AS1.16

Mountains and high elevation regions are often viewed as climate change “hotspots” which are responding particularly rapid to global climate forcing and may anticipate or amplify what is occurring elsewhere. Accelerating mountain climate change has widespread impacts ranging from an enhanced loss of snow and ice, through impacts on the hydrological regimes, to changes in ecological zonation as species move uphill. We examine global evidence for elevation contrasts in temperature trends (also known as elevation-dependent warming, EDW) and precipitation changes. We performed a meta-analysis of existing studies, which used in-situ station temperature and precipitation data from mountain regions as reported by the IPCC, and we analysed global gridded datasets (observations, reanalyses and model hindcasts). In both cases, we examined the elevation dependency of temperature and precipitation changes since 1900. The meta-analysis indicates that studies using pairs of station groups (in mountains and nearby low elevation areas) show a tendency towards enhanced warming at higher elevations. However, when all past studies of observations are combined globally, no systematic difference in warming rates for high vs. low elevation groups is found. Thus, on a global scale, local and regional features may obscure EDW. Precipitation changes in mountain areas based on station data are inconsistent, and a global elevational gradient in precipitation trends does not emerge. Gridded datasets (CRU, GISTEMP, GPCC, ERA5, CMIP5) show increased warming rates at higher elevations in specific regions (e.g. Andes for CMIP5 and Greater Alpine Region for ERA5), but again, there is no universal amplification of warming in mountains. The agreement between datasets is weak for temperature. Changes in precipitation show a tendency towards weaker (stronger) increase at higher (lower) elevations, especially in mid-latitudes. This means that the orographic effect may be weakening on a global scale, which may be a result of both thermodynamics and changes in atmospheric circulation.

How to cite: Schöner, W., Pepin, N., Arnone, E., Gobiet, A., Haslinger, K., Kotlarski, S., Notarnicola, C., Palazzi, E., Seibert, P., Serafin, S., Terzago, S., Thornton, J. M., Vuille, M., and Adler, C.: Elevational patterns of climate change – an assessment of temperature and precipitation for the mountain regions of the world, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11304, https://doi.org/10.5194/egusphere-egu22-11304, 2022.

Climate change has a strong impact on the environment in mountain areas, especially since mountain ecosystems depend on climatic conditions that vary with altitude. In recent years, it has become clear that warming strongly depends on elevation. In this study, we examine projected climate change in the Greater Alpine Region using the Weather Research Forecasting (WRF) model. Historical 30-year simulations (1979-2008) and climate change projections (2039-2068) were performed at high spatial resolution (4 km grid spacing) and with initial and boundary conditions provided by the global EC-Earth model. A focus on the altitudinal dependence of historical and future ETCCDI Climate Change indices is presented here: the results indicate that both temperature and precipitation are affected by climate change with an altitude dependence changing seasonally. Physical mechanism at the base of those differences are discussed.

How to cite: Napoli, A., von Hardenberg, J., Pasquero, C., and Parodi, A.: Altitudinal dependence of historical and future extreme events in the Great Alpine Region modelled with WRF, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2752, https://doi.org/10.5194/egusphere-egu22-2752, 2022.

Many mountainous environments and ecosystems around the world are responding rapidly to ongoing climate change. Long-term climatological time-series from such regions are crucial for developing improving understanding of the underlying mechanisms responsible for such changes, and generating more reliable future impact projections for environmental managers and other decision makers. Whilst it is already established that high elevation regions tend to be comparatively under-sampled, detailed spatial and other patterns in the coverage of mountain climatological data have not yet been comprehensively assessed on a global basis. To begin to address this deficiency, we analyse the coverage of records associated with the mountainous subset of the Global Historical Climatological Network-Daily (GHCNd) inventory with respect to space, time, and elevation. Three key climate-related variables – air temperature, precipitation, and snow depth – are considered across 292 named mountain ranges. To characterise data coverage relative to topographic, hydrological, and socio-economic factors, several additional datasets were introduced. Spatial mountain data coverage is highly uneven, and there are several mountain ranges whose elevational range is severely under-sampled by GHCNd stations. Crucially, the three "Water Tower Units" previously identified as having the greatest hydrological importance to society appear to have extremely low station densities. Mountain station density is weakly related to the human population or economic output of the corresponding downstream catchments. A script we developed enables detailed assessments of record temporal coverage and measurement quality information. This contribution should help international authorities and more regional stakeholders to identify areas, variables, and other aspects that should be prioritised for investment in infrastructure and capacity. Finally, the transparent and reproducible approach taken throughout will enable the work to be rapidly repeated for subsequent versions of GHCNd, and may furthermore enable similar analyses to be efficiently conducted on other spatial reporting boundaries and/or environmental monitoring station networks. 

