On 17 March 2019 Meteosat-11 captured imagery of stau clouds over the Alps.
By HansPeter Roesli (Switzerland)
With cold fronts approaching from the west (France) an airmass stagnating over the Po Valley was uplifted along the Alpine southside, in particular over the mountainous Piedemonte and Ticino areas. As a result a stau cloud deck appeared at low and mid-level, also known as ‘südstau’ in German or 'sbarramento’ in Italian.
Stau clouds are usually covered by higher pre-frontal clouds. Ice particles from the high clouds seed the stau clouds and make them drop snow or rain well before the cold front arrives over the western Alps. Due to the high clouds, satellites cannot usually see the stau clouds.
On a rare occasion on 19 March, in a relatively fast evolution a cold front crossed France and reached the Alps during the evening. Due to a rather shallow trough the advection at higher levels remained dry and no important prefrontal high clouds were formed.
On the southside of the Alps stau clouds began forming at the end of the night. The Meteosat-11 IR10.8 temperature (in yellow hues), laid over the HRV image (Figure 1), showed the cold front (blue arrows) and the stau clouds (red arrows) under an otherwise clear sky.
The animated sequence of this image type together with Meteosat-11 Airmass RGBs (Figure 2) gives an overview of the evolution in daylight. Significant precipitation set in only towards the evening, when the stau clouds grew bigger.
Figure 2: Meteosat-11 HRV with infrared overlay (left) and Airmass RGB (right), 17 March 06:00–16:00 UTC
Details at 375 m horizontal resolution can be gleaned from the Suomi-NPP VIIRS Natural Color RGB (Figure 3).
Due to the rather shallow trough the winds over the Alps were not very strong, which is the reason why the stau was rather moderate. The radiosonde from Milan and a wind profile from the GFS analysis, both at 12:00 UTC, confirm this. The cross section runs between Bologna and Basel, crossing the Alps half-way in the area of the Gotthard massif. Below 800h Pa (~2km) the winds were weak on the southside and got only stronger north of the Alps (foehn). Above this level the winds were reasonably strong, but way off what is usually the case for classical südstau situations.