Heini Wernli

Last Name

Wernli

First Name

Heini

ORCID

0000-0001-9674-4837

Organisational unit

03854 - Wernli, Johann Heinrich / Wernli, Johann Heinrich

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Publications 1 - 10 of 231

Predictability of extreme surface weather associated with Mediterranean cyclones in ECMWF ensemble forecasts - Part 1: Method and case studies
Hartmuth, Katharina; Büeler, Dominik; Wernli, Heini (2026)
Weather and Climate Dynamics
Extratropical cyclones are the main cause of extreme surface weather events in the Mediterranean such as heavy precipitation, floods, severe winds, and dust storms. However, the accuracy in predicting the timing, location, and intensity of such events is often insufficient, which is typically related to errors in cyclone position, propagation, and intensity. In this two-part study we use operational ensemble forecasts from the European Centre for Medium-Range Weather Forecasts to quantify the predictability of extreme surface weather conditions linked to Mediterranean cyclones. We apply an object-based approach to attribute events of extreme precipitation and surface winds to Mediterranean cyclones. Thereby, objects of extreme surface weather are identified at grid points that exceed the seasonal 99th percentile of these parameters and matched to cyclones based on their distance to the cyclone center. In this first part, we introduce the probabilistic method and three illustrative case studies of Mediterranean cyclones that occurred between November 2022 and September 2023, including the infamous Storm Daniel as well as Storms Denise and Jan. We find that the cyclones as well as their attributed objects of extreme surface weather are predicted well for lead times <= 48 h. However, for longer lead times there is large case-to-case variability in the ensemble performance. Predictions of extreme surface weather objects are found to be more uncertain (i) for smaller and less coherent objects, (ii) if the associated cyclone is captured by fewer ensemble members, and (iii) during the earlier stage of the cyclones' lifecycle. The methodological development and its application documented in this paper provide the basis for a multi-year investigation of the predictability of extreme weather linked to Mediterranean cyclones in the second part of this study.
Processes leading to heavy precipitation associated with two Mediterranean cyclones observed during the HyMeX SOP1
Flaounas, Emmanouil; Lagouvardos, Konstantinos; Kotroni, Vassiliki; et al. (2016)
Quarterly Journal of the Royal Meteorological Society
Case study of a long-lived Siberian summer cyclone that evolved from a heat low into an Arctic cyclone
Schnyder, Franziska; Lee, Franco; Wernli, Heini (2025)
Weather and Climate Dynamics
Extratropical cyclones are known for strongly influencing mid-latitude weather in particular during the cold season and for their association with high-impact weather such as destructive winds and heavy precipitation. Cyclones occur typically in the oceanic storm track regions, and most studies about cyclone dynamics focused on cyclones that developed over the ocean. In this study, we investigate a particularly long-lived example of the lesser known Siberian summer cyclones. Based on a climatological analysis of Siberian summer cyclone tracks in ERA5 reanalyses during the period 1979-2021, we identify 9 events that are initially identified as typical heat lows. While there is a large variability in surface cyclogenesis conditions of Siberian summer cyclones, the Siberian heat lows form in very dry and hot environments and exhibit deep, convectively well-mixed boundary layers at genesis. In a detailed case study of a long-lived Siberian summer cyclone in July 2021, we show how the cyclone forms as a heat low during a heat wave in Kazakhstan. The cyclone then interacts with an upper-level trough, propagates across the Asian continent and evolves into an Arctic cyclone, which experiences rapid intensification and produces a warm conveyor belt with a poleward outflow approaching the North Pole and leads to the formation of a tropospheric potential vorticity cutoff in the Arctic. This case is unusual since subtropical heat lows are not known to propagate far from their location of origin. This unusual cyclone has a track length of almost 4000 km and it is associated with a heatwave initially, heavy precipitation during intensification, and an important upper-level flow anomaly in the Arctic. Comparison with the other Siberian heat lows shows that a similar development can be observed for the other cases, although not as pronounced and long-lived. This extraordinary case study also indicates how compounding high-impact events in different locations may be related to one single weather system.
