Rainfall distribution of five landfalling tropical cyclones in the northwestern Australian region

Rain gauge data, satellite IR brightness temperature and radar-estimated rain rate for five tropical cyclones from the 2005–06 to 2009–10 seasons that made landfall along the northwestern coast of Australia are analysed. It is the first time that the spatial rainfall distribution of landfalling tropical cyclones in the southern hemisphere has been systematically investigated. It is found that the distributions of rainfall are more concentrated in the right side of the track of the landfall tropical cyclones, which is the offshore flow position. Potential mechanisms responsible for this observed asymmetry in rainfall distribution are discussed. These include the tropical cyclone motion direction, deep-tropospheric vertical wind shear and land-sea contrast in surface properties. Topography is considered to have less effect since Western Australia is relatively flat. The rainfall maxima are found in the front and downshear quadrants for these tropical cyclones, which is consistent with previous studies. The changes in vertical wind shear when these tropical cyclones moved to the south are largely attributed to the prevailing environmental flow. Three numerical simulations are performed; one with a realistic land surface, one with all topography removed and one with all land removed. These simulations show that the land surface effects play an important role in determining the asymmetry in rainfall distribution, which explains why in some cases the observed maximum rainfall does not follow closely the vertical wind shear direction when making landfall.