Mountains cover approximately one quarter of the total land surface on the planet, and a significant fraction of the world’s population lives in their vicinity. Orography critically affects weather and climate processes at all scales and, in connection with factors such as land-cover heterogeneity, is responsible for high spatial variability in mountain weather and climate.

Due to this high complexity, monitoring and modeling the atmosphere and the other components of the climate system in mountain regions is challenging both at short (meteorological) and long (climatological) time-scales. This session is devoted to the better understanding of weather and climate processes in mountain and high-elevation areas around the globe, as well as their modification induced by global environmental change.

We welcome contributions describing the influence of mountains on the atmosphere on meteorological time-scales, including terrain-induced airflow, orographic precipitation, land-atmosphere exchange over mountains, forecasting and predictability of mountain weather. Furthermore we invite studies that investigate climate processes and climate change in mountain areas and its impacts on dependent systems, based on monitoring and modeling activities. Particularly welcome are contributions that merge various sources of information and reach across disciplinary borders (atmospheric, hydrological, cryospheric, ecological and social sciences). In this respect the session invites also contributions on outcomes of the WMO "High Mountain Summit" taking place in October 2019.

Public information:
Online survey (including questions about a possible follow-up webinar): https://form.jotformeu.com/83462227858365

Co-organized by AS1/CR7/NH1
Convener: Wolfgang Schöner | Co-conveners: Carolina Adler, Maria Vittoria Guarino, Elisa Palazzi, Stefano Serafin
| Attendance Mon, 04 May, 14:00–15:45 (CEST)

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Chat time: Monday, 4 May 2020, 14:00–15:45

D3883 |
Enrico Arnone, Nick Pepin, Elisa Palazzi, Sven Kotlarski, Silvia Terzago, Petra Seibert, and Herbert Formayer

Mountain and high elevation regions often show distinct climate trends in temperature and precipitation, which can contrast those of adjacent lowland regions. In the context of temperature, this phenomenon is known as elevation-dependent warming (EDW). Past temperature trends can increase with elevation, but this is not always so, and they may peak in a critical elevation band, or show more complex elevation profiles. This is controlled by a variety of mechanisms which may be responsible for the observed patterns, including snow albedo feedback, vegetation change, cloud and moisture patterns, aerosol forcing and their interactions.

We here present a literature-based meta-analysis of elevation profiles in recent warming rates and, in a more general context, temperature change in mountain regions around the globe. For the recent historical period (~1960-2010) we find that when comparing like with like (i.e. high elevation regions with adjacent low elevation regions) warming rates are mostly stronger at higher elevations. Warming rates have also increased over time, with more recent decades showing stronger warming. On a global scale there is no significant difference between mean warming rates in mountains and in other areas. Thus, elevation-dependency within regions can be masked by differences in geographical location in global meta-analyses. Although there have been far fewer studies on vertical profiles of precipitation changes, we extend our meta-analysis to consider this parameter,  where information is available.

In addition to the meta-analysis, we compare past temperature and precipitation changes in mountain and lowland regions using global gridded observation-based and reanalysis datasets (e.g. CRU, ERA5, NCEP2) and global climate model simulations (CMIP5). Despite the uncertainties of these datasets (e.g. inhomogeneous underlying station coverage and related interpolation errors, biases, coarse spatial resolution), they allow us to compare different mountain regions globally with the same level of accuracy. There are only a few mountain areas that show distinct differences when their temperature trends are compared with lowland surroundings, but patterns vary by dataset and region. We also explore different extensions of adjacent lowlands, which may influence the quantification of differences in temperature and precipitation trends at high and low elevation.

This historical assessment is completed by an analysis of model projections (CMIP5) for studying the expected future evolution of climate change in mountains and contrasts to adjacent lowlands

How to cite: Arnone, E., Pepin, N., Palazzi, E., Kotlarski, S., Terzago, S., Seibert, P., and Formayer, H.: Climate change in mountains around the globe: Elevation dependencies and contrasts to adjacent lowlands, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15589, https://doi.org/10.5194/egusphere-egu2020-15589, 2020.

D3884 |
Julien Beaumet, Martin Menegoz, Hubert Gallée, Vincent Vionnet, Xavier Fettweis, Samuel Morin, Juliette Blanchet, Nicolas Jourdain, Bruno Wilhelm, and Sandrine Anquetin

The European Alps are particularly sensitive to climate change. Compared to temperature, changes in precipitation are more challenging to detect and attribute to ongoing anthropic climate change mainly as a result of large inter-annual variability, lack of reliable measurements at high elevations and opposite signals depending on the season or the elevation considered. However, changes in precipitation and snow cover have significant socio-environmental impact mostly trough water resource availability. These changes are investigated within the framework of the Trajectories initiative (). The variability and changes in precipitation and snow cover in the European Alps has been simulated with the MAR regional climate model at a 7 km horizontal resolution driven by ERA20C (1902-2010) and ERA5 (1979-2018) reanalyses.

For precipitation, MAR outputs were compared with EURO-4M, SAFRAN, SPAZM and E-OBS reanalyses as well as in-situ observations. The model was shown to reproduce correctly seasonal and inter-annual variability. The spatial biases of the model have the same order of magnitude as the differences between the three observational data sets. Model experiment has been used to detect precipitation changes over the last century. An increase in winter precipitation is simulated over the North-western part of the Alps at high altitudes (>1500m). Significant decreases in summer precipitation were found in many low elevation areas, especially the Po Plain while no significant trends where found at high elevations. Because of large internal variability, precipitation changes are significant (pvalue<0.05) only when considering their evolution over long period, typically 60-100 years in both model and observations.

Snow depth and water equivalent (SWE) in the French Alps simulated with MAR have been compared to the SAFRAN-Crocus reanalyses and to in-situ observations. MAR was found to simulate a realistic distribution of SWE as function of the elevation in the French Alpine massifs, although it underestimates SWE at low elevations in the Pre-Alps. Snow cover over the whole European Alps is evaluated using MODIS satellite data. Finally, trends in snow cover and snow depth are highlighted as well as their relationships with the precipitation and temperature changes over the last century.

How to cite: Beaumet, J., Menegoz, M., Gallée, H., Vionnet, V., Fettweis, X., Morin, S., Blanchet, J., Jourdain, N., Wilhelm, B., and Anquetin, S.: Detection of precipitation and snow cover trends in the the European Alps over the last century using model and observational data, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18274, https://doi.org/10.5194/egusphere-egu2020-18274, 2020.

D3885 |
Carolyne Pickler and Thomas Mölg

Anthropogenic influence on climate change has increased over time and has been detected in all major components of the climate system.  High altitude mountains constitute a highly-sensitive region.  This has lead to many studies, on varying scales, with detection of climate change as motivation.  However, questions persist as to how this anthropogenic influence is manifested in the mesoscale over these mountains and how it transfers between various scales.

