Unique climate features of the rainfall center along the northern coast of the Beibu Gulf

Based on the station rain gauge and ERA5 reanalysis data, the seasonal and diurnal variations of rainfall over South China have been analyzed and unique climate features of the rainfall center along the northern coast of the Beibu Gulf have been revealed. The average accumulated rainfall amount in Dongxing along the northern coast of the Beibu Gulf exceeds 2300 mm/a from 1981–2020, which is the largest annual rainfall amount in mainland China (under the density of the national-level station network) and the area with the highest summer water vapor content over China. Moreover, unlike the dominant rainfall amount during the pre-rainy season (autumn) in the eastern part of South China (over Hainan Island), rainfall along the northern coast of the Beibu Gulf reaches its highest level in June–August. The high morning peak at 8:00 Beijing time (BJT) is out phase with the significant unimodal afternoon peak over central Hainan Island and bimodal peaks during the early morning and afternoon in the eastern part of South China. In summer, the strong low-level southerly winds over the Beibu Gulf and local topography are crucial to the formation of this rainfall center. The strong southerly winds, blowing towards the coastlines, induce intense convergence at 900-1000 hPa in the coastal areas of South China. Under the influence of the Shiwa Mountain Range near the northern coast of the Beibu Gulf, the greatest divergence occurs here in the lower troposphere (700-900 hPa) at the same latitude in South China. The interaction between the divergence and convergence in the lower troposphere, combined with the abundant moisture, greatly contributes to the formation of the rainfall center. The ascending motion and water vapor transportation are even enhanced in the morning, favoring the diurnal rainfall peak.