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Orographic Precipitation Over the Island of Oahu.

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Title:Orographic Precipitation Over the Island of Oahu.
Authors:Robinson, Thomas E.
Contributors:Meteorology (department)
Keywords:Orographic Precipitation
Hawaiian Weather
Date Issued:Aug 2018
Publisher:University of Hawaiʻi at Mānoa
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.
Description:Ph.D. Thesis. University of Hawaiʻi at Mānoa 2018.
Rights:All UHM dissertations and theses are protected by copyright. They may be viewed from this source for any purpose, but reproduction or distribution in any format is prohibited without written permission from the copyright owner.
Appears in Collections: Ph.D. - Meteorology

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