A case of study of Kilimanjaro and the glaciers on its summit is undertaken to start addressing these questions.  Its unique location, an isolated peak with a summit at almost exactly 500 hPa, allows for the examination of the large and local scale climate change dynamics and how they are linked by the mesoscale circulation over the mountain.  Furthemore, it has been extensively studied on the large and local scale and has decadal automated weather station records.  A first step involves running the limited-area Weather and Research Forecasting (WRF) regional climate model over the East African region for the period of 1985-2015 using multiple grid nesting centred over Kilimanjaro.  The lateral boundaries of WRF will be forced with output from two simulations, historical and historicalNat, of a global climate model (BNU-ESM r1i1p1) from the Coupled Model Intercomparison Project Phase 5 (CMIP5).  These two simulations differ by the addition of anthropogenic forcing in the historical simulation.  The model was carefully selected by a rigorous testing procedure, where analysis of the top 5 ranked models yielded a first estimate of anthropogenic influence in East Africa.  Comparison of WRF output from both simulations will be undertaken to assess how anthropogenic forcing has affected dynamical (e.g. flow regimes) and microphysical processes (e.g. cloud composititon and stability) in the mesoscale over Kilimanjaro.

How to cite: Pickler, C. and Mölg, T.: Investigating the anthropogenic influence on the mesoscale over Kilimanjaro, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15536, https://doi.org/10.5194/egusphere-egu2020-15536, 2020.

D3886 |
Julia Curio, Reinhard Schiemann, Kevin Hodges, Andrew Turner, and Nicholas Klingaman

The Tibetan Plateau (TP) and surrounding high mountains constitute an important forcing of the atmospheric circulation due to their height and extent, and thereby impact weather and climate in East Asia. Mesoscale Tibetan Plateau Vortices (TPVs) form over the TP and are one of the major systems generating TP precipitation. The majority of TPVs remain on the TP throughout their lifetime, while a fraction moves east off the TP. These “moving-off” TPVs can trigger extreme precipitation and severe flooding over large parts of eastern and southern China, for example in Sichuan province and the Yangtze River valley. Due to their potentially severe impacts downstream of the TP, it is first important to understand the conditions under which TPVs can move east off the TP.

In this study, we examine the vertical and horizontal structure of TPVs moving off the TP in contrast to those that do not using reanalysis in order to understand which local and/or large-scale atmospheric conditions lead TPVs to move off the TP. We use composites of atmospheric fields at different stages of the TPV lifecycle (e.g. genesis, maximum intensity, and maximum precipitation) and at different locations over and downstream of the TP, to account for the heterogeneous topography. Preliminary results suggest that the large-scale background flow, characterised by the strength and position of the subtropical westerly jet, is one of the factors determining whether a TPV moves off the TP or not.

Another important question is how and where moving-off TPVs trigger precipitation. Do TPVs transport moisture from the TP to the downstream regions? Do they move off while already precipitating? Do they trigger precipitation dynamically east of the TP? Results from a case study suggest that the TPV triggers precipitation as it moves over the edge of the TP, which then stays locked to the orography while the system is moving further east. The TPV appears to change the local atmospheric circulation in the Sichuan basin while moving off, thereby directing a flow of moist air towards the eastern slope of the TP.

Understanding how the combination of the right large-scale atmospheric conditions and a TPV-induced change in the local circulation downstream of the TP can create an impactful TPV may enable improved forecasts of TPVs and their impacts in the densely populated regions downstream of the TP.

How to cite: Curio, J., Schiemann, R., Hodges, K., Turner, A., and Klingaman, N.: Impactful Tibetan Plateau Vortices: structure, lifecycle and environmental conditions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16856, https://doi.org/10.5194/egusphere-egu2020-16856, 2020.

D3887 |
Xuelong Chen, Yue Lai, and Yaoming Ma

The spatial-temporal structure of the Planetary Boundary Layer (PBL) over mountainous areas can be strongly modified by topography. The PBL over the mountainous terrain of the Tibetan Plateau (TP) is more complex than that observed over its flat areas. To date, there have been no detailed analyses which have taken into account the topography effects exerted on PBL growth over the Tibetan Plateau (TP). A clear understanding of the processes involved in the PBL growth and depth over the TP’s mountainous areas is therefore long overdue.
The PBL in the Himalayan region of the Tibetan Plateau (TP) is important to the study of interaction between the area’s topography and synoptic circulation. This study used radiosonde, in-situ measurements and ECMWF ERA5 reanalysis dataset to investigate the vertical structure of the PBL and the land surface energy balance in the Rongbuk Valley on the north of the central Himalaya, and their association with the Westerlies, which control the climate of the Himalaya in winters. Measurements show that the altitude of the PBL’s top in November was the highest of three intensive observation periods (i.e., June, August and November). The PBLs in November appeared to have been influenced by the Westerlies which prevails in this region during the non-monsoon season. We discovered that the deep PBLs seen in November correlate with the downward transmission of the Westerlies to the valley floor (DTWTV). It was found that DTWTV happened in the direction of southwest when the synoptic wind above the valley ridges height blow from southwest, which is parallel to the valley axis. DTWTV happened in the direction of southwest promotes a stronger near-surface wind, smaller aerodynamic resistance, and larger sensible heat flux, which cause PBLs grow high.

How to cite: Chen, X., Lai, Y., and Ma, Y.: The impact of the Westerlies on the PBL growth and land surface energy balance on the north of the central Himalaya, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1329, https://doi.org/10.5194/egusphere-egu2020-1329, 2020.

D3888 |
Francois Lott, Bruno Deremble, and Clément Soufflet

A non-hydrostatic theory for mountain flow with a boundary layer of constant eddy viscosity is presented. The theory predicts that dissipation impacts the dynamics over a an inner layer which depth δ is predicted by viscous critical level theory. In the near neutral case, the surface pressure decreases when the flow crosses the mountain to balance an increase in surface friction along the ground. This produces a form drag which can be predicted quantitatively. With stratification, internal waves start to control the dynamics and produce a wave drag that can also be predicted. For weak stratification, upward propagating mountain waves and reflected waves interact destructively and low drag states occur, whereas for moderate stability they interact constructively and high drag states are reached. In very stable cases the reflected waves do not affect the drag much.

The sign and vertical profiles of the Reynolds stress are profoundly affected by stability. In the neutral case and up to the point where internal waves interact constructively, the Reynolds stress in the flow is positive, with maximum around the top of the inner layer, decelerating the large scale flow in the inner layer and accelerating it above. In the stable case, the opposite occurs, and the large scale flow above the inner layer is decelerated as expected for dissipated mountain waves. These opposed behaviors challenge how mountain form drag and mountain wave drag should be parameterized in large-scale models.

The structure of the flow around the mountain is also strongly affected by stability: it is characterized by non separated sheltering in the neutral case, by upstream blocking in the very stable case, and at intermediate stability by the presence of a strong but isolated wave crest immediately downstream of the ridge.

How to cite: Lott, F., Deremble, B., and Soufflet, C.: Mountain waves produced by a stratified shear flow with a boundary layer: transition from downstream sheltering to upstream blocking, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5343, https://doi.org/10.5194/egusphere-egu2020-5343, 2020.

