Nonhydrostatic simulation of frontogenesis in a moist atmosphere. Part II: moist potential vorticity budget and wide rainbands

Abstract

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, an evolution of qe in the model is first successsfully validated. The parameterized turbulent subridge-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. 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. -from Authors