Rainfall microphysical characteristics observed in the Jakarta flood events

There has been a dramatic increase in flood events over Greater Jakarta since 2019. However, little study has been found regarding physical characteristics of rainfall during flood events. This study investigates the rainfall drop size distribution during the three cases of severe flood events in the Jakarta area on 31 December 2019-1 January 2020 (Case 1), 15-16 July 2022 (Case 2), and 6-12 October 2022 (Case 3). The characteristic of rainfall drop size is analysed and categorised based on stratiform, convective, and mixed convective-stratiform rainfall. This research used the Automatic Weather Station, Laser Precipitation Monitor, and ERA5 reanalysis datasets from December 2019 to October 2022. Overall, results show that raindrops with particle diameters up to 4 mm exist during the three flood cases over the lowland and mountainous regions. The mountainous region has a higher number of larger diameter sizes compared to the lowland. In Cases 1 and 2, the occurrence of stratiform rainfall dominated the lowland areas, while the convective rainfall was also present in the mountain during Case 1. In contrast to Cases 1 and 2, over the lowland during Case 3, mixed stratiform and convective rainfall have a significant proportion. Meanwhile, the stratiform and convective rainfall over the mountainous region occurred equally. The most prominent feature in Case 1, the stratiform rainfall persisted from 14 to 13 LT on the following day, coinciding with the severe flood over the Jakarta region. The anomaly of high moisture convergence and substantial negative vertical velocity over the lowland during Case 1 might generate a long duration stratiform rainfall that likely contributed significantly to floods.