Surface-flux regulation of the coupling between cumulus convection and baroclinic waves

Abstract

The authors examine the role of convection in the dynamics of eddy life cycles through numerical experiments using initial states that are baroclinically and conditionally unstable in midlatitudes. The location of wave-induced convection and its influence on the growing wave depends on how strongly the wave is coupled to the lower boundary through surface fluxes. For all three convective schemes used here (Emanuel, modified Grell, modified Kuo), convective destabilization is favored in the wave's warm sector when there are no surface fluxes included in the simulation and in the cold sector when there are. Convection is also shallower when it occurs in the cold sector, though still precipitating. For simulations using Emanuel convection, the relatively shallow convection plays a central role in a water cycle wherein 1) evaporation gives moisture to the cold sector's boundary layer, 2) convection pumps some of the moisture into the lower troposphere above the boundary layer, 3) the large-scale circulation transports the moisture eastward and upward into the wave's warm sector, and 4) stable precipitation condenses the moisture into precipitation. The additional condensation catalyzes a more energetic life cycle by inducing stronger vertical motion and, hence, a greater conversion of available potential energy to kinetic energy. This enhancement, however, is parameterization dependent, with the key factor being how much lower-tropospheric moistening a convection scheme produces.