Modelling the effects of land-sea contrast on tropical cyclone precipitation under environmental vertical wind shear

The precipitation pattern of a landfalling tropical cyclone (TC) with and without a weak environmental vertical wind shear (VWS) is investigated using WRF/NCAR model simulations under idealized conditions. In the simulations without VWS, results show that for the outer band (r ∼ 100-300 km), the cold and dry air originating over smooth land is advected offshore, reduces the stability and develops a band of rainfall on the eastern side of the TC, while the rough land surface tends to trigger more rainfall to the west. For the inner core (r<100 km), there is only small rainfall asymmetry when the land surface is smooth and dry, but with a rough land surface, the rainfall asymmetry becomes evident and generally stronger rainfall is found over the land areas to the west. Further experiments are performed to compare the effects from weak environmental VWS and land-sea contrast. It is found that the storm-scale (within 400 km from TC centre) VWS changes continuously in direction and magnitude due to asymmetric diabatic heating and accompanying upper-level winds. The rainfall pattern in the inner-core region follows closely the storm-scale VWS with a downshear-left relationship regardless of the surface properties, while in the outer-band region, rainfall distribution is first strongly affected by the surface roughness before landfall and by the environmental VWS afterwards. Therefore, an evolving rainfall-VWS (both environmental and due to storm-scale dynamics) relationship during TC landfall results.