Lee Cyclogenesis

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• Introduction
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• First variant: lee cyclogenesis induced by a cold front passage
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• Example of a cold front passing over a leeward vortex without the interaction of a jet streak 16
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• Second variant: lee cyclogenesis in the left exit region of a jet without the passage of a cold front
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• Example of a leeward vortex without the interaction of a jet streak

Example of a leeward vortex without the interaction of a jet streak

The next example shows a leeward vortex that does not develop nor transform into a mobile cyclone because none of the triggers from above (especially the interaction with the left exit region of a jet) are present. The leeward vortex is located at the anticyclonic side of a jet streak on the rear side of an upper-level trough. The development of the leeward vortex is not reinforced by any interaction with a driving force from above. Hence, the vortex remains stationary although an intense overflow across and around the Alps takes place. Such situations are very common throughout the year. A separation of the leeward vortex from the lee region of the mountains would deprive the vortex from its main forcing. In such a case, the vortex would dissipate its energy very rapidly.

Figure 10: IR10.8 loop (from March 15, 2019; 00:00 UTC to March 16, 2019; 06:00 UTC). Mean sea level pressure (black), isotachs at 300 hPa (yellow), cyclonic vorticity at 300 hPa (red) and geopotential height at 500 hPa (cyan).
Note: to access the gallery of images click here

The low-level flow (e.g., at 950 hPa) around the southern end of the Alps shows lee vortices that emerge and dissipate rapidly.

Figure 11: IR10.8 loop (from March 15, 2019; 00:00 UTC to March 16, 2019; 06:00 UTC). Mean sea level pressure (black), stream lines at 950 hPa (brown) and geopotential height at 500 hPa (cyan).
Note: to access the gallery of images click here