How to cite: Thornton, J., Pepin, N., Shahgedanova, M., and Adler, C.: Coverage of in situ climatological observations in the world's mountains, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1333, https://doi.org/10.5194/egusphere-egu22-1333, 2022.

Ten years of almost continuous observations at the highest Global Atmosphere Watch Regional station in the world are presented here. The Chacaltaya observatory (5240 m asl, 16.3ºS, 68.1ºW) was set up in December 2011. It is currently the only operational station characterizing optical and chemical properties of climate-relevant aerosol and gases in Bolivia and in a radius of about 1500 kilometers from the station. The observations show a clear influence of the well-marked dry and wet meteorological seasons. In addition, the impact on the Andean mountains of long and mid-range transport of biomass burning products from the lowlands is clearly recorded in different parameters measured at the station. Furthermore, the nearby presence of the largest metropolitan area in the region (~1.8 million inhabitants) is observed almost on a daily basis, and therefore different campaigns were carried out to characterize the area and its influence on our measurements. Specific results from these campaigns are discussed elsewhere. Finally, the topographic complexity represents an important challenge for modeling efforts in order to understand sources and sinks (and associated processes) of the observed parameters, requiring not only high spatial resolution and the correct choice of model options, but a novel way of interpreting these results. The decade of collaboration of an international consortium made it possible to keep the station running successfully. The challenge is now to preserve its functioning for the coming decades in a region with historically few high-quality observations while disrupting environmental and socio-economic changes take place.

How to cite: Andrade, M., Aliaga, D., Blacutt, L., Forno, R., Gutierrez, R., Velarde, F., Moreno, I., Ticona, L., Wiedensohler, A., Krejci, R., Ramonet, M., Laurent, O., Whiteman, D., Mohr, C., and Laj, P.: A decade of atmospheric composition observations in the undersampled Central Andes, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10801, https://doi.org/10.5194/egusphere-egu22-10801, 2022.

Air temperature is one of the most significant meteorological variables. Reconstruction of air temperature is necessary to analyse temperature variability during the recent global warming. So far, significant attention has been paid to the study of air temperature variability in Central European mountain regions. However, only minimum studies dealt with the Hrubý Jeseník and Králický Sněžník Mountains (the Czech-Polish border, northern Moravia). The paper aims to reconstruct mean, maximal and minimal air temperatures of four mountain stations above 1 000 m a. s. l. in the Jeseníky and Králický Sněžník Mountains between 1961–2020 and reveal the possible trends. To compile a consistent input dataset both in time and space, input data for the interpolation underwent thorough data quality control, homogenisation and filling of missing data. Input values were interpolated employing regression kriging via the SoilClim model into maps in 500m spatial resolution on a daily scale. Short-term temperature series had been reconstructed back to 1961 and consequently compared to Vysoká hole station (1463 m a. s. l.). Statistical significant increasing 10year annual and seasonal temperature trends were proved in the 1961–2020 period. However, the temperature of mountain peaks of the Hrubý Jeseník and Králický Sněžník within 10year annual trend increased slower in comparison with lowlands (0.3°C, respectively 0.4°C). The results highlight the importance of air temperature analysis in the mountain regions and contribute to a better understanding of temperature variability in the recent global warming.

How to cite: Dolák, L., Láska, K., Řehoř, J., Štěpánek, P., Zahradníček, P., and Lahoda, M.: Air temperature variability of the Hrubý Jeseník and Králický Sněžník Mountains peaks, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1612, https://doi.org/10.5194/egusphere-egu22-1612, 2022.

Mountain regions are areas characterized by great spatial variability in climate variables, due to the great differences in altitude, orientation and the abrupt topography that characterizes them. These features of the territory, together with a relative lower density of meteorological stations, make it difficult to characterize climate change in these areas.

This work describes the Spanish High Mountain Climate Database (SHMCDv1) consisting of daily quality controlled and homogenised records of maximum temperature, minimum temperature and precipitation for the territories that include the two highest altitude national parks in mainland Spain, the Sierra Nevada National Park (PNSN) and the Aigüestortes i Sant Maurici National Park (PNASM), and its area of ​​influence.

To build this database, 129 climatic series corresponding to the PPNASM area and 166 series in the PNSN area have been used, for the period between 1893 and 2020, obtained from various sources (AEMET, SMC, Climanevada database, LOOP Project). A systematic quality control has been applied to the series using the RClimdex-extraqc (see http://www.c3.urv.cat/data/manual/Manual_rclimdex_extraQC.r.pdf). This procedure has allowed the identification of 857 values considered erroneous, of which 10% has been recovered with the correct value. Its homogeneity has been tested and adjusted with a CLIMATOL homogenization method developed by J. Guijarro (2016) (see http://www.climatol.eu/index.html). 205 inhomogeneities have been detected and adjusted in the temperature series, which represents an average of 2 inhomogeneities for each series.

