Using EOF Analysis to Identify Important Surface Wind Patterns in Mountain Valleys

Abstract

Empirical orthogonal functions (EOFs) have been determined for three wind datasets from stations in valleys south of the Great Salt Lake in Utah. Two of the datasets were for summer months, with individual days selected from the MesoWest archive to represent conditions conducive to well-developed thermally driven flows. The remaining dataset was for the month of October 2000 and was derived from a combination of MesoWest data and data collected during intensive observation periods of the Vertical Transport and Mixing Experiment (VTMX) conducted in the Great Salt Lake area. This experiment investigated stable atmospheric conditions in the complex urban terrain around Salt Lake City, Utah. In all three datasets, the primary EOFs represented flows that were directed predominantly along valley axes and were caused by channeled or thermally driven flow. Diurnal variations in EOF intensity showed that thermal effects were the most common causal mechanism. The along-valley EOFs accounted for 43%–58% of the variance in the wind component datasets (8 or 10 stations each). The second EOFs accounted for 13%–18% of the variance. In the summer datasets, the second EOF appeared to represent day–night transition periods; there was evidence of both side canyon flows and day–night transitional effects in the October dataset. The EOF approach has promise for classifying wind patterns and selecting representative cases for simulation or for further detailed analysis.