Dynamical Meteorology: The Swiss Contribution
Davies, Huw C.; Wernli, Heini (2016)
From Weather Observations to Atmospheric and Climate Sciences in Switzerland - Celebrating 100 years of the Swiss Society for Meteorology
Seamless Multimodel Postprocessing for Air Temperature Forecasts in Complex Topography
Keller, Regula; Rajczak, Jan; Bhend, Jonas; et al. (2021)
Weather and Forecasting
Statistical postprocessing is applied in operational forecasting to correct systematic errors of numerical weather prediction models (NWP) and to automatically produce calibrated local forecasts for end-users. Postprocessing is particularly relevant in complex terrain, where even state-of-the-art high-resolution NWP systems cannot resolve many of the small-scale processes shaping local weather conditions. In addition, statistical postprocessing can also be used to combine forecasts from multiple NWP systems. Here we assess an ensemble model output statistics (EMOS) approach to produce seamless temperature forecasts based on a combination of short-term ensemble forecasts from a convection-permitting limited-area ensemble and a medium-range global ensemble forecasting model. We quantify the benefit of this approach compared to only postprocessing the high-resolution NWP. The multimodel EMOS approach (“mixed EMOS”) is able to improve forecasts by 30% with respect to direct model output from the high-resolution NWP. A detailed evaluation of mixed EMOS reveals that it outperforms either one of the single-model EMOS versions by 8%–12%. Temperature forecasts at valley locations profit in particular from the model combination. All forecast variants perform worst in winter (DJF); however, calibration and model combination improves forecast quality substantially. In addition to increasing skill as compared to single-model postprocessing, it also enables us to seamlessly combine multiple forecast sources with different time horizons (and horizontal resolutions) and thereby consolidates short-term to medium-range forecasting time horizons in one product without any user-relevant discontinuity. © 2021 American Meteorological Society
The 1-way on-line coupled atmospheric chemistry model system MECO(n) – Part 3: Meteorological evaluation of the on-line coupled system
Hofmann, C.; Kerkweg, Astrid; Wernli, Heini; et al. (2012)
Geoscientific Model Development
Three detailed meteorological case studies are conducted with the global and regional atmospheric chemistry model system ECHAM5/MESSy(→COSMO/MESSy)n, shortly named MECO(n). The aim of this article is to assess the general performance of the on-line coupling of the regional model COSMO to the global model ECHAM5. The cases are characterised by intense weather systems in Central Europe: a cold front passage in March 2010, a convective frontal event in July 2007, and the high impact winter storm "Kyrill" in January 2007. Simulations are performed with the new on-line-coupled model system and compared to classical, off-line COSMO hindcast simulations driven by ECMWF analyses. Precipitation observations from rain gauges and ECMWF analysis fields are used as reference, and both qualitative and quantitative measures are used to characterise the quality of the various simulations. It is shown that, not surprisingly, simulations with a shorter lead time generally produce more accurate simulations. Irrespective of lead time, the accuracy of the on-line and off-line COSMO simulations are comparable for the three cases. This result indicates that the new global and regional model system MECO(n) is able to simulate key mid-latitude weather systems, including cyclones, fronts, and convective precipitation, as accurately as present-day state-of-the-art regional weather prediction models in standard off-line configuration. Therefore, MECO(n) will be applied to simulate atmospheric chemistry exploring the model's full capabilities during meteorologically challenging conditions.
Insights from hailstorm track analysis in European climate change simulations
Brennan, Killian; Thurnherr, Iris; Sprenger, Michael; et al. (2025)
Natural Hazards and Earth System Sciences
