Pronounced high-level orographic cloud pattern appeared in the Severe Storm RGB behind the ridge of Pyrenees.
04 November 2020
02 July 2015
By Ivan Smiljanic (DHMZ)
The animation of the Severe Convection RGB imagery, 2 July 04:00–14:00 UTC shows the persistent high-level orographic wave cloud which developed on the lee-side of the Pyrenees mountains.
It is obvious from the synoptic view of the area (Figure 1) — especially looking at the position of high level jet (depicted by isotachs at 300hPa height) — that the formation of these clouds was connected to the strong high level flow. As is usual in such cases, the position of the cloud is to the right side of the jet axis.
On top of this large and long wave cloud (wavelength of hundreds of kilometres), which stretched almost perpendicular to the mountains, a formation of smaller orographic waves can also be seen. To depict these smaller patterns, images with better resolution are needed, such as the High Resolution Visible (HRV) (Figure 2).
Normally, Severe Convection RGB imagery is used for detection of convective systems, especially the most intensive cores or updrafts.
These cores are easily detected in this case because the most active parts of thunderstorms, especially the most intense cores/updrafts, have smaller ice particles at higher levels. However, in this case, no convective activity was present, only high level orographic clouds in the area of interest.
The typical yellow colour of these orographic clouds (also a typical colour of intense thunderstorm updrafts) comes from the small ice particles which form quickly at high levels.
These smaller particles reflect more sunlight back to the satellite than normal high-level ice clouds, at IR3.9. Therefore, in the RGB imagery (Figure 3) they appear as yellow shades not red.
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