Strong convective episodes occurred, at the boundary of a synoptic ridge, over a two day period in early June 2019, causing widespread damage in parts of Central Europe.
20, November 2020
By Ivan Smiljanic (SCISYS), Maria Putsay (Hungarian Meteorological Service), Jochen Kerkmann (EUMETSAT)
The synoptic view over Europe (Figure 1) reveals a wide ridge over Eastern Europe with a quasi-stationary low pressure over France and UK. Warm and moist air in the area of high pressure became unstable, causing deep, moist convection. These episodes were more organised and long lasting in the region closer to the boundary of two pressure systems, where more wind shear and less subsidence forcing was present.
Contours of anticyclonic movement in the high pressure system are apparent from the lines of low level convection over Belarus, as seen in Figure 1 and the infrared animation (Figure 2).
The dynamics of the convection, over the two-day period, are best seen on the animation in Figure 2, where several major convective systems are noticeable.
One such system kicked-off in eastern Germany and travelled for almost two days over the Baltic Sea, aided by an upper-level jet, along the 300 hPa geopotential isolines (Figure 1), before dissolving over Russia on 12 June.
This well organised mesoscale convective system (MCS) resembled the circular shape of typical tropical storms, due to highly developed, inherited circulation. This is most apparent through the shape of the radial outflow cirrus bands, as seen on the SEVIRI/VIIRS IR10.8 comparison images (Figure 3) and the Meteosat-11 sandwich product (Figure 4).
The AVHRR and SEVIRI instruments detected the complex structures and temperature gradients at the top of this system simultaneously, while the MCS system was situated over the Baltic Sea (Figure 5). The sandwich products in Figure 5 were blended from VIS0.6 and IR10.8 images in case of AVHRR, and from HRV and IR10.8 images in case of SEVIRI data.
Prior to that event, another convective system moved through southern Germany on 10 June, bringing hazardous weather to the Munich area. This system can be seen on the High Resolution Visible (HRV) and infrared sandwich product animation (Figure 6).
A less hazardous, but quite interesting system that developed over Slovenia can also be seen on that animation.
In Figure 6 the HRV Cloud RGB image is created from HRV (visualised in red and rreen colour beams) and from the IR10.8 channel (visualised in blue colour beam).
The crucial difference between the two systems is that the one south to the Alps was in an environment with less wind shear and weaker winds at the higher levels. This is apparent in data from the surrounding sounding stations (Figures 7 & 8).
For this reason, the storm over Slovenia had a more circular shape (versus the V-shape storm in southern Germany); less intense convective manifestation, and a shorter lifetime. However, both systems developed prominent cold regions on top of the clouds (cold V-shape, and cold ring), with apparent overshooting tops, gravity waves and above-anvil cirrus plumes (or ‘pancake cloud’ in the Slovenian storm, due to a less of wind shear at higher levels).
This is most apparent from the blend of the higher resolution visible and IR ‘temperature’ channel (Figure 9).
A closer look at the storm in Slovenia reveals that it started as two ‘twin updrafts’ that jointly contributed to complex patterns on top of the storm, together with other emerging updraft(s).
The complete lifecycle is described through a four-panel image that consists of annotated imagery from Figure 10. The figure shows (counter clockwise) the IR10.8 channel overlaid with contour lines of the Hungarian weather radar (column maximum) images (top left), the HRV channel (bottom left), the sandwich product blended from HRV and IR10.8 (bottom right), and a Hungarian weather radar image (top right).
It is interesting to note that the updrafts are seen in satellite imagery 20 minutes earlier than they show up on the weather radar.
Due to its proximity to the storm, the Slovenian weather radar provided more realistic dynamics of the system (Figure 11). It also provided an earlier signal on the two aforementioned updrafts, giving a better detection lead-time to a satellite image.
The thunderstorms that brought much more damage to Slovenia and parts of Croatia actually occurred a day later, 11 June (seen in the infrared animation in Figure 2).
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