Foehn Events Research Articles

Atmospheric-river-induced foehn events drain glaciers on Novaya Zemlya

Recently, climate extremes have been grabbing attention as important drivers of environmental change. Here, we assemble an observational inventory of energy and mass fluxes to quantify the ice loss from glaciers on the Russian High Arctic archipelago of Novaya Zemlya. Satellite altimetry reveals that 70 ± 19% of the 149 ± 29 Gt mass loss between 2011 and 2022 occurred in just four high-melt years. We find that 71 ± 3% of the melt, including the top melt cases, are driven by extreme energy imports from atmospheric rivers. The majority of ice loss occurs on leeward slopes due to foehn winds. 45 of the 54 high-melt days (>1 Gt d−1) in 1990 to 2022 show a combination of atmospheric rivers and foehn winds. Therefore, the frequency and intensity of atmospheric rivers demand accurate representation for reliable future glacier melt projections for the Russian High Arctic.

Subtropical Foehn Winds, Southeast Queensland, Australia

AbstractFoehn winds have been a focus of research in mid‐latitude mountainous regions for more than 150 years, where their onset is typically associated with warm, dry, and gusty winds. This research has now extended into high latitude regions, yet research of foehn winds in subtropical and tropical regions remains scarce. Here we present results from the first investigation of foehn winds in the subtropics of Southeast Queensland (SEQ), Australia. Analysis of meteorological records found that foehn winds occur throughout the year with peak frequency and duration in late winter (August) associated with the passage of shortwave troughs over southern Australia. Modeling of wind fields and atmospheric boundary layer conditions for three case studies was conducted using the Weather Research and Forecasting (WRF) model. Results showed foehn events in SEQ can be associated with mountain waves and hydraulic jump features in the lee of topographic barriers. Over lee slopes, acceleration of wind speeds and topographic channeling of foehn winds was found to occur, along with substantial increases in air temperature, and decreases in relative humidity. Warming of the foehn airstream is believed to occur primarily through isentropic drawdown with a likely contribution from surface sensible heat flux. Recommendations for future research are made in light of the importance of foehn winds to wildfire management and mitigation in SEQ.

A station‐based evaluation of near‐surface south foehn evolution in COSMO‐1

AbstractThis study investigates the skill of the Consortium for Small‐scale Modeling (COSMO) model (v5.7) at 1.1 km horizontal grid size in simulating the near‐surface foehn properties and evolution for five south foehn events and a five‐year‐long climatology. A significant near‐surface cold bias is found during foehn, with an average bias of K in the Rhine Valley in the five foehn cases and K in the major northern foehn valleys in the five‐year foehn climatology. The cold bias tends to be larger in the stronger and moister deep foehn events. Sensitivity experiments are carried out to examine the possible causes of the cold bias, including changes to the parameterization of the land–atmosphere interaction, 1D turbulence parameterization, and horizontal grid spacing. Most sensitivity experiments have only a very minor impact on the cold bias, except for the model run with a horizontal grid spacing of 550 m. The 550‐m COSMO run shows a reduced cold bias during foehn hours and also an improvement in the simulated foehn duration and northward foehn extent. By inspecting the vertical dimension, we found that the near‐surface cold bias downstream might partly originate upstream. A further contribution to the downstream cold bias is likely due to insufficient vertical mixing in the foehn flow. The latter is possibly enhanced in the 550‐m model run, leading to a less stably stratified atmosphere in the lower few hundred meters of the atmosphere and a reduction of the reported model cold bias.