In total, thermo-pluviometric series have been composed for a set of 98 meteorological stations, 27 of them located above 1500 meters of altitude.

Sen’s estimator of the slope have been used to estimate the temperature and precipitation trends corresponding to low mountain areas (<1500 m altitude) and high mountains (> 1500 m altitude) are calculated and analyzed to determine if there are differences in the evolution of recent temperature or precipitation due to altitude and between both mountain areas.

This work has been done thanks to funding from MINECO and MITECO, through the projects LACEN-CLI (ref: 2476-S/2017), MEROMONT (ref: CGL2017-85682-R) and Smart EcoMountains (LifeWatch-2019-10-UGR-01).

How to cite: Sigro, J., Pérez-Luque, A. J., Pérez-Martínez, C., Vegas-Vilarrubia, T., and Esteban-Parra, M. J.: Trends in temperature and precipitation in high mountain areas in Spain from the Spanish Hig Mountain Climate Database, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4257, https://doi.org/10.5194/egusphere-egu22-4257, 2022.

Major natural hazards in mountainous regions, such as flash floods and debris flows are mainly triggered by short-duration extreme precipitation. A better understanding of how these events are affected by orography can significantly improve risk management and adaptation to changing climate. Recently, significant progress has been made in high-resolution (particularly convection-permitting) modelling of precipitation over complex terrain, with the advantages of improved topographical features, physical representation of mountain-precipitation interactions, and avoided errors from convective parameterizations.

Here, we examine the mountain-precipitation interactions for subdaily precipitation extremes from three 10-year time slices (historical 1996-2005, near-future 2041-2050, and far future 2090-2099 – under the RCP8.5 scenario) of COSMO-crCLIM model simulations at 2.2 km resolution. We use the Upper Adige river basin in the Eastern Italian Alps, with a good coverage of high-quality precipitation data, as a case study. The ability of the convection-permitting model to represent the orographic impact on precipitation is examined based on a comparison between 2000-2009 simulations from the COSMO model run driven with ERA Interim, and observations from the local rain gauge network.

Given the availability of relatively short time-slices of model simulation, which prevent the use of conventional extreme value methods, we use here methods based on the concept of ordinary events, which are all the independent events that share the statistical properties of extremes. This offers now an opportunity for deriving frequency analyses from shorter data records, promising improved applications based on convection precipitation simulations.

How to cite: Lusito, L., Marra, F., Dallan, E., Zaramella, M., Troccoli, A., and Borga, M.: Impact of orography on current and future extreme sub-daily precipitation, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5453, https://doi.org/10.5194/egusphere-egu22-5453, 2022.

Mountains play a major role in shaping the weather and climate of the world. However, current under- standing of mountain climate and how it will change with further warming of the atmosphere is still very limited. The uncertainty is in large part related to the coarse grid spacing of current climate models (12-50 kilometres in regional and >50 kilometres in global climate models), which are not able to properly represent the complex mountainous orography and related processes. Thus, employing climate models with a kilometer-scale grid spacing provides a promising path. Here, we present simulations using the COSMO (COnsortium for Small-Scale MOdelling) climate model (COSMO-CLM) performed with a horizontal grid spacing of 2.2 km over all of High Mountain Asia. We evaluate model performance based on preliminary results from case study simulations of different precipitation events and year-long simulations, as well as the added value of kilometer-scale grid spacing. Our analysis lays the foundation for future applications of kilometer-scale runs for decadal simulations of both past and future climate, which comprise the ultimate goal of our project.

How to cite: Collier, E. and Ban, N.: Exploiting kilometer-scale grid spacing for climate simulations over High Mountain Asia, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12750, https://doi.org/10.5194/egusphere-egu22-12750, 2022.

Observations from six eddy-covariance stations in the Austrian Inn Valley reveal a strong spatial variability in near-surface turbulent fluxes. While the stations are located within an approximately 6.5 km long section of the valley and thus within an area similar or even smaller than a single grid cell in current global weather forecasting models, the sites strongly differ in terms of topography and land use. Observed magnitudes of sensible and latent heat fluxes are driven by the solar incoming radiation and thus affected by the local slope angle and orientation, with further influences from the land use on the partitioning of the available energy into sensible and latent heat fluxes. In addition, the locally induced thermal slope- and valley-wind circulation impact the diurnal cycles of the turbulent fluxes. To correctly represent turbulent exchange in mountainous terrain in numerical models, the models thus need to represent all these conditions and processes correctly. We are running WRF simulations with a 1-km grid spacing as part of an ongoing project to evaluate land-surface models and turbulence parameterizations over complex terrain. The simulations are used to determine how sensitive the modeled land-atmosphere exchange is to inaccuracies in the topography and land use, which are unavoidable at this spatial resolution, and whether the model can reproduce the observed spatial variability.

