Relationship between intraseasonal oscillation and diurnal variation of summer rainfall over the South China Sea

The diurnal variations in summer rainfall over the South China Sea (110 degrees E-120 degrees E, 10 degrees N-20 degrees N) are examined for active and inactive intraseasonal oscillation (ISO) periods that are characterized by strong and weak 850-hPa zonal winds, respectively. By analyzing the rainfall retrievals from the Tropical Rainfall Measuring Mission Precipitation Radar for the period 1999-2006, it is found that the peak rainfall during the active and inactive ISO periods occurs in the late morning and late evening, respectively. The morning peak in the active ISO period arises from the organized oceanic convective systems associated with the local convergence along the west coast of the Philippines. The evening peak in the inactive ISO period originates from a stratiform morphology that is initiated by land-based convective systems owing to the increased thermodynamic instability over the Philippines during daytime.