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

First variant: lee cyclogenesis induced by a cold front passage

This variant represents the classical lee cyclogenesis as it happens several times a year in the Gulf of Genoa. The passage of a baroclinic zone like a cold front from the north across the Alps, accompanied by an upper-level trough, leads to an intensification of the leeward vortex to form the classical variant of lee cyclogenesis.

The approaching trough goes hand in hand with an increase of cyclonic vorticity advection with height that provides a favourable environment for the leeward vortex to strengthen because upper-level divergence entails low-level convergence. In the course of cyclogenesis, the upper-level trough co-locates with the leeward vortex and they become locked in phase. A cut-off low over the Mediterranean Sea is often the end product of strong lee cyclogenesis.

When a cold front crosses the Alps, the sinking process of the air at the lee side of the mountains considerably weakens the temperature gradient at the lower parts of the front (foehn effect). Finally, there is no or only a weak temperature gradient near the ground due to the katabatic wind effect at the lee side of the Alps.

In contrast, the western part of the cold front passes over France between the Pyrenees and the Alps without hindrance and begins to wrap around the strengthened leeward vortex.

The example below shows such a classical lee cyclogenesis induced by the passage of a cold front over the Alps.

Figure 1: IR10.8 loop (from May 4, 2019, 09:00 UTC to May 5 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).
Note: to access the gallery of images click here

From the 850 hPa temperature field, one can clearly see how the cold spell from the north propagates unhindered over the Gulf of Lion and then turns eastward, while the eastern part of the cold front is blocked by the Alps and thus strongly delayed (see Figure 2).

Figure 2: IR10.8 loop (from May 4, 2019, 09:00 UTC to May 5 2019, 15:00 UTC). Mean sea level pressure (black) and temperature at 850 hPa (red, magenta and blue).
Note: to access the gallery of images click here

With the approaching cold front, we see another trigger acting; a strong PV anomaly associated with the trough that catches up from behind and becomes successively superimposed over the leeward vortex (see Figure 3). This results in a coupling of the PV-anomaly and the surface low that will persist until the end of the cyclones' life cycle.

Figure 3: IR10.8 loop (from May 4, 2019, 09:00 UTC to May 5 2019, 15:00 UTC). Mean sea level pressure (black), geopotential height at 500 hPa (cyan) and height of PV=1.5 (magenta).
Note: to access the gallery of images click here

The cold front is associated with a tropopause fold where upper-level (stratospheric) air is sinking to lower atmospheric levels. As shown in Chapter 2, subsiding air masses sink at a tropopause fold from stable to less stable regions, near to the ground. As the PV of an air parcel remains constant when moving along an isentropic surface, its cyclonic vorticity increases accordingly (see the Hoskins theory).

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