Orographic Effect on the Precipitation with Typhoon Washi in the Mindanao Island of the Philippines

Abstract

We examined the orographic effect of Mindanao Island in the Philippines on the distribution of precipitation from Typhoon Washi. The National Centers for Environmental Protection (NCEP) and US Department of Energy (DOE) Reanalysis data were downscaled using a regional spectral model to reproduce the typhoon in detail. A heat budget analysis revealed the reason for the absence of precipitation over the mountain in the center of Mindanao Island. Sensitivity experiments with various artificial terrains were conducted to estimate the orographic effect on the precipitation distribution. The peak rainfall, which occurred on the west side of Mt. Ragang, was caused by the interaction between the typhoon and both Mt. Ragang and Mt. Malindang. A rich source of heat and moisture is indispensable for typhoon rainfall. However, without a mountain, a typhoon would not cause heavy precipitation even after trespassing. Divergence of the typhoon wind due to one or more mountains is shown to be the key factor behind the heavy rainfall over land from Typhoon Washi in 2011.