Annual Changes in Rainfall Extremes over the Megacity Jakarta

Heavy rainfall has been known as one of the factors inducing hydrometeorological hazards over the Megacity Jakarta. However, research on changes in rainfall extremes (REs) is somehow very limited in the region. This study will investigate annual and seasonal of REs and how it varies with different topography. We used daily rainfall record at nine observational sites (1975–2016) and reanalysis data of ERA INTERIM (1979–2016). The result shows that the severest rainfall (maximum of consecutive 5-day rainfall/RX5day and 99th percentile/R99p) has strong positive trends particularly at Kemayoran (coastal site) although the increased trends are also found at any other stations over the inland and mountainous areas. The analysis of seasonal trends demonstrates that significant increasing trends only occur in the wet (Dec-Feb) and transitional season (Mar-May, Sep-Nov) over Kemayoran (the coastal station) while a positive trend is observed in all seasons over Citeko (the mountain site). Compared to the wet season, in the dry (Jun-Aug) and transitional (Sep-Nov) seasons, there is concentrated areas near Jakarta and its surroundings with a large standard deviation in Mean Sea Level Pressure (MSLP), Outgoing Longwave Radiation (OLR) as well as cloud cover indicating that this region has a larger variability than the average. A distinct season of increased rainfall trends between the coast and mountain along with high anomalies in the regional OLR, MSLP, and cloud cover in the dry season implies that the development of REs might as a result of an interaction between local topography and large-scale climate condition.