Convective Variability Associated with the Boreal Summer Intraseasonal Oscillation in the South China Sea Region

Abstract

AbstractThis study investigates the convective cloud population, precipitation microphysics, and lightning activity associated with the boreal summer intraseasonal oscillation (BSISO) over the South China Sea (SCS) and surrounding landmasses. SCS rainfall shows a marked 30?60-day intraseasonal variability. This variability is less evident over land. The population of mesoscale convective systems (MCSs) and the stratiform rain fraction over the SCS, Philippines, and Indochina increase remarkably after the onset of BSISO. Convection over the SCS during inactive periods exhibits a trimodal population including shallow cumulus, congestus, and deep convection, mirroring the situation over tropical open oceans. The shallow mode is absent over land. Shallow cumulus clouds rapidly transition to congestus clouds over the SCS under active BSISO conditions. Over land, deep convection and lightning lead total rainfall and MCSs by 2?3 BSISO phases, whereas they are somewhat in phase over the SCS. Although convective instability over the SCS is larger during active periods compared to inactive periods, variability in convective intensity and precipitation microphysics is minimal, with active periods showing only higher frequency of moderate ice scattering and 30-dBZ heights extending to ?10°C. Over the Philippines and Indochina, inactive phases exhibit substantially stronger ice scattering signatures, robust mixed-phase microphysics, and higher lightning flash rates, possibly due to greater convective instability and a stronger convective diurnal cycle. Total rainfall, convective environments, and convective structures over Borneo are all out of phase with that over the Philippines and Indochina, while southern China shows little BSISO variability on convective intensity and lightning frequency.