation in 2-km resolution runs matches better the observations. However, the overall impact of this factor is not always systematic. The comparison of several radiosounding-derived convective indices (e.g. lifted index, convective available potential energy, convective inhibition) with model data yield many non-systematic results. For instance, CCN concentrations do not seem to have any significant impact on any of the calculated indices. At the same time, runs with coarser resolution (2-km) often better depict the temporal development of CAPE but overestimate its amount.
How to cite: Czajka, B., Barthlott, C., Kohler, M., Wieser, A., and Hoose, C.: Analysis of the impact of selected sources of uncertainty on precipitation simultaions of summer convection over Central Europe , EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-14661, https://doi.org/10.5194/egusphere-egu23-14661, 2023.
To improve sub-seasonal forecasts, different global initiatives generate continental convection-permitting simulations for resolution less than 10 kilometres for multiple decades. These simulations, however, are based on land use maps with only single urban type. In this study, we explore how the density and height information of the urban canopy based on Local Climate Zones (LCZs) affect the dynamics among temperatures, precipitation and land use types for the 2022 summer heatwave in the Southwest UK. Four numerical experiments at a 3 km grid are run by switching off the parameterization of deep-convection in the Weather Research and Forecasting (WRF) models. These experiments are based on (i) the no urban scenario, (ii) the default MODIS land use scheme, (iii) the building environment parameterization (BEP), and (iv) the building energy model (BEM).
Results show that the cold advection over the UK led to downward motion according to a Q-vector analysis. The regional downward motion caused the formation of a heat dome. It is against the hypothesis that the 2022 summer heatwave was due to the hot circulation from Spain and equatorial Africa. Even though four land use schemes have similar simulated cold advection across the UK, our findings show that land use types affected water recycling due to local convection differently. These differences were related to the strength of rainstorms at the dissipating heatwave stage. Our results suggest that urban areas were more likely to have more persistent heatwaves since the intensity of rainstorms was affected by the lower local water recycling. This advanced understanding of the UK heatwave mechanism based on regional advection conditions and local convection processes will guide us on how to improve our sub-seasonal forecast in the urban area.
How to cite: Chun, K. P., Ezber, Y., Toker, E., Simões Reboita, M., Porfirio da Rocha, R., Risanto, B. C., Yetemen, O., Octavianti, T., Quinn, N., Sens, O. L., and Castro, C.: Investigating links among heatwaves, precipitation, and land use types using the Convection-Permitting Model in the Southwest UK for the 2022 boreal summer, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-9500, https://doi.org/10.5194/egusphere-egu23-9500, 2023.
Large hail, with a diameter of at least 20 mm, is a hazard associated with severe convective storms (SCS) that can cause significant damage. Understanding of atmospheric environments conducive to large hail is underpinned by catalogues of past events. Because of the small footprint of hail events, these often rely on crowdsourced reports. In the UK, the relative rarity of large hail and low public awareness of SCS hazards makes obtaining a complete set of reports difficult, and in many cases the precise time of the hail is not recorded. In this study, the two major databases of UK large hail reports are merged for the first time. Composite radar reflectivity data are used to verify and enhance 260 reports since 2006. Time of the hail and the basic storm mode (isolated, clustered or linear) are visually estimated from animations. Compared to the UK’s most severe historic hailstorms (1800–2004), our quality controlled climatology of all sizes of large hail shows a diurnal cycle with a slightly broader peak. Around 55% of large hail events are associated with isolated cells, while 34% have supercellular characteristics, a much lower proportion than found in the USA. The full event set (1979–2022), comprising over 850 reports, is used to update the seasonal, spatial and size distributions of large hail in the UK. We intend that this hail event set forms part of a multi-hazard analysis of UK SCS, also including tornadoes and extreme rainfall, and its relationship to background atmospheric conditions. The effect of climate change on UK SCS will be investigated through past and future trends in these background conditions.
How to cite: Wells, H., Hillier, J., Garry, F., Dunstone, N., Chen, H., Kahraman, A., Keat, W., and Clark, M.: Enhanced climatology of large hail in the UK: Radar-derived diurnal cycle and storm mode, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-3406, https://doi.org/10.5194/egusphere-egu23-3406, 2023.
