If the Jet streak approaches and crosses a frontal zone at a large acute angle, horizontal and vertical increase of frontal cloudiness in the left exit region of this crossing jet streak can be observed. These cloud phenomena are produced by the interaction between the front and the jet streak. The key parameters that better describe that influence 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 wind speed. The 'maximum' observed speed indicates the location and orientation of a Jet Streak.
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
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.
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.
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