Passage of Tropical Cyclones over Mountainous Islands—Part I: Control Experiment

The passage of a tropical cyclone (TC) over a mesoscale mountainous island, such as Puerto Rico, often brings heavy rainfall which could produce flooding and landslides. Factors that affect quantities and distribution of this type of orographic rainfall in this region are not well understood. The numerical mesoscale Advanced Research Weather Research and Forecast (ARW) model was adopted to conduct a study on Hurricane Jeanne's (2004) passage over the island. One control experiment (EXP1) with WSM 5-Class microphysics parameterization (MP) scheme was performed to obtain a simulation that closely compares with observations. Three sensitivity experiments on MP schemes were performed. Each sensitivity experiment was represented by a single MP scheme: Ferrier Microphysics (EXP2), WSM 6-Class (EXP3), and the Thompson Graupel scheme (EXP4).

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 regions. Only EXP3 reproduced acceptably both rainfall distribution and high peak locations. The cyclone's wind intensity and minimum sea-level pressure at model landfall differ significantly from reality. 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, respectively. In conclusion, the WSM 6-Class scheme (EXP3) appears to be strongly comparable with observations and is more reliable for further investigations of the production of heavy orographic rainfall. Estimates of some control parameters and common ingredients, such as Convective Available Potential Energy (CAPE) and Precipitation Efficiency (E), respectively, and major sensitivity tests with mountain height variations, were performed to help understand the dynamical and physical processes in the production and modification by the orography.