Strong Warming Over the Antarctic Peninsula During Combined Atmospheric River and Foehn Events: Contribution of Shortwave Radiation and Turbulence

AbstractThe Antarctica Peninsula (AP) has experienced more frequent and intense surface melting recently, jeopardizing the stability of ice shelves and ultimately leading to ice loss. Among the key phenomena that can initiate surface melting are atmospheric rivers (ARs) and leeside foehn; the combined impact of ARs and foehn led to moderate surface warming over the AP in December 2018 and record‐breaking surface melting in February 2022. Focusing on the more intense 2022 case, this study uses high‐resolution Polar WRF simulations with advanced model configurations, Reference Elevation Model of Antarctica topography, and observed surface albedo to better understand the relationship between ARs and foehn and their impacts on surface warming. With an intense AR (AR3) intrusion during the 2022 event, weak low‐level blocking and heavy orographic precipitation on the upwind side resulted in latent heat release, which led to a more deep‐foehn like case. On the leeside, sensible heat flux associated with the foehn magnitude was the major driver during the night and the secondary contributor during the day due to a stationary orographic gravity wave. Downward shortwave radiation was enhanced via cloud clearance and dominated surface melting during the daytime, especially after the peak of the AR/foehn events. However, due to the complex terrain of the AP, ARs can complicate the foehn event by transporting extra moisture to the leeside via gap flows. During the peak of the 2022 foehn warming, cloud formation on the leeside hampered the downward shortwave radiation and slightly increased the downward longwave radiation.

Characteristics and Establishment of Objective Identification Criteria and Predictors for Foehn Winds in Urumqi, China

The special terrain of Urumqi (in the valley area) often triggers strong foehn winds, causing huge losses to local people’s lives and social economies. By using the surface observation data of the hourly temperature, pressure, humidity, and wind from the downwind Urumqi Meteorological Station and the upwind Dabancheng Meteorological Station in the Middle Tianshan Canyon and the NCEP/NCAR reanalysis data during 2008–2022, this paper establishes the dataset of foehn processes at Urumqi Station in the past 15 years and analyzes the variation rules of the associated meteorological variables during the foehn processes. In addition, based on the physical mechanism of the occurrence of foehn, a three-element identification criterion (i.e., 94° ≤ 2 min average wind direction ≤ 168°, 2 min average wind speed ≥ 2.0 m/s, and Δθ between Urumqi station and Dabancheng station ≥ 0.29 K) for foehn in Urumqi is established by comparing and analyzing the variations of wind direction (WD), wind speed (WS), and the potential temperature difference (Δθ) between the two weather stations during the periods of foehn and non-foehn winds from 2013 to 2022. In addition, the performance of the three-element identification criterion is verified, and the results suggest that this criterion has an accuracy of 82.96% and a hit rate of 89.50% for the 2008–2012 foehn events in Urumqi. Moreover, the hit rate of this criterion for foehn wind of gale or above level (i.e., a 2 min wind ≥ 10.8 m/s on average) is 100%. In addition, combined with two predictors of sea-level pressure difference (ΔP) and Δθ between downwind stations and upwind stations, the foehn forecast can be more accurate than that provided by a single predictor. With ΔP ≤ −12 hPa and Δθ ≥ 5 K, the chances for foehn to occur are over 90%. This finding would have some reference and application values for the foehn forecasting.

The surface energy balance during foehn events at Joyce Glacier, McMurdo Dry Valleys, Antarctica

Abstract. The McMurdo Dry Valleys (MDV) are a polar desert, where glacial melt is the main source of water to streams and the ecosystem. Summer air temperatures are typically close to zero, and therefore foehn events can have a large impact on the meltwater production. A 14-month record of automatic weather station (AWS) data on Joyce Glacier is used to force a 1D surface energy balance model to study the impact of foehn events on the energy balance. AWS data and output of the Antarctic Mesoscale Prediction System (AMPS) on a 1.7 km grid are used to detect foehn events at the AWS site. Foehn events at Joyce Glacier occur under the presence of cyclones over the Ross Sea. The location of Joyce Glacier on the leeward side of the Royal Society Range during these synoptic events causes foehn warming through isentropic drawdown. This mechanism differs from the foehn warming through gap flow that was earlier found for other regions in the MDV and highlights the complex interaction of synoptic flow with local topography of the MDV. Shortwave radiation is the primary control on melt at Joyce Glacier, and melt often occurs with subzero air temperatures. During foehn events, melt rates are enhanced, contributing to 23 % of the total annual melt. Foehn winds cause a switch from a diurnal stability regime in the atmospheric surface layer to a continuous energy input from sensible heat flux throughout the day. The sensible heating during foehn, through an increase in turbulent mixing resulting from gustier and warmer wind conditions, is largely compensated for by extra heat losses through sublimation. Melt rates are enhanced through an additional energy surplus from a reduced albedo during foehn.

