In the first week of June 2021 there was an interesting distribution of severe convective events across Europe. Across the Benelux, Germany, and France, heavy rainfall with resulting flash floods dominated. At the same time, the storms in Spain were prolific producers of large hail.

The difference between the two regimes was the mid- to upper-tropospheric wind (Figure 3): strong wind shear across Spain led to hail-producing supercells moving fairly rapidly. The pulse-storms and multi-cells of Benelux, Germany and northern France moved very slowly creating heavy rainfall.

Figure 1 shows typical convective regimes during the week, in the two domains of interest. Notice the smaller number of convective systems over Spain (and France) being more persistent and organised, bigger in size, with intense convective processes (colder cloud tops). In contrast, the convection over Benelux (and north west Germany) takes the form of multiple singular, short-lived, unorganised convective cells.

Figure 1: Meteosat-11 SEVIRI IR10.8 channel, 3 June, 00:00-23:45 UTC. One-day loop (15-min time step) of convective activity over observed domains through enhanced infrared imagery.

Synoptically, two distinct modes of convection appeared due to the fact that, during the observed week, the wider Iberian peninsula region was under the influence of a passage of multiple baroclinic disturbances from the north west (region 1 in Figure 2). These disturbances are often related to more intense atmospheric dynamics (higher wind shear). Conversely, the wider Benelux region (region 2 in Figure 2) was under the influence of a low-gradient field, with weak dynamical forcing.

Figure 2: Meteosat-11 Airmass RGB overlaid by ECMWF H300, 3 June, 15:00 UTC. Synoptic view on high- and low-gradient regions of Geopotential height at 300hPa isobaric surface (two regions with two distinct convective regimes). Credit: ePort/EUMeTrain

Figure 3: Distribution of Convective Available Potential Energy (J/kg) and wind shear between the surface at 6 km above ground level on Friday 4 June, 1200 UTC. Source: ECMWF high resolution IFS model initialised at 00 UTC.

Figure 3 shows the severe weather events recorded in the European Severe Weather Database between 31 May and 6 June 2021. Green triangles represent hail > 2 cm, blue circles reports of flash flooding. Also shown are wind reports (> 25 m/s, yellow rectangles, and tornadoes and waterspouts (red triangles).