The Role of Boundary Layer Dynamics in Tropical Cyclone Intensification. Part I: Sensitivity to Surface Drag Coefficient

抄録

 This study examines the role of boundary layer dynamics in tropical cyclone (TC) intensification by numerical simulations. The hypothesis is that although surface friction has a negative effect on TC intensification because of frictional dissipation (direct effect), it contributes positively to TC intensification by determining the amplitude and radial location of eyewall updrafts/convection (indirect effect). Results from a boundary layer model show that TCs with larger surface drag coefficient (C_D) can induce stronger and more inwardly penetrated boundary-layer inflow and upward motion at the top of the boundary layer. This can lead to stronger and more inwardly located condensational heating inside the radius of maximum wind with higher inertial stability, and is favorable for more rapid intensification.

 Results from full-physics model simulations using the TC Model version 4 (TCM4) demonstrate that the intensification rate of a TC during the primary intensification stage is insensitive to C_D if C_D is changed in a reasonable range. This is because the increased/reduced positive contribution by the indirect effect of surface friction to TC intensification due to increased/reduced C_D is roughly offset by the increased/reduced negative (direct) dissipation effect due to surface friction. However, greater surface friction can significantly shorten the initial spin-up period through stronger frictional moisture convergence and Ekman pumping and thus faster moistening of the inner core column of the TC vortex, but would lead to a weaker storm in the mature stage.