Numerical Study on the Development of Asymmetric Convection during Tropical Cyclone Landfall

It has been observed and simulated in previous studies that landfalling tropical cyclones (TCs) develop asymmetry in convection. However, there is no definite conclusion on the location of convection and how convection interacts with the environment such as vertical wind shear development. Idealized numerical experiments are performed using the Weather Research and Forecasting model to study the effects of land surface on the wind structure and precipitation distribution of landfalling TCs. The simulation is on a f-plane, quiescent environment and without topography. Three nested grids are used with the finest horizontal resolution of 4 km, and the inner grid has no cumulus parameterization. The idealized cyclone vortex is initially over the ocean, and then drifts to the land surface due to asymmetric flow developed within the vortex by surface friction. Simulation results show that the low-level winds within the inner core are enhanced in northeasterlies during landfall, while the upper-level average winds have enhanced northwesterlies. This leads to a change in vertical wind shear to more westerlies and northwesterlies during landfall, which is similar to previous observational studies. The results also show that during landfall the precipitation concentrates in the front quadrants with respect to the vortex motion and with a slight preference to the front right where the surface winds are offshore. In other words, instead of generating heavy rainfall at the location with the largest frictional surface convergence, there are other mechanisms that determine the major locations of convection. Since there is no environmental flow in the simulations, the development of vertical wind shear is closely related to that of the asymmetric convection and associated precipitation. The interrelationships between asymmetric convection and vertical wind shear together with consideration of the distribution of vertical instability within the TC are being investigated, and will be presented during the conference.