Diurnal Cycle of Convection Over the Philippines as a Function of the Boreal Summer Intraseasonal Oscillation Activity

The PISTON field campaign, taking place in the South China Sea in late summer 2018, is in part motivated by a persistent off-shore rainfall maximum that appears west of coastal mountain ranges in Asia during the Asian Summer Monsoon (ASM). It is thought that the diurnal cycle of coastal precipitation plays an important role in the generation/persistence of this off-shore rainfall maximum. Several physical mechanisms have been proposed for this relation. Land/orographic precipitation can generate cold pools and gravity waves which, respectively, can converge with the southwest monsoon flow or destabilize the low-level air to generate off-shore precipitation. The goal of this study is to provide PISTON with prior knowledge of the climatology of the diurnal cycle of convection around the Philippines. Through analysis of TRMM 2A25 and 3B42 satellite radar products, characteristics of the diurnal cycle have been evaluated during both active and inactive phases of the boreal summer intraseasonal oscillation (BSISO), which is strongly coupled to the ASM. An initial analysis of TRMM precipitation features (PFs) indicate the afternoon orographic convection is more frequent and stronger during the inactive phases, when cloud shading from widespread monsoon precipitation is less significant and more daytime heating and destabilization can occur. However, rainfall data indicates that there is still a significant diurnal cycle in over-land rainfall during active BSISO phases when cloud shading is pronounced. So, while convection appears to be less intense and cloud shading is stronger during active BSISO periods, active-phase convection still may play a role in modulating the environment to produce the nocturnal off-shore precipitation maximum. PF sizes also increase from afternoon into night during both active and inactive periods, with a peak around 11 PM Local Time, indicating that individual convective elements may be growing upscale into a larger MCS-type feature and propagating off-shore.