D3889 |
Annelize VanNiekerk and Irina Sandu

Mountains are know to impact the atmospheric circulation on a variety of spatial scales and through a number of different processes. They exert a drag force on the atmosphere both locally through deflection of the flow and remotely through the generation of atmospheric gravity waves. The degree to which orographic drag parametrizations are able to capture the complex impacts on the circulation from realistic orography in high resolution simulations is examined here. We present results from COnstraing ORographic Drag Effects (COORDE), a project joint with the Working Group on Numerical Experimentation (WGNE) and Global Atmospheric System Studies (GASS). The aim of COORDE is to validate parametrized orographic drag in several operational models in order to determine both systematic and model dependent errors over complex terrain. To do this, we compare the effects of parametrized orographic drag on the circulation with those of the resolved orographic drag, deduced from km-scale resolution simulations which are able to resolve orographic low-level blocking and gravity-wave effects. We show that there is a large spread in the impact from parametrized orographic drag between the models but that the impact from resolved orography is much more robust. This is encouraging as it means that the km-scale simulations can be used to evaluate the caveats of the existing orographic drag parametrizations. Analysis of the parametrized drag tendencies and stresses shows that much of the spread in the parametrized orographic drag comes from differences in the partitioning of the drag into turbulent and flow blocking drag near the surface. What is more, much of the model error over complex terrain can be attributed to deficiencies in the parametrized orographic drag, particularly coming from the orographic gravity wave drag.

How to cite: VanNiekerk, A. and Sandu, I.: COnstraining ORographic Drag Effects (COORDE): A model intercomparison of resolved and parametrized orographic drag, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-75, https://doi.org/10.5194/egusphere-egu2020-75, 2020.

D3890 |
Pleun Bonekamp, Niko Wanders, Karin van der Wiel, Arthur Lutz, and Walter Immerzeel

Natural disasters in High Mountain Asia (HMA) are largely induced by precipitation and temperatures extremes. Precipitation extremes will change due to global warming, but these low frequency events are often difficult to analyse using (short) observed time series. In this study we analysed large  ensembles (2000 year) of present day climate and of a 2 °C and 3 °C warmer world produced with the EC-EARTH model. We performed a regional assessment of climate indicators related to temperature and precipitation (positive degree days, accumulated precipitation, (pre- and post-) monsoon precipitation), their sensitivity to temperature change and the change in return periods of extreme temperature and precipitation in a 2 and 3 °C warmer climate.

In general, the 2°C warmer world shows a rather homogeneous response of changes in climate indicators and return periods, while distinct differences between regions are present in a 3C warmer world and it no longer follows a general trend. This non-linear effect can indicate the presence of a tipping point in the climate system. The most affected regions are located in monsoon-dominated regions, where precipitation amounts, positive degree days, extreme temperature, extreme precipitation and compound events are projected to increase the most. Largest changes in climate indicators are found in the Hindu Kush and Himalaya regions. We also found that precipitation increases in HMA in a 3°C warmer world are substantially larger (13%) compared to the global average (5.9%). Additionally, the increase in weather extremes will exacerbate natural hazards with large possible impacts for the mountain people. The results of this study could provide importance guidance for formulating climate change adaptation strategies in HMA.

How to cite: Bonekamp, P., Wanders, N., van der Wiel, K., Lutz, A., and Immerzeel, W.: Shifts in in High-Mountain Asia’s mountain-specific climate indicators derived with large ensemble modelling, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9260, https://doi.org/10.5194/egusphere-egu2020-9260, 2020.

D3891 |
Remco de Kok and Walter Immerzeel

Glaciers are growing in a part of High Mountain Asia (HMA), contrary to the demise of glaciers worldwide. A proposed explanation for this behaviour is the decreasing strength of the "Western Tibetan Vortex" (WTV), a circular motion of air in the troposphere around northwestern High Mountain Asia, which is proposed to drive near-surface temperatures. Here, we show that the WTV is the change of wind field resulting from changes in near-surface temperature, and that it is not unique to northwestern HMA, but is generally applicable to large parts of the globe. Instead, we argue that net radiation is likely the main driver of near-surface temperatures in Western HMA in summer and autumn, and that the WTV is the response of the atmosphere to changes in temperature. The decreasing strength of the WTV, as seen during summer in the 20th century, is thus likely the result of changing net radiation, and not the main driver of cooling itself. We do argue that the WTV is a useful concept to understand large scale climate variability in the region, and that such an approach could yield important insights in other mid-latitude regions as well.

How to cite: de Kok, R. and Immerzeel, W.: The Western Tibetan Vortex as an emergent feature of near-surface temperature variations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-338, https://doi.org/10.5194/egusphere-egu2020-338, 2020.

D3892 |
xiao pan, Yunfei Fu, and Deqin Li

The characteristics including cloud occurrence frequencies, vertical structure, configuration of cloud type, and microphysical structure of single-layer and multi-layer clouds in Tibetan Plateau (TP) in summer (June-August) during 2007-2010 are investigated based on the CloudSat merged data. The results indicate that cloud over the TP is mainly in the form of single-layer cloud with occurrence frequency of 56.86%, and then followed by the form of double-layer cloud with 24.47%. The spatial distribution of occurrence frequency shows that the single-layer cloud is mainly located in the northern plateau, and fraction of multi-layer cloud decrease gradually from the southeast to the northwest. Single-layer clouds mainly consist of stratocumulus (22.71%), and then followed by altostratus (19.98%) and nimbostratus (19.42%). As for the multi-layer clouds, the upper layers mainly consist of cirrus and altostratus, and the middle layers are mainly dominated by altostratus, cirrus and altocumulus. The lower layers mainly consist of stratocumulus, altocumulus and cumulus. The vertical structure indicates that the averaged cloud thicknesses of single-layer are larger compared with multi-layer clouds. The distributions of microphysical characteristics of multi-level clouds and single-layer clouds are similar, while the averaged values of microphysical characteristics including particle number concentration, cloud water content and effective radius of single-layer are larger. Moreover, the microphysical variable values of upper cloud are lower compared with lower cloud, which are related to the cloud types. The precipitation is mainly in the form of liquid precipitation, and then followed by the solid precipitation, and the drizzle. Furthermore, the drizzle occurs mainly in the multi-layer clouds. The single-layer fraction in the daytime (62.99%) is larger than that at night (51.00%), whereas, multi-layer clouds are opposite. The fraction of liquid precipitation and deep convection are larger during the daytime than those at night. Conversely, the fractions of drizzle and nimbostratus are larger at night. In addition, higher surface temperature, larger surface specific humidity and higher surface pressure is found to be contributed to the formation of multi-layer clouds.

How to cite: pan, X., Fu, Y., and Li, D.: The Characteristics of Multi-layer Clouds in Summer over Tibetan Plateau Based on CloudSat Measurements, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12662, https://doi.org/10.5194/egusphere-egu2020-12662, 2020.

