This work is a collaborative effort among the University of Hawaii, Yonsei University, NCEP (National Centers for Environmental Predictions) and NWSFO-HNL (National Weather Service Forecast Office-Honolulu). The hydrostatic version of the Regional Spectral Model (RSM) with a 10-km resolution was implemented in early 1997 into the operational runstream for the state of Hawaii by NCEP. From preliminary analyses and feedback from forecasters, it is apparent that the 10-km RSM forecasts show improvement over Aviation runs (AVN). Nevertheless, (1) high winds and heavy rainfall over the Hawaiian Islands are localized in nature related to orographic effects, and (2) the complex island terrain is not adequately resolved by the 10-km RSM. In addition, large variations in local climate ranging from humid tropical climate on windward slopes to hot desert over bare lava soil with different vegetation covers are typical for the Hawaiian Islands. It is thus necessary to conduct high resolution (= 3 km) numerical weather predictions for the Hawaiian Islands using the non-hydrostatic mesoscale spectral model (MSM) coupled with an advanced land surface model with improved surface boundary conditions.
The f97 version of the RSM/MSM model contains only one soil type, sandy-clay loam, and one vegetation type, broadleave tree, with a constant (70%) vegetation fraction. In this work, the RSM and MSM have been coupled with the NOAH (NCEP, Oregon State University, Air Force, and National Weather Service Office of Hydrology) land surface model (LSM) for three regions of the state of Hawaii: the Hawaii-Maui-Molokai domain at a 3-km resolution, the Oahu domain at a 1.5-km resolution, and the Kauai domain at a 1.5-km resolution. The high resolution Land Use Land Cover data for the Hawaiian Islands from USGS are used with reference to the soil survey done by the United States Department of Agriculture in cooperation with the University of Hawaii. Since the summer of 2001, we have been conducting daily high resolution (1.5 km) experimental forecasts for the Oahu domain using the coupled MSM/LSM with improved surface boundary conditions.
We have assessed the impact of improved representation of the terrain and surface boundary conditions on simulating orographic and local effects under summer trade wind conditions, localized heavy rainfall and orographically amplified high winds based on a few cases. The results suggest that (1) over the island of Oahu, the inclusion of the LSM significantly improves the performance of the MSM in simulating 2-m temperature, 2-m dew point temperature, and 10-m wind speed and wind direction when compared to observations. The daytime cold bias and over-estimation of surface wind speed experienced by the RSM and the uncoupled MSM are especially corrected by coupling the MSM with the LSM; and (2) the high-resolution (= 3 km), coupled MSM/LSM shows considerable improvement over the 10-km RSM in localized heavy rainfall and high wind forecasts. Major model bias includes over-estimation (under-estimation) of precipitation on windward (lee) side of steep terrain.