During a winter storm in January 2007, a train derailed due to strong winds in a narrow valley in northeastern Switzerland. The accident was attributed to the Laseyer, a local windstorm characterized by flow reversal that manifests as easterly to southeasterly winds at the valley floor during strong prevailing northwesterly winds above. We analyze case studies of the local windstorm using sonic anemometer and Doppler lidar measurements. The data reveal a highly turbulent flow in the narrow valley and extreme wind speeds exceeding 45 m/s during Laseyer conditions.

Additionally, we use a newly developed large-eddy simulation (LES) atmospheric model to improve our understanding of the local windstorm. The model is implemented in a Python environment with the GT4Py (GridTools for Python) domain-specific library to enable performance portability.  Robust and efficient solution of the nonhydrostatic compressible equations is achieved using a finite-volume semi-implicit discretization following ECMWF’s IFS-FVM. LESs are performed above the highly complex terrain of northeastern Switzerland, which leads to extremely steep slopes exceeding 70°. With these LESs, we identify the mechanism behind the local windstorm, study its sensitivity to ambient flow conditions, and characterize the flow conditions in the narrow valley.

How to cite: Krieger, N., Kühnlein, C., Sprenger, M., Wernli, H., Bättig, P., Hervo, M., and Krieger, U.: Investigating a local windstorm using measurements and large-eddy simulations, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11038, https://doi.org/10.5194/egusphere-egu24-11038, 2024.

Previous studies have found a pronounced nocturnal low-level jet at the exit of the Inn Valley north of the valley contraction near Schwaigen which reaches into the Alpine foreland (e.g. Pamperin and Stilke, 1985 as part of the MERKUR experiment or a model study by Zängl, 2004). The exit jet forms under nocturnal stably stratified atmospheric conditions and is interpreted as a transition from subcritical to supercritical hydraulic flow.

As part of the pre-campaign of the TEAMx programme in June-August 2022, we have conducted measurements to corroborate the previous findings on the formation and maintenance of the Inn valley exit jet and learn more about its turbulence structure, which has not been studied in previous experiments. For this purpose, a wind lidar was deployed in Brannenburg, north of the valley constriction. TKE profiles were derived from the Lidar measurements using the method described in Smalikho and Banakh (2017).  Furthermore, 3-hourly radiosondes were launched at the site of the wind lidar, accompanied by drone measurements during an IOP (18/19 July 2022) in high pressure weather conditions with low cloud cover.  

Upper air and surface wind measurements during the IOP captured a well pronounced Inn valley exit jet which is analyzed in detail in this contribution. Additionally, a statistical analysis of the occurrence and characteristics of nocturnal low-level jets within the whole pre-campaign period is presented.

References:

TEAMx: http://www.teamx-programme.org/

Smalikho, I.N., and V.A. Banakh. "Measurements of wind turbulence parameters by a conically scanning coherent Doppler lidar in the atmospheric boundary layer." Atmospheric Measurement Techniques 10.11 (2017): 4191-4208.

Pamperin, H., and G. Stilke. "Nächtliche Grenzschicht und LLJ im Alpenvorland nahe dem Inntalausgang." Meteorologische Rundschau 38.5 (1985): 145-156

Zängl, G. "A reexamination of the valley wind system in the Alpine Inn Valley with numerical simulations." Meteorology and Atmospheric Physics 87.4 (2004): 241-256.

How to cite: Sedlmeier, K., Kossmann, M., Paunovic, I., Eichhorn-Müller, A., Nitsche, O., Leinweber, R., Päschke, E., and Mühlbacher, G.: The Inn Valley exit jet: results of the TEAMx pre-campaign, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12631, https://doi.org/10.5194/egusphere-egu24-12631, 2024.

Persistent inversions and associated low wind situations during the winter months often lead to air pollution problems in alpine basins and valleys, regardless of emission levels. The aim of this work is to determine how well high-resolution simulations with the Weather Research and Forecasting (WRF) model are able to reproduce the occurrence and weather conditions during temperature inversions in complex topography.