How to cite: Lehner, M., Simonet, G., Rotach, M. W., Obleitner, F., Giovannini, L., and Montagnani, L.: Simulating the land-atmosphere exchange over mountainous terrain, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2666, https://doi.org/10.5194/egusphere-egu22-2666, 2022.

Land surface models (LSMs), i.e. parameterization schemes for evaluating surface-atmosphere exchange implemented in meteorological models, usually prove inadequate over complex terrain,where orography strongly influences atmospheric processes and their interaction with the surface. In particular, LSMs use several parameters to suitably describe the surface and its interaction with the atmosphere, whose determination is often affected by many uncertainties. To this date, the sensitivity of meteorological model results to these parameters has not yet been studied systematically in complex terrain.The purpose of this work, which lies in the context of the TEAMx-related project ASTER, funded by the EGTC European Region Tyrol-South Tyrol-Trentino, is to evaluate the sensitivity of simulations with the Weather Research and Forecasting (WRF) meteorological model to variation of parameters describing land cover. Specifically, an idealized three-dimensional topography consisting of a valley-plain system is adopted and the analysis of the results focuses on the development of thermally-driven circulations. The analysis considers both the sensitivity to the type of vegetation cover and to the systematic variation of surface parameters based on typical values found in the literature. In particular, this analysis is carried out using the Global Sensitivity Analysis (GSA) methodology in order to quantify the uncertainty associated with the variation of each parameter evaluated and to estimate the optimal computational effort required for this type of study. The outcome of this analysis allows to evaluate which are the parameters that most influence model results and therefore should be estimated with particular attention in order to obtain reliable simulations over complex terrain.

How to cite: Di Santo, D., Zonato, A., and Giovannini, L.: Sensitivity of numerical simulations in an idealized valley to surface parameters, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1202, https://doi.org/10.5194/egusphere-egu22-1202, 2022.

Snow models rely on accurate meteorological input data at the spatial scales at which they operate. However, even the highest resolution operational atmospheric models often run at horizontal resolutions at least an order of magnitude coarser than most snow models. Different downscaling techniques can be employed to bridge this scale gap, typically being sorted into either statistical or dynamical techniques. Statistical techniques often rely on temporally invariant spatial patterns or simplistic conceptual relationships, making them computationally cheap but prone to errors. Dynamical downscaling generally offers a counterpoint to this tradeoff: stronger physical basis but more computational demand. Efforts have been made to optimize this tradeoff of dynamic downscaling, reducing computational demand while maintaining physical accuracy of predicted variables as well as the interdependency of downscaled variables such as winds and precipitation. The Intermediate Complexity Atmospheric Research (ICAR) model recently demonstrated an ability to match precipitation patterns from WRF, but with computational costs at least two orders of magnitude lower. While promising, these results from a 4km comparison did not translate to finer spatial resolutions often needed as input to snow models.

Thus, we introduce the High-resolution Intermediate Complexity Atmospheric Research Model (HICAR), a new variant of the ICAR model developed for spatial resolutions as high as 50m. Relative to a traditional atmospheric model like WRF, HICAR maintains the orders-of-magnitude reduction in computational demand which ICAR displayed, while resolving terrain-induced effects on the wind field not seen in ICAR. This is achieved through a novel combination of adjustments to a background wind field based on terrain descriptors with a wind solver. The solver enforces a mass-conservation constraint on the 3D wind field. These modifications successfully mimic dynamic effects such as flow blocking, ridge-crest speed up, and lee-side recirculation to be captured in the resulting wind field. These features are of particular importance for resolving snow deposition patterns, where the snow particles are particularly susceptible to advection by the near-surface flow field. We validate the accuracy of HICAR’s flow features using a wind LiDAR deployed in complex terrain and show a comparison between flow fields from HICAR and WRF at a horizontal resolution of 50 m. These comparisons demonstrate HICAR’s ability to resolve terrain-induced modifications to the flow field which result in increased heterogeneity of ridge-scale snowfall patterns. To this point, preliminary comparisons of snow deposition patterns in complex terrain between the HICAR and WRF models are offered. With this new model, physically-based downscaling of precipitation and other atmospheric variables which preserves their interdependencies is made available for high-resolutions (100m) and large-spatial extents (10,000 km²) which are often demanded by operational land-surface models.

How to cite: Reynolds, D., Kruyt, B., Gutmann, E., Jonas, T., Lehning, M., and Mott, R.: HICAR: An intermediate-complexity atmospheric model capable of resolving ridge-scale snow deposition processes over large mountain ranges, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11318, https://doi.org/10.5194/egusphere-egu22-11318, 2022.

