Statistical characteristics of pre-summer rainfall over South China before and after South China Sea monsoon onset from 1980 to 2017

Climatological characteristics of pre-summer (April-to-June) rainfall over South China (SC) are examined using 1980-2017 hourly rainfall observations at the national-level weather stations and the ERA-Interim reanalysis data.The short-(1-6 h), moderate-(7-12 h), and long-duration (>12 h) rainfall events are defined at individual stations. The rainfall amount, occurrence frequency, and intensity of the three rainfall types are compared between the two periods of the pre-summer rainy season, i.e., pre-and post-monsoon-onset periods, as well as among three designated subregions, namely, the west inland, east inland, and coastal regions. The major findings are as follows. (1) After the onset of South China Sea (SCS) monsoon, the average rainfall amount and hourly rainfall intensity increase over the entire SC irrespective of the rainfall-events'duration.The occurrence frequency increases for both the short-(1-6 h) and long-duration (> 12 h) rainfall types, but decreases for the moderate-duration (7-12 h) rainfall type. (2) The rainfall amount, frequency and intensity show pronounced regional variations and substantial sub-seasonal variations over SC. Owing to more favorable thermodynamic conditions after the monsoon onset, rainfall intensifies over entire SC, with larger increases of the longer-duration (> 6 h) events over the west-inland region. Moreover, the rainfall events occur more frequently over the west-inland and coastal regions, but less over the east-inland region. As a result, the total rainfall amounts increase over the west-inland and coastal regions after the onset of monsoon, while changing little over the east-inland region. (3) The rainfall events over the west-inland region mainly occur in nocturnal hours during both periods, with the longer-duration events beginning earlier and the beginning and peaking hours of each rainfall type delaying gradually from west to east. The short-duration rainfall events dominantly initiate in the afternoon (about 14:00 BT) over the east-inland, which is more prominent after the SCS monsoon onset, while the beginning and peaking hours of the longer-duration events over the east-inland lack a regional coherence. The short-duration rainfall events over the coastal region mainly initiate during 05:00-08:00 BT in the pre-monsoon-onset period. After the onset of monsoon, this feature remains within 50 km from the coastal line, but changes to around 14:00 BT farther away from the coastal line in the coastal region. The long-duration events over the coastal region tend to initiate in nighttime and peak in daytime during both periods.