Rainfall Microphysical Characteristics Observed in The Jakarta Flood Events

There has been a dramatic increase in flood events over the Greater Jakarta since 2019. However, little evidence has been found regarding the type of rainfall that is responsible the most to flood events. This study investigates physical characteristics of the rainfall, that is the rainfall drop size distribution during the three cases of severe flood events on 31 December 2019–1 January 2020 (Case 1), 15–16 July 2022 (Case 2), and 6–12 October 2022 (Case 3) over Jakarta and its surrounding regions. The characteristic of rainfall drop size is analysed and categorized based on stratiform, convective, and mixed stratiform and convective 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 a total number of raindrops (n > 100) with diameter particle up to 4 mm exists during the three flood Cases over the lowland and mountainous regions. The mountainous region has a higher number of larger diameter sizes (approximately up to 4 mm) compared to the lowland (up to 3 mm). In the wet (Case 1) and dry (Case 2) seasons, the occurrence of the flood is much related to the stratiform rainfall indicated by a larger drop number (n > 100) with a small diameter (≤ 2 mm) over the lowland, with the relative contribution around 60% in Case 1 and 46% in Case 2. Over the mountain, in Case 1, although the stratiform is still predominant, the convective rainfall with a large drop number (n > 100) with a big diameter (2–3 mm) exists. Unlike Cases 1 and 2, over the lowland during Case 3, mixed stratiform and convective rainfall have a significant proportion (50%). Meanwhile, stratiform and convective rainfall over the mountainous region has the same percentage during the flood event in Case 3. The most prominent part is that in Case 1, the stratiform rainfall occurs continuously with a long-duration from 14 LT to 13 LT on the following day, and those days are recognised as the severest flood which has happened over the Jakarta region in a decade. The anomaly of high moisture convergence and substantial negative vertical velocity over the lowland during Case 1 might generate a long-duration stratiform rainfall inducing severe floods. The flood is likely exacerbated by an overflowing river over the downstream (near Jakarta) from the upper stream (mountain) water flow originated from a combined sustained stratiform rainfall and a large diameter of convective rainfall.