熱帯低気圧の発達における境界層力学の役割　Part I： 海面抵抗係数に対する感度

抄録

This study examines the role of boundary layer dynamics in tropical cyclone (TC) intensification using numerical simulations. The hypothesis is that although surface friction has a negative effect on TC intensification due to 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 indicate that TCs with a 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 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 over 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 spinup period through stronger frictional moisture convergence and Ekman pumping and thus expedite moistening of the innercore column of the TC vortex but is likely to lead to a weaker storm in the mature stage.