Despite the large socio-economic and ecologic relevance of snow in Austria, no comprehensive assessment of the impact of climate change on snow in Austria existed until recently. Within the project „Future Snow Cover Evolution in Austria” (FuSE-AT, https://fuse-at.ccca.ac.at/) gridded observational datasets and the national climate scenarios (ÖKS15) have been extended by basic variables and user oriented indicators around the topic snow. This has been realized by developing a gridded snow model for climatological time-scales, based on the operational snow model of ZAMG (SNOWGRID-CL) and driving it with gridded meteorological datasets for the past (1961 – 2019) and with the full ensemble of ÖKS15 (including the emission pathways RCP2.6, RCP4.5 and RCP8.5) into the future (1961 – 2100)  to generate daily snow variables on a 1 km x 1 km grid. The results are available for users via the Data Centre of the Climate Change Centre Austria (https://fuse-at.ccca.ac.at/).

This new dataset includes snow water equivalent, snow depth, new snow, run-off from snow melt and the number of hours with suitable meteorological conditions for technical snow generation (using different wet-bulb-temperatures as threshold criteria). In addition, numerous user-oriented indicators have been analyzed. In close cooperation with stakeholders from the sectors winter tourism, hydropower generation and water supply, case studies to demonstrate socio-economically relevant  applications of this new dataset have been conducted.

The results show that the natural snow season length has significantly decreased already in the past in virtually all areas and altitude levels of Austria. Future scenarios of snow heavily depend on the emission pathway. The snow season length is expected to decrease by about three weeks (corresponds to -20% to -30% around 1500 m a.s.l.) until the mid-21st century in all scenarios, but it stabilizes on this level in RCP2.6, while it drastically further decreases in RCP8.5 to losses around -80% to -90% below 1500 m a.s.l. Further, we could demonstrate that the meteorological potential for generation of technical snow responds less sensitive to climate change than natural snow, but strongly depends on  altitude, exposition, time horizon and emission pathway. More detailed results will be given in the presentation.

How to cite: Gobiet, A., Abbeg, B., Koch, R., Olefs, M., Seitner, V., Strasser, U., and Warscher, M.: Future perspectives of natural and technical snow in Austria, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11753, https://doi.org/10.5194/egusphere-egu22-11753, 2022.

Mountains are specific systems, very sensitive to climate change. Most mountain glaciers around the world are shrinking, which is often associated with global warming over the last decades. Identifying the impact of climate changes on mountain glaciers and possible consequences on surrounding ecosystems and biodiversity is a prerequisite to better enhance adaptation and mitigation capacities at local and regional scales. The overall objective of the study is to investigate the climatic variability on annual and seasonal time scales during the last decades. Trends and breakpoints in time-series are analyzed in rainfall, solar irradiance, maximum and minimum temperature, and potential evapotranspiration in twenty-one contrasting locations situated in temperate zones (in the European Alps, the Pyrenees), in the Andean tropical and subtropical zones (Ecuador, Bolivia, Venezuela, Colombia, Peru), tropical Southeast Asia (Indonesia), equatorial Africa (Rwenzori) and, arid and high latitude zones (Argentina). We analyzed how the trends are associated with cloud cover properties (e.g., mean cloud amount, mean cloud pressure, radiatively-weighted average cloud visible optical thickness, and mean cloud temperature) and various climate variability indices: the Atlantic multi-decadal oscillation (AMO), El Niño Southern Oscillation (ENSO), Pacific Decal Oscillation (PDO), the Equatorial Southern Oscillation Index (SOI), and the North Atlantic Oscillation (NAO). Within this work, we used high resolution gridded datasets: Terraclimate (Monthly Climate and Climatic Water Balance for Global Terrestrial Surfaces), CHIRPS (Climate Hazards Group InfraRed Precipitation with Stations), MSWEP (Multi-Source Weighted-Ensemble Precipitation), regional simulations from 12 models provided by the Coordinated Regional Climate Downscaling Experiment (CORDEX), and the Cloud_cci Along-Track Scanning Radiometer and Advanced data set. Long-term (i.e. 1958-2020) significant trends of increased (decreased) annual and seasonal Tmax were identified over all European, Andean, Indonesian, and African glaciers. Over the Argentinian glaciers, long-term trend analysis shows a non-significant increasing trend in Tmax. Over all glaciers, long-term trend analysis shows a significant increasing trend in Tmin. Long-term significant trends of decreased annual rainfall were identified over African and most Alps and Pyrenees glaciers. On the other hand, no significant trends of rainfall were identified over the other glaciers. European glaciers were more influenced by the cloud cover properties than the tropical glaciers, with negative correlations between mean cloud amount and Tmax and solar irradiance. AMO plays a greater role than ENSO and PDO in causing climatological changes on glaciers in temperate and African zones. While the Bolivian and Argentinian glaciers were the least influenced by AMO and NAO, most of the glaciers in Ecuador and Colombia were the most influenced by SOI. These preliminary results highlight strong regional contrasts in climate variability (and its response to the influence of large-scale climatic variability patterns) between the different regions of the world.

