On the mechanism of the Hainan Island modulating extreme rainfall along the South China coast via thermodynamic forcing

Abstract Warm moist airflow from the northern South China Sea produces a large amount rainfall along the South China coast (SCC) in early summers. Currently, the prediction skill for coastal extreme rainfall (ER) remains very poor, which can be partly attributed to the lack of observations over sea. This study provides a new perspective that the Hainan Island modulates the warm moist airflow in the planetary boundary layer (PBL) and thus fulfills an important role in the production of coastal ER. Based on observations in 30 early summers during 1988–2017, this study defines 372 days as the coastal ER days using the 99th percentile threshold of hourly rainfall. On the ER days, the diabatic heating renders the PBL over Hainan Island much warmer than that over the SCC, and the strong southwest wind over sea induces warm advection from the island. The heat budget analysis indicates Hainan Island as the main heat source for the sea areas near SCC. Moreover, the strong zonal temperature gradient east of the island enhances the low-level atmospheric baroclinicity, which is the key reason for the strengthening of the south wind component within the PBL. A typical early-summer heavy rainfall event is studied using the Weather Research and Forecasting (WRF) model to further discuss the impact of Hainan Island on the coastal ER. In the control experiment, the WRF model reasonably reproduces the distribution and evolution of coastal ER. Two additional sensitivity experiments are conducted to reduce the PBL temperature over Hainan Island using data assimilation. The presence of a colder island yields a weaker zonal temperature gradient to the east, which reduces the south wind component in the PBL and thus the moisture convergence along SCC. The weaker warm advection from Hainan Island also results in a colder PBL over sea and a weaker land-sea thermal contrast. Notably, ER along the SCC is decreased by 38% and 80% with the PBL over Hainan Island cooled by ~ 0.3 and ~ 0.5 K, respectively. These results highlight the importance of deepening the understanding of upstream thermodynamic processes in the monsoon airflow.