Orographic Precipitation Over the Island of Oahu.

Abstract

To demonstrate the relationship between orographic vertical motion and precipitation, a linear diagnostic upslope model has been developed to provide a high-resolution picture of terrain induced vertical motion and precipitation over the Island of Oahu. The model is called the Hawaiian Orographic Precipitation (HOP) model. The HOP model demonstrates that vertical motions often exist near the surface that are on the order of 10 m s-1, agreeing with direct observations along the Ko‘olau Mountain ridge. The HOP model vertical motion and Orographic Rainfall Index (ORI) are correlated with precipitation during the month of February 2015, but the microphysics of the HOP model is insufficient to produce a meaningful diagnosis of the actual precipitation during that time period. The ORI and orographic vertical motion are closely related indicating that the vertical motion is more important than the moisture in determining where precipitation occurs in the area of the mountains. Three case studies demonstrate that the linear model performs well when the Froude Number is above 1, and is at its best when the precipitation is light and falls out over the mountain. The HOP model does not perform well in light wind and synoptically forced weather conditions. The high-resolution terrain gradient from the HOP model is smoothed and applied to the Weather Research and Forecasting (WRF) model in a new method for calculating the lowest level vertical motion. During the month of January 2016, the WRF model produces less precipitation than is observed at rain gauges around the Island of Oahu, but the new method produces more precipitation than the original. During a stormy period on 09 March 2012, the new method produces a different vertical motion pattern that affects the predicted weather over the island. The skill of the modified WRF model is improved during the 09 March 2012 storm by using the new method.