Orographic Effects on Rainfall induced by the Passage of Tropical Cyclones over Mountainous Islands

The passage of a tropical cyclone (TC) over a mesoscale mountainous island, such as Puerto Rico, often brings heavy rainfall which produces flooding and landslides. Factors that affect the amount and distribution of this type of orographic rainfall in this region are not well understood. This study investigates the impact of microphysics (MP) schemes in a TC simulation using the Advanced Research Weather Research and Forecast (ARW) model. The numerical mesoscale ARW model was adopted to conduct a study of Hurricane Jeanne's (2004) passage over the island. Sensitivity experiments using four different MP schemes were performed. Each sensitivity experiment was represented by a single MP scheme, i.e.: WSM 5-Class (EXP1), Eta Ferrier Microphysics, WSM 6-Class, and the Thompson Graupel scheme. Results show strong consistency for the cyclonic track among all experiments with a significant landfall time difference of ~4 hours ahead of observations. Rainfall distribution was well represented, with maxima on the southeastern and higher mountain regions. EXP3 produced the best simulation in terms of both rainfall distribution and locations with high rainfall magnitude. However, there were sizeable differences between the model cyclone's wind intensity and minimum sea-level pressure at model landfall and those of the observed storm. EXP2 and EXP3 produced a more realistic sea-level pressure between 988 and 992 hPa, and sustained winds of less than 87 mph, compared with 991 mbar and 60 mph of the observed storm, respectively. In conclusion, the simulation using the WSM 6-Class scheme appears to be strongly comparable with observations, and is the best choice for further investigations of the production of heavy orographic rainfall. Future study will examine some control parameters and common ingredients, such as Convective Available Potential Energy, Precipitation Efficiency, and the impact of mountain height variations, to help understand the dynamical and physical processes associated with orographic precipitation induced by the passage of a TC.