D3893 |
Xuelin Hu

Accurate simulation and prediction of intense precipitation events require better understanding of their physical mechanisms. This study uses Yaan—a place with regional maximum rainfall in central China—to investigate the cause and process of intense precipitation. Hourly rain gauge records and the new ERA5 reanalysis are used to characterize the evolution process of warm season intense regional rainfall events (RREs) in Yaan and its associated three-dimensional circulation. Results show that before the start of the Yaan intense RREs, moderate rainfall amount (frequency) appears northeast of the key region. The rainfall then moves southward in the following several hours along the eastern periphery of the Tibetan Plateau where it reaches peak. It then moves to and end up in the south and southeast Sichuan Basin. The progression of the RREs is found to be associated with a counter-clockwise rotation of anomalous surface winds associated with a developing mesoscale surface low-pressure center, which is further associated with the southeastward progression of a large-scale synoptic scale wave. The easterly phase of the winds in the counter-clockwise rotation causes upslope motion perpendicularly toward the terrain that leads to maximum rainfall. The findings illustrate how large-scale circulations, mesoscale systems, and specific topographic features interact to create the RREs evolution in Yaan.

How to cite: Hu, X.: The Evolution Process of Warm Season Intense Regional Rainfall Events in Yaan, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1349, https://doi.org/10.5194/egusphere-egu2020-1349, 2020.

D3894 |
Bibhasvata Dasgupta and Prasanta Sanyal

The anomalous behaviour of Karakoram Glaciers (Hewitt, 2005) in the backdrop of a warming planet has been a decade long debate baffling climatologists worldwide. While a lot of effort has been given to understand this behaviour, very little has been explored with respect to the factors that favour glaciation rates. A fundamental approach to glacial mass budget calculation involves a simplistic assessment of accumulation and melt. Analysis of meteorological datasets over the last 40 years yields conflicting scenarios. On one hand, we have observed a significant negative trend in winter rainfall and snowfall amount coupled with increasing surface temperatures and vertical mixing of atmospheric vapour. On the other hand, parameters that reflect the bulk of a cryospheric reservoir such as snow depth, dry snow/wet snow percentages show stable to increasing trend. Between lower moisture input and potential ablation rates, the steady-state nature of Karakoram glaciers have emulated optimism in the works of climatologists worldwide. In this study, we have tried to formulate an ‘accumulation index’ as a function of moisture input, surface temperature and atmospheric vertical circulation. Precipitation trends are negative yet periodic which suffices a positive accumulation rate. At the same time, local factors such as debris field and wet snow cover area help preserve the accumulated bulk of a given winter through the upcoming warm summers. However, in a potentially warming planet, accumulation rates aren't proportional to ambient temperature. Studies show that the mass balance turns sharply negative at temperatures above -10 °C due to accelerated ablation which overcompensates accumulation. This makes the Karakoram phenomenon a function of global meteorology rather than local factors i.e. debris cover, vorticity, etc. Therefore, we suggest that the Karakoram Glaciers aren’t behaving anomalously, but lagging in phase with central and eastern Himalayan glaciated regions.

How to cite: Dasgupta, B. and Sanyal, P.: The Karakoram Predicament, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-628, https://doi.org/10.5194/egusphere-egu2020-628, 2020.

D3895 |
Muhammad Saleem Pomee, Elke Hertig, and Bashir Ahmad

The Indus River system originates within high mountain ranges of Hindukush, Karakoram and Himalayans (HKH) and contains the largest cryosphere outside the Polar Regions. It assures livelihood of millions of people, before descending into the Arabian Sea. Different processes, which involve complex interplays of contrasting synoptic-scale circulations and regional topography, largely govern precipitation, which varies significantly with space-time and altitudes in upper Indus basin (UIB). In contrast, the Lower Indus (LI) has arid to semi-arid climate and depends heavily on melt-dominated water supply from the UIB. Considering climate hotspot nature of this basin, a pragmatic assessment of future precipitation and temperature changes at basin-scale are fundamental to provide effective policy advice.

However, long-term, reliable and consistent data to effectively simulate orographic climatology within UIB that largely governs the basin hydrology is scarce. Consequently, even the mean direction of regional climate is highly controversial and ranging from rapidly retreating glaciers to the so-called “Karakoram anomaly”. While the provision of additional useful data is still an ongoing process, improvements in simulation methodologies using the available observational network, can still offer some opportunities to reduce uncertainties. One way is to make use of large-scale atmospheric circulations, which are modeled more reliably than precipitation itself. Moreover, the circulation-precipitation relationships can additionally explain governing mechanisms to improve confidence in resulting simulations.

In our study, we modeled observed precipitation and temperature (Tmax and Tmin) dynamics of the entire basin. A seasonally and spatially differentiated analysis was done using improved UIB monitoring, which provide enhanced spatio-altitudinal information. By taking advantage of the recent high-altitudes (HA) installations within UIB, we argue that precipitation at relatively low-altitudes only quantitatively differ from HA rates, but share a significant joint variability at sub-regional scales. Therefore, the low-altitude stations (historic) can provide reasonable inferences about more uncertain orographic structure of UIB. We adapted generalized linear models (GLMs) with Tweedie and Gamma distributions to model precipitation and multiple linear regressions (MLRs) for temperature simulations using time-series of carefully selected regionally representatives, as predictand and principal component scores of different larger-scale dynamical and thermodynamic variables from ERA-Interim reanalysis, as predictors. The final regression models, which were identified through a cross validation framework, showed significant statistical skills and physical consistency to simulate observed seasonal precipitation and temperature variability over larger spatio-altitudinal scales.    

We further used the predictors to identify better performing regional and seasonal CIMP5- GCMs by comparing predictors through Taylor diagrams in the historical period. ERA-Interim predictors served as a basis for evaluation. Reanalysis uncertainties were assessed by using also NCEP-NCAR-II and ERA5 reanalysis. We considered two radiative forcings (RCP4.5 and RCP8.5) to analyze median change signals of precipitation (temperature) during mid (2041-2070) and end of 21st century (2071-2100). The signal to noise ratio was computed to evaluate future changes compared to observed natural variability. 


How to cite: Pomee, M. S., Hertig, E., and Ahmad, B.: Precipitation and temperature projections for the Indus River basin of Pakistan during 21st century using statistical downscaling , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5828, https://doi.org/10.5194/egusphere-egu2020-5828, 2020.

D3896 |
Osmar Bonfim, Luca Mortarini, Ivan Toro, and Elisa Palazzi

How to cite: Bonfim, O., Mortarini, L., Toro, I., and Palazzi, E.: Elevation-dependent warming in the tropical and subtropical Andes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3079, https://doi.org/10.5194/egusphere-egu2020-3079, 2020.

D3897 |
Héctor Navarrete, Marcelo Somos-Valenzuela, and Ivo Fustos-Toribio

Changes in climate are dramatically shaping the planet, imposing new conditions, and constraints on water systems that are not easy to foresee. The need for an integrated application of methodologies that links advances in climatology with hydrology and water management is now undeniably necessary. Given the latitude of the Chilean Patagonia, the effects of climate change are beginning to show more dramatically. However, there are analogous examples of glaciated systems in other parts of the world that can offer valuable insight into the effects that increases in temperature would have on the evolution of river flows in Patagonia. Where glaciers are present, stream flows are positively/negatively correlated with temperature/precipitation. In equilibrium, glaciers regulate river flows by smoothing the annual streamflow variations.  In the Chilean Patagonia, significant attention has been given to the evaluation of lake formation and the impacts of potential Glacier Lakes Outburst Floods, and studies of glacier melting contribution to sea-level rise. In this study, we will use the Weather Research Forecast Hydro (WRF-Hydro) with the Crocus snow/glacier module to model the Paulina glacier hydrology and to estimate the streamflow in the NEF river basin. We use a nested watershed to study and parameterize the models in high resolution. We carried fieldwork to collect streamflow and climate data to calibrate and correct the model results. We used an Unmanned Aerial Vehicle (UAV) to generate a high-resolution elevation model of the glacier terminal, as well as the use of Landsat imagery to determine changes in the glacier area, snow line, and ASTER imagery to determine changes in thickness.

