The Meteosat-8 image below shows two types of waves above the Czech Republic, interfering with each other.
22 October 2020
21 December 2008
Yellow arrows point to the first wave of a series of lee waves, forming downwind of the Krusne Hory/Erzgebirge mountains on the Czech-German border.
by Martin Setvak, Milan Salek, Petr Novak and Pavla Sknvankova (CHMI )
This type of low-level waves is quite common in the area, forming under suitable stratification and northwesterly winds (for a similar case see e.g. the case from 27 January 2008 ).
However, the second set of waves, labeled by the red arrows, probably represents gravity waves propagating across the country from north to south, interfering with the lee waves. Higher resolution MODIS images show further details of the clouds formed by these waves, namely their turbulent cloud-top structure — see 500 m resolution true-color image or 250 m resolution band 1 image .
The evolution and propagation of the waves can be seen nicely in the animation obtained from the Rapid Scan Service of Meteosat-8 imagery . The gravity waves seem to be associated with the passage of a cold front across the area (see surface map , source: DWD).
The position and shape of the gravity waves roughly corresponds to the location and shape of the cold front itself, also shown in the animations of the Airmass RGB product (see overview animation , or more detailed zoom on Central Europe ). Furthermore, a loop of radar reflectivity images shows that the location of the main precipitation band corresponds to the shape and location of the cold front.
However, detailed analysis of the radar and MSG imagery shows somewhat more complex patterns. The shape of the main precipitation band (see 12:10 UTC radar image ) shows that the eastern part of this band has different curvature from that of the gravity waves shown in the corresponding MSG image below.
This can be even better seen in the composite image loop showing MSG rapid scan imagery, overlaid with partially transparent radar imagery, both scanned at corresponding 5 minute intervals. Here it is obvious that the precipitation band starts to develop (at about 10 UTC) well ahead of the gravity waves, roughly matching them in the western part around 12 UTC, and staying behind them later on. In the eastern part the precipitation band and the gravity waves are not at all co-located throughout the entire episode.
So, if the precipitation band is related to the cold front, it is probable that the gravity waves were generated at higher levels by some other mechanism, and not directly by the propagation of the cold front itself. The Prague Libus 12:00 UTC sounding shows the presence of a very strong inversion layer between the 2,5 to 3.0 km levels, at or inside which the gravity waves most likely have propagated southward.
The most plausible mechanism to generate the gravity waves seems to be the rapid descent of the dynamic tropopause (determined as 1.5 PVU) above the Baltic Sea and Poland between 00 and 12 UTC (see Aladin model analysis ). This rapid decrease of the dynamic tropopause height in combination with a jet stream may generate strong vertical motion, creating waves in the stable air of the temperature inversion and above it. Here, at 12 UTC, the gradient of the dynamic tropopause height in the area of the Czech-Polish border closely matches the shape and propagation direction of the gravity waves, spreading southward, which tends to confirm this hypothesis.
Meteosat-8 HRV Image
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