The predictability of deep moist convection is subject to large uncertainties, mainly due to inaccurate initial and boundary data, incomplete description of physical processes, or uncertainties in microphysical parameterizations. In this study, we present hindcasts of a supercell storm that occurred during the Swabian MOSES field campaign in southwestern Germany in summer 2021. The supercell storm of 23 June 2021 passed directly over the main observation site equipped with various instruments, allowing a detailed comparison of simulations and observations. The preconvective radiosonde observations revealed suitable conditions for supercell development, i.e., low convective inhibition, moderate convective available potential energy, sufficient deep-layer shear, and a Bulk Richardson number of 22. Numerical simulations were performed with the ICOsahedral Non-hydrostatic (ICON) model using two horizontal grid spacings (i.e., 2 km/1 km) with a single-moment and a double-moment microphysics scheme. The double-moment scheme allows us to study aerosol effects on clouds and precipitation with cloud condensation nuclei (CCN) concentrations ranging from low to very high. Numerical results show that all 2-km model realizations do not simulate convective precipitation at the correct location and time. For the 1-km grid spacing, changes in aerosol concentration resulted in large changes in convective precipitation, causing the supercell to disappear completely in some simulations. Only the 1-km model run, which assumes a clean environment, is able to realistically capture the supercell storm. During the Swabian MOSES field campaign, aerosol particle concentrations and size distributions were continuously measured with an optical particle counter from June to August 2021. The day of the supercell storm was characterized by the lowest potential CCN values of the month, suggesting that the low aerosol concentration in the successful model run is a reasonable assumption for this case study. Possible reasons for the discrepant model results, i.e., effects of grid spacing on convection initiation and detailed analyses of microphysical process rates, are discussed. These results demonstrate the benefits of using an aerosol-aware double-moment microphysics scheme at high model resolution for convection initiation and cloud evolution, and that the use of different CCN concentrations can determine whether a supercell is successfully simulated or not.
How to cite: Barthlott, C., Czajka, B., Kohler, M., Hoose, C., Kunz, M., Saathoff, H., and Zhang, H.: Grid spacing effects on convection initiation and aerosol-cloud interactions: A case study of a supercell storm from the Swabian MOSES 2021 field campaign, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-6994, https://doi.org/10.5194/egusphere-egu23-6994, 2023.
On the western coastal region of the Korean Peninsula in the winter, heavy snowfall occurs due to air mass modification which is called the western coast snowfall. The western coast snowfall occurs through a mechanism similar to the lake effect snowfall that is formed over the Great Lakes in the North America. In the winter season, cold and dry northwesterly wind blowing from the south-eastern flank of the Siberian High is formed over the Yellow Sea. The cold and dry air mass from the continent gets heat and moisture from the ocean when it passes over the relatively warm sea surface, , which invigorate snow clouds. The snow clouds generated over the ocean flow into the western coastal region by the westerly winds, hence predicting when the snow clouds flow into the land is important for snowfall forecast. Although the western coast snowfall can persist for several days when the synoptic environment is maintained, diurnal fluctuations in snowfall inflow appears during the snowfall cases. In this study, the diurnal variation of snowfall inflow on the western coastal region was investigated by analyzing of the dynamic/thermodynamic factors affecting the diurnal variation. The western coast snowfall shows snowfall inflow into the land in the evening, then inflow decreases after sunrise, with the snowfall becoming concentrated over the ocean, and the snowfall inflow increases after sunset. The diurnal variation of the snowfall inflow structure appears with the diurnal variation of the dynamic/thermodynamic structure according to the solar radiation diurnal cycle. During the evening, as the temperature of the lower troposphere over the land decreases due to radiative cooling, the lower troposphere thermal stability increases. After sunrise, the planetary boundary layer (PBL) height grows due to radiative heating, and the wind in the lower troposphere weakens, limiting the snowfall inflow to 9 LST, where both factors affect simultaneously. In addition, as the lower troposphere temperature over the land decreases, the land-sea horizontal temperature contrast increases, and the density wall of the colder land blocks the inflow of the snowfall. On the other hand, during the daytime, the lower troposphere temperature of the land rises due to radiative heating and the thermal stability decreases. As the horizontal temperature contrast decreases and the PBL height decreases after sunset, the lower troposphere wind becomes stronger, which allows the snowfall penetrates into the land. According to the time lag in heating/cooling by radiation of the lower troposphere, it is analyzed that the time point of snowfall inflow interruption (increasing) appears after sunrise (sunset).
Acknowledgement
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government(MSIT) (No. 2021R1A4A1032646).
How to cite: Yoo, S., Chang, E.-C., and Lee, G.: A study on the diurnal variation of the snowfall structure in the western coastal region of the Korean Peninsula., EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-11509, https://doi.org/10.5194/egusphere-egu23-11509, 2023.