Machine learning-based prediction of Alpine foehn events using GNSS troposphere products: first results for Altdorf, Switzerland

Abstract. Remote sensing of water vapour using the Global Navigation Satellite System (GNSS) is a well-established technique and reliable data source for numerical weather prediction (NWP). However, one of the phenomena rarely studied using GNSS are foehn winds. Since foehn winds are associated with significant humidity gradients between two sides of a mountain range, tropospheric estimates from GNSS are also affected by their occurrence. Time series reveal characteristic features like distinctive minima and maxima as well as a significant decrease in the correlation between the stations. However, detecting such signals becomes increasingly difficult for large datasets. Therefore, we suggest the application of machine learning algorithms for the detection and prediction of foehn events by means of GNSS troposphere products. This initial study develops a new, machine learning-based method for detection and prediction of foehn events at the Swiss station Altdorf by utilising long-term time series of high-quality GNSS troposphere products. Data from the Automated GNSS Network Switzerland (AGNES) and various GNSS sites from neighbouring countries as well as records of an operational foehn index are used to investigate the performance of several different classification algorithms based on appropriate statistical metrics. The two best-performing algorithms are fine tuned and tested in four dedicated experiments using different feature setups. The results are promising, especially when reprocessed GNSS products are utilised and the most dense station setup is used. Detection- and alarm-based measures reach levels between 66 %–80 % for both tested algorithms and thus are comparable to those from studies using data from meteorological stations and NWP. For operational prediction, limitations due to the availability and quality of GNSS products in near-real time (NRT) exist. However, they might be mitigated to a significant extent by provision of additional NRT products and improved data processing in the future. Results also outline benefits for the results when including geographically relevant stations (e.g. high-altitude stations) in the utilised datasets.

Temperature and moisture transport during atmospheric blocking patterns around the Antarctic Peninsula

We assess temperature and moisture transport in and around the Antarctic Peninsula (AP) associated with atmospheric blocking over two domains, one located to the west (150–90°W, 50–70°S, Western AP (WAP)) and the other over and to the east (90–30°W, 50–70°S, Eastern AP (EAP)) of the AP. We make use of surface meteorological observations, ERA5 reanalysis data, and a state-of-the-art atmospheric river (AR) database. Observed temperature anomalies indicate that the WAP and EAP blocking patterns are characterized by significant cold and warm anomalies over the AP, respectively, particularly in austral autumn, winter and spring. Consistent with these changes, cold anomalies depicted by ERA5 are associated with the transport of cold and dry air from the Antarctic continent by southerly and southeasterly flow over the eastern flank of the WAP blocking. ERA5 results highlight the importance of blocking days over the EAP domain (largely centered over the Drake Passage) to the occurrence of warm events associated with northerly and northwesterly warm air transport. Significant increases in integrated vapor transport (IVT) and AR frequency are also evident during the EAP blocking, particularly on the windward side of the AP. During the most extreme blocking days in this domain, there exist significant increases in latent and sensible heat fluxes on the windward side of the AP and the Larsen C Ice Shelf, respectively, indicating the contribution of foehn events to warm anomalies, especially in austral autumn and winter. The co-occurrences between landfalling ARs and blocking are found to amplify foehn effect due to higher IVT and associated latent heat condensation compared to blocking days without ARs. We conclude that blocking patterns are important to understand the occurrence of extremely warm events and landfalling ARs in the AP and their potential impacts on the surface cryospheric processes.

