B) Second variant: lee cyclogenesis in the left exit region of a jet without the passage of a cold front
The second variant is characterized by the intensification of the leeward vortex without an air mass boundary like a cold front being involved. In this case, the additional effect of warm air advection or the approaching upper-level trough that increases vertical motion is missing. Still the dynamic effect of the jet as sole remaining trigger produces some effect on the leeward vortex. This effect might be strong enough for a decoupling of the vortex from the lee region, but this trigger is not sufficient to develop the vortex into a mesoscale frontal system because temperature gradients are missing.
Such situations typically occur at the rear side of an upper-level trough, in a north-westerly stream that passes over the Alps (see example Figure 8).
The role of the jet streak
The life cycle of a cyclone is strongly dictated by upper-level dynamics (mainly by upper-level divergence) from the initial stage until its final dissipating stage. Divergence aloft (below the tropopause) induces rising air due to mass conservation which results in surface convergence (see Figure 6). The varying intensity of upper-level divergence has an immediate impact on lower levels and influences the development of the surface low accordingly.
When upper-level divergence intensifies, vertical transport of air molecules increases and the surface low deepens. As a consequence, cyclonic circulation around the surface low intensifies.
Figure 6: Upper-level divergence induces surface convergence due to mass conservation.
Upper-level divergence fields are found near jet streaks, more precisely in the left exit and in the right entrance region of a jet streak (see the jet cross circulation pattern in Figure 7). In order to intensify the surface low and to induce cyclogenesis, a jet streak needs to interact with a leeward vortex.
Figure 7: Schematic of the cross circulation at the jet entrance and exit region. © COMET.
The next example shows the impact of the left exit region of a jet on the development of a leeward vortex located over south-eastern France. The vortex, though growing stronger, remains a low-level phenomenon in a warm air mass.
Figure 8: IR10.8 loop (from April 13, 2019, 12:00 UTC to April 14, 2019, 18:00 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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The development of the leeward vortex located over south-eastern France into a low-level cyclone that moves across the Mediterranean Sea is not influenced by air mass boundaries nor do any fronts develop (see Figure 9). The core of the cyclone remains warm during the whole process of cyclogenesis.
Figure 9: IR10.8 loop (from April 13, 2019, 12:00 UTC to April 14, 2019, 18:00 UTC). Mean sea level pressure (black) and temperature at 850 hPa (red, magenta and blue).
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