Tropical cyclogenesis and vertical shear in a moist Boussinesq model

Tropical cyclogenesis is studied in the context of idealized three-dimensional Boussinesq dynamics with perhaps the simplest possible model for bulk cloud physics. With low-altitude input of water vapour on realistic length and time scales, numerical simulations capture the formation of vortical hot towers. From measurements of water vapour, vertical velocity, vertical vorticity and rain, it is demonstrated that the structure, strength and lifetime of the hot towers are similar to results from models including more detailed cloud microphysics. The effects of low-altitude vertical shear are investigated by varying the initial zonal velocity profile. In the presence of weak low-level vertical shear, the hot towers retain the low-altitude monopole cyclonic structure characteristic of the zero-shear case (starting from zero velocity). Some initial velocity profiles with small vertical shear can have the effect of increasing cyclonic predominance of individual hot towers in a statistical sense, as measured by the skewness of vertical vorticity. Convergence of horizontal winds in the atmospheric boundary layer is mimicked by increasing the frequency of the moisture forcing in a horizontal subdomain. When the moisture forcing is turned off, and again for zero shear or weak low-level shear, merger of cyclonic activity results in the formation of a larger-scale cyclonic vortex. An effect of the shear is to limit the vertical extent of the resulting emergent moist vortex. For stronger low-altitude vertical shear, the individual hot towers have a low-altitude vorticity dipole rather than a cyclonic monopole. The dipoles are not conducive to the formation of larger-scale vortices, and thus sufficiently strong low-level shear prevents the vortical-hot-tower route to cyclogenesis. The results indicate that the simplest condensation and evaporation schemes are useful for exploratory numerical simulations aimed at better understanding of competing effects such as low-level moisture and vertical shear.