Met-11 WV (Aspot)

Signatures of foehn and bora situations in water vapour images

14 April 2020 05:00-18:00 UTC

Met-11 WV (Aspot)
Met-11 WV (Aspot)

Investigating the foehn situation on 14 April 2020 and on how it is seen in water vapour imagery.

Last Updated

11 February 2021

Published on

09 February 2021

By Jochen Kerkmann (EUMETSAT), HansPeter Roesli (Switzerland) and Ivan Smiljanic (CGI)

In the case study Retour d’est or extended Bora flow along Po Valley, the 'Retour d’est' or extended Bora flow along the Po Valley is discussed using visible, infrared and near-infrared satellite imagery.  The synoptic situation is a typical north-foehn situation over the alpine southside and the Po Valley. Figures 2 and 5 of the case study show the two main foehn corridors in northern Italy/southern Switzerland, the foehn which is later consumed by the Retour d'est.

This case study concentrates on the foehn situation on 14 April 2020 and on how it is seen in WV7.3 (and WV6.2) imagery. Figure 1 shows the WV7.3 image corresponding to the Airmass RGB image of the Retour d'est case study (Figure 2 in that case). One can see distinct warm (dark) stripes extending from the main crest of the Alps southward into the Po Valley. Our interpretation of these stripes is that these swaths represent the main foehn corridors on the alpine southside, very dry air at mid and low levels that replaces the moist air in the Po Valley. The broadest foehn swath is the one over Ticino, further to the east the swaths are smaller and less pronounced.

Meteosat-11 Water Vapour 7.3 imagery

Annotated Meteosat-11 WV7.3 compare1
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Figure 1: Meteosat-11 WV7.3 image from 14 April 2020, 10:45 UTC.

To our knowledge, this is the first documented case of foehn signatures seen in water vapour imagery. So far, foehn (or chinook, zonda or berg wind) has only been seen in infrared imagery. This case does not refer to foehn waves/clouds or the foehn wall, which can be seen in visible, infrared and water vapour imagery, but about the dry, warm foehn air that descends into the valleys, and warms up land surfaces.

More details of the foehn development may be gleaned from the spectacular WV7.3 animation (Figure 2). The foehn starts to develop at around 05:00 UTC (start of the loop), and it continues until it is destroyed by the Retour d'est (not shown here).

Figure 2: Meteosat-11 WV7.3 images, 14 April 2020, 05:00-12:00 UTC.

 

The forward/backward (rocking) animation is shown here as it allows better tracking of the features. Note that the foehn streaks are not visible in the WV6.2 animation (Figure 3), because it is not the right level for the foehn signal. Only some high-level mountain waves and the Retour d'est can be faintly seen.

Figure 3: Meteosat-11 WV6.2 images, 14 April 2020, 05:00-18:00 UTC.

On 14 April 2020, foehn streaks were also observed over France, to the lee of the Massif Central (see WV7.3 animation from 05:00 to 16:00 UTC).

The discovery of the foehn streaks in the case of 14 April 2020 led to the question of whether this phenomenon can be observed in other foehn situations? To answer this, all north foehn situations of the years 2019 and 2020 (using EUMeTrain ePort), and all north foehn cases reported on the EUMETSAT and EUMeTrain case study libraries, were checked. The only other case of distinct foehn streaks was the case of 4 February 2020 shown in Figure 4. This case is actually a west foehn case, i.e. strong westerly flow over the Alps with foehn in northwest Italy (Piemonte). The narrow foehn streaks extend far downstream across the Po Valley up to the Adriatic Sea.

Figure 4: Meteosat-11 WV7.3 images, 4 February 2020, 05:00-10:00 UTC.

Although the other north foehn cases did not exhibit foehn streaks (or only very weak signatures), we would like to show the best examples of WV7.3 loops to get an overview of the various features you can expect to find.

