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 cold front passing over a leeward vortex without the interaction of a jet streak

The next example shows that the passage of a cold front alone without the interaction of additional upper-level triggers on the leeward vortex does not necessarily trigger lee cyclogenesis.

The chosen example shows again the passage of a cold front across the Alps. This time, orography does not strongly impede the cold front as in the previous example, mainly because the front only comes into effect above 700 hPa (see the cross section in Figure 5). Moreover, the interaction of the surface low with the left exit region of a jet is missing.

In the case under consideration, the cold front passage enhances the leeward vortex, but a stronger development resulting in the creation of a new frontal system does not occur nor does the leeward vortex leave its initial position. The short-lived intensification of the leeward vortex arises from the increased wind speed during the cold frontal passage.

Figure 4: IR10.8 loop (from December 6, 2019, 15:00 UTC to December 7 2019, 15:00 UTC). Mean sea level pressure (black), isotachs at 300 hPa (yellow), cyclonic vorticity at 300 hPa (red) and geopotential height at 500 hPa (cyan).
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Figure 5: Left: SEVIRI IR10.8μm image with a red line indicating the position of the vertical cross section. Right: Cross section (northwest to southeast) from the ECMWF model.