A high-resolution modelling and observational analysis of an extreme rainfall event driven by the Northerly Cold Surge and intraseasonal tropical variability in Jakarta: January 2020

Moist convection in the Maritime Continent (MC) is typically driven by synoptic disturbances: Northerly Cold Surge (NCS), Borneo Vortex, and Madden-Julian Oscillation (MJO). One or more of these tropical disturbances can control the convective behaviour in the MC, resulting in changes in the diurnally forced convection, cloud populations and diurnal precipitation. This investigation analyses a record extreme rainfall event on Java Island around New Year's Eve 2020, the highest amount of rainfall recorded in the capital city of Indonesia, Jakarta. We use reanalysis data from ECMWF Reanalysis v5 (ERA5) to identify and analyse the southward propagation of the NCS. Satellite measurements from the Himawari-8 Advanced Himawari Imager and satellite-derived cloud physical properties reveal the cloud signatures of the NCS. High-resolution Weather Research & Forecasting Model (WRF) simulations were performed to understand the mesoscale dynamic process of the NCS's interaction with the enhanced precipitation at the diurnal scale.

Our results suggest that this extreme event resulted from the interaction of an NCS event and the diurnally forced convection. A persistent northwesterly wind near the surface over the Java Sea induced an intense low-level wind convergence from the meridional moisture transport associated with the NCS and the equatorial trough over Java. This promoted the necessary unstable conditions for organised convection during the afternoon-evening. The cloud populations and diurnal cycle of heavy rainfall in western Java were affected by the frontal region of the NCS with the offshore propagating land breeze from Java and Sumatra, as well as the intense convergence of moisture air in the internal seas of the MC. Our analysis also suggests that the presence of this strong cross-equatorial flow in the MC induced moisture transport from the southern part of Sumatra to the western region of Java. The findings outlined here could be utilised to enhance our understanding of severe weather in the MC.