Strong winds from south-westerly directions created a Foehn situation over Austria on several days from 15 December 2019, with increased intensity on 17 December. The involved cloud types were very complex and changed during the day .
By Yasmin Markl and Andreas Wirth (ZAMG) and Ivan Smiljanic (SCISYS)
The Met-11 Airmass RGB product, overlaid with the wind streamlines at 500 hPa (Figure 1, top right, click to expand), indicates that the air mass advection over the Alps, mostly formed in the southwesterly direction.
The Meteosat-11 HRV and IR10.8 RGB of 17 December 500 hPa geopotential shows the typical foehn 'nose' south of the Alps (see Figure 2). The complex cloud structure is also clearly visible in the Meteosat-11 HRV and IR10.8 RGB of 17 December.
In the morning of the 17 December, the HRV image (Figure 3, left) shows the typical stripe pattern of lee waves north of the Alps, but also in the south. The lower lee clouds located north of the Alps are superimposed by a cirrus veil. This cirrus veil prevents height assessment of the lee waves below as it is opaque for IR radiation (Figure 3, right). Given their black and white structure in the HRVIS image, however, it is highly likely that they consist of water droplets as these evaporate when being advected to lower levels (from wave crest to wave trough) while ice crystals usually prevail in this process.
The rather smooth cloud tops over the central Alps in the HRV image originate from higher, optically thick cirrus clouds. They consist of small ice particles as can be seen in the Severe Convection RGB with its bright yellow colour. This cloud shield propagates northward during the day and covers successively the lower lee waves.
The HRV loop (Figure 5) also shows a north-south lee wave pattern in the cirrus shield over the eastern part of Austria.
Figure 5: Meteosat-11 High Resolution Visible (HRV) animation, 17 Dec 08:00 UTC to 14:00 UTC
Lee waves show a different orientation when wind direction changes with height. This can be seen for the lee waves located north of the Alps and for those located in the eastern part of Austria (Figure 6 and 7).
SZAC Wind barbs at 900 hpa overlaid
SZAC Wind barbs at 700 hpa overlaid
Unusual high temperatures and strong wind gusts go hand-in-hand with this weather condition. Wind gusts up to 150 km/h and temperatures above 20 °C were recorded.
This video shows the some typical Foehn clouds near Salzburg in the morning of 17 December 2019 with a distinct cloud gap towards the south.