Drivers for the deepening of severe European windstorms and their impacts on forecast quality

Abstract

European windstorms are a high-impact weather phenomenon, regularly inflicting substantial damages, both human and economic. This study examines a set of objectively selected intense European windstorms from the 1979-2015 period using re-analysis and forecast products from the European Centre for Medium-Range Weather Forecasts (ECMWF). The storms are first categorized with respect to their diabatic relative to baroclinic contribution to deepening using the pressure tendency equation, and secondly with respect to their track relative to the jet stream as the large-scale element controlling storm deepening and propagation. As expected, baroclinic processes dominate the majority of storms, such that deepening is closely related to warm advection ahead of the cyclone centre. Contributions from diabatic processes vary strongly and exceed those from horizontal temperature advection in 10 out of 58 cases, with values of up to 60%. Remarkably, in several cases, planetary waves in the stratosphere appear to facilitate cyclogenesis but can also act to oppose deepening in a few cases. The diabatic contribution is significantly correlated with the time a given storm spends on the equatorward side of the jet, where there is greater potential for diabatic processes in the warm, moist air. In terms of forecast quality and consistent with previous studies, the storm core pressure is generally underestimated and storms tend to be too slow and shifted south in the forecast, particularly for longer lead times. These biases, however, reduce markedly with the improvement of the operational system over time. There is no systematic dependence of forecast behaviour on the diabatic contribution or track relative to the jet. In the future, some of these analyses should be repeated with homogeneous reforecast data to substantiate these findings better.

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

“…At longer lead times, it systematically underestimates the speed of motion and the depth of the storms. Previous studies also found a slow bias in the track forecasts of extratropical cyclones in general (Froude et al, 2007b) and a negative bias in the intensity forecasts of severe storms only (Pirret et al, 2017). This suggests that the underestimation of the speed of motion is systematic but that of the depth is specific to deep cyclones.…”

“…Froude et al (2007b) also found a slow bias in a systematic evaluation of the track of extratropical cyclones, while Pirret et al (2017) further found a low bias in intensity for severe European storms. This suggests that the speed is systematically underestimated for extratropical cyclones in general, while the intensity is underestimated for deep cyclones only.…”

“…For this large sample ranging from shallow to deep cyclones, they further found large errors in the intensity forecasts but contrasting biases that depend on the region and on the model. Pirret et al (2017) recently applied this tracking approach to severe European storms in operational ECMWF ensemble forecasts and found a negative bias in intensity in addition to the slow bias in track. They further investigated the relative contribution of diabatic and baroclinic processes to the intensification of the storms.…”

“…However, this dataset appears not to perform as well as its ECMWF counterpart for predicting wind over central Europe (Dabernig et al, 2015). The operational ECMWF ensemble forecast is initialised twice a day and contains 50 members but Pirret et al (2017) struggled to find a relation between the predictability and the intensity, track or physical processes of storms because of the steady increase in skill with more recent model versions. This illustrates the difficulty in systematically investigating the predictability of severe storms, even with an extended dataset such as the 20-year reforecast used here.…”

New insights into Rossby wave packet properties in the extratropical UTLS using GNSS radio occultations

“…Odell et al (2013) noted large stratospheric geopotential height tendencies during the 1993 Braer storm, the deepest extratropical low on record with 914 hPa minimum surface pressure. A study of 60 severe European windstorms (Pirret et al, 2017) found that in 20 cases the stratosphere contributed 10 or more to their deepening. The pressure tendency equation used in Pirret et al (2017) used the 100 hPa level (∼16 km) as the upper boundary, with the stratospheric contribution being dependent 15 on geopotential height tendencies on this boundary (their dPhi term).…”