Diurnal Cycle and Dipolar Pattern of Precipitation over Borneo during an MJO Event: Lee Convergence and Offshore Propagation

Abstract

Surface precipitation anomalies over Maritime Continent islands typically lead oceanic precipitation by a week in the form of dipolar pattern before the arrival of Madden–Julian oscillation (MJO) convective phase. The authors study this dipolar pattern over Borneo during the boreal winter MJO event in January–February 2017 using cloud-permitting modeling, observation, and reanalysis datasets. The diurnal cycles of precipitation are analyzed during the local growing and decaying stages of this MJO event. Both the observation and simulation show positive precipitation anomaly over southwestern Borneo and negative anomaly over northeastern Borneo associated with the MJO easterly in the growing stage, whereas the pattern reverses in the decaying stage. Due to relatively high terrain, the low-level flows over Borneo split near the topography on the diurnal time scale. During the late afternoon and night (1700–2000 local solar time), the splitting-flow-induced wake vortices and thermally driven sea breezes tend to converge at the leeside, both contributing to leeward convergence and precipitation, which peaks at midnight. Subsequent offshore propagation during midnight and early morning develops from the leeward inland convection, and propagates northwestwards in the growing stage over west Borneo, and eastward in the decaying stage over east Borneo. Offshore propagation lasts until the next noon when sea breezes and island convection initiate. The timing and location of the offshore propagation suggest that it is not an independent convective mode. Instead, it is tied to the dipolar distribution of island precipitation modulated by the MJO.