Thermodynamics and airstreams of a south foehn event in different Alpine valleys

AbstractFoehn flows are associated with a characteristic warming of the respective air in lee‐side valleys. Recent studies about the physical mechanisms question the traditional thermodynamic foehn theory, pointing out the potential role of adiabatic descent and other diabatic processes, besides upstream latent heating in clouds, for the warming. This study applies a kilometre‐scale simulation together with online trajectories and a Lagrangian heat budget to investigate the foehn air warming for an intense, long‐lasting Alpine south foehn period in November 2016. Thereby, the focus lies on the attribution of the warming to adiabatic descent and the different diabatic processes in six foehn valleys, as well as the linkage of the warming processes to different airstreams. Overall, adiabatic warming emerges as the most important process for 57% of all air parcels arriving in the foehn valleys. However, upstream latent heating in clouds is the dominant mechanism for a considerable amount of air parcels (35%). The heat budget analysis reveals a clear transition along the Alps from west to east, whereat diabatic warming constitutes the driving mechanism for western valleys during the central phase of the event, and adiabatic warming dominates the foehn air warming in eastern valleys. The foehn trajectories can be separated into different air‐stream categories according to their diabatic temperature change. Air parcels experiencing strong diabatic heating are transported in an easterly barrier jet in the Po Valley before crossing the Alpine crest. Air parcels experiencing diabatic cooling, in contrast, are advected quasi‐horizontally from the south towards the Alpine crest prior to their descent. The relative strength of the different airstreams therefore determines the dominating warming process. These findings strongly point to the combined importance of adiabatic and diabatic warming mechanisms, which co‐occur with varying relative contributions, depending on the valley and time period of the event.

Energy and mass exchange at an urban site in mountainous terrain – the Alpine city of Innsbruck

Abstract. This study represents the first detailed analysis of multi-year, near-surface turbulence observations for an urban area located in highly complex terrain. Using 4 years of eddy covariance measurements over the Alpine city of Innsbruck, Austria, the effects of the urban surface, orographic setting and mountain weather on energy and mass exchange are investigated. In terms of surface controls, the findings for Innsbruck are in accordance with previous studies at city centre sites. The available energy is partitioned mainly into net storage heat flux and sensible heat flux (each comprising about 40 % of the net radiation, Q*, during the daytime in summer). The latent heat flux is small by comparison (only about 10 % of Q*) due to the small amount of vegetation present but increases for short periods (6–12 h) following rainfall. Additional energy supplied by anthropogenic activities and heat released from the large thermal mass of the urban surface helps to support positive sensible heat fluxes in the city all year round. Annual observed CO2 fluxes (5.1 kg C m−2 yr−1) correspond well to modelled emissions and expectations based on findings at other sites with a similar proportion of vegetation. The net CO2 exchange is dominated by anthropogenic emissions from traffic in summer and building heating in winter. In contrast to previous urban observational studies, the effect of the orography is examined here. Innsbruck's location in a steep-sided valley results in marked diurnal and seasonal patterns in flow conditions. A typical valley wind circulation is observed (in the absence of strong synoptic forcing) with moderate up-valley winds during daytime, weaker down-valley winds at night (and in winter) and near-zero wind speeds around the times of the twice-daily wind reversal. Due to Innsbruck's location north of the main Alpine crest, southerly foehn events frequently have a marked effect on temperature, wind speed, turbulence and pollutant concentration. Warm, dry foehn air advected over the surface can lead to negative sensible heat fluxes both inside and outside the city. Increased wind speeds and intense mixing during foehn (turbulent kinetic energy often exceeds 5 m2 s−2) help to ventilate the city, illustrated here by low CO2 mixing ratios. Radiative exchange is also affected by the orography – incoming shortwave radiation is blocked by the terrain at low solar elevation. The interpretation of the dataset is complicated by distinct temporal patterns in flow conditions and the combined influences of the urban environment, terrain and atmospheric conditions. The analysis presented here reveals how Innsbruck's mountainous setting impacts the near-surface conditions in multiple ways, highlighting the similarities with previous studies in much flatter terrain and examining the differences in order to begin to understand interactions between urban and orographic processes.

