The Effect of Surface Friction on the Mesoscale Organization of Cumulus Convection in Tropical Cyclones

Abstract

In order to study the effect of surface friction on the properties of mesoscale convection which constitute the rainbands of tropical cyclones, numerical experiments are performed with an axi-symmetric non-hydrostatic model. As discussed by Yamasaki (1983), when the tangential wind velocity is not very strong (10ms^-1 or less), the typical time scale of mesoscale convection is a few hours. When the tangential wind intensifies and frictional inflow becomes very significant, the lifetime of mesoscale convection becomes 11-12 hours. In contrast to the convection with time scale of a few hours, it does not decay owing to an outward shift of low-level convergence associated with the intensification of cold downdraft outflow. Its decay is caused by its inward shift due to strong frictional inflow. In this situation, rainwater falling from the cloud, which tilts outward with height at this stage, acts to prevent the low-level warm moist air from flowing into the cloud. A new mesoscale convection is formed at some distance from the old one. Structure, time evolution and the mechanism of the mesoscale convection with the longer lifetime are discussed in detail.