A Further Study of the Tropical Cyclone without Parameterizing the Effects of Cumulus Convection

Abstract

   As a continuation of previous studies (Yamasaki, 1977a, b) numerical experiments of axially symmetric tropical cyclones are performed. As in the previous studies, a fine resolution model is used in which convective clouds are not parameterized but explicitly resolved. A recent advance in the computer has enabled us to deal with the tropical cyclone with realistic horizontal scale even when a sufficiently small grid size is used. Although assumption of axial symmetry restricts very realistic simulation of real tropical cyclones, it seems that the numerical experiments have revealed many important aspects of the formation and intensification processes and the structures of the tropical cyclone and their mechanisms.
   At the early stage before the tangential velocity attains about 10 ms^-1, the area of convective activity and the vortex size expand with time because convection at the outermost part of the convective area propagates outward. Individual convective clouds are usually organized as a convective system with a time scale of about 3 hours, which is referred to as ‘mesoscale convection’ in this paper. As one mesoscale convection weakens, another forms at some distance. As a result of successive formation, convective activity and rainfall propagate outward or inward and persist for a long period of time. A large-scale (cyclone-scale) meridional circulation is intensified by an ensemble of several mesoscale convections, whereas the continuous formation of mesoscale convection is maintained by the large-scale circulation. The mechanism of such cooperative interaction between moist convection and large-scale motion at this stage, however, is different from that of the original CISK found by Ooyama (1964) and Charney and Eliassen (1964). That is, surface friction does not play any significant role, but instead, the downdraft and cooling due to evaporation of rainwater play an essential role. Such a new type of CISK was also discussed in Yamasaki (1975, 1979).
   When the rotational winds are intensified, surface friction becomes important. That is, the radial positions of the outermost convection and of maximum tangential velocity begin to shift inward by frictional inflow. Such an inward shift occurs when the tangential velocity attains 10-15 ms^-1. Even at this stage it appears that cooling due to evaporation of rainwater and downdraft have significant effects on the large-scale dynamics.
   When the tangential velocity near the vortex center exceeds about 20 ms^-1, an eye and eyewall are formed. Then rapid fall of the central surface pressure as well as rapid intensification of the tangential winds takes place. Surface friction plays an essential role in the formation and maintenance of the eye and eyewall. Several small-scale features, which have not been studied with coarse grid models with parameterized convection, are found in the eye and eyewall, including time variation with a period of about 10 minutes.
   It is suggested that the long-lasting convections obtained in this study may correspond to observed spiral rainbands, which have been interpreted by many authors as internal gravity waves modified by convective heating. The structure and the phase velocity of the long-lasting convections are different from those of internal gravity waves.