Orographic Precipitation Index (OPI) for Heavy Precipitation Associated with the Passage of Hurricanes over the Southern-Central Appalachian Mountains

Hurricanes Hugo (1989) and Isabel (2003), which have similar impinging angle and different total rainfall amounts, are chosen for studying the common ingredients for heavy orographic rainfall over the Appalachian Mountains. These two storms are simulated by the Weather Research and Forecast (WRF) model for thorough comparison of each ingredient. Seven common ingredients: the high precipitation efficiency of the incoming airstream, the low-level jet, the steep mountain orography, the orographically induced vertical motion, the high moisture upstream, the large convective system, and the slow movement of the convective system are found playing major roles in enhancing the orographic rainfall. Based on the common ingredient, an Orographic Rain Index (ORI) for heavy orographic rainfall is then proposed: ORI= (Vn h/L)RH(Ls/U), where Vn is the total upstream horizontal wind normal to the mountain range, h the mountain height, L the mountain width, h/L the mountain steepness, RH the relative humidity, U the storm propagation speed, and Ls the horizontal scale of the tropical cyclone or precipitating system. The ORI is then tested for six local regions of each storm, based on local rainfall maximum received during the passages of Hugo and Isabel. The 1-km resolution simulated ORI value for each local region correlates well with the total rainfall for both storms. Thus, ORI may serve as a predictor for the occurrence of heavy orographic rainfall associated with tropical cyclones or precipitating system passing over a mesoscale mountain range.