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Christian Rocken, Teresa Van Hove, James Johnson, Fred Solheim, Randolph Ware, Mike Bevis, Steve Chiswell, and Steve Businger

Abstract

Atmospheric water vapor was measured with six Global Positioning System (GPS) receivers for 1 month at sites in Colorado, Kansas, and Oklahoma. During the time of the experiment from 7 May to 2 June 1993, the area experienced severe weather. The experiment, called “GPS/STORM,” used GPS signals to sense water vapor and tested the accuracy of the method for meteorological applications. Zenith wet delay and precipitable water (PW) were estimated, relative to Platteville, Colorado, every 30 min at five sites. At three of these five sites the authors compared GPS estimates of PW to water vapor radiometer (WVR) measurements. GPS and WVR estimates agree to 1–2 mm rms. For GPS/STORM site spacing of 500–900 km, high-accuracy GPS satellite orbits are required to estimate 1–2-mm-level PW. Broadcast orbits do not have sufficient accuracy. It is possible, however, to estimate orbit improvements simultaneously with PW. Therefore, it is feasible that future meteorological GPS networks provide near-real-time high-resolution PW for weather forecasting.

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Jian-Jian Wang, Hann-Ming Henry Juang, Kevin Kodama, Steve Businger, Yi-Leng Chen, and James Partain

Abstract

The operational implementation of the National Centers for Environmental Prediction (NCEP) Regional Spectral Model (RSM) in Hawaii is the first application of a mesoscale model to improve weather forecasts in the Pacific region. The primary model guidance for the National Weather Service Pacific region has been provided by the NCEP Aviation (AVN) run of the Global Spectral Model (GSM). In this paper, three recent synoptic-scale disturbances that affected the Hawaiian Islands are selected to demonstrate the potential utility of model guidance produced by the RSM and contrast it qualitatively with that from the AVN. NCEP RSM simulations, with enhanced grid resolution, can resolve convective rainbands and the interaction between the environmental airflow and the complex island topography, features the GSM cannot capture.

RSM model performance in reproducing mesoscale structures associated with the synoptic-scale systems is encouraging. For the first simulation, a kona low case on 3 November 1995, the RSM predicted a northeast–southwest-oriented rainband that closely matched a convective cloud band in the satellite imagery and maximum rainfall over Kauai. The second RSM simulation, a cyclogenesis event on 3 March 1996, shows remarkable agreement with observations. Important features such as the heavy rains and high winds over portions of Maui and Hawaii are accurately forecast. The third RSM simulation, a heavy rain event on 13 November 1996, is associated with convergence along a trailing cold-frontal trough. In this case the RSM correctly forecast the timing and distribution of heavy rainfall on the island of Oahu. Subjective comparisons between RSM output and observations demonstrate the potential utility of the model guidance for local weather forecasts in Hawaii.

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Clifford F. Mass, Steve Businger, Mark D. Albright, and Zena A. Tucker

Abstract

This paper describes a localized windstorm that struck some areas of northwest Washington State on 28 December 1990 with winds exceeding 45 m s−1, resulting in extensive property damage, treefalls, and power outages. Arctic air, originating within the interior of British Columbia, descended into a mesoscale gap in the Coast/Cascade Mountains and then accelerated ageostrophically to the west. This gap acceleration is explained quantitatively by a three-way balance among the pressure gradient force, friction, and inertia. The flow maintained its integrity as a narrow current of high-speed air as it exited the gap and subsequently accelerated over water. Troughing in the lee of the Cascade Mountains enhanced the horizontal pressure gradient over northwest Washington; this pressure gradient approximately balanced frictional drag resulting in only minimal acceleration. Farther south the flow decelerated as the current spread out horizontally.

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