Moist potential vorticity generation 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 moisture gradients. Negative (positive) MPV can be generated in the region where baroclinic vectors have a component along (against) the direction of moisture gradients. Numerical simulations of extratropical cyclones with different moisture distributions show that at the different stages of cvclogenesis, negative MPV usually appears in the warm sector near the north part of the cold front, the bent-back warm front, the warm core and the cold front. The effects of the Boussinesq approximation on the distribution of vorticity and M PV are examined. The Boussinesq approximation neglects several components of the solenoidal term in the vorticity equation. As a result, it 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. Consequently, the influence of the Boussinesq approximation on the M PV field is very significant. Evidence of the presence of conditional symmetric instability in the negative MPV region of the extratropical cyclone is also presented.Ph.D