Overview and first results of the Wind and Storms Experiment (WASTEX): a field campaign to observe the formation of gusts using a Doppler lidar

Wind gusts are responsible for most damages in winter storms over central Europe, but capturing their small scale and short duration is a challenge for both models and observations. This motivated theWind and Storms Experiment (WASTEX) dedicated to investigate the formation of gusts during the passage of extratropical cyclones. The field campaign took place during the winter 2016–2017 on a former waste deposit located close to Karlsruhe in the Upper Rhine Valley in southwest Germany. Twelve extratropical cyclones were sampled during WASTEX with a Doppler lidar system performing vertical scans in the mean wind direction and complemented with a Doppler C-band radar and a 200m instrumented tower. First results are provided here for the three most intense storms and include a potential sting jet, a unique direct observation of a convective gust and coherent boundary-layer structures of strong winds

Revisiting the synoptic-scale predictability of severe European winter storms using ECMWF ensemble reforecasts

Abstract. New insights into the synoptic-scale predictability of 25 severe European winter storms of the 1995–2015 period are obtained using the homogeneous ensemble reforecast dataset from the European Centre for Medium-Range Weather Forecasts. The predictability of the storms is assessed with different metrics including (a) the track and intensity to investigate the storms\u27 dynamics and (b) the Storm Severity Index to estimate the impact of the associated wind gusts. The storms are well predicted by the whole ensemble up to 2–4 days ahead. At longer lead times, the number of members predicting the observed storms decreases and the ensemble average is not clearly defined for the track and intensity. The Extreme Forecast Index and Shift of Tails are therefore computed from the deviation of the ensemble from the model climate. Based on these indices, the model has some skill in forecasting the area covered by extreme wind gusts up to 10 days, which indicates a clear potential for early warnings. However, large variability is found between the individual storms. The poor predictability of outliers appears related to their physical characteristics such as explosive intensification or small size. Longer datasets with more cases would be needed to further substantiate these points

Understanding summer wind systems over the eastern Mediterranean in a high‐resolution climate simulation

Regional and local wind systems are often complex, particularly near coastal areas with a highly variable orography. Thus, the realistic representation of regional wind systems in weather and climate models is of strong relevance. Here, we evaluate the ability of a 13-year convection-permitting climate simulation in reproducing the interaction of several regional summer wind systems over the complex orography in the eastern Mediterranean region. The COSMO-CLM simulations are driven by hourly ERA-5 reanalysis and have a spatial resolution of 2.8 and 7.0 km. The simulated near-surface wind fields are compared with unique very high-resolution wind observations collected within the “Dead Sea Research Venue” project (DESERVE) and data from the Israel Meteorological Service synop network. The high-resolution COSMO-CLM simulations largely reproduce the main characteristics of the regional wind systems (Mediterranean and Dead Sea breeze, slope winds in the Judean Mountains and winds along the Jordan Rift valley), whereas ERA-5 is only able to represent the Mediterranean Sea breeze. The high-resolution simulations substantially improve the representation of regional winds, particularly over complex orography. Indeed, the 2.8 km simulation outperforms the 7.0 km run, on 88% of the days. Two mid-July 2015 case studies show that only the 2.8 simulation can realistically simulate the penetration of the Mediterranean Sea Breeze into the Jordan Rift valley and complex interactions with other wind systems like the Dead Sea breeze. Our results may have profound implications for regional weather and climate prediction since very high-resolution information seems to be necessary to reproduce the main summertime climatic features in this region. We envisage that such simulations may also be required at other regions with complex orography

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

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

Vermischungsvorgänge in der unteren Troposphäre über orographisch strukturiertem Gelände. Ein Beitrag zum EUROTRAC-Teilprojekt TRACT

Meteorologische und luftchemische Messungen, die während der TRACT Feldmeßkampagne im September 1992 in Südwestdeutschland, Ostfrankreich und der Nordschweiz durchgeführt wurden, zeigen einen starken Einfluß der Orographie auf die vertikale Durchmischung und den Horizontaltransport von Luftmassen in der unteren Troposphäre während Schönwetterlagen. Detaillierte Untersuchungen der konvektiven Grenzschichtentwicklung werden sowohl anhand von Meßdaten als auch mittels einfacher Modellrechnungen durchgeführt. Es werden verschiedene Parametrisierungen der Grenzschichtentwicklung diskutiert, die orographisch bedingte Advektionseffekte berücksichtigen und somit zu einer besseren Beschreibung der Grenzschichthöhe über unebenem Gelände beitragen. Über hügeligem Gelände wird der Austausch zwischen der atmosphärischen Grenzschicht und der freien Troposphäre (Handover) durch Stufen in der Grenzschichthöhe und Durchbrüche in der Grenzschichtinversion verstärkt. Regionale Windsysteme entlang der großen Täler zwischen den Gebirgen bewirken einen Transport von Luftverunreinigungen zwischen verschiedenen städtischen Ballungszentren. Diesen regionalen Luftströmungen sind thermisch induzierte Zirkulationssysteme an den Berghängen und in den Seitentälern der Gebirge überlagert, die zu einem Luftmassenaustausch zwischen den dicht besiedelten Tälern mit hohen Emissionen und den ländlichen Bergregionen führen. Es wird gezeigt, daß hierbei die nächtlichen Bergwinde zu sekundären Ozonmaxima an Talstationen führen können. Geländebeeinflußte Nebelverteilungen bewirken starke räumliche Unterschiede in den Spurengasverteilungen und in der Grenzschichthöhe und begünstigen somit das Handover an Grenzschichtstufen

