18th Conference on Weather and Forecasting, 14th Conference on Numerical Weather Prediction, and Ninth Conference on Mesoscale Processes

Wednesday, 1 August 2001: 8:30 AM
Does Increasing Horizontal Resolution Produce Better Forecasts? The Results of Two Years of Real-Time Numerical Weather Prediction in the Pacific Northwest
Cliff F. Mass, University of Washington, Seattle, WA; and D. Ovens, K. Westrick, and B. A. Colle
This paper examines the effects of increasing horizontal resolution on the performance of mesoscale numerical weather prediction models. A review of previous subjective studies suggests that decreasing grid spacing to 10 km or less generally produces more realistic mesoscale structures, with particular benefits for orographic and diurnally driven flows. There are only a few long-term objective verification studies in the literature and these indicate diminishing returns as horizontal grid spacing decreases below approximately 10-km. A multi-year verification of the University of Washington MM5 real-time forecasting system is presented that objectively compares the realism of predicted surface parameters at 36, 12, and 4-km grid spacing over western Washington State. Verification statistics are calculated by interpolating model forecasts to the observation sites. For precipitation it is shown that model skill over western Washington improves as grid spacing decreases from 36 to 12 km. Verification scores generally degrade as resolution is increased from 12 to 4 km as overprediction develops over the windward slopes and crests of terrain. However, for heavy precipitation amounts on windward slopes the transition from 12 to 4-km does appear to enhance forecast accuracy. Temperature and wind statistics indicate substantially improved skill as horizontal resolution decreases from 36 to 12-km, while only minor increases in skill are evident as grid spacing decreases to 4 km. In contrast, verification of sea-level pressure suggests little improvement as resolution is increased. The benefits of resolution are not uniform over the 4-km domain. For all parameters, the region downwind of the Olympic barrier--the southeastern corner of Vancouver Island and the eastern Strait of Juan de Fuca-- enjoys the largest enhancement of forecast skill as resolution is increased from 36 to 12 km, with noticeable, but lesser, improvements as grid spacing decreases to 4 km. Filtering the verification times so as to consider only periods with relatively good synoptic scale forecasts enhances forecast skill almost equally across the three resolutions. A case study illustrating the advantages and problems associated with high resolution is then presented. Finally, the paper examines the benefits of resolution, some pitfalls associated with traditional verification approaches, and suggests future directions for numerical weather prediction.

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