Convective Cloud Population, Precipitation Microphysics, and Lightning Activity Associated with the Boreal Summer Intraseasonal Oscillation

Weather forecasting and climate models have difficulty in simulating the boreal summer intraseasonal oscillation (BSISO), a dominant mode of the tropical intraseaonal oscillation in boreal summer over the Indian Ocean and South China Sea (SCS). This is largely due to incomplete understanding of the underlying multiscale physical processes, which motivates the PISTON field campaign to be held in the SCS in the late summer of 2018. This study prepares for PISTON by developing a climatology on the convective cloud population, precipitation microphysics, and lightning activity associated with the BSISO cycles (both the 30-60 day and 10-20 day modes). The active BSISO1 (30-60 day mode) periods are featured by a northwest-southeast-oriented rain band between Bay of Bengal and SCS, whereas a southwest-northeastward tilted rain band between SCS and Japan marks the active BSISO2 (10-20 day oscillation) phases. In general, convective intensity (e.g., radar echo-top height) and lightning activity are in phase with rainfall over the SCS, but they are out of phase with precipitation over landmasses along the BSISO1 rain band (i.e., Indochina and Philippines). Alternatively, convective intensity and precipitation microphysics show the least variability to the south (Borneo) and north (southern China) of the BSISO1 rain band. During active BSISO1 phases (5-7), convective systems (both land and ocean) along the rain band are characterized by larger size, colder cloud tops (IR), and larger fractions of stratiform precipitation. However, convection over land (Indochina and Philippines) during active phases shows reduced radar echo-top (30 dBZ) heights, weaker microwave ice scattering signatures, and lower lightning density compared to inactive phases. Statistics of environmental conditions suggest that the peak convection over land is due to stronger surface heating during inactive phases. We suggest that larger sea surface heat fluxes during active phases enhances convective intensity over the SCS. On the other hand, convection over both land and ocean, except for southern China, show only marginal 10-20 days (BSISO2) variability. Mesoscale organization, convective intensity, and microphysical properties of precipitation systems over southern China all vary significantly between active and inactive periods of BSISO2. This is possibly because southern China is more influenced by the southwest-northeastward rain band (or the so-call Meiyu/Baiu rain band) during active BSISO2 phases.