NONHYDROSTATIC SIMULATION OF FRONTOGENESIS IN A MOIST ATMOSPHERE .3. THERMAL WIND IMBALANCE AND RAINBANDS

The dynamical mechanisms contributing to the cross-front ageostrophic circulation are identified in high-resolution (40 to 5 km) nonhydrosta tic simulations of moist frontogenesis. In a first step, the importanc e of the alongfront ageostrophic circulation is assessed. The structur e of the intense thermal wind imbalance (TWI) occurring in the vicinit y of the surface cold front is diagnosed and explained using a budget of the alon,ofront vorticity eta. It allows one to propose a new balan ce in terms of the steadiness of the eta field in the system moving fr amework. The TWI is thus found nearly equal to the total cross-front e ta transport by resolved and subgrid scales. It is shown that, first, the deviation of the prefrontal air toward the front, enhanced by the surface friction and cloud diabatic processes, allows generation of a layer of positive eta near at the top of the PBL. Second, the frontal lifting of this eta structure is responsible for the basic structure o f the TWI. In a second step, a general form of the Sawyer-Eliassen (SE ) diagnostic equation is used, including diabatic effects as well as e ffects of thermal wind imbalances (or ''ageostrophic residue''). This latter effect is evaluated using the steadiness balance, which is conf irmed by a budget diagnosis. The solution of this SE equation provides an accurate diagnostic of the causes of the secondary circulation, bo th qualitatively and quantitatively, down to small scales. Finally, th e SE equation is used to explain the formation and Idealization of rai nbands in regions of effective symmetric stability. In particular, it is shown that the ''ageostrophic residue'' plays a crucial role on the behavior of the bands. It explains about 25% and 60% of the intensity of the warm sector-wide rainband and of the narrow cold-frontal rainb and, respectively, for a case with intense surface friction.