A 20‐Year Study of Melt Processes Over Larsen C Ice Shelf Using a High‐Resolution Regional Atmospheric Model: 2. Drivers of Surface Melting

AbstractQuantifying the relative importance of the atmospheric drivers of surface melting on the Larsen C ice shelf is critical in the context of recent and future climate change. Here, we present analysis of a new multidecadal, high‐resolution model hindcast using the Met Office Unified Model, described in Part 1 of this study. We evaluate the contribution of various atmospheric conditions in order to identify and rank, for the first time, the most significant causes of melting over the recent past. We find the primary driver of surface melting on Larsen C is solar radiation. Foehn events are the second most important contributor to surface melting, especially in nonsummer seasons when less solar radiation is received at the surface of the ice shelf. Third, cloud influences surface melting via its impact on the surface energy balance (SEB); when the surface temperature is warm enough, cloud can initiate or prolong periods of melting. Lastly, large‐scale circulation patterns such as the Southern Annular Mode (SAM), El Niño Southern Oscillation, and Amundsen Sea Low control surface melting on Larsen C by influencing the local meteorological conditions and SEB. These drivers of melting interact and overlap, e.g., the SAM influences the frequency of foehn, commonly associated with leeside cloud clearances and sunnier conditions. Ultimately, these drivers matter because sustained surface melting on Larsen C could destabilize the ice shelf via hydrofracturing, which would have consequences for the fate of the ice shelf and sea levels worldwide.

Is it north or west foehn? A Lagrangian analysis of Penetration and Interruption of Alpine Foehn intensive observation period 1 (PIANO IOP 1)

Abstract. A case study of a foehn event in the Inn Valley near Innsbruck, Austria, that occurred on 29 October 2017 in the framework of the first intensive observation period (IOP) of the Penetration and Interruption of Alpine Foehn (PIANO) field campaign is investigated. Accompanied with northwesterly crest-level flow, foehn broke through at the valley floor as strong westerly winds in the morning and was terminated in the afternoon by a cold front arriving from the north. The difference between local and large-scale wind direction raises the question of whether the event should be classified as north or west foehn – a question that has not been convincingly answered in the past for similar events based on Eulerian approaches. Hence, the goal of this study is to assess the air mass origin and the mechanisms of foehn penetration to the valley floor based on a Lagrangian perspective. For this purpose a mesoscale simulation with the Weather Research and Forecasting (WRF) model and a backward trajectory analysis with LAGRANTO are conducted. The trajectory analysis shows that the major part of the air arriving in Innsbruck originates 6 h earlier over eastern France, crosses the two mountain ranges of the Vosges and the Black Forest, and finally impinges on the Alps near Lake Constance and the Rhine Valley. Orographic precipitation over the mountains leads to a net diabatic heating of about 2.5 K and to a moisture loss of about 1 g kg−1 along the trajectories. A secondary air stream originates further south over the Swiss Plateau and contributes about 10 % to 40 % of the trajectories to the foehn air in Innsbruck. Corresponding trajectories are initially nearly parallel to the northern Alpine rim and get lifted above crest level in the same region as the main trajectory branch. Air parcels within this branch experience a net diabatic heating of about 2 K and, in contrast to the ones of the main branch, an overall moisture uptake due to evaporation of precipitation formed above these air parcels. Penetration into the Inn Valley mainly occurs in the lee of three local mountain ranges – the Lechtal Alps, the Wetterstein Mountains, and the Mieming Chain – and is associated with a gravity wave and a persistent atmospheric rotor. A secondary penetration takes place in the western end of the Inn Valley via the Arlberg Pass and Silvretta Pass. Changes in the upstream flow conditions cause a shift in the contributions of the associated penetration branches. From a Lagrangian perspective this shift can be interpreted on the valley scale as a gradual transition from west to northwest foehn despite the persistent local west wind in Innsbruck. However, a clear classification in one or the other categories remains subjective even with the Lagrangian approach and, given the complexity of the trajectory pattern, is nearly impossible with the traditional Eulerian view.

