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

Cloud resolving simulations are used to evaluate the importance of topography to the diurnal cycle (DC) of precipitation over Luzon, Philippines, and the nearby South China and Philippine Seas during the July-August 2016 boreal summer intraseasonal oscillation (BSISO) event. The mean DC of precipitation is compared across three simulations with varying topography - flat, true, and doubled topographic height - for suppressed and active BSISO conditions separately. Over land during both suppressed and active conditions, the peak and amplitude of the mean DC of convection increases as the topography increases. During suppressed conditions, the mean DC peak over land is delayed in the flat run relative to the true and doubled topography run DC peak due to weaker mechanical and thermodynamic forcing at midday. Additionally, the true and doubled topography runs have a delayed DC peak during active vs. suppressed conditions, while the DC peak in the flat run remains about the same during active vs. suppressed conditions. The change in DC peak timing from suppressed to active conditions seems to be caused by differences in the longevity of precipitation. Precipitation in the true and doubled topography runs maintains peak intensity longer during active vs. suppressed conditions, while peak precipitation intensity remains the same in the flat run in active vs. suppressed conditions. Elevated topography focuses precipitation over the coastal mountains during suppressed conditions, while dictating which side of the domain (i.e., east Luzon and the Philippine Sea vs. west Luzon and the South China Sea) more precipitation occurs in during active conditions. This change in precipitation location is mainly due to the blocking effect of topography.