AS1.16

Several studies report on elevation-dependent warming (EDW), i.e., when warming rates change with elevation. This study assesses future EDW in the Andes, using an ensemble of regional climate model simulations belonging to the CORDEX experiment. EDW was assessed by calculating the (minimum and maximum) temperature difference between the end of the century (2071-2100) and the period 1976-2005 and relating it to the elevation. For the maximum temperatures, a positive EDW (enhancement of warming rates with elevation) was identified in both the western and eastern side of the tropical and subtropical Andes and in all seasons. For the minimum temperature, while a positive EDW was identified in the Subtropics (particularly in the western side of the chain), the Tropics are characterized by a negative EDW throughout the year. The tropical boundary marks a transition between discordant EDW behaviours in the minimum temperature. In the Tropics, EDW drivers were found to be different for the minimum temperature (Tmin) and for the maximum temperature (Tmax). Changes in Tmin  are mostly associated with changes in downward longwave radiation, while changes in Tmax are mainly driven by changes in downward shortwave radiation. This might explain the opposite EDW signal found in the tropical Andes during daytime and nighttime. Changes in albedo are an ubiquitous driver for positive EDW in the Subtropics, for both the minimum and the maximum temperature. Changes in longwave radiation and humidity are also EDW drivers in the Subtropics but with different relevance throughout the seasons and during daytime and nighttime. Besides the dependence on the latitude, we found that the western and eastern sides of the Cordillera might be influenced by different EDW drivers.

How to cite: Palazzi, E., Toledo, O., Cely Toro, I. M., and Mortarini, L.: Elevation-dependent warming in the tropical and subtropical Andes with CORDEX models, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10385, https://doi.org/10.5194/egusphere-egu22-10385, 2022.

On average, surface temperatures are rising globally, but the pace of warming varies with regional factors. Rates of warming are expected to increase with elevation, a phenomenon referred to as elevation-dependent warming (EDW). Drivers of EDW include albedo changes due to an upward migration of snow- and treelines, as well as a rise of the condensation level and water vapour changes.

Amplified warming in high altitudes can have a great impact on mountain ecosystems and agriculture, which are particularly sensitive to changes in climate. The cryosphere is also impacted by EDW, with consequences for downstream water availability. While various studies have reported the presence of EDW, it is still unclear whether the phenomenon occurs in all mountain ranges or at all elevations. Research on EDW is made more difficult by sparse station observations: satellite data can be used to overcome these limits and facilitate analysis on the scale of whole mountain ranges and for longer time periods.

In this study, we used 20 years of land surface temperature (LST) observations from the Moderate Resolution Imaging Spectroradiometers (MODIS) on board of the TERRA satellite. The Andes were chosen as study area due to their latitudinal and altitudinal extent, which covers a wide range of climate and socio-economic zones.

We found warming to occur predominantly in the midlatitudes, while in the tropical Andes both, cooling and warming patterns occur. Additionally, seasonal variations of the magnitude and sign of the trends are more pronounced in the tropical latitudes than in the southern Andes. On average, EDW occurs in the western part of the Andes (Pacific watershed), while we find no elevation-dependence or even an opposite pattern (less warming at higher elevations) for the eastern side (Atlantic watershed). Our results depict the complex nature of EDW and call for further process-based studies supported by remote sensing data.

How to cite: Stöckhardt, M., Hänchen, L., Thomas, C., Maussion, F., and Wohlfahrt, G.: Elevation-dependent surface temperature changes in the Andes, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12324, https://doi.org/10.5194/egusphere-egu22-12324, 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.

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.

This contribution aims at presenting results from the project “Atmospheric boundary-layer modeling over complex terrain”, a collaboration between the University of Trento, the University of Bolzano and the University of Innsbruck with the objective to evaluate the performance of turbulence and land surface parameterizations over mountainous terrain and to identify potential issues that have a large impact on model results and consequently on the quality of weather forecasts.

