Diurnal Variation of Summer Monsoon Season Precipitation Over Southern Hainan Island, China: The Role of Boundary Layer Inertial Oscillations Over Indochina Peninsula

Abstract

AbstractIn June after the onset of the East Asian summer monsoon season, rainfall on the south side of Hainan Island (HNI) of China exhibits a trimodal pattern with an unusual peak near noon. A typical case of 20 June 2017 is chosen to analyze the mechanisms through a convection‐permitting simulation together with rainfall observations. Clockwise rotation in low‐level winds is found at and off the coast of south HNI, leading to maximum onshore southwesterly winds in the late morning that is closely linked to the diurnal variation of rainfall. The offshore diurnal variations of low‐level winds are tracked upstream and attributed to boundary layer inertial oscillations over the inland plain of Indochina Peninsula. In addition, a short wide gap comprised mostly of hills lower than 700 m is located across the higher Annamite Range along the east coast of the Peninsula, right upstream of HNI. Nocturnal boundary layer low‐level jet (LLJ) forming over the plain passes through the gap with significant acceleration due to gap channeling and downslope flow effects, reaching peak intensity off the Peninsula coast at around 09 local standard time (LST). The enhanced LLJ propagates downstream towards HNI and impinges on the mountains at around 12 LST when rainfall reaches peak there. As the inertial‐oscillation‐indued perturbation winds approaching the Island weaken and eventually reverse direction, lifting weakens and low‐level stability increases as descending motion develops offshore. Rainfall on the southern Island therefore dissipates in late afternoon.