The city of Graz in south-eastern Austria often experiences increased strength and persistence of winter inversions due to its location and local topography. Experiments in this area with the WRF model show a better reproduction of these weather conditions when the shortwave radiation scheme Dudhia is used instead of the RRTMG, while variations in the microphysics and planetary boundary schemes did not lead to relevant changes in the model results.

In a next step, additional basins and alpine valleys will be investigated to determine the influence of shape and extent of the topography on the results.

The model is forced with the ECMWF-IFS analysis data with a spatial resolution of 9 km. Two one-way nested domains with resolutions of 3 and 1 km are used to investigate what resolution is required to adequately represent the local topographic effects. The model results are compared with station and radiosonde observations as well as with the analysis and nowcasting system INCA for Austria.

How to cite: Perny, K., Formayer, H., and Nadeem, I.: Testing the capability of the WRF model on representing temperature inversions in alpine basins and valleys, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18557, https://doi.org/10.5194/egusphere-egu24-18557, 2024.

Analysis of surface precipitation accumulations upstream, near-shore, and adjacent to the Olympic mountains from the 17 December 2015 case during OLYMPEX using Weather Research and Forecasting (WRF) simulations, the NPOL dual-polarization radar, and high-resolution soundings investigates the role of low-level blocking on upstream precipitation enhancement. Past work shows that frontal systems often slow while approaching complex terrain if the Froude number is sufficiently low. Low-level blocking of stable air ahead of a front can modify precipitation distributions by frontal deformation, slowing, splitting, or merging. Observed coastal sounding-derived vertical stability profiles indicate high levels of low-level stability and significant vertical wind shear, which showed little change while a warm front propagated northeastward and stalled as the stable air mass likely dammed against the terrain. Radial velocity from the NPOL radar and simulated wind fields indicate strong down-valley flow coupled with a frontal jet also contributed to long-lasting Kelvin-Helmholtz (KH) waves extending offshore.

Using WRF simulations along with OLYMPEX observations, we examined the evolution of precipitation upstream of complex terrain by breaking down the distribution of pre-frontal and frontal precipitation accumulations as the warm front approached the Olympic Peninsula. Through dividing the event into regions upstream of NPOL and into timeframes relative to landfall, results indicate pre-warm frontal precipitation accumulations decrease with distance upstream of the coast with the highest accumulations present over the terrain. As the front's translation speed slowed and eventually stalled, the warm frontal period accumulations are highest far upstream of the coast and over the terrain, with lesser accumulations in the middle region. These results indicate that upstream precipitation enhancement upstream is an indirect effect of the terrain influencing the frontal shape and propagation, resulting in enhanced frontal precipitation accumulations.

How to cite: Hence, D. and James, S.: Evolution of Surface Precipitation Accumulations Upstream of the Olympic Mountains using Observations and Simulations: An OLYMPEX Case Study, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20124, https://doi.org/10.5194/egusphere-egu24-20124, 2024.

Complex mountain orography induces sharp gradients in precipitation accumulation locally. The associated complexity in understanding these events depends on local orographic, microphysical, and dynamical conditions, which makes simulating snowfall a major challenge for regional atmospheric models. This study addresses these deficiencies by using a unique repository of snowfall measurements at a range of ‘super sites’ in the European Alps and Himalayas, which are used to produce a precipitation-optimised version of the atmosphere-only UK Met Office Unified Model (MetUM) at a spatial resolution of 1.5 km. The snowfall measurements involve using the winter time-series of water pressure in frozen lakes to measure the mass of falling snow during extreme precipitation events directly over the lake area, which are comparable in size to the model’s grid cells. Development of the precipitation-optimised version of the MetUM involves undertaking a series of model sensitivity experiments focused on varying the physical representation of cloud and precipitation microphysics, with the aim of better capturing the onset and end periods, and amounts of received snowfall during these extreme events. The MetUM is configured to use a double moment cloud microphysical scheme (CASIM: Cloud AeroSol Interacting Microphysics) with prescribed hydrometeor spectral attributes necessary to quantify both the auto-conversion rates and thresholds for the cloud conversion to take place. Results from these experiments suggest that local microphysical processes, often subsumed within small spatial scales, can influence dynamics at larger scales, impacting gradients in precipitation. Cloud radiative properties, including the hydrometeor effective radii and optical depths are further validated against satellite-based observations.

