Nonhydrostatic Simulation of Frontogenesis in a Moist Atmosphere. Part II: Moist Potential Vorticity Budget and Wide Rainbands

Abstract

The different processes responsible for the occurrence of wide rainbands, as obtained by high-resolution (5-km) nonhydrostatic two-dimensional simulations of frontogenesis induced by shear, with an explicit representation of the convection are discussed. The study is restricted to a case of strong friction at surface without any surface heat flux. A budget of the moist potential vorticity (qe) has been implemented for a rigorous investigation of generating mechanisms of wide rainbands. The balance between sources, transport, and evolution of qe in the model is first successfully validated. The parameterized turbulent subgrid-scale processes represent the main qe source in these simulations, especially at the PBL top. It is shown that friction acts as a source of intense qe vertical flux at the ground, maximum below the alongfront low-level jets in both warm and cold air masses. An intense positive qe anomaly is obtained in the warm sector, appears to be generated by frictional processes in the far prefrontal zone, and is then transported towards the frontal system. This anomaly induces an intensification of the alongfront low-level jet on its warm flank. In the present shear-driven case, this jet corresponds to a maximum of warm moist advection: the warm conveyor belt, resulting in the formation of an intense warm sector wide rainband located 300 km ahead the surface cold front, lies in a region of strong to weak moist symmetric stability. Wide cold-frontal rainbands, on the other hand, occur in a region of moist symmetric instability, which thus seems to enhance the circulation forced by the geostrophic shearing deformation and frictional convergence in the frontal zone and favors the development of these bands. They efficiently transport the lowest qe values upward and are thus diffusive. It is suggested that these bands were initiated by the dissipation of convective cells generated during the previous convective stage.