Diabatic Potential Vortiticity Modification, the Low-Level Jet, and Moisture Transport in Cyclones

The problem of quantitative precipitation forecasting extends into the broader realm of atmospheric predictability via dynamical feedbacks associated with latent heat release. For example, in situations where forecast models fail to adequately represent heavy precipitation, errors are translated into the thermodynamic and dynamic fields as well. An excellent diagnostic tool for addressing this type of feedback mechanism is the potential vorticity (PV), which is conserved following adiabatic frictionless flow. The specific problem to be examined in the current study involves PV redistribution in the vicinity of pronounced cold-frontal rain bands in cold-season cyclones.

The presence of a well-developed rain band along a cold front (such as an ana-cold front) is shown to lead to the formation of a lower-tropospheric PV maximum via diabatic redistribution. This PV maximum is associated with southerly perturbation flow to the east, in the cyclone warm sector. If the PV anomaly is pronounced, and isolated from the larger upper-tropospheric PV reservoir, then a distinct low-level jet (LLJ) may be observed. The linkage between precipitation intensity and the intensity of the LLJ is analyzed using PV diagnostics for a recent cold-season cyclone event in the south-central U.S.

Diabatic PV redistribution is shown to contribute to the development and eastward propagation of an elongated band of PV in the vicinity of the cold front. Computations of the diabatic PV tendency confirm that latent heat release is sufficient to generate the observed PV feature. The LLJ and moisture transport are enhanced to the east of this PV feature, as quantified via PV inversion. The ability of numerical models to represent this feedback process is dictated by the ability of the model to resolve and represent the cold-frontal rain band. Predictability in these situations is shown to be sensitive to the choice of model convective parameterization scheme (CPS).