How to cite: Gumber, S., Orr, A., Field, P., Pritchard, H., Covi, F., Deb, P., Girona-Mata, M., Widmann, M., and Potter, E.: Using Novel Lake-based Snowfall Measurements in the Alps and Himalayas to optimise Cloud and Precipitation processes in a Regional Atmospheric Model (MetUM), EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10043, https://doi.org/10.5194/egusphere-egu24-10043, 2024.

The Yarlung Tsangbo Grand Canyon (YGC), one of the world’s deepest canyons, is located in the southeastern Tibetan Plateau (SETP). The YGC exhibits the highest frequency of convective activity in China. Due to frequent rainstorms in the wet season, natural disasters such as landslides and debris flows frequently occur, and often block traffic corridors. Thus, understanding the relationship between water vapor changes, convective cloud activity, and extreme rainfall events in the YGC is critical. A comprehensive observation network for water vapor variations, cloud activity, local circulation, and land-air interactions in the YGC was installed to help us to determine the relationship between the water vapor transport and heavy precipitation in the YGC and the physical process that determines the precipitation intensity, especially for cases of strong precipitation.

More than three years data collected from a rain gauge network, disclose that the spatial pattern of rainfall distribution. There are two regions (500 m and 2500 m AMSL) with high precipitation in the YGC. Diurnal cycles showed some variations among sites, but a clear floor was visible around afternoon and peak values exhibited in the early morning. The monthly precipitation in the YGC region shows two peaks in April and July. Vertical convection and vapor transport are important for extreme rainfall in this region.

We analyzed 35 years observation data of daily precipitation to objectively classify the weather systems responsible for the SETP heavy precipitation. Hierarchical clustering method divided the atmospheric circulation of the regional heavy precipitation into two representative patterns: the Tibetan Plateau vortex type (TPVT, accounting for 56.6% of the heavy precipitation events) and the mid-latitude trough type (MLTT，43.4%). The comprehensive analysis of the two patterns shows a clear connection between the heavy precipitation and positive vorticity anomaly, moisture convergence and the southeastward shift of the westerly jet core. Specifically, TPVT heavy precipitation events are caused by potential vorticity dry-to-wet processes during its eastward movement, while MLTT events are associated with the intrusion of deeply extratropical trough-ridge circulations into the SETP.

We used the Weather Research and Forecasting (WRF) model to simulate the water vapor flux during extreme rainfall events. The general shortcoming of the WRF precipitation simulation nudged with the European Centre for Medium-Range Weather Forecasts’ reanalysis dataset version 5 (ERA5), is that it cannot capture strong rainfall period. We tested many WRF parameterization schemes at a 1 km grid resolution. It was found that when an optimized combination of parameterization schemes in WRF can better capture the variations in the wind and water vapor concentration in the YGC channel, the model produced the best simulation results for extreme rainfall in the YGC.

These analyses have help us understanding the impacts of YGC valley on the water vapor transport and extreme rainfall outbreak mechanism.

How to cite: Chen, X., Cao, D., Zhang, Q., Xu, X., and Ma, Y.: Extreme rainfall characteristics and its simulations in the Yarlung Tsangbo Grand Canyon, China, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2067, https://doi.org/10.5194/egusphere-egu24-2067, 2024.

The Tibetan Plateau (TP) directly heats the middle tropospheric atmosphere, and accurate simulation of its surface temperature is of great concern for improving climatic prediction and projection capabilities, but climate models always exhibit a cold bias. Based on the Coupled Model Intercomparison Project Phase 6 (CMIP6) models and in-situ observations during 1981-2014, this study elucidates the impact of the snow overestimation on the temperature simulation over the TP in CMIP6 from the perspective of local radiation processes and atmospheric circulation. On the one hand, more snow in the CMIP6 models not only directly cools the surface more, but also makes the surface receive less shortwave radiation due to the higher surface albedo, and thus has lower ground surface temperature (GST), and the more snow/albedo-low temperature process is particularly evident in the westerly region due to more uncertainty at high elevations. This process contributes 87% to the annual GST cold bias. Lower GST corresponds to less latent heat transfer and thereby lower surface air temperature (SAT). In addition, the more snow in the CMIP6 models leads to the weaker the South Asian summer monsoon and the westerlies, and brings less warm and moist air (less integrated water vapor flux), as well as less clear-sky downward longwave radiation (less water vapor amount and lower tropospheric air temperature) to the TP (contributing 58% to the annual GST cold bias). These processes will result in less both precipitation and surface latent heat loss, which offsets a 35% annual GST cold bias. Besides, the physical mechanism of snow on GST and SAT differs with season over the westerly and monsoon regions of the TP. The research highlights the importance of topography and snow parameterization schemes for optimizing CMIP6 models.

