Understanding the role of topography on the diurnal cycle of precipitation in the Maritime Continent during MJO propagation

The role of topography in the diurnal cycle of precipitation is analyzed during the propagation of a Madden–Julian Oscillation (MJO) event over the Islands of the Maritime Continent using cloud-permitting simulations. The control simulation (CTL) using realistic topography captures the timing, magnitude, and location of the observed diurnal cycle of precipitation. The idealized simulation (FLAT) without topography delays the arrival of peak precipitation by about an hour compared to CTL. The magnitude of area-averaged precipitation remains unchanged in both simulations over areas with altitude < 500 m. The largest difference is found over areas with relatively high topography (> 1000 m), where diurnal rainfall was significantly reduced in FLAT. The comparison between both simulations from a moisture budget analysis shows that 62% of the reduction in precipitation in FLAT is associated with a reduction in vertical advection (VADV), and 31% with a reduction in horizontal advection (HADV) of moisture. Furthermore, the reduction in VADV was equally contributed by the planetary boundary layer (PBL) and the free troposphere, whereas the reduction in HADV was mostly (~ 80%) confined within the PBL. The moisture budget analysis of the MJO event shows that the changes in background winds significantly impact precipitation over lower topography areas. These results also indicate that the changes in moisture advection above PBL are important to better understand and monitor the moisture budget in the troposphere over land in the MC.