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APRIL 2002 RIFE ET AL. 921 Mechanisms for Diurnal Boundary Layer Circulations in the Great Basin Desert DARAN L. RIFE National Center for Atmospheric Research, Boulder, Colorado* THOMAS T. W ARNER National Center for Atmospheric Research,* and Program in Atmospheric and Oceanic Sciences, University of Colorado, Boulder, Colorado FEI CHEN National Center for Atmospheric Research, Boulder, Colorado* ELFORD G. ASTLING U.S. Army West Desert Test Center, Dugway Proving Ground, Dugway, Utah (Manuscript received 6 April 2001, in ®nal form 6 September 2001) ABSTRACT 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 ¯ats (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 ¯ow at night and off-playa ¯ow during daylight. There is also a multiplicity of drainage ¯ows that in¯uence 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 ¯ow near sunset. 1. Introduction sociated with coastlines (Dalu and Pielke 1989), snow- There is plentiful evidence that regional landscape ®eld boundaries (Segal et al. 1991), surface moisture variability and the adequacy with which its effects are gradients (Yan and Anthes 1988; Chang and Wetzel represented in atmospheric models have a potentially 1991), and anthropogenic landscape changes (Segal et strong impact on simulated mesoscale weather and cli- al. 1989; Chase et al. 1999). Even though these few mate. Many sources of such local landscape forcing, examples are representative of a long history of study such as associated with terrain and coastlines, are fairly of locally forced mesoscale circulations in temperate well understood and have been thoroughly studied. environments, there has been little similar attention to Speci®cally, numerous modeling and empirical studies arid areas. This is perhaps because of the perceived document mesoscale impacts of surface variability as- remoteness, sparse population, or low economic value of such areas. However, warm-climate arid and semi- *The National Center for Atmospheric Research is sponsored by arid areas compose almost 40% of earth's land surface, the National Science Foundation. and thus the aggregate effect of small-scale, local land± atmosphere interaction over deserts should have global impacts. In addition, population increases in some arid Corresponding author address: Daran L. Rife, NCAR/RAP, P.O. Box 3000, Boulder, CO 80307-3000. areas (e.g., the southwest United States) far exceed E-mail: [email protected] those in more temperate climates, so desert weather q 2002 American Meteorological Society Unauthenticated | Downloaded 09/24/21 10:07 AM UTC 922 MONTHLY WEATHER REVIEW VOLUME 130 processes are having a direct effect on a growing num- 2. The properties of salt ¯ats, the salt breeze, and ber of people. other thermally forced circulations in desert There is a variety of arid land, land surface contrasts environments that can force mesoscale circulations. Indeed, the well- studied mountain±valley circulations and sea/lake a. Properties of salt ¯ats, or playas breezes are as common in arid environments as else- Salt ¯ats are commonly termed playas (Spanish for where, and some limited work has been done to doc- beach) in North America, so that term will be used ument their characteristics (Lieman and Alpert 1992; hereafter. Playas have properties that can be quite dis- Steedman and Ashour 1976; Warner and Sheu 2000). tinct from other desert and nondesert substrates. Most However, a type of thermally forced circulation that is of the larger playas were lakes during previous pluvial associated only with deserts is the so-called salt breeze, periods (e.g., the Pleistocene), and the smaller ones orig- which develops near the edge of salt ¯ats that typically inated as a result of regional erosion patterns. There are have higher albedo, thermal conductivity, and moisture over 50 000 playas on earth (Rosen 1994). In North content than does the surrounding desert. These con- America there are approximately 300 playas with areas trasts cause a thermally direct circulation, with the low- of more than 5 km2, virtually all being in the desert level ¯ow away from the salt ¯at during the day and environments of the West (Neal 1965), with thousands toward it at night. Salt ¯ats of various properties are of smaller playas existing in the semiarid high plains common in deserts worldwide, so salt breezes should grasslands (Steiert 1995). be ubiquitous, as should be their effect on day and night Playas have a number of properties that cause their boundary layer structure. surface energy budget to differ from that of their sur- In this study, special surface and upper-air data, in roundings. combination with a numerical model, were used to help de®ne the structure of a salt breeze, as well as lake and Thermal conductivity: The often-moist playa sub- mountain±valley circulations, in the Great Basin Desert strate, and its compactness relative to sand, often (or Great Salt Lake Desert) to the southwest of Salt cause its thermal conductivity to be greater than Lake City, Utah. This desert area includes much com- that of the surrounding nonplaya sandy sub- plex terrain and large salt ¯ats, and is potentially in¯u- strates. Albedo: The albedo of playas with salt crusts can enced by the thermally driven circulations associated contrast greatly with the surroundings, and there with the Great Salt Lake and Utah Lake to the northeast can be a large seasonal and diurnal variation in the and east, respectively. An analysis of observations for contrast. In the summer, the salt crust can have a the study period is ®rst performed, and this is then com- relatively high albedo. However, in the winter, or pared to a surface wind and temperature climatology after heavy rain in the summer, a thin salt crust can for an area near the edge of a salt ¯at. Numerical model dissolve and the albedo decreases signi®cantly. simulations are then described, to help de®ne the three- McCurdy (1989) reports that when a Great Salt dimensional boundary layer structure associated with Lake Desert playa salt crust was moistened from the salt breeze as well as mountain±valley and lake rain its albedo was 0.24, but when it was com- breezes. To an observer on the ground in the center of pletely dry it had a late afternoon albedo that ex- this desert study area, the landscape looks ¯at, barren, ceeded 0.75. Even signi®cant diurnal variations in and generally unremarkable, except for elevated terrain albedo result from the fact that, when the water in the distance. Nevertheless, it will be shown that the table is high, the crust can rehydrate at night, and local boundary layer wind ®eld exhibits a complex spa- then become progressively drier and more re¯ec- tial and diurnal structure that results from both local and tive during the day. Finally, wind speed and di- distant forcing mechanisms. rection can in¯uence the inner-playa distribution of Because salt breezes have been relatively less well albedo. When, as is sometimes the case, 5±10 cm studied and documented than lake or mountain±valley of water cover the playa, wind can shift the location breezes, section 2 will summarize the characteristics of of the shallow pond of water from one location to salt ¯ats and the associated salt breezes, as well as other another, and thus the albedo distribution shifts as thermally forced circulations that are common in des- the changes in wind speed and direction dry and erts. Section 3 will describe the modeling system em- moisten different areas of the playa. ployed, and provide a summary of the model experi- Vegetation: The playa often contrasts with its sur- ments. In section 4, the meteorological conditions dur- roundings in terms of the vegetation cover. Because ing the study period will be presented, as will a simple of the salinity, vegetation is generally very sparse climatology of the diurnal variability of the local surface over the playa, but not necessarily so over the sur- wind and temperature ®elds near a salt ¯at. Section 5 rounding desert. This difference produces gradients presents the results of the model simulations, and this in the transpiration and albedo. is followed in section 6 by a summary and discussion Latent heat ¯ux: Because playas are often characterized of the results. by a high water table, the surface may be moist, but Unauthenticated | Downloaded 09/24/21 10:07 AM UTC APRIL 2002 RIFE ET AL. 923 the surface of the surrounding sandy or rocky desert that the salt-breeze circulation could be attributed main- is generally quite dry, except after a rain.

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