Our expected result is the quantification of the volume contribution of freshwater from a small mountain glacier. Further, we will use this parameterization at a regional setting to evaluate the potential of transferring parameters from a small glacier watershed to a broader context in the Baker River Basin in the Chilean Patagonia.

How to cite: Navarrete, H., Somos-Valenzuela, M., and Fustos-Toribio, I.: High Resolution Modeling of a Mountain Glacier in the Chilean Patagonia, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12093, https://doi.org/10.5194/egusphere-egu2020-12093, 2020.

D3898 |
Bert Van Schaeybroeck, Céline Van den Hende, Jan Nyssen, Sander Van Vooren, Michiel Van Ginderachter, and Piet Termonia

Recent work highlight the ambiguities in the definition and difficulties in quantification of the rain shadow effect. According to this phenomenon  there is a reduced rainfall on the leeward side of the mountains as compared to the windward side. We present a statistical approach to study this effect in case climatological time series of model data are available in geographically complex regions. Our approach requires only gridded rainfall, wind and  model elevation. We disentangle the aspects that contribute to the rainfall enhancement at the windward side. We apply the approach on the summer mountain precipitation (kerimt) over the Ethiopian Highlands based a new 21-year long climate run with the regional climate model ALARO-0 at a resolution of 4 km. There is an overall increased rainfall of 20% for windward events as compared to leeward events, but locally this can exceed 150%. This increase can be attributed to the positive differences between windward and leeward events in their frequency of occurrence, and, in the rainfall quantity during rainfall events. Differences in rainfall frequency, on the other hand, are spatially inhomogeneous and smaller than the spatial variations of the rainfall frequencies themselves.

How to cite: Van Schaeybroeck, B., Van den Hende, C., Nyssen, J., Van Vooren, S., Van Ginderachter, M., and Termonia, P.: The rain-shadow effect for the Ethiopian Highlands, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1924, https://doi.org/10.5194/egusphere-egu2020-1924, 2020.

D3899 |
Simon C. Scherrer and Sven Kotlarski

The monitoring of near-surface temperature is a fundamental task of climatology that remains especially challenging in mountain regions. Here we assess the regional monitoring capabilities of modern reanalysis products in the well-monitored northern Swiss Alps during the last 20 to almost 60 years. Monthly and seasonal 2 m air temperature (T2m) anomalies of the global ERA5 and the three regional reanalysis products HARMONIE, MESCAN-SURFEX and COSMO-REA6 are evaluated against high quality in situ observational data for a low elevation (foothills) mean, and a high elevation (Alpine) mean. All reanalysis products show a good year-round performance for the foothills with the global reanalysis ERA5 showing the best overall performance. The high-resolution regional reanalysis COSMO-REA6 clearly performs best for the Alpine mean, especially in winter. Most reanalysis data sets show deficiencies at high elevations in winter and considerably overestimate recent T2m trends in winter. This stresses the fact that even in the most recent decades utmost care is required when using reanalysis data for near-surface temperature trend assessments in mountain regions. Our results indicate that a high-resolution model topography is an important prerequisite for an adequate monitoring of winter T2m using reanalysis data at high elevations in the Alps. Assimilating T2m remains challenging in highly complex terrain. The remaining shortcomings of modern reanalyses also highlight the continued need for a reliable and dense in situ observational monitoring network in mountain regions.


How to cite: Scherrer, S. C. and Kotlarski, S.: Temperature monitoring in mountain regions using reanalyses: Lessons from the Alps, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8945, https://doi.org/10.5194/egusphere-egu2020-8945, 2020.

D3900 |
Cristina Vegas Cañas, J. Fidel González Rouco, Jorge Navarro Montesinos, Etor E. Lucio Eceiza, Elena García Bustamante, Inés Álvarez Arévalo, Ernesto Rodríguez Camino, Andrés Chazarra Bernabé, and Félix García Pereira

This work provides a first assessment of temperature variability from interannual to multidecadal timescales in the Sierra de Guadarrama, located in central Spain, from observations and regional climate model (RCM) simulations. Observational data are provided by the Guadarrama Monitoring Network (GuMNet; www.ucm.es/gumnet) at higher altitudes and by the Spanish Meteorological Agency (AEMet) at lower sites. An experiment at high horizontal resolution of 1 km using the Weather Research and Forecasting (WRF) RCM, feeding from ERA Interim inputs, is used. Through model-data comparison, it is shown that the simulations are annually and seasonally highly representative of the observations, although there is a tendency in the model to underestimate observational temperatures, mostly at low altitudes. Results show that WRF provides an added value in relation to the reanalysis, with improved correlation and error metrics relative to observations.

The analysis of long term trends shows no significant temperature trends in the area during the last 20 years. However, when spanning the analysis to the whole observational period, back to the beginning of the 20th century at some sites, significant annual and seasonal temperature increases of ca. 1degC/century develop, most of it happening during de 1970s.

The temporal variability of temperature anomalies in the Sierra de Guadarrama is highly correlated with the temperatures in the interior of the Iberian Peninsula. This relationship can be extended broadly over south-western Europe.

How to cite: Vegas Cañas, C., González Rouco, J. F., Navarro Montesinos, J., Lucio Eceiza, E. E., García Bustamante, E., Álvarez Arévalo, I., Rodríguez Camino, E., Chazarra Bernabé, A., and García Pereira, F.: An Assessment of Long-Term Temperature Variability in the Sierra de Guadarrama (Spain), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18693, https://doi.org/10.5194/egusphere-egu2020-18693, 2020.

D3901 |
Guillaume Gastineau, Francois Lott, Juliette Mignot, and Frederic Hourdin

In the IPSL-CM6A-LR model, the subgrid scale orography (SSO) parameterization imposes at low level a blocked flow drag opposed to the local flow and a lift that is perpendicular to the local flow. We suggest that their tuning impacts of the Arctic sea ice coverage and the large scale oceanic circulation in climate models. In forced atmospheric mode, increasing the blocking and reducing the lift leads to an equatorward shift of the Northern Hemisphere subtropical jet, and a reduction of the mid latitude eddy-driven jet. It improves the simulated variability, with a reduced storm-track, and increased blocking frequency over Greenland and Scandinavia. Second, it contributes to cool the polar low-troposphere in winter. We show that the reduction in eddy activity yields a reduction of the poleward heat fluxes in the low troposphere of the mid-latitudes and polar regions. Transformed Eulerian Mean diagnostics also show that there is a reduction of the low-level eddy-driven subsidence in the polar region consistent with the simulated cooling. The changes are amplified in the coupled model, as the eddy-driven jet shift further south. The low-troposphere polar cooling is further amplified by the temperature and albedo feedbacks in link with the Arctic sea-ice. This corrects the warm winter bias and the lack of sea-ice that were present over the Arctic without changing the SSO parameters. This also impacts the ocean, with an equatorward shift of the Northern Hemisphere oceanic gyre, and a weakening of the AMOC.