As a first example, we show the WV7.3  animation of the famous 21 January 2005 case, our first major Meteosat Second Generation (MSG) foehn case study, see Figure 5. It does not show foehn streaks, but lee mountain waves over Piemonte and southern France (stationary waves attached to the mountain barrier). The detection of lee waves is an important element of operational meteorology because lee waves can indicate the presence of areas of turbulence. Usually, lee waves can be quite easily detected in visible and infrared satellite images, although not when there aren't any clouds, as in this case. Hence, WV7.3 images can be essential to identify areas of mountain wave turbulence.

Figure 5: Meteosat-8 WV7.3 images, 21 January 2005, 03:00-12:00 UTC.

The second case is a EUMeTrain north foehn case from 27 January 2008 (Figure 6). As the focus is on warm foehn signatures, not on clouds, mid- and high-level clouds are over-enhanced (white). For all cases we have used the same brightness temperature (BT) range, namely from 230 K (white) to 270 K (black). The dominating feature in this case is the large, high-level lee cloud. Foehn streaks are not visible, only some lee waves. Interestingly, the distance of the lee cloud to the main crest of the Alps increases from west to east. This has been observed in many other cases,  such as this wave cloud case from 7 April 2017 and this wave cloud case from 7 September 2007, which show, among other images, ground pictures and a time lapse movie of high-level wave clouds.

Figure 6: Meteosat-9 WV7.3 images, 27 January 2008, 03:00-10:00 UTC.

Figure 7  (2 January 2019) is again an example where foehn streaks can be seen over the Po Valley, though not as pronounced as in Figures 2 and 4. Mountain waves can also be seen to the lee of the Alps and the Appenine mountains, but no high-level lee clouds.

Figure 7: Meteosat-11 WV7.3 images, 2 January 2019, 04:00-16:00 UTC.

The last two cases are not from the Alps, but from the Pyrenees (Spain) and the Dinaric mountains (Croatia). Figure 8 shows the WV7.3 animation from 9 January 2019, from 08:00 UTC to 15:00 UTC, over the Pyrenees (see synoptic situation, Credit: EUMeTrain). Some high-level wave clouds can be seen in the western part of the image; mountain waves in the cloud free areas over the Mediterranean Sea. As regards foehn signatures, a distinct warming of the WV7.3 channel can be observed south of the Central Pyrenees, a broad warm area with one warm streak as shown on the 11:00 UTC image.

Figure 8: Meteosat-11 WV7.3 images, 9 January 2019, 08:00-15:00 UTC.

Figure 9 shows a very strong bora case over the Adriatic Sea (see synoptic situation, Credit: EUMeTrain). Numerous orographic waves can be seen to the lee of the Dinaric mountains and the Appenines. Some of these waves intersect in the middle of the Adriatic Sea. 'Weak' bora swaths can be observed in the beginning of the loop over the south Dalmatian coast, as shown in the 02:30 UTC image. The main bora corridors are embedded somewhere in the areas of strong orographic waves.

Figure 9: Meteosat-11 WV7.3 images, 23 February 2019, 01:00-07:00 UTC.

As this is a case over the Adriatic Sea, it was worth checking if the strong bora winds caused any ocean signatures. For this, high-resolution visible imagery was used to find areas of increased ocean surface reflectance related to rough sea, foam and white caps (see the case of strong Etesian winds). Figure 10 shows the enhanced, high-resolution VIIRS True Color RGB from the same day over the Adriatic Sea. Most of the Adriatic Sea is cloud free. Over the northern Adriatic Sea two swaths with wave-like increased reflectance can be seen (marked by thick yellow arrows). These are probably the two largest bora corridors. Further south, south of Sibenik, elongated streaks of higher reflectivity indicate smaller bora corridors (marked by thin yellow arrows). Note that the streaks on the Makarska coast and the one close to Dubrovnik are also seen on the WV7.3 images.

 

Suomi-NPP True Color imagery

Annotated Suomi NPP True Color RGB compare1
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Figure 10: Suomi NPP, VIIRS, True Color RGB, 23 February 2019 (Credit: NASA).


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