Generation of Moist Potential Vorticity in Extratropical Cyclones

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Abstract
The mechanism of moist potential vorticity (MPV) generation in a three-dimensional moist adiabatic and frictionless flow is investigated. It is found that MPV generation is governed by baroclinic vectors and moisture gradients. Negative (positive) MPV can be generated in regions where baroclinic vectors have a component along (against) the direction of moisture gradients. The numerical simulation of extratropical cyclones shows that negative MPV first appears in the warm sector near the north end of the cloud frontal zone at the development stage and then intensifies in the bent-back warm front at the mature stage. After the cyclone matures, the negative MPV region moves toward the warm core, and another region of negative MPV begins to develop along the cold front. The effects of the Boussinesq approximation on the distribution of vorticity and MPV are examined because this approximation, through widely used in theoretical studies, simplifies the solenoidal term in the vorticity equation. Results show that the Boussinesq approximation underestimates the thermally direct circulations in the cold front and the bent-back warm front by 25% to 30%. This effect is more pronounced when latent heat release is taken into account. It is found that the main deficiency of the Boussinesq approximation is that it will not give the correct MPV features in the intense portions of extratropical cyclones.

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