A 3-DIMENSIONAL SIMULATION OF AIR-FLOW AND OROGRAPHIC RAIN OVER THE ISLAND OF HAWAII

A three-dimensional mesoscale numerical model is presented, designed with the capability of simulating the airflow and orographically-induced rain in the presence of steep irregular terrain. The model utilizes a fourth-order accurate version of Arakawa's potential enstrophy and total energy conserving scheme to improve the simulation of nonlinear aspects of the airflow over steep topography, along with an adiabatic reference atmosphere to reduce the effects of orographic truncation errors. The precipitation processes are represented by large-scale condensation. The model is applied to the island of Hawaii. The results of a 24-h simulation indicate the model is generally successful in reproducing leeward eddies depicted in the composite surface airflow patterns. A nondimensional analysis of vortex parameters suggests that the simulated vortex pattern is an atmospheric analog of the Karman vortex street and compares favorably with an observed case of Hawaii vortices. Spatial distribution of simulated rainfall is also generally in good agreement with observations. However, the lack of precipitation in the lowland along the windward as well as leeward coast is apparent. The excess rainfall over the southern peninsula suggests the need for an finer grid mesh to improve representation of the effects of subgrid-scale mountain forcing. An application of the model to regional climate modeling through the use of large-scale objective analyses is also discussed.