How to cite: Gastineau, G., Lott, F., Mignot, J., and Hourdin, F.: Alleviation of an arctic sea ice bias in a coupled model through modifications of the subgrid scale orographic parameterization, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9905, https://doi.org/10.5194/egusphere-egu2020-9905, 2020.

D3902 |
Johannes Horak, Marlis Hofer, and Alexander Gohm

The output of general circulation models is too coarse to adequately capture the features influencing local climate and weather, particularly in complex topography. To asses the long-term impact of a changing global climate in mountainous regions, regional climate models need to be run on a fine spatial and temporal grid. Here the Intermediate Complexity Atmospheric Research (ICAR) model is a computationally frugal and physics based alternative to full physics regional climate models such as the Weather Research and Forecasting (WRF) model. A sizable portion of the computational efficiency of ICAR stems from its application of linear mountain wave theory to determine the wind field in the domain, thereby avoiding a numerical solution of the Navier-Stokes equations of motion. Heat, moisture and other atmospheric quantities are then advected in this wind field. Microphysical conversion processes between water vapor and various hydrometeor species are handled by a complex microphysics scheme. Altogether ICAR does not require measurements and enables computationally cheap downscaling, particularly in mountainous regions with complex topography, yielding a physically consistent set of atmospheric variables. However, in a real-world application and evaluation of ICAR we observed a strong sensitivity of the model performance to the elevation of the model top (Horak et al., 2019).

We present three recommendations, derived from idealized simulations, that improve different aspects of ICAR simulations. The simulations constitute an idealized ridge experiment with a non-dimensional mountain height of 0.5. The ridge is specified by a witch of Agnesi function and the sounding characterized by a saturated, horizontally and vertically homogeneous atmosphere with constant and stable stratification. The wind field calculated by ICAR is compared to the exact analytical solution. Furthermore, the water vapor, suspended hydrometeor and precipitating hydrometeor fields are used as proxies to identify inconsistencies in the model output, such as the dependence of the results on the elevation of the model top. To highlight the deviations of ICAR results from a full physics model, resulting from non-linearities in the wind field, the ICAR output was additionally compared to that of a WRF simulation. The results of our investigation strongly suggest that ICAR simulations can be significantly improved by (i) calculating the Brunt-Väisälä frequency from the forcing data set instead of the perturbed state of the atmosphere, (ii) setting the model top to an elevation of at least 11.4 km and, (iii) by applying a zero value boundary condition to the water vapor and hydrometeor species at the model top. To our knowledge none of the preceding studies employing ICAR satisfied these three conditions. Overall our investigation deepens the understanding of the ICAR model sensitivity to crucial model components, thereby increasing the potential of the model as a tool for long-term impact studies in data-sparse regions with complex topography.

Horak, J., Hofer, M., Maussion, F., Gutmann, E., Gohm, A., and Rotach, M. W. (2019), Assessing the added value of the Intermediate Complexity Atmospheric Research (ICAR) model for precipitation in complex topography. Hydrology and Earth System Sciences, 23(6), 2715-2734. 

How to cite: Horak, J., Hofer, M., and Gohm, A.: Three recommendations to improve simulations with the Intermediate Complexity Atmospheric Research (ICAR) model, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4767, https://doi.org/10.5194/egusphere-egu2020-4767, 2020.

D3903 |
Christian Steger, Jesus Vergara-Temprado, Nikolina Ban, and Christoph Schär

Weather and climate in alpine areas are strongly modulated by complex topography. Besides its influence on atmospheric flow and thermodynamics (such as orographic precipitation and foehn winds), topography also affects incoming surface radiation in various ways. Direct shortwave radiation might be blocked due to shading effects from neighbouring terrain. Diffuse shortwave radiation can be altered by a reduced sky view factor and reflectance of radiation from surrounding terrain. Similar, the net longwave radiation is affected by emissions from neighbouring terrain.

Radiation in virtually all state-of-the-art weather and climate models is only computed in the vertical direction using the column approximation, and the above-mentioned effects are usually not represented. Still, a few models consider topographic effects by correcting incoming radiation fluxes based on topographic parameters like slope aspect and angle, elevation of horizon, and sky view factor. The Consortium for Small-scale Modeling (COSMO) model includes such a scheme, which is currently only used in the Numerical Weather Prediction mode of the model.

In this study, we apply the surface radiation correction scheme in the climate mode of COSMO. To study its impacts in detail, we force COSMO’s land-surface model (TERRA) offline with output from a COSMO simulation, which was run without radiation correction at a horizontal resolution of 2.2 km and for a domain covering the Alps. A useful proxy to study the impact of the correction scheme is snow cover duration (SCD), because snow cover length is, amongst other factors, strongly controlled by incoming surface radiation that drives ablation. A comparison of SCD simulated by COSMO with satellite-derived snow cover data (MODIS and AVHRR) reveals a distinctive bias, where SCD is overestimated for south-facing grid cells and underestimated for north-facing cells. Applying the radiation correction in the offline TERRA simulation shows only a moderate reduction of the bias. One reason for this minor improvement is the fact that the topographic parameters are computed from a smoothed digital elevation model (DEM) – thus the impact of the radiation correction scheme is damped. If topographic parameters are computed from unsmoothed DEM, biases in SCD are further reduced. Currently, further sensitivity experiments are conducted to investigate the effect of computing the topographic parameters from a sub-grid DEM and to assess the energy conservation of the radiation correction scheme.

How to cite: Steger, C., Vergara-Temprado, J., Ban, N., and Schär, C.: Topographic effects on longwave and shortwave surface radiation in a kilometre-scale regional climate model, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12080, https://doi.org/10.5194/egusphere-egu2020-12080, 2020.

D3904 |
Juerg Schmidli and Abouzar Ghasemi

In fair weather conditions, thermally driven local winds often dominate the wind climatology in deep Alpine valleys resulting in a unique wind climatology for any given valley. The accurate forecasting of these local wind systems is challenging, as they are the result of complex and multi-scale interactions. Even more so, if the aim is an accurate forecast of the winds from the near-surface to the free atmosphere, which can be considered a prerequisite for the accurate prediction of mountain weather.  This study investigates the skill of a high-resolution numerical weather prediction (NWP) model, the most current version of the COSMO-DWD model,  at 1.1 km grid spacing in simulating the thermally driven local winds in the Swiss Alps for a month-long period in September 2016. The study combines the evaluation of the surface winds in several Alpine valleys with a more detailed evaluation of the wind evolution throughout the valley depth for a particular site in the Swiss Rhone valley. The former is based on a comparison with observations from the operational measurement network of MeteoSwiss, while the latter uses data from a wind profiler stationed at Sion airport.

How to cite: Schmidli, J. and Ghasemi, A.: Evaluation of thermally driven local winds in the Swiss Alps simulated by a high-resolution NWP model, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5631, https://doi.org/10.5194/egusphere-egu2020-5631, 2020.

