Factors, I: Baroclinic instability (PV anomalies approach)

One way of explaining baroclinic instability is via potential vorticity considerations (IPV= Isentropic potential vorticity). According to Hoskins et Al, 1985, when a positive IPV anomaly (normally associated to a tropopause fold, with stratospheric air intrusion in the troposphere) approaches a baroclinic zone at low levels, it will induce a low level cyclonic circulation. This low level circulation, will amplify, via thermal advections over the baroclinic zone, the upper level disturbance, thus reinforcing the positive IPV anomaly, and so on...

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In isentropic coordinates, potential vorticity (PV) can be writen as seen in the equation. A value of PV of 10-6 K kg-1 m2 s-1 corresponds to a PV unit (PVU=1.0). Values close to 1 PVU may be associated to the tropopause, with higher values above. [Question]: Which of the following staments concerning with potential vorticity is true?

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Sorry! Stratospheric air PV is usually large mainly because its large static stability. (check the equation again!)

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Not always!, the answer depends on the value of the Coriolis parameter f. For the same stability, the higher the latitude from where the air comes, the larger the PV of the air.

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That is right!!

[CM of symmetric PV anomaly]: The wind is usually strong around the PV anomaly. Partly because of thermal wind balance must be satisfied, and thermal gradients are strong between tropospheric and stratospheric air.
[First stage]: Upper level PV anomaly induces a cyclonic circulation at low levels over a baroclinic zone.

[Second stage]: Thermal advections start taking place (WA= warm advection, CA=cold advection), and if the static stability is low, they will be very efficient in amplifying the upper level disturbance or anomaly. The so called self-development (upper -low level interaction) process will further promote the cyclogenesis.
An example of an upper level PV anomaly situated over a strong baroclinic zone at low levels. [Left]: Surface pressure plus temperature fields at 925 hpa level, 21-12-1979, 00 UTC. [Right] Potential vorticity field at 250 hpa. We can see west of Sardinia Island, a very strong baroclinic zone at 925 hpa, and in the same position, but at upper levels a 9 PVU potential vorticity anomaly. That matches very much Hoskins conceptual model, an accounts for an important part of the deepening of the cyclone.
An example of an upper level PV anomaly and its effect on surface development. In this case a low level PV anomaly is also generated, probably via latent heat release processes (see latent heat release unit of study). After some time both anomalies become coupled, which is usually a sign of strong or explosive cyclogenesis. The surface represented corresponds to the 6.41x10exp(-5) s-1 geostrophic vorticity constant surface. Images in animation by Catalina Estarellas, Illes Balears Meteorological Center, INM.