Common Ingredients for Heavy Precipitation associated with the Passage of Hurricanes Hugo (1998) and Isabel (2003) 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, i.e. 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. In both cases, CAPEs are very small and ∂θ_e/∂z is not negative near surface, indicating a lack of the existence of conditional or potential instability. Although Hugo has 7.5%more RH, and 33%faster low level jet compared to those of Isabel, it has 42% faster propagation speed and 34% lower precipitation efficiency, the combined effect of common ingredients makes Hugo's total rainfall 25% lower than that of Isabel due to faster propagation speed of the storm and lower precipitation efficiency. It is proposed to measure the combined effect of common ingredients by consolidating the ingredients for heavy orographic rain into the Orographic Rain Index (ORI): ORI= (V_N Dh/Dx)RH(L_s/U), where〖 V〗_N is the total upstream horizontal wind normal to the mountain range, Dh/Dx the mountain steepness, RH the relative humidity, U the storm propagation speed, and Ls the horizontal scale of the tropical cyclone. 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 passing over a mesoscale mountain range.