D3905 |
Jan Weinkaemmerer, Ivan Bašták Ďurán, and Jürg Schmidli

In the convective boundary layer over mountainous regions, the mean values and the fluxes of quantities like heat, mass, and momentum are strongly influenced by thermally induced flows. Several studies have pointed out that the enhanced warming of the air inside a valley can be explained by the valley-volume effect whereas the cross-valley circulation leads to a net export of heat to the free atmosphere. We are interested in the influence of an upper-level wind on the local circulations and the boundary-layer properties, both locally and in terms of the horizontal mean, as this aspect has not yet received much attention. LES are carried out over idealized, two-dimensional topographies using the CM1 numerical model. For the analysis, turbulent, mean-circulation, and large-scale contributions are systematically distinguished. Also, budget analyses are performed for the turbulence kinetic energy and the turbulent heat and mass flux. Based on the first results for periodic topographies, no crucial influence on the horizontally averaged heat-flux and temperature profile can be observed, even though the flow pattern of the thermal wind is qualitatively changed. In addition to that, the impact on moisture transport will be evaluated and simulations over different topographies as well as for different atmospheric conditions and surface properties will be presented.

How to cite: Weinkaemmerer, J., Bašták Ďurán, I., and Schmidli, J.: The impact of large-scale winds on thermally driven flows and exchange processes over mountainous terrain, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8101, https://doi.org/10.5194/egusphere-egu2020-8101, 2020.

D3906 |
Lukas Jansing and Michael Sprenger

Foehn-related research has a long-standing tradition in mountain meteorology. In this context, the reason for Foehn air warming and the factors determining the descent of the air into the valleys have gained particular interest. Here, we readdress those research questions by combining a COSMO model hindcast at 1 km horizontal and 10 min temporal resolution with air parcel trajectories for a South Foehn case study in November 2016. The sub-synoptic situation in the model is studied using horizonal cross sections at different levels. Vertical cross sections in the Po valley and along the axes of major Foehn valleys complement the Eulerian analysis.

The selected event is characterized by its long duration, a far northern extent and exceptionally strong gusts. A low-level jet is discernible west of the Alps and a pronounced north-south pressure gradient develops. A striking feature is the strong tilt of the isentropes downstream of the Alpine crest. Trajectories reveal the versatile pathways of air parcels over major Alpine passes before they descend into the Foehn valleys. Differences with respect to upstream ascent and descent are observed for the different valleys. By tracing meteorological variables along the trajectories, the relative importance of adiabatic and diabatic processes for the Foehn air warming is quantified. The properties of air parcels that descend into the valleys or stay at higher levels are contrasted in order to identify factors that determine the descent.

The case study will set the scene for a forthcoming detailed analysis of Foehn flows based on online trajectories that make use of the wind fields at every model time step. The analysis will be extended to a number of cases representing the different South Foehn varieties. We will trace the temperature tendencies due to all diabatic processes (cloud processes, radiation, turbulence) along the trajectories in order to quantify their respective importance for Foehn air warming. First results in this extended framework will be presented.

How to cite: Jansing, L. and Sprenger, M.: Eulerian and Lagrangian perspectives on a Foehn event in the Alpine region, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4713, https://doi.org/10.5194/egusphere-egu2020-4713, 2020.

D3907 |
Dorita Rostkier-Edelstein, Pavel Kunin, and Pinhas Alpert

The atmospheric dynamics in the Dead Sea Valley has been studied for decades. However, the studies relied mostly on surface observations and simple coarse-model simulations, insufficient to elucidate the complex flow in the area. In this seminar I will present a first study using high resolution (temporal and spatial) and sophisticate both, measurements and modeling tools. We focused on afternoon hours during summer time, when the Mediterranean Sea breeze penetrates into the Dead Sea Valley and sudden changes of wind, temperature and humidity occur in the valley.

An intense observations period in the area, including ground-based remote sensing and in-situ observations, took place during August and November 2014. The measurements were conducted as part of the Virtual Institute DEad SEa Research Venue (DESERVE) project using the KITcube profiling instruments (wind lidars, radiometer and soundings) along with surface Energy Balance Station. These observations enabled analysis of the vertical profile of the atmosphere at one single location at the foothills of Masada, about 1 km west of the Dead Sea shore.

High resolution (1.1 km grid size) model simulations were conducted using the Advanced Research Weather version of the Weather Forecast and Research mesoscale model (WRF). The simulations enabled analysis of the 3D flow at the Dead Sea Valley, information not provided by the observations at a single location. Sensitivity tests were run to determine the best model configuration for this study.

Our study shows that foehn develops in the lee side of the Judean Mountains and Dead Sea Valley in the afternoon hours when the Mediterranean Sea breeze reaches the area. The characteristics of the Mediterranean Sea breeze penetration into the valley and of the foehn (e.g. their depth) and the impact they have on the boundary layer flow in the Dead Sea Valley (e.g. sudden changes in temperature, humidity and wind) are conditioned to the daily synoptic and mesosocale conditions. In the synoptic scale, the depth of the seasonal pressure trough at sea level and the height of inversion layers play a significant role in determining the breeze and foehn characteristics. In the mesoscale, the intensity of the Dead Sea breeze and the humidity brought by it determines the outcomes at the time of Mediterranean Sea breeze penetration and foehn development. Dynamically, the foehn is associated with a hydraulic jump.

Hypothetical model simulations with modified terrain and with warmer Mediterranean Sea surface temperature were conducted to reveal the relative contribution of each of these factors and of their synergism on the observed phenomena. The information provided by the factor separation study can be useful in future climate projections, when a warmer Mediterranean Sea is expected.

The forecasting feasibility of foehn and the sudden changes in the Dead Sea valley 24 hours in advance using WRF is suggested following the present study. These forecasts can be most valuable for the region affected by pollution penetration from the metropolitan coastal zone.

How to cite: Rostkier-Edelstein, D., Kunin, P., and Alpert, P.: Investigation of sea breeze and foehn in the Dead Sea valley with remote sensing observations and WRF model simulations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1708, https://doi.org/10.5194/egusphere-egu2020-1708, 2020.

D3908 |
Deon van der Mescht, Markus Geldenhuys, and Liesl Dyson

A fatal crash of a light aircraft occurred in the complex coastal mountainous terrain along the South African South Cape in December 2015. An investigation of the meteorological conditions on that day revealed the interaction between mountain waves, gap flow and blocking near a cold front and terrain. The crash made it clear that it is necessary to provide forecasters with knowledge of the turbulence that will arise under these circumstances. Against this background, experiments were carried out near the crash site, with automatic weather stations and radio stations to answer this question. Turbulence has been successfully characterized by the Froude number, Froude altitude scale and thermal wind equation. The Bernoulli equation, which classifies the gap flow, was not helpful due to the effect of the upwind blocking area. Phenomena in descending order of the generated wind force were, compression effect above the peak (44.7 ms-1), blocking (26 ms-1) and finally gap flow. The gap flow had a negative impact on the strength of the barrier jet. Phenomena in descending order of the turbulence intensity were; gap flow, mountain wave/rotors and finally blocking. Gap flow generated higher vertical speeds than mountain waves. These mountain waves generated the highest vertical speeds measured in South Africa to date, combined with the waves of the shortest wavelength. A blocking jet with a depth of 600 m and a width of 80 km changed the formation of mountain waves significantly. The blocking jet was so strong, that it extended up to 30 km beyond the end of the mountain range. Most likely, a combination of mountain waves, gap flow and blocking contributed to the crash, which shows that these three features cannot be seen as separate processes.

