Understanding the development of inter-monsoon convective storms over the Western Peninsular Malaysia

The western Peninsular Malaysia experiences biannual maxima in total rainfall. The first maximum is between October and December and the second is between March and May. During these periods, localised severe rainfall events are common. They are usually short, intense and sometime cause flash floods and landslides. Forecasting these local events is difficult, and continuous study to understand the mechanism of the rainfall events is important for the advancement of tropical weather forecasting in the future. This study focuses on inter-monsoons, which are April-May and September-October. Heavy rainfall days (daily total rainfall ≥ the 90th percentile value) are determined by the average regional mean of three Peninsular Malaysia regions (northwest coast, west coast and inland). The analysis shows that a stronger easterly wind anomaly is common during these heavy rainfall days. Another common feature is a stronger ascending motion over the peninsula and also a high amount of moisture in the mid troposphere ( around 700hPa pressure level) over the east side of the peninsula. To investigate the mechanisms responsible for severe inter-monsoon convection in greater detail, two case studies were selected and simulated using the limitedarea high-resolution UK MetOffice Unified Model. The Peninsular Malaysia and Sumatra Island influence the development of overnight rainfall over the Malacca Strait by constraining land-sea breezes and low-level flow. The Titiwangsa mountains over the Peninsular Malaysia then induce rainfall development at noon. The combination of these two events influences the development of severe convective storms of 2 May 2012 over the western Peninsular Malaysia. Sensitivity studies were carried out to investigate the influence of the local orography on this event. The flattened Peninsular Malaysia orography causes a lack of rainfall over the inland part of the Peninsular Malaysia and Sumatra Island yet still produces overnight rainfall over Malacca Strait but weaker. By removing Sumatra Island, it can be seen that the western and inland parts of the Peninsular Malaysia receive more rainfall, as it is influenced by the westerly wind and the Indian Ocean. The second case study of 24 September 2014 was inaccurately simulated thus further analysis on the mechanism of the convective storm development is rather impossible. However, the sensitivity experiments were still conducted and the results showed a significant difference in the rainfall pattern as the local orography was modified and Sumatra Island was removed. These results suggest the importance of the interaction between land masses, orography, low level flow and the diurnal cycle on the development of the severe rainfall events