Comparison of kilometre and sub‐kilometre scale simulations of a foehn wind event over the Larsen C Ice Shelf, Antarctic Peninsula using the Met Office Unified Model (MetUM)

AbstractA foehn event on 27 January 2011 over the Larsen C Ice Shelf (LCIS), Antarctic Peninsula and its interaction with an exisiting ground‐based cold‐air pool is simulated using the MetUM atmospheric model at kilometre and sub‐kilometre scale grid spacing. Atmospheric model simulations at kilometre grid scales are an important tool for understanding the detailed circulation and temperature structure over the LCIS, especially the occurrence of foehn‐induced surface melting, erosion of cold‐air pools, and low‐level wind jets (so‐called foehn jets). But whether there is an improvement/convergence in the model representation of these features at sub‐kilometre grid scales has yet to be established. The foehn event was simulated at grid spacings of 4, 1.5 and 0.5 km, with the results compared to automatic weather station and radiosonde measurements. The features commonly associated with foehn, such as a leeside hydraulic jump and enhanced leeside warming, were comparatively insensitive to resolution in the 4 to 0.5 km range, although the 0.5 km simulation shows a slightly sharper and larger hydraulic jump. By contrast, during the event the simulation of fine‐scale foehn jets above the cold‐air pool showed considerable dependence on grid spacing, although no evidence of convergence at higher resolution. During the foehn event, the MetUM model is characterised by a nocturnal cold bias of around 8 °C and an underestimate of the near‐surface stability of the cold‐air pool, neither of which improved with increased resolution. This finding identifies a key model limitation, at both kilometre and sub‐kilometre scales, to realistically capture the vertical mixing in the boundary layer and its impact on thermodynamics, through either daytime heating from below or the downward penetration of foehn jet winds from above. Detailed model‐resolved foehn jet dynamics thus plays a crucial role in controlling the near‐surface temperature structure over the LCIS, as well as sub‐grid turbulent mixing.

The foehn wind east of the Andes in a 20-year climate simulation

This study investigates the spatial structure and the seasonal occurrence of foehn wind to the east of the Andes using a flow blocking analysis in a 20-year climate simulation. The latter was performed by the Eta-CPTEC regional model at 50-km horizontal resolution. This version of the model includes a cut-cell scheme to represent topography and a finite-volume vertical advection scheme for dynamic variables. The results indicate that foehn wind more frequently blows during winter and spring on the eastern slopes of the Andes, except to the south of 37° S where it blows at all seasons. Higher mountains of the Central Andes (27° S–35° S) and the High Plateau (15° S–27° S) result in blocked foehn events, with a weak adjustment to the geostrophic balance. On the Central Andes, rain and snow on mountain tops may also contribute to generate foehn wind on the eastern slopes. The results show that a low pressure develops to the east of the Central Andes, and also to the east of the High Plateau when foehn blows. Lower mountains in Patagonia (to the south of 37° S) result in more frequent non-blocked foehn event, with better adjustment to the geostrophic balance.

