The Importance of Topography to the Luzon Diurnal Cycle During a BSISO Event

One of the foci of PISTON is how local processes such as topography and the diurnal cycle in the Philippines and other parts of the Maritime Continent interact with the boreal summer intraseasonal oscillation (BSISO). We address this issue by examining changes to the diurnal cycle over Luzon, Philippines and its surrounding oceans during active vs. suppressed days of a BSISO event, including alterations to topography in a set of cloud resolving model (CRM) experiments. Simulations are done with the Regional Atmospheric Modeling System (RAMS) CRM for several days of the July-August 2007 BSISO event. The domain is 1000 km x 1000 km centered over Luzon with 2 km horizontal grid spacing. Lateral boundary nudging to reanalysis is used to realistically capture the BSISO event. The CRM experiments include a control run with realistic topography and two sensitivity runs where topography is doubled in one run and flattened in the other.

Preliminary results indicate that topography helps determine the timing and intensity of diurnal precipitation over land during suppressed days of the BSISO event. Diurnal cycle precipitation over land during suppressed BSISO days becomes progressively more intense with an earlier peak when topography increases from flat to doubled. In contrast, during the active BSISO days, topography helps dictate the location of precipitation over the ocean. As topography increases from flat to doubled, the amount of precipitation falling over the South China Sea increases, whereas the amount falling over the Philippine Sea decreases. We will discuss the implications of changes in the sea- and land-breeze circulations for producing the differences in results across the various CRM experiments.