Convective Characteristics of the Boreal Summer Intraseasonal Oscillation over and around the South China Sea

One of the dominant modes of the tropical intraseaonal oscillation in boreal summer over the Indian Ocean and South China is the boreal summer intraseasonal oscillation (BSISO). Weather forecasting and climate models have difficulty in simulating the BSISO due to incomplete understanding of the underlying multiscale physical processes. This study investigates the regional, intraseasonal, and diurnal variability of BSISO-associated convection over the SCS and surrounding landmasses. Large-scale rainfall, environmental variables, TRMM precipitation features, and lightning data during boreal summer (June-Sept) have been stratified by the BSISO index (the 30-60 days mode). The SCS is characterized by suppressed precipitation and weak southerlies during inactive BSISO phases (phases 1-3), while a substantial northwest-southeast oriented rainband and strong low-level westerlies dominate active BSISO phases (phases 5-7). In general, convective intensity (e.g., radar echo-top height) and lightning activity are in phase with rainfall over the SCS. However, convective intensity and lightning are out of phase with rainfall over landmasses along the BSISO rainband (e.g., Indochina and Philippines). During active BSISO phases, convective systems over both land and ocean are characterized by larger size, colder cloud tops (IR), and greater fraction of stratiform precipitation. Convection over the SCS during active BSISO phases has taller precipitation echoes (20-dBZ echo top heights), higher lightning density, stronger microwave ice scattering signatures, and more robust mixed-phase microphysics (larger 30/40 dBZ echo volume above the freezing level). These same parameters maximize over land masses of Indochina and the Philippines during BSISO inactive periods. Statistics of environmental conditions suggest that the peak convection over land is due to stronger surface heating (thus higher CAPE) during inactive phases, whereas larger sea surface heat fluxes (leading to higher CAPE) during active phases enhances convective intensity over the SCS. On the other hand, mesoscale organization, convective intensity, and microphysical properties of precipitation systems to the south and north of the BSISO key rainband region have only negligible intraseasonal variability. Land convection shows a strong diurnal cycle (maximizing at afternoon and early evening) across all BSISO phases, even though cloud shading is strong during active phases. Offshore convection peaks at midnight and early morning times during inactive BSISO phases. It is interesting that nocturnal offshore convection is not obvious in active BSISO periods, opposing to the hypothesis that offshore convection could be enhanced by the convergence between the nighttime offshore flows and the strong southwest-west winds of active BSISO.