<p>Both 2021 and 2022 broke records in terms of the amount of large (≥ 2 cm) and very large (≥ 5 cm) hail reports across Europe. 24 June 2021 featured the highest number of large hail reports per day (860) in the history of the European Severe Weather Database and giant (≥ 10 cm) hail was reported in three countries. In 2022, the insured damage exceeded € 4 billion in France alone while 215 people were injured that year. Furthermore, the Catalonian hailstorm on 30 August 2022 caused the first direct hail fatality in Europe since 1997.  </p> <p>In this work, we studied storm-scale processes of severe hailstorms and their larger-scale environment in relation to the maximum observed hail diameter and hailstorm lifetime. The storm-scale properties include storm type, the occurrence of storm mergers, and the type of storm dissipation.  The larger-scale environment was addressed using CAPE-shear parameter space, hodograph properties (shape, longest segment in the hodograph, and storm-relative winds), and the presence of boundaries near the storms. We selected the most impactful hailstorms of 2021 and 2022, all of which featured very large hail and caused considerable damage to property or agriculture, or caused injuries. 79 hailstorms were selected from both years, spanning maximum hail diameters of 5 to 14 cm and hailstorm lifetimes of 10 to 420 minutes.</p> <p>We found that most hailstorm hodographs had a straight shape with the longest segment between 1 and 3 km and storm-relative inflow typically exceeded 10 m/s. Hodograph properties and the amount of CAPE had no relation to the duration of the hailstorm. Hailstorms forming near boundaries had average lifetimes twice as long as hailstorms forming elsewhere. For hail > 5 cm, CAPE had the strongest correlation with the observed diameter, even higher than the CAPE-shear product. Hodographs suggest that the inflow magnitude into the deviant moving storms stays almost the same (around 10 m/s) for 10 to 22 m/s of 0-6 km bulk shear. In some cases, very large hail occurred in marginally favorable environments only after a storm merger occurred. This shows that storm-scale processes (merger, deviant motion of the storm) and interaction with boundaries can be as important as the background environment.</p>

<p>The development of additive logistic regression models (AR-CHaMo) for large hail, severe convective wind gusts, and F1 or stronger tornadoes for Europe and parts of North America allowed us to identify how the best predictors vary among different threats and different forecast domains. The best predictors were identified using the variance explained, based on the skill of logistic models for individual parameters as well as on investigating pairs of different parameters and their relation to hazard frequency.</p> <p>For the models, we have chosen predictors that perform well over both domains and could thus be used to develop a global convective hazard model. In the case of large hail, CAPE was found to be a better predictor across Europe than across North America, where mid-tropospheric lapse rates discriminate better between environments with and without large hail. We found that CAPE below the -10 °C level was a skillful predictor in both domains. For severe convective wind gusts, it was found that they occurred with lower CAPE and lower amounts of absolute moisture in Europe than in North America. Height of the LCL or a parameter that predicts the cold pool strength worked better in Europe than in North America. Strong mean wind in the bottom troposphere was found among the best predictors of severe wind gusts in both domains. Regional differences among the best predictors were also found for F1 and stronger tornadoes, even though the amount of SRH in the lower troposphere is universally a skillful predictor.</p> <p>We applied models using the best predictors of large hail across North America and Europe to the ERA-5 reanalysis to obtain a global model of large hail hazard. Then, we compare the model to existing hail climatologies worldwide and discuss its limitations and potential improvements.</p>

<p>Additive Logistic Regression Models for different convective hazards were developed across Europe and a portion of North America using lightning observations, severe weather reports and convective parameters from the ERA5 reanalysis. To model convective hazards, convective initiation was taken explicitly into account by computing, for instance, <em>P<sub>hail </sub></em>(probability of hail) as the product of <em>P­<sub>storm</sub></em> (probability of convective initiation) and <em>P<sub>hailstorm </sub></em>(conditional probability of hail given a storm). We will report on the development of models that are skilful across Europe and North America, and on the regionally dependent skill of convective parameters used as model predictors. To reconstruct the probability of lightning, large hail, and very large hail from 1950 to 2021, the models were applied to the ERA5 reanalysis, at one hourly intervals across both regions. The modelled hazard climatologies are in strong agreement with observed patterns and can accurately resolve local-scale features thanks to the (0.25 x 0.25° degree) spatial resolution of the ERA5 reanalysis. We analysed long-term trends over the 71 year period and detected important differences between the regions. Across North America, 1950–2021 hail trends were found to be weak and mostly non-significant, but a period of enhanced lightning activity (+30% to 1950–2021 average) was detected between 1980 and 1990 across the Central Plains. In Europe, trends are mostly positive and significant, with the highest trend modelled across Northern Italy. Here, the convective activity has seen an abrupt increase with very large hail 3 times as likely in recent years (2012–2021) than in the 1950s. Apart from a sharp increase in frequency, the year-to-year variability has also increased with yearly differences in occurrence exceeding 100% for large and 200% for very large hail compared to the long-term average. In addition to (very) large hail, preliminary results on the development of models for severe wind gusts and tornadoes models along with the corresponding long-term trends will be presented.</p>

<p>The representation of convection in ECMWF’s forecasting system has been improved in recent years by advances in computing, substantial upgrades of both horizontal and vertical resolutions, and by major changes in the moist processes in the model. These developments have also opened up the opportunity for improvements of existing convective products and the development of new ones, such as lightning density diagnostics. As part of this ongoing initiative ECMWF is partnering with the European Severe Storms Laboratory (ESSL) on a number of projects. The computation of Convective Available Potential Energy (CAPE) and Convective Inhibition (CIN) parameters from the model has been revised and more versions of CAPE and CIN have been implemented and are made available to the forecasting community. CAPE and composite CAPE-shear parameters have been included in ECMWF’s Extreme Forecast Index (EFI) to help forecasting outbreaks of severe convection in the medium range. Alongside objective statistical verification, the convective EFI has been evaluated at ESSL’s Testbed recently. ECMWF has implemented ensemble vertical profiles to facilitate forecasting convection among other applications. ECMWF is working with ESSL on providing more parameters and products for forecasting deep, moist convection and its attendant severe weather. These include Storm Relative Helicity (SRH) and post-processed probabilities of various convective hazards such as large hail and severe wind gusts. Following its open data policy, ECMWF has also provided more probabilistic and deterministic graphical convective products on its website. This presentation will provide a brief overview of all these recent developments.</p>