Multi‐day valley cold‐air pools in the western United States as derived from NARR

Abstract

ABSTRACTValleys can trap cold air to form Valley Cold Pools (VCPs). Characterized by stable stratification and weak winds, VCPs, especially those that last over multiple days, can produce adverse effects such as poor visibility and severe air pollution. Using the gridded data set of the North American Regional Reanalysis (NARR) for the period of 1979 through 2012, this study examines the climatology of VCPs in the western United States with a focus on spatial and temporal variability. The results reveal a widespread occurrence of short‐lived (1–2 days) VCPs over both the mountainous areas of the West and the Northern Plains. Longer VCP episodes, however, tend to be limited to large basins/valleys in the Northwest and the Intermountain West. The leading mode of variability in the annual number of cold‐season multi‐day VCP event anomalies appears to be linked to a sea‐surface temperature anomaly pattern typically found during the warm phase of Pacific Decadal Oscillation. Corresponding to the positive phase of the leading mode of variability is an anomalous 500‐hPa ridge over the western United States induced by a Rossby wave train, which, by blocking cold‐air intrusions and producing mid‐level subsidence warming, leads to more persistent VCP episodes in the western United States. In addition to large‐scale wintertime circulation anomalies, local surface temperature anomalies also contribute to the variability.