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Elford G. Astling


A multi-level diagnostic model is used to study the physical and dynamical mechanisms that produce low-level clouds and light precipitation within the cold air sector on the west side of a Midwest cyclone. Computational results show frictional effects to be the primary mechanism in the absence of secondary upper level trough mechanisms or intense heating from the underlying surface. Also, this study shows that the vertical motion field may be substantially modified by secondary effects when the usually dominant contributions by vorticity and thermal effects are weak.

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The excellent North American radiosonde network is used to calculate the poleward energy transport for the continental area during the period January–March 1966. The transport of sensible and latent heat and geopotential and kinetic energy is partitioned according to four circulation modes—mean and transient meridional circulations and stationary and transient eddy circulations. In addition, the roles of various synoptic features in the transient eddy flux are examined.

The mean meridional transport was computed in two ways. One involved a calculation of the contribution of the North American sector to the hemispheric mean meridional transport. Because of strong meridional flow at high levels and a lack of compensating flow at low levels, very large transports were obtained. The transports were much greater than the average for the entire hemisphere and point up the helical structure of the meridional cells. To obtain comparisons with other modes of transport, we made another calculation of the mean meridional transport by subtracting the vertical mean component from the longitudinal average. The results show that the energy transports were large and positive in subtropical latitudes and were zero or small and negative in middle latitudes.

Of the remaining modes, the transient eddy mode was the most effective in transporting energy poleward. The maximum transport occurred at 40°N for both the hemisphere and for North America; however, the value for North America was about 50 percent larger and the latitudinal variation was considerably greater than for the hemisphere. Sensible heat transport was largest, with the maximum latent transport amounting to one-half the sensible heat. Energy fluxes by the standing eddy and transient meridional modes were relatively small.

A brief study of the importance of various large-scale synoptic features in transporting energy indicated that large-amplitude troughs with closed 500-mb Lows are most effective in the transient eddy transport. Indications exist that the largest poleward energy transport is accomplished during the intensifying stage of baroclinic disturbances associated with the 500-mb Lows.

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Daran L. Rife, Thomas T. Warner, Fei Chen, and Elford G. Astling


The purpose of this observation- and model-based study of the Great Basin Desert boundary layer is to illustrate the variety of locally forced circulations that can affect such an area during a diurnal cycle. The area of the Great Basin Desert (or Great Salt Lake Desert) that is studied is located to the southwest of Salt Lake City, Utah. It is characteristic of the arid “basin and range” province of North America in that it contains complex terrain, varied vegetation and substrates, and high water tables associated with salt-encrusted basin flats (playas). The study area is especially well instrumented with surface meteorological stations operated by the U.S. Army's West Desert Test Center and a collection of cooperating mesonets in northeastern Utah. The study period was chosen based on the availability of special radiosonde data in this area.

One of the processes that is documented here that is unique to desert environments is the salt breeze that forms around the edge of playas as a result of differential heating. The data and model solution depict the diurnal cycle of the salt breeze, wherein there is on-playa flow at night and off-playa flow during daylight. There is also a multiplicity of drainage flows that influence the study area at different times of the night, from both local and distant terrain. Finally, the lake-breeze front from the Great Salt Lake and Utah Lake progresses through the complex terrain during the day, to interact with early mountain drainage flow near sunset.

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