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Abstract
Abstract. A major linear mesoscale convective system caused severe weather over northern France, Belgium, the Netherlands and northwestern Germany on 3 January 2014. The storm was classified as a cold-season derecho with widespread wind gusts exceeding 25 m s−1. While such derechos occasionally develop along cold fronts of extratropical cyclones, this system formed in a postfrontal air mass along a baroclinic surface pressure trough and was favoured by a strong large-scale air ascent induced by an intense mid-level jet. The lower-tropospheric environment was characterised by weak latent instability and strong vertical wind shear. Given the poor operational forecast of the storm, we analyse the role of initial and lateral boundary conditions to the storm's development by performing convection-resolving limited-area simulations with operational analysis and reanalysis datasets. The storm is best represented in simulations with high temporally and spatially resolved initial and lateral boundary conditions derived from ERA5, which provide the most realistic development of the essential surface pressure trough. Moreover, simulations at convection-resolving resolution enable a better representation of the observed derecho intensity. This case study is testimony to the usefulness of ensembles of convection-resolving simulations in overcoming the current shortcomings of forecasting cold-season convective storms, particularly for cases not associated with a cold front.
Highlights
Mesoscale convective systems (MCSs) often occur in central Europe, in late spring and summer
The ERAI and ECAN simulations are both downscaled to a final grid spacing of 2.8 km (D2) in order to analyse the differences in the atmospheric conditions during the development of the derecho-producing mesoscale convective system (DMCS) in comparison to the ERA5 reference simulation
Considerable sensitivity was found when modifying the update frequency of the lateral boundary conditions (LBCs) in CCLM-ERA5: The ERA5-driven CCLM simulation with 6-hourly LBCs did not simulate the surface pressure trough associated with the development of the DMCS, which extends from southeastern England to northern France at 14:00 UTC in the reference simulation with hourly LBCs (Fig. 9a, b)
Summary
Mesoscale convective systems (MCSs) often occur in central Europe, in late spring and summer. MCSs exhibit a linear structure, last for several hours and lead to both intense wind gusts and precipitation over large areas and are sometimes classified as derechos (Johns and Hirt, 1987) While such events primarily occur over western Europe during the summer half of the year (Gatzen, 2004), they may occur during wintertime (Gatzen et al, 2011). F1-rated wind damage was reported in western Belgium and northwestern Germany (Fig. 1) According to these observations, this convective event can be classified as a cold-season derecho following the definition of Johns and Hirt (1987), which includes four essential points: (1) a concentrated area with convective gusts >25.7 m s−1 having a major axis length of at least 400 km must be observed, (2) the gust reports. The last section includes a short summary and our conclusions
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