Representation of the rain shadow effect in Patagonia using an orographic‐derived regional climate model

AbstractThis study focuses on Patagonia, where Foehn events observed in the lee of the Andes mountains are not yet well simulated by state‐of‐the‐art climate models. It has been agreed that one source of this shortcoming is related to the poor relief representation in models. To resolve this need, a common method used is to enhance the spatial resolution of the model to retrieve a more complex surface elevation, at the expense of calculation time or surface area covered. This paper tackles the problem from a different angle by addressing the Digital Elevation Model (DEM) generalization, that is, the altitudes generalization from a high‐resolution DEM to a coarser resolution grid model. Most current climate models use DEM generalization methods that smooth the relief, a key controlling factor in Foehn events modelling. The aim of this study is to compare three original methods of DEM generalization (percentile 90 [P90], envelope maximum [EM], and thalweg and crests [TC]) and to evaluate their impact on simulated precipitation and temperature fields on the eastern part of Patagonia, where warm and dry air masses are expected. Thanks to MAR, a Regional Climate Model, we validate the models at 10 and 5 km resolutions against the Climate Research Unit and perform three sensitivity experiments involving a change in the DEM generalization. Our results show that (a) a finer spatial resolution can slightly improve the temperature biases, however, it cannot resolve the precipitation biases and (b) a more appropriate use of DEM generalization induces a significant decrease in precipitation for the P90 and EM methods and an increase in mean temperature for all three methods in the study area. This study serves as a recommendation for a better use of DEM generalization in climate models performing in Patagonia, but also regions sharing the same orographic features as the Patagonian relief.

Characteristics and evolution of diurnal foehn events in the Dead Sea valley

Abstract. This paper investigates frequently occurring foehn in the Dead Sea valley. For the first time, sophisticated, high-resolution measurements were performed to investigate the horizontal and vertical flow field. In up to 72 % of the days in summer, foehn was observed at the eastern slope of the Judean Mountains around sunset. Furthermore, the results also revealed that in approximately 10 % of the cases the foehn detached from the slope and only affected elevated layers of the valley atmosphere. Lidar measurements showed that there are two main types of foehn. Type I has a duration of approximately 2–3 h and a mean maximum velocity of 5.5 m s−1 and does not propagate far into the valley, whereas type II affects the whole valley, as it propagates across the valley to the eastern side. Type II reaches mean maximum wind velocities of 11 m s−1 and has a duration of about 4–5 h. A case study of a type II foehn shows that foehn is initiated by the horizontal temperature gradient across the mountain range. In the investigated case this was caused by an amplified heating and delayed cooling of the valley boundary layer in the afternoon, compared to the upstream boundary layer over the mountain ridge. The foehn was further intensified by the advection of cool maritime air masses upstream over the coastal plains, leading to a transition of subcritical to supercritical flow conditions downstream and the formation of a hydraulic jump and rotor beneath. These foehn events are of particular importance for the local climatic conditions, as they modify the temperature and humidity fields in the valley and, furthermore, they are important because they enhance evaporation from the Dead Sea and influence the aerosol distribution in the valley.

Modeling Study of Foehn Wind Events in Antarctic Peninsula with WRF Forced by CCSM

Significant changes have occurred in the Antarctic Peninsula (AP) including warmer temperatures, accelerated melting of glaciers, and breakup of ice shelves. This study uses the Weather Research and Forecasting model (WRF) forced by the Community Climate System Model 4 (CCSM) simulations to study foehn wind warming in AP. Weather systems responsible for generating the foehn events are two cyclonic systems that move toward and/or cross over AP. WRF simulates the movement of cyclonic systems and the resulting foehn wind warming that is absent in CCSM. It is found that the warming extent along a transect across the central AP toward Larsen C Ice Shelf (LCIS) varies during the simulation period and the maximum warming moves from near the base of leeward slopes to over 40 km away extending toward the attached LCIS. Our analysis suggests that the foehn wind warming is negatively correlated with the incoming air temperature and the mountain top temperature during periods without significant precipitation, in which isentropic drawdown is the dominant heating mechanism. On the other hand, when significant precipitation occurs along the windward side of AP, latent heating is the major heating mechanism evidenced by positive relations between the foehn wind warming and 1) incoming air temperature, 2) windward precipitation, and 3) latent heating. Foehn wind warming caused by isentropic drawdown also tends to be stronger than that caused by latent heating. Comparison of WRF simulations forced by original and corrected CCSM data indicates that foehn wind warming is stronger in the original CCSM forced simulation when no significant windward precipitation is present. The foehn wind warming becomes weaker in both simulations when there is significant windward precipitation. This suggests that model’s ability to resolve the foehn warming varies with the forcing data, but the precipitation impact on the leeward warming is consistent.

