The visible, infrared and water vapour images below show an example of high lee wave clouds (type 2) forming in the lee of the Massif Central (France) on 20 February.
21 October 2020
19 February 2002
Atmospheric circulations resulting from airflow over mountains exist on many scales. The size of a given circulation is generally related to the size of the forcing obstacle, but also the atmospheric conditions play an important role, for example, vertical stability and wind variation with height.
Depending on the atmospheric conditions two types of lee wave cloudiness can be observed:
- Lee wave clouds — characterised by narrow middle level cloud bands perpendicular to the wind direction and parallel to the mountain chain. The wave length ranges from about 3 to 40 km.
- Lee wave clouds consisting of high cirrus clouds extending from the mountain chain over the leeward side, and continuing (in some cases) for up to 1000 km (large wave length).
The latter type often occurs along the right (anticyclonic) side of a jet stream, and sometimes in the jet exit region.
Key parameters for the nowcasting/forecasting of lee wave clouds are:
- the wind direction with respect to the orientation of the mountain chain (the wind direction need not necessarily be perpendicular to the mountain chain but there must be a normal component);
- the stability of the atmosphere (a stable layer in the middle or upper troposphere);
- the humidity profile (sufficient humidity near the wave crests).
In the images below the lee wave clouds, which extend downstream down to the Alps, can be easily distinguished from the frontal cloud band that covers large part of France and Germany.
The lee clouds are whiter (colder and denser) and remain attached to the obstacle (Massif Central). They appear in the right exit region of a strong jet stream over Western Europe (see absolute topography at 300 hPa) in an area of marked atmospheric stability above 700 hPa (see sounding Lyon-Satolas).
A second, smaller area of lee wave clouds can be observed downstream (to the east) of the Western Alps. Within the lee cloudiness over France, some cloud stripes aligned parallel to the wind direction can bee seen.
They look like waves, but they are, in fact, perpendicular to the typical direction of lee waves. A possible explanation is that the single cloud stripes are generated separately by the highest peaks of the Massif Central, i.e. the cloudiness follows the orography, as indicated in the conceptual model below.
Figure 1: Meteosat-6 (visible)
Figure 2: Meteosat-6 (infrared)
Figure 3: Meteosat-6 (water vapour)
The animated infrared images below show a second interesting example of high-level lee wave clouds, which was observed on 23 April 2002. As in the first example, lee clouds form along the anticyclonic side of a jet stream, but this time to the south of the Massif Central being the wind at high levels from the north. Some lee wave clouds also form on the southern side of the Pyrenees.
An impressive development of lee wave clouds (type 2) can also be observed in the lee of the Norwegian mountains. They extend downstream to the Czech Republic covering a vast area of about 500,000 km2. These lee clouds are quite common in Scandinavia during the whole year.
Meteosat-6 (infrared channel)
Animation (23 Apr 2002, 09:00–15:00 UTC)
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