GENERATION OF MOIST POTENTIAL VORTICITY IN EXTRATROPICAL CYCLONES

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 mo isture gradients. Negative (positive) MPV can be generated in regions where baroclinic vectors have a component along (against) the directio n of moisture gradients. The numerical simulation of extratropical cyc lones shows that negative MPV first appears in the warm sector near th e north end of the cold frontal zone at the development stage and then intensifies in the bent-back warm front at the mature stage. After th e cyclone matures, the negative MPV region moves toward the warm core, and another region of negative MPV begins to develop along the cold f ront. The effects of the Boussinesq approximation on the distribution of vorticity and MPV are examined because this approximation, though w idely used in theoretical studies, simplifies the solenoidal term in t he 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 pronounce d when latent heat release is taken into account. It is found that the main deficiency of the Boussinesq approximation is that it will not g ive the correct MPV features in the intense portions of extratropical cyclones.