Numerical case study of an extreme rainfall event during 9–11 December 2004 over the east coast of Peninsular Malaysia

Summary An exceptional rainstorm affected the eastern coast of Peninsular Malaysia during 9–11 December 2004 as a result of a westward propagating tropical disturbance known as the Borneo vortex. Rainfall totals near the storm center exceeded 600 mm and led to flash floods, loss of life and severe damage in the area. This study presents the results of a numerical simulation of this event using the fifth generation of the Penn State – NCAR Mesoscale Model (MM5). The model successfully simulated the synoptic circulation and reproduced the episode with comparable spatial patterns and total accumulated amount of precipitation to the observed. Various sensitivity experiments showed that the local topography is decisive in shaping the rainfall distribution during the storm episode. The role of the terrain elevation appears to be to block the westward progression of the system and inhibit excessive rainfall in the inland areas of Peninsular Malaysia. To the north of the storm center where coastal terrain elevation is relatively high, orography plays an important role in the rainfall by providing an additional forcing for moist air lifting. An additional fake dry simulation suggested that latent heat release is crucial for the development of the storm. Without latent heating, the vertical coupling of low-level convergence and upper level divergence is weakened and the vertical motion associated with the storm is suppressed.