Deep Stable Layers in the Intermountain Western United States

Abstract

A deep stable layer (DSL) is a layer much deeper than a typical nocturnal inversion with stabilities not frequently found over a sizable portion of the lowest 1.5 km. They have traits that can cause the stagnation of cold air in basins, i.e., light winds at the surface even if moderately strong winds aloft are present, and the restriction of the growth of daytime convective boundary layers. The objective definition used in this study is that, if 65% of the lowest 1.5 km of the 1200 UTC [0500 mountain standard time (MST)] sounding has a lapse rate of 2.5°C km?1 or less, then the day is under the influence of a DSL. A climatology of days under the influence of a DSL was performed at four sites in the intermountain western United States: Grand Junction, Colorado; Salt Lake City, Utah; Winnemucca, Nevada; and Boise, Idaho. The DSL is a wintertime phenomenon with 10% to 20% of the days in December and January at the four stations being under the influence of a DSL. Successive days with a DSL present lead to episodes of varying lengths. Episodes of three days or longer occurred at least once each year at Boise and Grand Junction and at least once every two years at Salt Lake City and Winnemucca. An episode of 8 days duration occurred at Grand Junction. A DSL episode that occurred in December 1980 was examined in depth to gain insight into the life cycle of a DSL. Synoptic-scale warming above 1 to 1.5 km and weak surface heating were important for the initiation of the episode. The longwave radiation properties of a persistent fog layer and weak surface heating were important physical processes for maintaining and prolonging the episode. From the episode it is hypothesized that DSLs form when warming aloft traps relatively cold air near the surface, decoupling it from the rest of the atmosphere. The barriers surrounding the basins are important for the formation of a DSL because they prevent the horizontal movement of the cold air. DSLs have important implications for air quality, episodes of persistent fog, and surface temperature forecasts.