Diurnal Variations of Pre-Summer Rainfall over Southern China

In this study, the diurnal cycle of pre-summer rainfall over southern China is examined using the merged 0.1°-resolution gridded, hourly rain gauge and satellite rainfall dataset and the National Centers for Environmental Prediction Final Global Analysis during April to June of 2008-2012. Results show pronounced diurnal variations in rainfall amount, frequency and intensity over southern China, with substantially different amplitudes from southwestern to southeastern China, and from the pre- to post-summer monsoon period. Southwestern China often encounters significant nocturnal-to-morning rainfall, most of which is generated by travelling mesoscale convective systems (MCSs) from upstream, under the influence of enhanced nocturnal southwesterly low-level jets. Southeastern China is dominated by afternoon rainfall, as a result of surface heating, likely aided by local topographical lifting. Both the pre- and post-summer monsoon periods exhibit two diurnal rainfall peaks: one in the early morning and the other in the late afternoon. But the latter shows the two peaks with nearly equal amplitude whereas the former displays a much larger early morning peak than that in the late afternoon. Three propagating modes of MCSs accounting for the pre-summer rainfall are found: (i) an eastward or southeastward propagating mode occurs mostly over southwestern China that is associated with midlatitude westerly traveling disturbances having notable baroclinicity; (ii) a quasi-stationary mode over southeastern China appears locally in the warm sector with weak gradient flows; and (iii) an inland-propagating mode occurs during the daytime in association with sea breezes along the southern coastal regions during the post-summer monsoon period.