How to cite: Al-YAARI, A., Condom, T., Junquas, C., Rabatel, A., Sicart, J., Cauvy, S., and Dangles, O.: Climate variability of glacierized areas under contrasted climate conditions over the last 60 years, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10345, https://doi.org/10.5194/egusphere-egu22-10345, 2022.

It is well known that atmospheric inversions may be decisive for the response of the atmospheric flow to an encounter with mountains.  In the literature, there has indeed been focus on this impact of inversions on the flow, but less focus on the inversions themselves; when, where and why the occur.  In this study, a large set of upper-air data from Iceland is explored to assess the climatology of inversions, and to some extent, the characteristics of the flow associated with statically stable layers in the troposphere.  The data reveal high frequency of tropospheric inversions, typically at 800-900 hPa.  The maximum frequency is from late winter until late autumn, with a minimum in mid-winter.  In the summer, the mean elevation of the inversions is lower than in the late winter and in the autumn.  Inversions in southerly flow are typically associated with moderate baroclinicity and advection of relatively warm airmasses above the inversion.  Inversions in northerly flow do not show this characteristic.   Case studies indicate substantial variability in synoptic-scale flow patterns leading to inversions.

How to cite: Jónsdóttir, L. S. and Ólafsson, H.: Climatology and some dynamic features of inversions in Iceland, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11743, https://doi.org/10.5194/egusphere-egu22-11743, 2022.

Exchange and transport processes in the atmospheric boundary layer (ABL) are driven by turbulence on a wide range of scales. Their adequate parameterization in numerical weather prediction (NWP) models is essential for a high predictive skill of forecasts. In heterogenous and complex terrain, the common simplification of turbulence to statistical models does not necessarily hold. Coherent structures such as convective cells, secondary circulations, gusts, slope and valley flows can be summarized to sub-mesoscale structures which are not well represented in models. A reason for the lack of understanding of these flow features is the challenge to adequately sample their three-dimensional, spatio-temporal structure and their contribution to the energy budget of the ABL.
We present a system to achieve simultaneous spatial measurements with a fleet of multirotor unmanned aircraft systems (UAS). The major benefit of this approach is, that true simultaneous measurements can be obtained without the need of expensive infrastructure such as masts or lidar instruments. In field campaigns with more than 1000 single flights at the Meteorological Observatory Lindenberg - Richard Aßmann-Observatory (MOL-RAO), the system was validated in 2020 and 2021 to provide reliable measurements of the horizontal wind vector. We showed that turbulent eddies can be resolved with a time resolution of up to 2~Hz, unless the overall TKE level is below the noise threshold of the UAS measurements, which can be the case in stable atmospheric stratification. Additionally to the wind vector estimation that is based only on avionic data from the autopilot, pressure, temperature and humidity sensors are carried by each UAS.
In future, within the project ESTABLIS-UAS, the fleet of UAS shall be expanded and capabilities for flights beyond visual line of sight and throughout the whole ABL shall be developed. The project includes a three-fold approach to validate single UAS measurements, fleet observations and methods to derive spatial averages and fluxes. Wind tunnel tests, field experiments and virtual measurements in numerical simulations will be performed to gain confidence in the achievable accuracy in a wide range of conditions. Also, measurement strategies are to be developed that allow the derivation of meaningful fluxes in the mountain boundary layer (MoBL). 
The UAS fleet is planned to be deployed in two campaigns in the framework of the TEAMx research programme. The ESTABLIS-UAS measurements will fill observational gaps in the sub-mesoscale. The analysis of the UAS fleet data in synthesis with continuous ground observations and remote sensing will provide unprecedented new insights into the complex MoBL flow. The results will foster the development of new and better parameterization of the ABL in complex terrain.

How to cite: Wildmann, N., Wetz, T., and Zink, J.: Towards spatio-temporal measurements in the mountain boundary layer with a fleet of UAS, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6490, https://doi.org/10.5194/egusphere-egu22-6490, 2022.

Banner clouds are clouds that appear to be attached on the leeward side of a steep mountain or ridge on otherwise cloud-free days. The current work considers fundamental questions associated with the formation of this type of clouds using large-eddy simulations. Previous work was based on an idalized model configuration with  pyramid-shaped orography; there, it was shown that the shear of the oncoming flow plays a key role for the geometry of the lee-side vortex and, hence, for the shape of the banner cloud. 