A set of Reynolds-averaged Navier-Stokes (RANS) simulations at 1 km horizontal resolution is performed in an idealized three-dimensional valley-plain topography, using typical geometrical features of a north-south Alpine valley, with ridges up to 1500 m above the valley floor and a distance of 20 km from crest to crest. Simulations are initialized with a linear and stable vertical profile of potential temperature, dry air and an atmosphere at rest. The aim of the modeling experiment is to evaluate the sensitivity of model results to planetary boundary layer (PBL) parameterizations, exploring the performance of the PBL schemes implemented in the Weather Research and Forecasting (WRF) model, including a newly developed k-ε closure. Results from the RANS simulations are compared against a large-eddy simulation (LES) with a resolution of 100 m, which is taken as the benchmark. A full diurnal cycle has been considered for the evaluation of numerical results, focusing on the development of along- and cross-valley thermally-driven circulations and on the associated thermal field both in the nighttime and in the daytime phases. The sensitivity of model results to the change of the PBL scheme is assessed using as key metrics the strength and the timing of the thermally-driven circulations, as well as the vertical profiles of mean and turbulent quantities, when available. Results show that in most cases there is a good agreement between RANS simulations and the LES considering the main features of both along- and cross-valley circulations and the diurnal evolution of the thermal stratification. In particular, the intensity of the along-valley wind is generally well-reproduced by all the RANS simulations, while higher discrepancies are found for the timing of the evening transition. On the other hand, RANS simulations are in good agreement with the LES considering the timing of slope winds, whereas the simulation of their intensity presents much more variability, especially during nighttime.

How to cite: Giovannini, L., Zonato, A., Di Santo, D., Bisignano, A., and Zardi, D.: Sensitivity of the simulation of thermally-driven circulations in an idealized valley to planetary boundary layer parameterizations, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5840, https://doi.org/10.5194/egusphere-egu22-5840, 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.

In mountainous regions, incoming surface radiation is strongly influenced by surrounding and local terrain. The direct beam part of incoming shortwave radiation depends both on local slope angle and azimuth as well as on neighbouring terrain, which can induce topographic shading. Shortwave radiation can be reflected (multiple times) by terrain, which leads to enhanced incoming diffuse shortwave radiation for locations with a reduced sky view factor (SVF) – particularly under snow-covered conditions when surface reflectivity is high. Finally, incoming longwave radiation can also be modulated by neighbouring terrain due to radiation exchange between facing slopes.

Considering these effects in spatially distributed land surface models – either stand-alone or embedded in weather and climate models – typically requires the following topographic quantities: slope angle, slope aspect, terrain horizon and SVF. The first two quantities can be computed rapidly because they only depend on local terrain. The computation of the latter two quantities is however expensive, particularly for high-resolution (~30 m) digital elevation models (DEMs), because a large quantity of non-local DEM information has to be processed. We developed a new efficient algorithm for terrain horizon computation, which is based on a high-performance ray-tracing library. A benchmark against conventional algorithms confirmed its high performance – particularly for DEMs with very high resolution and for large terrain horizon search distances. Furthermore, due to the smooth representation of terrain by a triangle mesh, the new algorithm does not reveal artefacts in the computed horizon line in cases where the horizon is formed by proximal terrain. Finally, we demonstrate that the new algorithm is also eligible to compute sub-grid SVF for large spatial domains in a very efficient way. Sub-grid SVF is a useful quantity to parameterise above-mentioned topographic effects on surface radiation in weather and climate models applied on regional or even global scales.

How to cite: Steger, C. R., Steger, B., and Schär, C.: An efficient ray-tracing based algorithm to compute terrain horizon and sky view factor to consider topographic effects on surface radiation in spatially distributed land surface models, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6719, https://doi.org/10.5194/egusphere-egu22-6719, 2022.

Considerable knowledge gaps remain in understanding the spatial and temporal patterns of precipitation in the Tianshan, a large system of mountain ranges located in Central Asia. Based on the Global Precipitation Climatology Centre (GPCC) data set and NCEP/NACR reanalysis data, this study investigates the precipitation variations over the Tianshan Mountains on different time scales using the Ensemble Empirical Mode Decomposition (EEMD), and a subsequent attribution analysis with respect to large-scale climate modes.

During 1950-2016, the annual precipitation in most of the Tianshan regions showed an increasing trend with the exception of its wettest sub-regions, the Western Tianshan. In addition to the overall trend, the annual precipitation in Tianshan shows high-frequency variations of 3-year and 6-year quasi-periods and low-frequency variations of 12-year and 27-year quasi-periods. Winter precipitation in the Tianshan Mountains exhibits multi-decadal oscillations with periods of 26.8 and 44.7 years, with similar multi-decadal variability as the East Atlantic-Western Russia (EATL/WRUS) teleconnection pattern. The enhanced meridional characteristics of the EATL/WRUS trigger more water vapor fluxes from low-latitude oceanic regions, resulting in a wet period of Tianshan in winter after 1988. Similarly, summer precipitation in the Tianshan Mountains entered a wet period after 1986. The Scandinavian (SCAND) teleconnection pattern is significantly negatively correlated with Tianshan summer precipitation. During the negative phase of SCAND in summer, strong high pressure over the Ural Mountains and low pressure over Central Asia combine to induce enhanced conveying of water vapour to the Tianshan from the Arctic Ocean. Furthermore, the Silk Road pattern (SRP) and East Asia-Pacific teleconnection (EAP) have affected Tianshan summer precipitation for the periods 1964-1984 and 1985-2004.