How to cite: van der Mescht, D., Geldenhuys, M., and Dyson, L.: Blocking, gap flow and mountain waves along the coastal escarpment of South Africa, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13367, https://doi.org/10.5194/egusphere-egu2020-13367, 2020.

D3909 |
Helen Ward, Mathias Rotach, Alexander Gohm, Martin Graus, Thomas Karl, Lukas Umek, Maren Haid, and Thomas Muschinski

Accurate prediction of meteorological conditions is particularly important for cities in mountainous terrain, where populations are frequently exposed to extreme weather and poor air quality. However, the wide range of processes that interact across different scales and considerable spatiotemporal variability in these regions present challenges to measurement and modelling. Analysis of turbulence observations in and around Innsbruck reveals similarities and differences in the climate of this alpine city compared to previously-studied urban sites. In particular, the effect of the wind regime (e.g. thermally-driven circulation, foehn) on the timing and magnitude of the surface energy budget is explored. These observations are then used in a detailed assessment of the performance of the Weather Research and Forecasting (WRF) model (at 1-km grid spacing) including the multi-level urban surface scheme. It is found that WRF captures the valley-wind circulation reasonably well, although underestimates the turbulent kinetic energy both inside and outside the city. Even in this complex mountainous setting, the multi-level urban scheme is able to improve model performance.

How to cite: Ward, H., Rotach, M., Gohm, A., Graus, M., Karl, T., Umek, L., Haid, M., and Muschinski, T.: Evaluating WRF in highly complex terrain – a city surrounded by mountains, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9508, https://doi.org/10.5194/egusphere-egu2020-9508, 2020.

D3910 |
Bart Geerts, Coltin Grasmick, and Robert Rauber

Stratiform clouds over mountains are subject to locally strong updrafts that impact snow growth. These vertical drafts occur at a range of horizontal scales and depth, and include vertically propagating gravity waves, shallow terrain-driven (evanescent) waves, embedded convection, and shear-driven overturning cells. The latter essentially are Kelvin-Helmholtz (KH) waves; we find them to be remarkably common in deep stratiform precipitation systems associated with frontal disturbances over complex terrain, as is evident from transects of vertical velocity and 2D circulation, obtained from a 3-mm airborne Doppler radar. The high range resolution of this radar (~40 m) allows detection and depiction of KH waves in fine detail. These waves are observed in a variety of wavelengths (<100 m to > 1 km), depths, amplitudes, and turbulence intensities. Proximity rawinsonde data confirm that they are triggered in layers where the Richardson number is very small. Complex terrain may locally enhance wind shear, leading to KH instability.  In some KH waves, the flow remains mostly laminar, while in other cases it breaks down into turbulence. KH waves are frequently locked to the terrain, and occur at various heights, including within the free troposphere, at the boundary layer top, and close to the surface. They are observed not only upwind of terrain barriers, as has been documented before, but also in the wake of steep terrain, where the waves can be highly turbulent.  Doppler radar data and flight-level cloud probe data are used to explore the dynamics of KH waves and the response in terms of droplet growth, ice initiation, and snow growth.

How to cite: Geerts, B., Grasmick, C., and Rauber, R.: small-scale updrafts and snow growth in stratiform orographic clouds , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6338, https://doi.org/10.5194/egusphere-egu2020-6338, 2020.

D3911 |
Shizuo Fu, Richard Rotunno, and Huiwen Xue

Orographic precipitation is, on the one hand, an important source of fresh water, and on the other hand, a potential cause of floods and other disasters. Previous studies have focused on the situation where the whole atmosphere is saturated and nearly moist-neutral. However, there are times when subsaturated low-level layers are observed to be below saturated, nearly moist-neutral, upper-level layers.

A series of idealized two-dimensional simulations are performed here to investigate the impact of this subsaturated low-level layer on orographic precipitation. It is found that the impact is mainly controlled by a nondimensional parameter and two competing effects. The nondimensional parameter is N2zt/U, where N2 and zt are, respectively, the dry Brunt–Väisälä frequency and depth of the subsaturated low-level layer, and U the cross-mountain wind speed. When the nondimensional parameter exceeds a critical value, the decelerated region on the upwind side of the mountain moves upwind, resulting in weak surface precipitation near the mountain peak. When it is smaller than the critical value, surface precipitation occurs near the mountain peak.

The two competing effects are: 1) the vapor-transport effect, meaning that increasing zt decreases the amount of vapor transported to the mountain, and hence tends to decrease surface precipitation; and 2) the updraft width effect, meaning that increasing zt enhances flow blocking, producing a wider updraft over the upwind slope, and hence tends to increase surface precipitation. When the vapor-transport effect dominates, surface precipitation decreases with zt. When the updraft-width effect dominates, surface precipitation increases with zt.

How to cite: Fu, S., Rotunno, R., and Xue, H.: Response of Orographic Precipitation to Subsaturated Low-Level Layers, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3266, https://doi.org/10.5194/egusphere-egu2020-3266, 2020.

D3912 |
Soojin Yoo and Eun-Chul Chang

 In the winter season of the Gangwon region, where is located in the mid-eastern part of the Korean Peninsula, easterly wind that is induced by Siberian-high frequently causes heavy snowfall. When dry and cold air mass from continent is advected over the East sea of Korea that is relatively warmer than the continental air mass, thermal instability in the lower troposphere increases, which can induce convective cloud rolls. The clouds accompanied by the snowfall are penetrated to inland by the prevailing easterly wind. The Korean Peninsula has the geographical characteristics that mountain ranges exist along the eastern coastline, that can block easterly wind and induce upward motion over the upstream region. Previous studies presented key factors which can affect the snowfall in Gangwon region are air-sea temperature difference, wind turning layer, Froud number (FN), and the horizontal temperature contrast between land and sea. In this study, the idealized experiment is conducted by utilizing the Weather Research and Forecasting (WRF) model to examine effects of each key factor on the snowfall structure. The individual impact of each key factor is investigated by changing the variables while other factors were controlled. When the height of the wind turning layer is higher than the mountain, the maximum snowfall is located over the mountain ridge in the large FN, whereas the snowfall is limited to the windward area in the small FN. On the other hand, when the wind turning layer is lower than the mountain, it shows that the snowfall cannot cross the mountain regardless of the FN. When the horizontal temperature contrast between the land and the sea is large enough, the snowfall is limited to the seaward area off the coastal line.

How to cite: Yoo, S. and Chang, E.-C.: A Study on the Key Factors of Snowfall structure in mid-eastern region of the Korean Peninsula by Using idealized numerical Experiment, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20854, https://doi.org/10.5194/egusphere-egu2020-20854, 2020.