In the current work, the scope is extended from an idealized pyramid to the realistic orography of Mt Matterhorn. The simulations show that the wind shear of the oncoming flow is less essential than before, because the underlying rough orography creates "its own" flow profile by the time the flow reaches the windward side of the mountain. By contrast, the wind speed turns out to be quite relevant, because large windspeed is associated with strong turbulence, turbulence reduces stratification, and reduced stratification helps to form the lee vortex. However, at the same time, the flow field for realistic Matterhorn orography makes it much harder to identify a coherent lee vortex to be associated with the banner cloud. 

How to cite: Thomas, M. L., Wirth, V., and Piotrowski, Z.: What flow conditions are conducive to banner cloud formation at Mt. Matterhorn?, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-878, https://doi.org/10.5194/egusphere-egu22-878, 2022.

This study investigates the skill of the COSMO model (v5.7) at 1.1 km horizontal resolution (COSMO-1) in simulating the near-surface foehn evolution, with a focus on surface temperature, for a set of five south foehn events and a 5-year-long analysis dataset based on COSMO-1. A significant cold bias during foehn hours is found in the Rhine Valley as well as other northern Alpine valleys for all five cases and the 5-year climatology. Several possible causes of the cold bias are examined using sensitivity experiments for the five foehn cases. The sensitivity experiments include changes to the parameterization of the land-atmosphere interface (i.e. adoption of a skin temperature, a change of the heat resistance in the laminar sublayer, and a new formulation of the bare soil evaporation), to the 1D turbulence parameterization (including horizontal shear production of turbulence as a first step towards 3D effects), and to the horizontal grid spacing (1.1 km versus 550 m). While several of the sensitivity experiments impact the 2-m temperature during non-foehn hours, only a change in the horizontal grid spacing has a significant impact on the 2-m temperature during foehn hours. The 550-m run shows also an improvement in the simulated foehn duration and northward foehn extent. Possible reasons for the improvements and the remaining bias will be discussed.

How to cite: Tian, Y., Schmidli, J., and Quimbayo-Duarte, J.: A station-based evaluation of south foehn forecasting with COSMO-1, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3680, https://doi.org/10.5194/egusphere-egu22-3680, 2022.

Various research has been conducted into the turbulent momentum fluxes in orographic flow, and is in some models parameterised to simulate orographic form drag, yet the role of turbulent exchange of heat and moisture presents a somewhat opaque picture, as these processes are generally not resolved or parameterised in NWP models.  We present results from rare, low-level leeside turbulence observations in a strong mountain wind event.  These observations were obtained in 2018 during the Iceland-Greenland Seas Project (IGP) field campaign, a coupled atmosphere-ocean project, which included two research flights over the steep and complex orography of two Icelandic peninsulas with the aim of investigating leeside turbulent exchange processes.  High resolution regional model MetOffice Unified Model (MetUM) forecasts are run and compared to in-situ observations from 12^th and 19^th March 2018 over the Westfjords and Snaesfellsness peninsula in north-western Iceland.  With sub-kilometre horizontal grid-cell lengths flow features such as a downslope windstorm coupled with a hydraulic jump and a wave-breaking region directly aloft are well resolved and provide suitable cases for testing different MetUM science configurations.  The evaluation of control forecasts has shown a consistent 2 K bias in the lower atmospheric boundary layer sourced from the global driving model.  Sensitivity tests are run with the aim to ultimately parameterise scalar transport of heat and moisture in the leeside orographic flow.  This presentation will outline current progress and, to a degree, will attempt to answer the question of the significance and importance of turbulent scalar exchange within a strong wind event.

Keywords: MetUM, tuurtbulent exchange, orographic, NWP, Iceland

How to cite: Hodder, W., Renfrew, I., Elvidge, A., Sheridan, P., and Petersen, G. N.: Examining turbulent scalar exchange during a strong wind event, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8009, https://doi.org/10.5194/egusphere-egu22-8009, 2022.

Katabatic winds are generated by the combination of a vertical density gradient, slope and gravity, when the surface radiative budget is negative. We presently analyse some results of a campaign led in the French Alps in 2019 (Charrondière et al. 2022) in order to study katabatic flows over a steep snowy alpine slope of about 30°, that develop during winter anticyclonic conditions. In the topographic and meteorological configuration of the experiment, these downslope flows have a jet shape, with a maximum wind speed height zj very close to the surface, at about 30 cm height.

A 3D pitot type sensor allowed measurements of wind speed down to 3 cm height above the surface, at a high sampling frequency of 1250 Hz. Sonic anemometers placed on a fixed bracket allowed to capture for the first time the 3D velocity of the katabatic flows (f=20 Hz) in the topographic coordinate system, whereas previous studies are in the streamline coordinate system.

