A comparison of convergence-and surface-flux-based convective parameterizations with applications to tropical cyclogenesis

Abstract

The evolutions of radially symmetric vortices and idealized easterly waves are compared using three simple convective parameterization schemes. The parameterizations are formulated for a model atmosphere consisting of three coupled, shallow, constant-density layers. The first was developed by Ooyama and later refined by DeMaria and Pickle (the ODP scheme). Their scheme uses horizontal convergence in the boundary layer to define a vertical mass flux, along with a closure relation based on conservation of moist static energy that determines the vertical redistribution of mass. The second scheme is a modification of the ODP scheme in that convection is allowed to stabilize the profile. A third scheme has the convective mass flux from the boundary layer determined by the assumption that convective up- and downdrafts keep the equivalent potential temperature of the boundary layer in near equilibrium (the BLQ scheme). For vortices that are initially radially symmetric, the ODP scheme produces a hurricane-like vortex, provided that the middle atmosphere is sufficiently moist. However, the intensification is slow unless an unrealistically weak density stratification is used. For the modified ODP scheme, the vortex intensifies on a shorter timescale in an atmosphere with a realistic stratification, regardless of the midlevel moisture profile. After the intensification, convective transport of air with high equivalent potential temperature stabilizes the profile, and the vortex begins to decay. For the BLQ scheme, the vortex will intensify, provided that the middle atmosphere is sufficiently moist and after the atmosphere has been conditioned for deep convection by the shallow convection. For the idealized easterly waves, the development between the schemes is quite different. Only the BLQ scheme allows convection to condition the atmosphere for further deep convection, which results in a developing disturbance.