HyMeX: A 10-Year Multidisciplinary Program on the Mediterranean Water Cycle

Drobinski, P. ... et. al.-- 20 pages, 10 figures, 1 table, supplement material http://journals.ametsoc.org/doi/suppl/10.1175/BAMS-D-12-00244.1HyMeX strives to improve our understanding of the Mediterranean water cycle, its variability from the weather-scale events to the seasonal and interannual scales, and its characteristics over one decade (2010–20), with a special focus on hydrometeorological extremes and the associated social and economic vulnerability of the Mediterranean territoriesHyMeX was developed by an international group of scientists and is currently funded by a large number of agencies. It has been the beneficiary of financial contributions from CNRS; Météo-France; CNES; IRSTEA; INRA; ANR; Collectivité Territoriale de Corse; KIT; CNR; Université de Toulouse; Grenoble Universités; EUMETSAT; EUMETNET; AEMet; Université Blaise Pascal, Clermont Ferrand; Université de la Méditerranée (Aix-Marseille II); Université Montpellier 2; CETEMPS; Italian Civil Protection Department; Université Paris- Sud 11; IGN; EPFL; NASA; New Mexico Tech; IFSTTAR; Mercator Ocean; NOAA; ENEA; TU Delft; CEA; ONERA; IMEDEA; SOCIB; ETH; MeteoCat; Consorzio LAMMA; IRD; National Observatory of Athens; Ministerio de Ciencia e Innovación; CIMA; BRGM; Wageningen University and Research Center; Department of Geophysics, University of Zagreb; Institute of Oceanography and Fisheries, Split, Croatia; INGV; OGS; Maroc Météo; DHMZ; ARPA Piemonte; ARPA-SIMC Emilia-Romagna; ARPA Calabria; ARPA Friuli Venezia Giulia; ARPA Liguria; ISPRA; University of Connecticut; Università degli Studi dell'Aquila; Università di Bologna; Università degli Studi di Torino; Università degli Studi della Basilicata; Università La Sapienza di Roma; Università degli Studi di Padova; Università del Salento; Universitat de Barcelona; Universitat de les Illes Balears; Universidad de Castilla-La Mancha; Universidad Complutense de Madrid; MeteoSwiss; and DLR. It also received support from the European Community's Seventh Framework Programme (e.g., PERSEUS, CLIM-RUN)Peer reviewe

Forecasting wind gusts in winter storms using a calibrated convection-permitting ensemble

Windstorms associated with low‐pressure systems from the North Atlantic are the most important natural hazards for central Europe. Although their predictability has generally improved over the last decades, forecasting wind gusts is still challenging, due to the multiple scales involved. One of the first ensemble prediction systems at convection‐permitting resolution, COSMO‐DE‐EPS, offers a novel 2.8‐km dataset over Germany for the 2011–2016 period. The high resolution allows representation of mesoscale features that are barely captured by global models, while the long period allows both investigation of rare storms and application of statistical post‐processing. Ensemble model output statistics based on a truncated logistic distribution substantially improve forecasts of wind gusts in the whole dataset. However, some winter storms exhibit uncharacteristic forecast errors that cannot be reduced by post‐processing. During the passage of the most severe storm, gusts related to a cold jet are predicted relatively well at the time of maximum intensity, whereas those related to a warm jet are poorly predicted at an early phase. Wind gusts are overestimated during two cases of frontal convection, which suggests that even higher resolution is needed to resolve fully the downward mixing of momentum and the stabilization resulting from convective dynamics. In contrast, extreme gusts are underestimated during a rare case involving a possible sting jet, but this arises from the representation of the synoptic rather than the mesoscale. The synoptic scale also controls the ensemble spread, which is inherited mostly from the initial and boundary conditions. This is unsurprising, but leads to high forecast uncertainty in the case of a small, fast‐moving cyclone crossing the model domain. These results illustrate how statistical post‐processing can help identify the limits of predictability across scales in convection‐permitting ensemble forecasts. They may guide the development of regime‐dependent statistical methods to improve forecasts of wind gusts in winter storms further

Latent Heat Flux Measurements over Complex Terrain by Airborne Water Vapour and Wind Lidars

Vertical profiles of the latent heat flux in a convective boundary layer (CBL) are obtained for the first time over complex terrain with airborne water vapour differential absorption lidar and Doppler wind lidar. During the Convective and Orographically-induced Precipitation Study (COPS) over the Black Forest Mountains in south-western Germany both lidars were installed nadir-viewing onboard the Falcon research aircraft of the Deutsches Zentrum f�¼r Luft- und Raumfahrt (DLR). On 30 July 2007, additional in-situ measurements by the Karlsruhe Institute of Technology (KIT) were performed with a Dornier-128 aircraft that flew below the Falcon. This unique instrument configuration allows to validate the lidar-derived fluxes and to assess lidar-specific issues such as instrument noise and data gaps that impinge on the results. The cospectra of in-situ humidity and vertical velocity peak at wavelengths between 1 - 3 km and reveal that the dominant scales of turbulent transport are larger than 700 m in space. Consequently the airborne lidarsâ�� horizontal and vertical resolution of ~200 m is sufficient to seize most of the flux. The lidar and in-situ fluxes of five collocated 45-km flight legs agree within �±20 %, the average difference over the total distance of 225 km is 3 %. A flux comparison with ground-based water vapour Raman and wind lidars shows agreement within the instrumentsâ�� accuracies under low-wind conditions. All latent heat fluxes vary between 100 - 500 W/m�² in the CBL and have small vertical divergences. Vertical velocity spectra in the mid-CBL enable to estimate the dissipation rate of turbulent kinetic energy that amounts to 5â�¢10-4 m2 s-3 in the Rhine Valley and 10-3 m2 s-3 over the Black Forest Mountains. This new airborne lidar instrumentation proves to be a valuable tool for the study of CBL processes and variability, particularly over complex terrain