Diurnal Cycle of Coastal Convection in the South China Sea Region and Modulation by the Boreal Summer Intraseasonal Oscillation

One intriguing feature of the Asian Summer Monsoon (ASM) is that when monsoon winds impinge upon the coastal mountains, the precipitation preferentially falls off shore, rather than directly over the mountains where orographic enhancement is the strongest. Several mechanisms have been proposed in the literature closely tied to the diurnal cycle, pinpointing diurnal cycle’s potential role in regulating the offshore rainfall maxima. Also, interactions between the large-scale circulation and the diurnal cycle are thought to influence the development and propagation of the boreal summer intraseasonal oscillation (BSISO), a dominant intraseasonal mode within the ASM. Motivated by these outstanding problems, this study has investigated the diurnal cycles of coastal precipitation and clouds in the South China Sea (SCS) region and their modulations by the BSISO. As such, over 20 years of three-dimensional radar observations from TRMM and GPM are used, as well as scatterometer-based surface winds (CCMP product). Our results show that offshore rainfall over the Philippines maximize during the active phases of BSISO, whereas offshore rain maximum over Borneo occurs during inactive BSISO periods. Offshore precipitation in the SCS region usually peaks in early morning. However, over Luzon and central Philippines, it rains off shore round the clock during active BSISO periods, possibly caused by local convergence due to substantial weakening of large-scale westerlies/southwesterlies when impinging on coasts. Analyses further show that off-shore rainfall are largely contributed by large systems with high fraction of stratiform rain, originated from off-shore propagating MCSs. On the other hand, diurnal cycle over land/island is still strong (peaking at afternoon) during active BSISO periods, although precipitation gets much extended by long- living large MCSs. It looks like surface heating and flux over land still strong enough to trigger intense afternoon convection, provided the enhanced cloud shading during active BSISO phases. During the nighttime, onshore (or downstream) propagating convection may not survive because of precipitation cold pool, mesoscale descending in the wake of afternoon intense convection, and radiative cooling over land, leading nocturnal heavy rain to mainly fall off shore. Lastly, vertical radar cross sections perpendicular to the coast show salient features of precipitation vertical structures for propagating and non-propagating convection, and their diurnal variations.