Intraseasonal Variability in the diurnal cycle of precipitation in the Philippines

Precipitation in the region surrounding the South China Sea (SCS) over land and coastal waters exhibits a strong diurnal cycle associated with a land-sea temperature contrast that drives a sea-breeze circulation. The boreal summer intraseasonal oscillation (BSISO) is an important modulator of the daily mean precipitation rate and the amplitude of the diurnal cycle, an understanding of which is a primary goal of the PISTON field program. Using 19 years of the CMORPH precipitation product for Luzon Island in the northern Philippines, it is shown that in aggregate the daily mean precipitation and diurnal cycle amplitude are maximized over Luzon Island before the arrival of the broader oceanic convective envelope associated with the BSISO. A strong oceanic diurnal cycle near the coast due to offshore propagation of convection is evident in the transition from the inactive to active phase.

This behavior is dramatically different on small spatial scales within the Philippine archipelago, depending strongly on topography. For example, the BSISO influence on the diurnal cycle on the eastern side of the high mountains of Luzon is nearly opposite to the western side. It is proposed, using wind, moisture, and radiation budget products from the ERA-Interim reanalysis, that the enhanced diurnal cycle west of the mountains during BSISO suppressed phases is a consequence of increased insolation combining with weaker prevailing onshore winds that promotes a stronger sea-breeze circulation. Offshore propagation is suppressed until ambient mid-level moisture increases over the surrounding oceans during the transition to the active BSISO phase. In the BSISO enhanced phases, strong low level winds out of the southwest combine with increased cloudiness to suppress the sea-breeze circulation in the SCS region. Results are consistent when examined in other precipitation products, and support conclusions derived from studies focusing on different regions of the Maritime Continent, with some important local distinctions owed to geography.