How to cite: Guan, X., Yao, J., and Schneider, C.: Ensemble Empirical Mode Decomposition of the variability of precipitation over the Tianshan Mountains, Central Asia, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7023, https://doi.org/10.5194/egusphere-egu22-7023, 2022.

Snow cover phenology (onset, melting) and duration are expected to react to temperature trends. Quantification of snow cover changes is an essential step for further climate change impact evaluations given their multiple meteorological, hydrological, ecological, and societal implications. This study revisits the snow trends across Romania, using data from 114 weather stations (14 located in the Carpathian Mountains, at above 1,000 m), with complete and long-term series of snow observations,covering the 1-2,504 m elevation range. The trends in the dates of snow cover onset (SCO), snow cover melting (SCM) and snow cover duration (SCD) over the past 60 years (1961–2020) have been investigated over five elevation bands (<500, 501-1,000, 1,001-1,500, 1,501-2,000, and 2,001-2,500 m) for identifying hot-spots of snow cover change and retrieving evidence of elevation-dependency under climate warming. A declining SCD was systematically observed country-wide, statistically significant at only 25% of the weather stations included in this study. The decline is more accelerated in the lowlands, generallybelow 500 m. On the opposite, there is no statistically significance SCD change at above 2,000 m. Overall, SCD trends show no statistical dependency on elevation. The SCD decline is driven by the negative changes in SCM, due to the stronger warming in the late-winter and spring than in the fall and early winter. The snowmelting date advanced the most by over 7 day decade^-1at mid-to-high elevations (1,500-2,000 m) and in the lowlands (below 1,000 m). In the mountains, the most notable delays occurred in the Western and Southern Carpathians (~4-7 days decade^-1). At above 2,000 m, the negative SCM trends are weaker (~4 days decade^-1) and not statistically significant. Unlike SCM, over most parts of the country (63% of stations) SCO advanced towards winter, although only about 7% of trends were statistically significant (mostly at stations below 300 m). The SCO advance evolved at fairly comparable rates when comparing the highlands (above 1,000 m, ~4 day decade^-1) and the lowlands (below 1,000 m, 4 to 5 days decade^-1). An opposite climate signal, indicating earlier snow onsets was also observed at about 30% of stations (including the highest elevation stations, located at above 2,200 m), although not statistically significant. Overall, SCO and SCM trends show a weak-to-moderate but statistically significant relationship with elevation (r=.23-.25, p<0.05), suggesting, on the one hand, that warming effects on snow phenology are particularly strong in the lowlands and, on the other hand, that there are other driving factors influencing the snow phenology(i.e., atmospheric circulation, local factors).Our results are linked to the rising temperature, particularly strong in winter and spring, which in our case is more accelerated in the low elevation areas.

This study was funded by the Ministry of Environment, Water and Forests, in the framework of research project A.III.10 (Spatial-temporal climate variability in Romania).

How to cite: Amihăesei, V., Micu, D. M., Dumitrescu, A., and Cheval, S.: Observed trends in snow phenology and duration across Romania (1961 to 2020), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11963, https://doi.org/10.5194/egusphere-egu22-11963, 2022.

The tendency to form a sea of clouds (SOC) with surface meteorological conditions was observed for 3-year warm periods at the foot of Mt. Yatsugatake by time-lapse camera and meteorological instruments at Fujimi Panorama ski resort, Nagano prefecture. In situ observation revealed large- and small-scale SOCs in the valley. Large-scale SOCs were commonly observed in the early morning, while small-scale SOCs in the eastern valley corresponded with low-level orographic clouds ascending over the slopes of Yatsugatake. An empirical algorithm was developed to detect the occurrence of nocturnal low-level clouds, corresponding to large-scale morning SOCs, using Hiamari-8 images on hourly basis with references to in situ camera observation. SOCs frequently occurred in the large-scale valley or basin in the inland areas in the Japanese Alps region, referred as 12 target areas, and they were infrequent in the coastal areas or high elevations over 2000 m. When we defined days of wide-ranging SOC occurrence, in which SOCs occurred in the half or more of target areas, 67% were associated with a subsidence inversion layer by a synoptic-scale high pressure system. The low-level cloud-top height determined by two camera images at different altitudes almost corresponded with the height of the inversion layer observed by radio-sounding data at Wajima station. We concluded that the synoptic-scale subsidence inversion layer plays an important role in forming large-scale SOCs in the Japanese Alps region in addition to nocturnal radiative cooling conditions.

