Numerical study on the development of asymmetric convection and vertical wind shear during tropical cyclone landfall

Idealized simulations on an f-plane of tropical cyclone (TC) landfall under a quiescent environment in the Southern Hemisphere are performed to investigate the effects of land–sea surface contrast on precipitation. In the control simulation, with realistic roughness and moisture over land to the south of the TC, the simulated vortex moves toward land due to a land-induced steering flow. The abrupt decrease (increase) of tangential wind at the surface leads to convergence (divergence) on the onshore (offshore) flow side. Enhanced convergence at the top of the planetary boundary layer is found on both onshore and offshore sides and is caused by advection from the surface and the enhanced offshore radial wind. The boundary-layer top convergence pattern is consistent with the rainfall distribution. Vertical wind shear develops during the landfall process associated with the low- and upper-level asymmetric flows across the model domain. The wavenumber-1-like low-level asymmetric flow is introduced by an asymmetric geopotential height field that is generated by the large area of frictionally induced convergence on the onshore side and divergence on the offshore side. The upper-level asymmetric flow is attributed to asymmetric convection and associated diabatic heating after landfall. Most rainfall is found in the down-shear right quadrant, which is consistent with previous studies that focused on the effect of environmental shear. Although the existence of feedback from the vertical wind shear to rainfall remains an open question, the relation between maximum rainfall and vertical wind shear is robust, especially when the shear magnitude is large after landfall.