A Numerical Simulation Study on the Genesis of a Tropical Storm

Abstract

The genesis of a tropical storm is studied using a numerical simulation model. The model used is an 11-level primitive equation model covering a channel domain of 25° span with open lateral boundaries at latitudes 5.5 and 30.5°N. The initial basic flow field is based on the mean condition at 80°W during Phase III of GATE. The superposed wave disturbance is initially confined in the lower troposphere. The time integration of the model is carried out to 96 h, during which a tropical storm develops accompanied by an upper level anticyclone. The genetic sequence of the disturbance system, from a shallow easterly wave into a tropical depression and further into a tropical storm, is described. The minimum surface pressure of the system deepens from 1008.4 to 1002.6 mb at 96 h. The maximum surface wind at 96 h is above 17 m s?1. The relative vorticity at 950 mb intensifies from 43 ? 10?6 s?1 at the initial time to 237 ? 10?6 s?1 at 96 h. The surface convergence increases from 24 ? 10?6 10?6 s?1 to 71 ? 10?6 s?1. The processes involved in the above transformation are extensively discussed. Attention is given to the change in the area of rainfall and cloud from a zonal pattern to a cluster-type, the deepening of the cloud within the system, the appearance of horizontal tilt of the trough axis and the time variation of its vertical tilt, the evolution of the vertical motion field, the thickening of the convergence layer around the depression center, the formation of a warm core at 335 mb and its downward extension, the appearance of a cold core at a higher level, etc. The intensification of the vortex and the growth of a warm core are analyzed by examining the budgets of vorticity and heat at the tropical depression stage. The vorticity increase at low levels is due to stretching of the vortex. Relative horizontal advection causes a decrease of vorticity in some outer areas. At upper levels, the upward protrusion of positive vorticity from below and relative horizontal advection cause a positive tendency. Both the effect due to horizontal divergence and the twisting up of a horizontal vortex make negative contributions. The net effect at upper levels is to produce a compact positive vorticity area within a large region of negative vorticity. Upper level warming is largely due to the excess of the condensation-convection heating over the cooling effect associated with the upward motion. The appearance of an upper level disturbance in the present model is caused entirely by the forcing from below. Supplemental experiments confirm that, although the diabatic heating effect of radiation plays an important role, the heating due to the condensation of water vapor is essential for the formation of a tropical storm in the present case.