Toward an Understanding of Tropical Cyclone Formation with a Nonhydrostatic, Mesoscale-Convection-Resolving Model

摘要

This paper describes results from numerical experiments which have been made toward a better understandingof tropical cyclone formation. This study uses a nonhydrostatic version of the author’s mesoscale-convection-resolvingmodel that was developed in the 1980s to improve paramerization schemes of moist convection. In this study thehorizontal grid size is taken to be 20 km in an area of 6,000 km x 3,000 km, and a non-uniform coarse grid is used in twoareas to its north and south.Results from two numerical experiments are presented; one (case 1) without any environmental flow, and the other (case2) with an easterly flow without low-level vertical shear. Three circular buoyancy perturbations are placed in the west-eastdirection at the initial time. Convection is initiated in the imposed latently unstable (positive CAPE) area. In both cases, avortex with a pressure low is formed, and two band-shaped convective systems are formed to the north and the south ofthe vortex center. The vortex and two convective systems are oriented in the westsouthwest – eastnortheast direction, andtheir horizontal scales are nearly 2,000 km.In case 1, the band-shaped convective system on the southern side is stronger, and winds are stronger just to its south. Incontrast, in case 2, the northern convective system is stronger, and winds are stronger just to its north. Therefore, thedistributions of the equivalent potential temperature in the boundary layer and latent instability (positive buoyancy of therising air) are also quite different between cases 1 and 2. The TC formation processes in these different cases arediscussed, with an emphasis on the importance of examining the time change of latent instability field.