A Time-Lagged Ensemble Simulation on the Modulation of Precipitation over West Java in January–February 2007

Abstract

A numerical experiment using a regional nonhydrostatic model is performed to investigate the synoptic condition related to the heavy precipitation event that occurred at Jakarta in West Java, Indonesia, in January–February 2007. A time-lagged ensemble forecast method is employed with nine ensemble members. The ensemble mean well reproduces the temporal modulation of the spatial distributions of precipitation obtained from the Tropical Rainfall Measuring Mission data. During the simulated two months, several monsoon surges are observed, but only the surge event during which the Jakarta flood event occurred is associated with a cold anomaly. The top of the cold northerly is about 1.5 km. The cold surge event is preceded by the so-called Borneo vortex event, which is dominated by a cyclonic vortex around Borneo, Indonesia, with a horizontal scale of 1000 km and a vertical scale of 3 km. An analysis of cumulative distribution functions in a pentad time scale shows the modulation of the probability of rainfall rate. In pentad 7 (31 January–4 February), which includes the heavy rainfall event, the fraction of the area with precipitation is the highest and the contribution of heavy rainfall to the total amount is one of the highest in the two-month period. The diurnal cycle of occurrence of heavy rainfall is also modulated; in pentad 7, semidiurnal variation becomes dominant, and the largest peak appears in the early morning.