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

Lee Cyclogenesis, as we use the term here, is the development of a leeward vortex as described in Chapter 3 into a mesoscale cyclone. The starting vortex forms at the lee side descent of an air flow that crossed a mountain barrier. The further development of this vortex into a mesoscale cyclone consists not only of the deepening of the surface low but also the transition from a stationary lee vortex into a mobile cyclone.

Classic theories on lee cyclogenesis investigated the orographic forcing of baroclinic waves (Rossby waves) to explain lee cyclogenesis with the help of the quasi-geostrophic theory. In the following chapters, we will focus on the upper-level dynamic triggers that are involved in any cyclogenetic process:

1. Cyclonic vorticity advection on the leading edge of an upper-level trough;
2. Upper-level divergence in the left exit region of a jet streak;
3. A potential vorticity (PV) anomaly superimposed over the leeward vortex;

These three upper-level driving forces can act either alone or in combination with each other. All three driving forces are often connected to the passage of a cold front. Nevertheless, the intensification of the leeward vortex that occurs in the left exit (or right entrance region) of a jet is not necessarily connected to the passage of a cold front - they can also trigger low-level vortices inside a uniform airmass (most commonly at the rearward side of a trough).

As soon as the vortex moves away from the lee of the mountain range, orographic effects do not support it anymore. The "feeding mechanisms" for the vortex to grow and survive outside the lee region are provided by the above mentioned upper-level triggers.

In the following chapters, we will analyse the triggers that bring a leeward vortex to develop from a local, stationary phenomenon into a mesoscale cyclone by examining real examples. Therefore, we divide the cyclones into two categories:

1. The classical lee cyclogenesis: leeward vortices that develop into a mesoscale frontal system.
2. Leeward vortices that move away from their place of origin but do not develop into mesoscale cyclones. Instead, they preserve their characteristic as a low-level cyclone.