Unraveling the physics behind severe flash floods in Indonesia’s new capital

Since the establishment of Indonesia’s new capital, Nusantara (IKN), hydroclimate extremes have emerged as a significant environmental concern. One of the most notable events was the devastating flash flood on March 15–16, 2022, which was triggered by 4–6 hours of prolonged heavy rainfall, causing severe damage and substantial economic loss.

An international research team from Indonesia, the UK, the Netherlands, and Australia has identified mesoscale convective systems (MCSs) as the primary cause of this heavy rainfall. The study is published in the journal Advances in Atmospheric Sciences.

Using high-resolution GSMaP (Global Satellite Mapping of Precipitation) data, the team found that the rainfall peaked during the MCS’s mature stage on the evening of March 15, 2022, and diminished as it entered the dissipation stage.

The study, led by Prof. Eddy Hermawan from Indonesia’s National Research and Innovation Agency also examined various environmental factors that influenced the MCS event, including the Madden-Julian Oscillation (MJO, a major atmospheric phenomenon characterized by an eastward-moving pulse of cloud and rainfall near the equator that typically recurs every 30 to 60 days), equatorial waves, and low-frequency variability.

“Our findings indicate that the MJO and equatorial waves play a crucial role in the early stages of MCS development by enhancing moisture convergence in the lower boundary layer, while local factors become more influential during the mature and later stages of the MCS evolution,” said Prof. Hermawan. “These results are supported by the backward trajectory of moisture transport analyses.”

The study revealed that the MJO and equatorial waves contributed significantly to lower-level meridional moisture flux convergence during the pre-MCS stage and initiation, with their contributions accounting for up to 80% during the growth phase. While La Niña and the Asian monsoon had negligible impacts on MCS moisture supply, a substantial contribution from the residual term of the water vapor budget during the maturation and decay phases of MCSs was observed.

“This suggests that local forcing, such as small-scale convection, local evaporation, land-surface feedback, and topography, also plays a crucial role in modulating the intensity and duration of the MCS,” said Ainur Ridho, a scientist from the University of Reading, UK.

This study enhances our understanding of the potential causes of extreme rainfall in Nusantara and could aid in improving rainstorm forecasting and risk management in the region. Looking ahead, the team plans to apply deep learning techniques to simulate and predict extreme weather events, such as heavy rainfall, associated with the development of MCSs in IKN and its surrounding areas.

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