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Examination of deep stable layers in the intermountain region of the western United States




Wolyn, Paul G., author
Mckee, Thomas B., author

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The definition of a deep stable layer sed in this report is 65% of the lowest 1.5km of the 1200 GMT sounding having a lapse rate of 2.5°Ckm-1 or less. Deep stable layers are associated with one important group of days which can potentially cause poor regional air quality in the intermountain region of the western United States. At Grand Junction, CO, Salt Lake City, UT, Winnemucca, NV, and Boise, ID they cause low daytime convective boundary layer heights and can allow for light winds near the surface even if moderate or strong synoptic scale winds aloft are present. A climatology of deep stable layer days showed that at the four intermountain region stations most of the days with deep stable layers occurred in December and January. Using a strict deep stable layer definition and episode criteria, episodes of three days or longer occurred on the average at least once every two years at Salt Lake City and Winnemucca, and at least once a year at Boise and Grand Junction. An analysis of the mixing volumes for five consecutive Decembers at the four intermountain region stations shows that all the deep stable layer days had low mixing volumes. A deep stable layer episode, which occurred from December 6 to December 23, 1980 at the four intermountain region stations, was examined in-depth to study the life cycle of a deep stable layer episode and to study the importance of different meteorological factors to the initiation, continuation, and termination of the episode. The initiation of the episode is associated with the movement of a warm ridge aloft into the region and is accompanied by a descending region of rapid warming and strong stability. Synoptic-scale warm air advection and subsidence are both important mechanisms for causing the warming aloft. Weak incoming solar radiation resulting in modest surface heating is important to prevent the destruction of the descending stable region. When the region of rapid warming descended to 0.5km-1.5km it formed a capping stable layer. In this part of the episode called the continuation phase, a disturbance was able to weaken the deep stable layer but not terminate it. The longwave radiative effects of fog may be important in this phase of the episode. The termination of the episode is associated with the destruction of the warm ridge aloft and the movement of disturbances into the region. Surface heating may be important for aiding in the termination of the episode. The presence of a thick fog layer can require a stronger disturbance to terminate the episode.


December 1986.
Also issued as Paul G. Wolyn's thesis (M.S.) -- Colorado State University, 1986.

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Air -- Pollution potential -- West (U.S.)


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