We focus mainly on the inner region of the jet, below zj. The turbulent momentum flux is decreasing with height, and its variation can be derived from a simplification of the along-slope momentum equation where the gravity term balances the turbulent momentum flux gradient to first order, as shown in Denby and Smeets (2000).

We compare the inner region of the jet with a neutral turbulent boundary layer in terms of wind speed profile, and derive a correction of the classical log-law that considers the gravity effect on the along-slope velocity. This correction is different from the well-known Monin-Obukhov stability correction, which is negligible for the present flow because of relative low turbulent sensible heat fluxes compared to turbulent momentum fluxes.

We also show that the slope-normal velocity is negative and as high as 10-15% of the maximum wind speed in the inner region of the jet. The slope-normal momentum equation behavior in this region of the jet is consistent with the observations and confirms that a gravity source term directs the flow to the ground.

We finally analyze the impact of gravity on the temperature equation: the mean temperature profile and the turbulent sensible heat flux are also modified by it. All these modifications have implications on the turbulent Prandtl number, which behaves differently from what we expect on a neutral turbulent boundary layer cooled at the surface.

Charrondière, C., Brun, C., Cohard, JM. et al. Katabatic Winds over Steep Slopes: Overview of a Field Experiment Designed to Investigate Slope-Normal Velocity and Near-Surface Turbulence. Boundary-Layer Meteorol 182, 29–54 (2022). 

Denby B, Smeets CJPP (2000) Derivation of turbulent flux profiles and roughness lengths from katabatic flow dynamics. Journal of Applied Meteorology 39(9):1601–1612

How to cite: Brun, C., Charrondière, C., Hopfinger, E., Cohard, J.-M., and Sicart, J.-E.: Turbulent flow in the inner layer of a katabatic jet along a steep alpine slope, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6441, https://doi.org/10.5194/egusphere-egu22-6441, 2022.

The climatological frequency of deep moist convection is enhanced in the vicinity of mountainous regions. Most studies to date focus on convection initiation (CI) on the windward side of mountains, but uplift and CI can also occur on their lee side. The factors controlling this phenomenon are only partially understood, but it is frequently hypothesized that a lee-side hydraulic jump may provide the uplift required to initiate lee-side convection.

Here we argue that lee-side CI is best understood as a consequence of low-level convergence along an orographic dryline. The dryline marks the boundary between relatively dry air desceding from the mountains and conditionally unstable air over an adjacent plain. The stronger the convergence along the dryline, the more likely is CI to occur.

We initially focus on the atmospheric conditions leading to strong lee-side convergence and low-level uplift in an unsaturated atmosphere. A variety of scenarios are investigated using an idealised modelling setup, exploring a range of linearity and hydrostaticity of the cross-mountain flow and varying surface fluxes. By computing a convergence budget, we determine the dominant forcings affecting lee-side convergence and how these vary across flow regimes. A relationship is determined between the level of hydrostaticity and linerity of the flow, the strength of lee side convergence, and the corresponding boundary-layer uplift.

We then turn to considering flows with a conditionally unstable leeside environment. We replicate the scenarios in which strong lee-side convergence and low-level uplift are expected, and we determine whether the uplift is actually sufficient to initiate deep moist convection.

How to cite: Serafin, S. and Potter, E.: An idealized study of convection initiation along orographic drylines, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9456, https://doi.org/10.5194/egusphere-egu22-9456, 2022.

As the atmospheric circulation is the only media brings moisture from above the ocean to high mountains on the western Tibet Plateau (TP), the wind variability is of great importance to the water cycle centering at the western TP. This study thereby examines the leading modes of the wind fields over the western TP. The multivariate empirical orthogonal function (MV-EOF) analysis method is employed in this study to detect the dominant wind patterns above the western TP, which extracts the leading modes of the combined meridional and zonal wind variability at 200-hpa level in the region of 22°N-50°N, 50°E-92°E. Here, we find the first leading mode of the combined zonal and meridional wind field in annual mean and in most seasons (spring, summer and autumn) over the western TP show high similarity to the Western Tibetan Vortex (WTV), a large-scale atmospheric pattern recently recognized over the western TP. In winter, the WTV, however, is closer to the second leading mode. By moving the position of the same area surrounding the western TP and re-checking, we find the WTV is tied closely to geographical location of the western TP. In short, the WTV generally represents the first leading mode of the wind field in most seasons over the western TP. This study augments our knowledge on the wind variability over the western TP.

How to cite: Wang, J., Li, X.-F., Liu, S., Liu, T., Dai, Y., and Yang, S.: Leading Modes of Wind Field Variability over the Western Tibet Plateau, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13107, https://doi.org/10.5194/egusphere-egu22-13107, 2022.