The case presented below is a fine example of a north foehn over the Alps.
It is also a follow-up case study to one on melting snow and Genoa Low from 19 January 2005 (see Melting snow over North West Italy).
More information and detailed analysis of the feature can be found in the In Depth section.
by Jochen Kerkmann, Gordon Bridge, HansPeter Roesli, Stéphanie Guedj (EUMETSAT) and Danny Rosenfeld (HUJI)
From the Meteosat-8 images one can see that the stau cloudiness over the Alps extends some 50 km downwind of the main mountain range (Locarno is overcast with weak precipitation, Lugano is clear). Experienced forecasters from the area suggest that a wind of at least 60 knots at 500 hPa over the Central Alps is needed to have Locarno cloud-covered during a foehn event. At the same time, the Po Valley is cloud-free with a strong and gusty foehn wind and temperatures above +15 °C. In the previous night (20–21 January), at Stabio, the southernmost point of Ticino and Switzerland, with the arrival of the foehn wind the surface temperature had risen from -2.6 °C to above +13 °C.
In the RGB VIS0.8, IR3.9r, IR10.8 (upper left) image, it can be seen that water droplets and small ice particles quickly evaporate on the lee side of the Alps in the downwind area and large ice particles are slightly more persistent, as seen in the form of a dark red stripe (low reflectivity in the IR3.9 channel) on the cloud edge. The location of the edge of the clouds appears to be dictated by strong descending wind on the lee side of the Alps, which then almost immediately rebounds forming lee waves as seen in the water vapour images.
In the WV7.3 image, there is a particularly clear indication of lee waves in the generally cloud-free areas over Northern Italy. Such a precise indication of lee waves is very important for aviation forecasting since there could be areas of clear air turbulence (CAT) associated with them (see also Detection of mountain waves over Spain and the Balearic Islands in the WV7.3 channel of MSG).
An additional interesting feature is the narrow gap in the clouds along the southern (Swiss) side of the Jura mountains. This gap is produced by the Jorât wind, a mini-foehn effect, well known to sailors on the lakes of Neuchâtel and Geneva. It might be reasonable to expect that such a narrow gap in the clouds south of the Jura mountains exists in situations where the clouds contain mainly small (drizzle-producing) droplets that then completely evaporate in the mini-foehn, thus creating the gap. The gap closes to the southwest and the clouds take on a more pink/magenta colour (indicating larger cloud droplets).
Furthermore, in the HRV image and in the animations one can see low-level mountain waves to the lee of the Massive Central and the mountains of Sardinia. Also, thin, high-level orographic clouds can be seen to the lee of the Pyrenees and Eastern Alps, with greenish colour (indicating small ice particles).
Another striking feature in the RGB VIS0.8, IR3.9r, IR10.8 image (upper left) is the yellow cloud band spreading downwind from England across Belgium and Germany to the Alps. This colour indicates that the majority of droplets are small, a classic sign of continental (polluted) air with a high number of condensation nuclei. On either side of this yellow cloud band the cloud colour (pink/magenta) indicates air of a maritime origin (less condensation nuclei and larger droplets). The source of the airmass may be critical for the stau and foehn effects over the Alps as continental (polluted) airmasses would result in less precipitation in the stau zone and bigger cloud gaps in the foehn zone.
RGB Composite VIS0.8, IR3.9r, IR10.8
Full Resolution (713 KB)
Close-up Look (544 KB)
Colour Interpretation (511 KB)
Animation 1 (09:00–15:00 UTC, AVI, 10 MB)
Animation 2 (10:00–13:00 UTC, AVI, 13 MB)
Channel 12 (HRV)
Full Resolution (554 KB)
Close-up Look (240 KB)
Colour Interpretation (105 KB)
HRV, HRV, IR12.0-IR10.8 (763 KB)
HRV+VIS0.8, HRV+IR3.9r, HRV+IR10.8 (356 KB)
Channel 06 (WV7.3)
Full Resolution (395 KB)
Close-up Look (317 KB)
Channel 05 (WV6.2)
Full Resolution (409 KB)
Close-up Look (327 KB)
Channel 09 (IR10.8)
Full Resolution (429 KB)
Close-up Look (277 KB)
Stau and Foehn effects over the Alps (8 October 2003)
Detection of mountain waves over Spain and the Balearic Islands (28 December 2004)
Melting snow over North West Italy (19 January 2005)
Surface Chart (21 Jan 2005, 00:00 UTC, GIF, 65 KB, source: Met Office)
Absolute topography 700 hPa (21 Jan 2005, 12:00 UTC, PDF, 64 KB, source: ECMWF)
Temperature 700 hPa (21 Jan 2005, 12:00 UTC, PDF, 64 KB, source: ECMWF)
Vertical velocity 700 hPa (21 Jan 2005, 12:00 UTC, PDF, 73 KB, source: ECMWF)
Relative humidity 700 hPa (21 Jan 2005, 12:00 UTC, PDF, 89 KB, source: ECMWF)
Cross-section of equivalent potential temperature (21 Jan 2005, 12:00 UTC, PDF, 7 KB, source: ECMWF)
Cross-section surface parameters (21 Jan 2005, 12:30 UTC, PNG, 28 KB)
Map showing position of cross-sections (JPG, 311 KB, source: Atlas of the world PHILIP's)
Rapport "Caractérisation de l'effet de Foehn dans les Alpes: Cas du 21 janvier 2005"
(PDF, 5 MB, author: Stéphanie Guedj)
RGB Composite NIR1.6, VIS0.8, VIS0.6
Full Resolution (679 KB)
Close-up Look (478 KB)
RGB Composite VIS0.8, NIR1.6, IR3.9r
Full Resolution (718 KB)
Close-up Look (517 KB)
WV6.2–WV7.3, IR9.7–IR10.8, WV6.2
Full Resolution (637 KB)
Colour Interpretation (295 KB)
Close-up Look (435 KB)