Positive vorticity advection
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Positive vorticity advection, or PVA, is the result of higher values of vorticity advecting into lower values of vorticity.
[edit] Development
Vorticity in the atmosphere is created in three different ways, which are named in their resultant vorticity. These are; Coriolis vorticity, curvature vorticity, and shear vorticity. For example, at the base of a trough, there is curvature and shear vorticity. Air entering the base of the trough does not have this vorticity and so there is negative vorticity advection (NVA). This produces convergence because of the way the air gains vorticity while entering the base of the trough. The opposite happens when air is exiting the base of a trough. This air has more vorticity than the air it is entering and therefore produces PVA. PVA produces divergence as a result of how there is a loss of vorticity. Coriolis vorticity in this situation is ignored because it acts about the same on all the air flowing through the base of the trough.
[edit] Significance in forecasting
The divergence with PVA is significant because it creates forced lift in the atmosphere. This forced lift, in the presence of conditions favorable for atmospheric convection, can cause clouds or precipitation. NVA will do the opposite and lead to a stable atmosphere. In combination with a jet streak, PVA can lead to the amplification of a trough which is significant for forecasting many conditions of the atmosphere.