A first-of-its-kind multi-model convection permitting ensemble for investigating convective phenomena over Europe and the Mediterranean

A recently launched project under the auspices of the World Climate Research Program’s (WCRP) Coordinated Regional Downscaling Experiments Flagship Pilot Studies program (CORDEX-FPS) is presented. This initiative aims to build first-of-its-kind ensemble climate experiments of convection permitting models to investigate present and future convective processes and related extremes over Europe and the Mediterranean. In this manuscript the rationale, scientific aims and approaches are presented along with some preliminary results from the testing phase of the project. Three test cases were selected in order to obtain a first look at the ensemble performance. The test cases covered a summertime extreme precipitation event over Austria, a fall Foehn event over the Swiss Alps and an intensively documented fall event along the Mediterranean coast. The test cases were run in both “weather-like” (WL, initialized just before the event in question) and “climate” (CM, initialized 1 month before the event) modes. Ensembles of 18–21 members, representing six different modeling systems with different physics and modelling chain options, was generated for the test cases (27 modeling teams have committed to perform the longer climate simulations). Results indicate that, when run in WL mode, the ensemble captures all three events quite well with ensemble correlation skill scores of 0.67, 0.82 and 0.91. They suggest that the more the event is driven by large-scale conditions, the closer the agreement between the ensemble members. Even in climate mode the large-scale driven events over the Swiss Alps and the Mediterranean coasts are still captured (ensemble correlation skill scores of 0.90 and 0.62, respectively), but the inter-model spread increases as expected. In the case over Mediterranean the effects of local-scale interactions between flow and orography and land–ocean contrasts are readily apparent. However, there is a much larger, though not surprising, increase in the spread for the Austrian event, which was weakly forced by the large-scale flow. Though the ensemble correlation skill score is still quite high (0.80). The preliminary results illustrate both the promise and the challenges that convection permitting modeling faces and make a strong argument for an ensemble-based approach to investigating high impact convective processes.

Foehn Event Triggered by an Atmospheric River Underlies Record‐Setting Temperature Along Continental Antarctica

AbstractA record‐setting temperature of 17.5°C occurred on 24 March 2015 at the Esperanza station located near the northern tip of the Antarctic Peninsula (AP). We studied the event using surface station data, satellite imagery, reanalysis data, and numerical simulations. The Moderate Resolution Imaging Spectroradiometer Antarctic Ice Shelf Image Archive provides clear evidence for disintegration and advection of sea ice, as well as the formation of melt ponds on the ice sheet surface at the base of the AP mountain range. A deep low‐pressure center over the Amundsen‐Bellingshausen Sea and a blocking ridge over the southeast Pacific provided favorable conditions for the development of an atmospheric river with a northwest‐southeast orientation, directing warm and moist air toward the AP, and triggering a widespread foehn episode. A control simulation using a regional climate model shows the existence of local topographically induced warming along the northern tip of the AP (∼60% of the full temperature signal) and the central part of the eastern AP (>90% of the full temperature signal) with respect to a simulation without topography. These modeling results suggest that more than half of the warming experienced at Esperanza can be attributed to the foehn effect (a local process), rather than to the large‐scale advection of warm air from the midlatitudes. Nevertheless, the local foehn effect also has a large‐scale advection component, since the atmospheric river provides water vapor for orographic precipitation enhancement and latent heat release, which makes it difficult to completely disentangle the role of local versus large‐scale processes in explaining the extreme event.

Dynamics of a Puelche foehn event in the Andes

Dynamics of a Puelche foehn event in the Andes