(Publlished in Japanese on Tenki 68, 2021: https://www.metsoc.jp/tenki/year.php)

How to cite: Ueno, K. and Kobayashi, Y.: The genesis tendency for a sea of clouds to occur at night in the Japanese Alps region derived by surface observation and satellite data, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1245, https://doi.org/10.5194/egusphere-egu22-1245, 2022.

Foehn air parcels are typically associated with a pronounced warming within lee-side valleys. While the physical mechanism of this warming has been disputed for over a century, recent studies emphasize the key role of both adiabatic descent (isentropic drawdown), but also turbulent mixing and upstream latent heating in clouds, depending on the Foehn case and the region. This study aims to attribute the warming to adiabatic descent and key diabatic processes for six major Alpine Foehn valleys in Switzerland and Austria. To this end, a mesoscale model simulation including online trajectories is combined with a Lagrangian heat budget to investigate how relevant the different processes are for an intense and long-lasting South Foehn event in November 2016.

In agreement with earlier findings for the Alpine Foehn, adiabatic descent constitutes the most important process for the majority (57%) of air parcels arriving within the six Foehn valleys. Nonetheless, upstream latent heating in clouds is more important for a considerable number (35%) of air parcels. On the one hand, the Lagrangian analysis reveals a clear difference between western and eastern Alpine valleys, as adiabatic warming gradually becomes more important for the eastern valleys. On the other hand, a distinct temporal evolution is identified, where diabatic processes emerge as the main warming mechanism for the western valleys during the central phase of the Foehn event.

As the contribution for diabatic heating varies strongly for the different Foehn valleys, it is used to subdivide the Foehn trajectories into three different airstreams. Air parcels associated with intense diabatic heating are typically advected within a low-level easterly barrier jet in the Po Valley before traversing the Alps. Diabatically cooled air parcels, on the other hand, originate at higher levels and are quasi-horizontally advected from the south towards the Alpine crest. Hence, the varying intensity of the contributing airstreams dictates the dominating warming mechanism. The results prevent a clear separation into ‘Swiss Foehn’ and ‘Austrian Foehn’, as, in our case study, both varieties either simultaneously occur in the different valleys, or distinct time periods of the Foehn within a valley are more or less dominated by either or the other airstream.

How to cite: Jansing, L. and Sprenger, M.: Foehn air warming in six Alpine valleys: Lagrangian heat budget analysis and relation to airstreams, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1483, https://doi.org/10.5194/egusphere-egu22-1483, 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.

Orographic gravity waves (also known as mountain waves) cause the atmosphere to exert a drag force on mountains. By Newton’s 3^rd law, the mountains exert an equal and opposite force on the atmosphere. It is clear from linear wave theory how to develop a framework for representing this reaction force in parametrizations for vertically propagating waves in climate and weather prediction models: the waves break and dissipate either due to critical levels (where the wind speed is perpendicular to the horizontal wavenumber vector, or zero), or due to the progressive decrease of density with height. But the situation is more complicated for trapped lee waves, which propagate horizontally near the surface, and where the wave energy is alternately reflected at the ground and at an elevated layer where the waves become evanescent. It is clear that boundary layer friction should be responsible for most of the dissipation of trapped lee waves, but it is not clear, even in the inviscid approximation, what form the wave momentum flux profiles that force the large-scale mean flow will take. This is due to the complications associated with the fact that trapped lee waves have both horizontal and vertical momentum (and pseudo-momentum) fluxes, which oscillate indefinitely with the wave phase downstream of the orography. No mechanism equivalent to critical levels, or density decay with height, acting on vertically propagating mountain waves, is available for trapped lee waves. In this study, this limitation is overcome by accounting for the effects of weak friction. While for an inviscid trapped lee wave train, the horizontally integrated momentum flux is ill-defined (except at the surface), in a dissipative problem where friction exists, no matter how small, the wave train necessarily decays downstream, and so is spatially bounded. This allows the areally integrated effect of the trapped lee wave to be expressed in terms of the divergence of the vertical flux of horizontal wave momentum (as for vertically-propagating waves). On the other hand, the form of the momentum flux profile (which defines this divergence) is different from any form that could be inferred from inviscid theory, although it is independent of the magnitude of friction, as long as this is small. These results from linear theory are compared with high-resolution numerical simulations of trapped lee waves for the two-layer atmosphere of Scorer, which confirm the form of the momentum flux profiles, and suggest that these may be independent of the adopted form of friction, at least to some extent. The results therefore facilitate the formulation of parametrizations for trapped lee waves with a much more solid physical  basis, and are likely to be generalizable to other atmospheric profiles.

How to cite: Teixeira, M. A. C. and Argaín, J. L.: The momentum flux profiles produced by trapped lee waves, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1927, https://doi.org/10.5194/egusphere-egu22-1927, 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.