The diurnal cycle of convection over the northern South China Sea

Understanding the diurnal cycle of convection in coastal environments is important because so much precipitation occurs there and global models do not properly represent the diurnal cycle. In the region of the Indian monsoon, satellite data indicate southward propagation of precipitation systems from India over the Bay of Bengal, as observed during JASMINE. Precipitation systems (inferred from the cold cloud tops) propagate all the way from the India coast near 20N to the equator. Radar data from the R/V Ron Brown in the Bay of Bengal indicate that the convection associated with the diurnal signal has characteristics of squall lines.

Similar southward-propagating convective systems were observed during the 1998 South China Sea Monsoon Experiment (SCSMEX). GMS satellite, BMRC C-Pol radar, and TRMM Microwave Imager data have been used to investigate the diurnal cycle of convection over the northern South China Sea during SCSMEX. As observed over the Bay of Bengal, there is a prominent signal of convecton propagating southward from the southern coast of China during the summer monsoon season. During the SCSMEX monsoon onset period in mid-May, convection formed in the early morning along the coast and moved southward at about 15 m/s, reaching the Dongsha Island area by early afternoon. As the convection moved southward from China, the stratiform rain fraction increased, consistent with the idea that maturing mesoscale convective systems are integral features of the southward-propagating convection. Similar southward propagation was also evident when the convective envelope shifted to the centeral South China Sea in late May, suggesting that a coastline is not required for the initiation and propagation of the precipitation features. The mechanisms for this propagation may be related to gravity waves, although further study is needed.