Propagation of Convective Systems Associated with Early Morning Precipitation and Different Northerly Background Winds over Western Java

Abstract

Early morning precipitation (EMP) events occur most frequently during January and February over the northern coast of West Java and are characterized by propagating systems originating from both inland and offshore. The timing of EMP is determined by the initial location, direction, and speed of the propagating precipitating system. This study explores processes that characterize such propagating precipitation systems by performing composite analysis and real-case numerical simulations of selected events using the Weather and Research Forecasting (WRF) model with a cloud-permitting horizontal resolution of 3 km. In the composite analysis, EMP events are classified according to the strength of the northerly background wind (VBG), defined as the 925 hPa meridional wind averaged over an area covering western Java and the adjacent sea. We find that under both strong northerly (SN) and weak northerly (WN) wind conditions, EMP is mainly induced by a precipitation system that propagates from sea to land. For WN cases, however, precipitating systems that propagate from inland areas to the sea also play a role. The WRF simulations suggest that mechanisms akin to cold pool propagation and advection by prevailing winds are responsible for the propagating convection that induces EMP, which also explains the dependence of EMP frequency on the strength of VBG. On the basis of the WRF simulations, we also discuss the roles of sea breeze and gravity waves in the initiation of convection.