Cyclogenesis in connection with the left exit and the right entrance region of a jet streak

In the Case Study Cold front over middle Europe, two waves were seperately identified and discussed. Both these showed intensification due to Front intensification in connection with the left exit region of a jet streak. To make clear what influence FI by Jet has on the strengthening of a Wave we will parse this phenomena using its keyparameters. For a ‘FI by Jet’ these are:

The Intensification of the front is represented by the PVA which indicates cloud enhancement. The presence and location of the Jet are represented by the Isotachs, which indicate the lines of equal speed. The 'maximum' observed speed indicates the location and orientation of a Jet Streak. When a Jet Streak crosses a frontal zone with a large acute angle an increase of cloudiness or PVA in the right entrance and left exit region of this crossing jet streak can be observed.

To simplify this the following schematics of an idealised Jet Stream that you normally see in the text books might help.

Fig. 1 Jet Stream orientated between isobars

In this figure the height contours between a low and high pressure system and a Jet, orientated from west to east, are pictured. The geostrophic component of the wind is represented by the blue arrows, which are parallel to the isobars, and the isotachs are pictured in yellow. Due to an increase in gradient of height contours, air will accelerate in the entrance region, whereas on the exit side, the gradient of height contours slackens, leading up to a deccelleration of the air flow. Now, according to the equation of motion:

It follows that an ageostrophic wind, Vag, will develop. On the entrance side of the Jet Streak the ageostrophic component will be positive and blow from south to north, wheras on the exit side Vag is negative, e.g. blow from north to south. The further away from the jet the more this component of the wind will weaken.

The total wind speed, pictured as the magenta arrows, is the sum of the geostrophic and the ageostrophic components of the wind. Focusing on these arrows it becomes clear that convergence is associated to NVA and can be found at the left entrance and right exit region of a Jet Streak and divergence is associated to PVA and can be found at the right entrance and left exit region of a Jet Streak.

Fig. 2 Jet Stream orientation and position of the PVA and NVA maxima.

These regions of divergence and convergence at the upper levels of the troposhere (300 hPa) also induce a pair of horizontal divergence and convergence at the surface causing a secundary circulation (Holton, 1982). The associated upward motion causes cyclogenesis. It is therefor that cyclogenesis and cloud enhancement is favoured in the left exit and right entrance region of a Jet Streak.

A final step is to present a vertical cross section which is laid from position A to B in Figure 2.

Fig. 3 Vertical cross section in the exit region.

The vertical Cross Sections shows thare areas which are characterised by their PVA and NVA maxima. Subsequently the cyclonic motions associated to the PVA maximum causes the air to rise, which explains the convergence near the surface and the divergence at jet level. Over the area of the NVA convergence takes place at jet level and due to the decending air Near the surface divergence is observed.