High-level wave clouds over the Andes in June 2005.
24 August 2021
29 June 2005
By Jochen Kerkmann and Jose Prieto (EUMETSAT)
The Meteosat-8 images below show a case of stationary, high-level wave clouds over Argentina produced by the Andes mountains during a situation of strong westerly winds (it should be noted that the Andes mountains are located very close to the limit of the Meteosat-8 field of view). Typically, during such a situation strong foehn winds can occur on the eastern slope of the Andes, also called 'zonda' winds (in Spanish, viento zonda).
Although this phenomenon can occur along the entire length of the extratropical Andes, it is most frequently detected near the cities of Mendoza [32°S, 69°W , 704m above sea level] and San Juan (32°S, 68°W, 598m above sea level), which represent important urban regions of western Argentina (see map of Argentina , source: Marcelo E. Seluchi et al., 2003).
One interesting aspect in the images below is the high values of the brightness temperature difference (BTD) between the IR3.9 and the IR10.8 channels (see lower left image) with values reaching more than +70K. Also other channels, and differences between channels, reach extremely high values for these high-level ice louds, for example:
- -19K for both the BTD IR12.0–IR10.8 and the BTD IR10.8–IR8.7, indicating thin, very high ice clouds.
- +26K for the BTD IR9.7–IR10.8, indicating very high clouds.
- +33% reflectivity for the IR3.9 channel, indicating very small ice particles.
- +50% reflectivity for the NIR1.6 channel, indicating very small ice particles.
It is interesting to note that the high-level wave clouds do not form right above the Andes, but at a certain distance on the downwind side extending far downstream into Paraguay and even into Brazil. Similar cases of large downstream cirrus wave clouds (slightly detached from the mountain barrier) have been observed over the Alps and the Norwegian mountains.
However, in Europe the wave clouds usually do not reach such a high altitude (probably 16km in this case). The extremely high values of reflectivity in the IR3.9 channel can only be explained by homogeneous freezing processes at very low temperatures, which lead to the formation of very small ice particles. A similar case has been observed over the Carpathian mountains (Romania) in November 2003. In the RGB composite VIS0.8, IR3.9r, IR10.8 these clouds appear in an intense green (thin cloud) to yellow (thick cloud) colour and could be confused with a super-cooled water cloud. However, it is clear from the IR10.8 temperature well below -40°C that these clouds are actually ice clouds.
Remapped Meteosat-8 images
Mercator projection centered at 30°S, 55°W
RGB composite VIS0.8, IR3.9r, IR10.8
RGB composite WV6.2-WV7.3, IR9.7-IR10.8, WV6.2 (Airmass)
RGB composite IR12.0-IR10.8, IR10.8-IR8.7, IR10.8 (Dust)
Eta–CPTEC Regional Model (Marcelo E. Seluchi et al., 2003)
Remapped IR10.8 image with 500 hPa geopotential field
Remapped IR10.8 image with surface pressure field
Remapped VIS0.8 image
Remapped Difference image IR3.9–IR10.8
Remapped RGB composite WV6.2–WV7.3, IR9.7–IR10.8, WV6.2 (Airmass)
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Wave clouds over the Alpine region
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High-level mountain wave clouds over the Po Valley
High-level mountain wave clouds over the Po Valley in Meteosat imagery, September 2007.
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