Hailstorms are among the most destructive weather events, posing significant threats to infrastructure, agriculture, and human life. This study applies hailstorm-tracking diagnostics to kilometer-scale, decade-long climate simulations over Europe using the COSMO v6 model driven by ERA5 reanalyses. Convection is treated explicitly, and hail is modeled online with the HAILCAST parameterization. Simulations represent current and future climate simulations, the latter corresponding to a +3 K global temperature increase implemented via a pseudo-global warming approach. We analyze high-frequency hail output at 5 min intervals, which enables tracking ∼40000 hailstorms in Europe in current and future climate simulations separately. Storm track properties include length, duration, hail size, and spatial distribution, while three-dimensional environmental variables along these tracks yield storm-centered composites of hailstorm structure and allow for the examination of storm inflow environments. Our analysis reveals significant shifts in the characteristics of hailstorms under the future climate scenario. Notably, hail frequency trends vary across Europe, but the trends in hailstorm environments are comparatively uniform. The most striking results are as follows: (i) hail swath areas are projected to change in terms of both frequency and spatial extent, with a 2-fold increased frequency of storms producing ∼50 mm and larger hail diameters. Per-storm hail swath areas generally expand by 15 %-30 %, with swath area increases being more important for smaller hail, while frequency changes dominate for larger hail. (ii) The effect of increased hail melting due to the higher elevation of the 0 °C level on the storm maximum hail diameters is found to be minor. (iii) Precipitation and wind hazards accompanying hailstorms are expected to increase on average by 20 % and 5 %, respectively, whereas extreme hail-precipitation compound events, i.e., hail with a diameter of at least 30 mm followed by 50 mm h-1 of rainfall, are projected to be twice as frequent in the future.
A European Hail and Lightning Climatology From an 11-Year Kilometer-Scale Regional Climate Simulation
Cui, Ruoyi; Thurnherr, Iris; Velasquez Alvarez, Patricio; et al. (2025)
Journal of Geophysical Research: Atmospheres
Hail and lightning, associated with severe convective storms, can cause extensive damage to infrastructure, agriculture, and ecosystems. Because of the small scale of these storms and the complexity of the involved processes, observing and modeling convective storms is challenging. The potential of online diagnostics in convection-permitting models to simulate hail and lightning, especially over climatic time scales and extended regions, has not yet been fully exploited. To address this gap, we present a European-wide hail and lightning climatology (2011–2021) using the Consortium for Small Scale Modeling (COSMO) regional climate model with a horizontal grid spacing of 2.2 km, coupled with a hail growth model (HAILCAST) and the lightning potential index (LPI) diagnostics. We further developed a new European-wide hail product based on the Operational Program for the Exchange of Weather Radar Information (OPERA) composite. Model validation against observations demonstrates an overall good performance in simulating hail and lightning on spatial, seasonal, and diurnal scales. The highest hail frequencies occur during summer along the slopes of high mountain ridges, such as the Alps, Pyrenees, and the Carpathians, aligning with observed lightning hotspots in Europe. In autumn, hail and lightning occur predominantly over the Mediterranean and along the Adriatic coast. Severe hail events with a maximum hail diameter larger than 20 mm mainly occur in the Po Valley, western Spain, and Eastern Europe. This 11-year simulation provides a European-wide data set of severe convective storms and their properties, serving as a basis for further studies of convective events and their impacts.
Quantifying the relevance of atmospheric blocking for co-located temperature extremes in the Northern Hemisphere on (sub-)daily time scales
Pfahl, S.; Wernli, Heini (2012)
Geophysical Research Letters
Atmospheric blocking can influence near-surface temperature via circulation and radiative forcing. This study investigates the relevance of blocking for co-located (sub-)daily temperature extremes and the spatial variability of this relationship in the Northern Hemisphere. It is shown that over large parts of the high-latitude continents warm temperature extremes often occur simultaneously with atmospheric blocking at the same location. Taking also weak blocks into account, more than 80% of the six-hourly warm extremes are associated with blocking, e.g., in eastern Canada, Scandinavia and parts of Siberia. On the contrary, cold extremes typically are not related to co-located atmospheric blocking. This difference between warm and cold extremes points to differences also in the physical driving mechanisms of the extremes. The strong linkage of warm temperature extremes and blocking should be considered when investigating changes of temperature extremes with global warming.
Quantifying the Relevance of Cyclones for Precipitation Extremes
Pfahl, Stephan; Wernli, Heini (2012)
Journal of Climate

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