Tropical cyclone rainfall: An observational and numerical study of the structure and governing physical processes.

摘要

Fresh water flooding has become the largest threat to life at hurricane landfall in the United States, in part because of a lack of skill of current Quantitative Precipitation Forcast (QPF) methods. This study aims to develop a global climatology of tropical cyclone (TC) rainfall and to improve our understanding of physical processes that affect the TC rainfall structure and distribution, using satellite observations and numerical simulations. First, the TC rainfall distributions with respect to the storm intensity and location are examined, using global satellite observations from the Tropical Rainfall Measuring Mission (TRMM). Secondly, numerical simulations using the Penn State/National Centers for Atmospheric Research (NCAR) mesoscale modelling system, version 5 (MM5), are performed to study how specific processes affect the cyclone rainfall. The presence of moisture and momentum sources on a storm's inner core is investigated using a method that modifies the environmental conditions in the model.; The mean TC rainfall distribution and the first order asymmetry vary with storm intensity and geographical location among the six oceanic basins. The mean rainfall increases with storm intensity within 250 km of the storm center while the radius of maximum rainfall decreases. The asymmetric component is determined by the first-order Fourier decomposition in a coordinate system relative to storm motion and shear. The rainfall asymmetry with TC motion varies significantly with both storm intensity and geographic location. For the global average of all TCs, the maximum rainfall is located in the front quadrants. However, the global composite asymmetry is larger when analysed with respect to shear. The asymmetry is observed down-shear left (right) in the Northern (Southern) Hemisphere for shear values >7.5 m s -1. The analysis is further extended to examine the net effect of the storm motion and the vertical wind shear. It is found that the storm-motion induced rainfall asymmetry is comparable to that induced by the shear when the shear is 5 m s-1. TC propagation speed becomes more important in the relatively low shear environment. The overall rainfall asymmetry, in all oceanic basins, depends on the angle and relative magnitude between the storm motion and shear vectors.; The combined effect of shear and TC propagation speed is further investigated using numerical simulations of Hurricane Floyd (1999). Floyd was a well-observed intense storm that experienced variable environmental shear, from relatively low shear to close to 15 m s-1 high shear during its life cycle. (Abstract shortened by UMI.)