Dynamical Downscaling of the Climate for the Hawaiian Islands. Part I: Present Day

Hawaii’s high and steep topography leads to pronounced small scale variations in climate and this makes comprehensive modeling of the weather and climate particularly challenging. This paper describes a regional model formulation designed for simulations of the microclimates in Hawaii, and then documents and analyzes an extended retrospective simulation for near present day conditions. Part II will apply the model to projected climate conditions near the end of the present century.\udA nested version of the advanced Weather Research and Forecasting model with fine horizontal resolution and improved physics for the Hawaiian region has been configured. A 20-year triply-nested simulation of the atmospheric flow was undertaken with a 3-km Resolution mesh covering all main Hawaiian Islands and a 1-km mesh over Maui. Ocean surface temperatures are prescribed from observations and meteorological fields at the boundaries of the outermost domain are taken from global reanalyses. The simulations are compared to surface, balloon and satellite observations over the same period. The 3-km version of the model simulates realistically the frequency of trade wind inversions, time-mean rainfall and other variables on relatively small scales over Hawaii Island. There is a reasonable agreement between observed and simulated mean rainfall patterns over the other islands as well. However, the simulated distribution of mean rainfall over Kauai and (most particularly) Maui and Oahu reveals some significant deficiencies which we attribute to inadequate resolution of the topography on these islands. The 1-km simulation over Maui shows clear improvement in the mean rainfall over the 3-km version