Impacts of the Madden‐Julian Oscillation on Widespread Heavy Rainfall Over the Western Region of the Maritime Continent: Sumatra

The Precipitation Severity Index (PSI) is used to examine the occurrence of and mechanisms behind widespread persistent rainfall events in Sumatra. Analysis of cloud properties from Himawari‐8 is combined with data from IMERG and ERA5 to explore new results about changes in cloud properties relating to heavy rainfall and intraseasonal variability. This study brings a fresh angle to the challenge of understanding the Madden‐Julian Oscillation (MJO) modulation of cloud and precipitation over Sumatra by examining changes in the total cloud area and, more importantly, changes in the cold cloud tops that indicate the deepest convection. Although widespread cold clouds are likely regardless of MJO phase, the enhanced prevalence of cold tops requires the support of the large‐scale convective envelope. These results provide insight about how the MJO influences the nature as well as the amount of convection. Heavy rainfall is shown to make a larger percentage contribution to the total rainfall during the MJO‐enhanced convection phases (2‐3‐4) over Sumatra. This contribution is suggested to be related to a higher prevalence of cold cloud core areas in phases 2 and 4. In phase 3, there is a remarkable increase in the percentage contribution of heavy rainfall in the cold cloud area but a relatively lower prevalence of the coldest cloud core area, suggesting large‐scale environmental support for heavy rainfall but not necessarily the most intense convection. These results exemplify the role of mesoscale processes and cloud microphysics in modulating the MJO‐scale modulation of rainfall.