Lee Cyclogenesis

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.

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).
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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).
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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).
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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).

N.B.:

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.

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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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).
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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