How to cite: Wu, F. and You, Q.: Understanding of CMIP6 surface temperature cold bias over the westerly and monsoon regions of the Tibetan Plateau, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2463, https://doi.org/10.5194/egusphere-egu24-2463, 2024.

Mountains play a crucial role in the climate system at various temporal and spatial scales. Additionally, they serve as vital sources of resources, such as fresh water, and host a diverse range of biodiversity. This influence on development and natural ecosystems is particularly significant in semi-arid regions like the Sierra de Guadarrama. This mountain range is located in the Iberian Peninsula and has been the subject of official meteorological observations since the mid-20th century. Nevertheless, there is a gap in the knowledge of the rainfall and temperature variability and its drivers in this important region. TROPA-UCM group has been intensively observing and studying this range since 1998 and recently, a methodology has been developed to extend the observations from 1900 to present using data from the ERA20C and in-situ observations. This has enabled longer time series and a deeper analysis of large-scale teleconnection patterns and climate variability, unlike ever before. The analysis includes trends in temperature, snow precipitation, and snowpack duration. Variations in precipitation and temperature have been identified, providing valuable information for estimating potential changes in seasonal runoff and rainfall intensity. This information can be of great use to organizations responsible for the management of this area, for developing adaptation strategies for new scenarios and to improve seasonal to decadal predictions.

How to cite: Durán, L., González-Cervera, Á., and Rodríguez-Fonseca, B.: Analysis of climate variability and teleconnection patterns in Sierra de Guadarrama (Iberian Peninsula) using 120-year observed and reconstructed time series, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19545, https://doi.org/10.5194/egusphere-egu24-19545, 2024.

Several observational products of key climate variables have been widely used to evaluate the extent of the ongoing effects of climate change in the Alpine area, one of the most vulnerable and sensitive regions to the continuous warming of climate. However, a limited spatial coverage in most observational products and quality issues of data may strongly impact climate and hydrological studies results in terms of reliability, accuracy and precision. Even though the collection and management of meteorological data for the whole Alpine area is a challenging task due to strong fragmentation and diversity of data sources, further efforts need to be dedicated to produce new harmonised, high-quality and high-resolution products able to permit a more robust assessment of climate change and its impacts.  

Here we present a new observational dataset gathering in-situ measurements of meteo-climatic variables provided by a variety of meteorological and hydrological services within the extended Alpine region. The observational network consists of about 10000 in-situ weather stations, measuring key climate variables up to 2020 at daily time resolution, resulting in an extended and homogeneous coverage, both in space and elevation. Data collected are screened, inspecting the presence of most important critical issues in terms of data quality. A deep quality control of collected time series has been performed by checking internal, temporal and spatial consistency of time series, exploiting the problem of outlier removal. Inhomogeneities in time series are detected by a multi-methods approach and significant inhomogeneous periods are corrected. 

A climatological and trend analysis, in terms of both mean and extreme values, was carried out on a selection of homogenised time series extending over the period 1961-2020. The most common climate indices and statistics are used to perform the analysis at different time frequencies and spatial scales. A further analysis concerned the relationship between climate variables and main teleconnection patterns.

The present dataset addresses the most important issues affecting state-of-the-art observational products and it represents a powerful tool for better understanding Alpine climate changes over the last decades and improving the reliability of future scenarios.

How to cite: Bongiovanni, G., Matiu, M., Crespi, A., Napoli, A., Majone, B., and Zardi, D.: A new dataset of daily observations from a dense network of weather stations covering the Extended Alpine Region, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13294, https://doi.org/10.5194/egusphere-egu24-13294, 2024.