The diurnal variations of rainfall and winds over Malaysia

Abstract

This study focuses on the diurnal variations of rainfall and winds over Malaysia. Data from 35 ground stations, combined with other remote-sensing products, are analyzed. Rainfall distribution has large spatial variations with higher rainfall on the windward side, especially during the winter northeast monsoon season. The timing of the season with heaviest rainfall occurs when the northeast monsoon has the largest impinging angle on the coastal mountain ranges. The driest season occurs during the southwest monsoon season, with low total precipitable water over the area, and when the monsoon trough is farthest to the north over the South China Sea. Land-sea breezes and/or upslope-downslope winds occur throughout the year for most areas. The timing of the diurnal rainfall maximum and horizontal rainfall distributions is closely related to terrain, local winds and the interaction of the diurnally driven winds with the prevailing flow. On the windward side, sea breezes/upslope winds combine with the prevailing flow resulting in a rainfall maximum during the late afternoon due to orographic lifting. On the leeside, sea breezes/upslope winds result in a secondary rainfall maximum during the late afternoon. The afternoon rainfall maximum mainly occurs near the foothills of the mountain ranges and propagates inland during the late afternoon/early evening. Over the northwest coast of East Malaysia, nocturnal rainfall is most significant during the late northeast monsoon season due to convergence of land breezes and the northeasterly monsoon flow. Throughout the year, convergence between land breezes and the prevailing flow or between land breezes from both sides of the coast within the strait results in nocturnal rainfall over the coastal areas. Over the east coast of West Malaysia, land breezes are evident, except during the early northeast monsoon season under strong northeasterlies (7~8 m s-1). During the early northeast monsoon season, nocturnal rainfall is most significant due to the orographic blocking of the strong northeasterly monsoon flow. Flow deceleration of the incoming northeasterly monsoon flow is most significant in the early morning.