The Bekasi River Basin is highly vulnerable to severe and recurrent flooding, as evidenced by significant infrastructure and environmental damage during major events. This study investigates the catastrophic floods of 2016, 2020, 2022, and 2025 by implementing the Rainfall-Runoff-Inundation (RRI) model to simulate key hydrological processes. After validation using historical water level data, the model performed effectively, achieving the highest coefficient of determination (R² = 0.75) and lowest root mean square error (RMSE = 0.66) at Cileungsi Station. In contrast, the lowest R² = 0.02, and the highest RMSE = 3.74 at Pondok Gede Permai (PGP) Station. The results reveal a concerning trend of worsening 5-year flood events, with the 2025 flood reaching a peak inundation depth exceeding 3 m and affecting an area of 2.97 km², caused by a rainfall threshold of more than 180 mm/day. Furthermore, the model shows a rapid hydrological response, with a time lag of approximately 7 h or less between peak rainfall and flood onset across three monitoring stations. Analysis indicates these severe floods were primarily triggered by heavy rainfall combined with significant land cover changes. The findings provide valuable insights for flood prediction and mitigation strategies in this vulnerable region.

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.

The megacity Jakarta, Indonesia, encounters hydrometeorological hazards driven by intense and/or frequent rainfall. The Madden–Julian oscillation (MJO), a large-scale atmospheric disturbance propagating eastward through the tropics every 30–60 days, influences rainfall variability in Jakarta. Existing studies often rely on coarse-resolution data and neglect the distinct roles of stratiform versus convective rain rates, especially in relation to the MJO. This study examines the locally diurnal and seasonal characteristics of stratiform and convective rain rates and their variations with the MJO, using high-resolution C-band Doppler radar data (2009–12). Results show that although convective rain rates peak in December–February (DJF), the intensity calculated over rainy pixels is higher in September–November (SON). The diurnal afternoon peak of total rain rate is primarily attributed to the convective rain rate over mountains (24%) and lowlands, whereas the morning peak of overall rain rate is affected by the stratiform rain rate (11%). There is a higher proportion of stratiform rain rate during the MJO active phase, and this rain rate is not particularly tied to the diurnal cycle. However, in June–August (JJA) and SON, the suppressed MJO phases produce a larger ratio of stratiform to convective rain rates, with a more localized signature. The stratiform rain rate in the suppressed phase is likely associated with anvils from the isolated convection, while in the active phase, it is linked to the large-scale MJO envelope. The high convective rain rate during SON, despite lower overall totals, implies greater potential for flash flooding due to its intensity. Therefore, understanding the seasonal predominance of rain types by MJO phase can improve early warning accuracy for Jakarta. Significance Statement: There is an urgent need to effectively monitor and predict the timing and location of peak rainfall in megacities within the Indonesian Maritime Continent region to minimize the risk of severe flooding, which frequently occurs during the wet season. Our results indicate that high-intensity convective rainfall is not the sole contributor to total rainfall during this time. In fact, lighter, more frequent, and long-lasting stratiform rainfall also plays a significant role in the average rainfall total. This highlights the importance of understanding the characteristics of both types of rainfall, including their intensity and occurrence, to accurately model the tropical rainfall climate. Such understanding is crucial for improving and interpreting global circulation simulations.