If stratospheric air is penetrating into the troposphere, a PV anomaly associated with a vorticity maximum will be observed. The reason for this is that the static stability in the troposphere is significantly lower than in the stratosphere.
In the troposphere, the distance between isentropic surfaces is much larger than in the stratosphere. As a result, relative vorticity will increase. (see Additional Parameters Indicative Of The Diagnosis Of Cloudiness: Potential Vorticity ). In the case of a propagation of the PV anomaly, there will be advection of positive vorticity ahead of the anomaly and advection of negative vorticity behind it. The advection of positive vorticity, if increasing with height, causes divergence in the upper troposphere and, as a consequence, upward motion.

Although potential vorticity (PV) and vorticity advection (VA) are strongly connected, the use of PV as a key parameter, together with the WV image, is often more illustrative than using PVA patterns alone.
When a PV anomaly approaches an area of moist and potentially unstable air, strong convection is initiated and convection systems such as EC, Comma, Cb Clusters or MCS can develop. Additionally decaying systems can make intensify through this process. Locations favourable for these phenomena are in cold air to the rear of frontal cloud bands.

In the WV imagery, strong convective development can be seen near northeast Spain at 42N/02E. The PV maximum coincides with the dark stripe in the WV image. A Cb Cluster has developed on the forward edge of the Dark Stripe.
The cross section shows a distinct PV anomaly, with the PV-2 unit protruding to 400 hPa. A distinct maximum of PVA can be found along the forward edge of the PV anomaly, causing upward motion and development of the Cb Clusters.