1654 JOURNAL OF APPLIED METEOROLOGY AND CLIMATOLOGY VOLUME 51 Episodic Dust Events of Utah’s Wasatch Front and Adjoining Region W. JAMES STEENBURGH AND JEFFREY D. MASSEY Department of Atmospheric Sciences, University of Utah, Salt Lake City, Utah THOMAS H. PAINTER Jet Propulsion Laboratory, Pasadena, California (Manuscript received 4 January 2012, in final form 4 April 2012) ABSTRACT Episodic dust events cause hazardous air quality along Utah’s Wasatch Front and dust loading of the snowpack in the adjacent Wasatch Mountains. This paper presents a climatology of episodic dust events of the Wasatch Front and adjoining region that is based on surface weather observations from the Salt Lake City International Airport (KSLC), Geostationary Operational Environmental Satellite (GOES) imag- ery, and additional meteorological datasets. Dust events at KSLC—defined as any day [mountain standard time (MST)] with at least one report of a dust storm, blowing dust, and/or dust in suspension with a visi- bility of 10 km or less—average 4.3 per water year (WY: October–September), with considerable in- terannual variability and a general decline in frequency during the 1930–2010 observational record. The distributions of monthly dust-event frequency and total dust flux are bimodal, with primary and secondary maxima in April and September, respectively. Dust reports are most common in the late afternoon and evening. An analysis of the 33 most recent (2001–10 WY) events at KSLC indicates that 11 were associated with airmass convection, 16 were associated with a cold front or baroclinic trough entering Utah from the west or northwest, 4 were associated with a stationaryorslowlymovingfrontorbaroclinictroughwestof Utah, and 2 were associated with other synoptic patterns. GOES imagery from these 33 events, as well as 61 additional events from the surrounding region, illustrates that emission sources are located primarily in low-elevation Late Pleistocene–Holocene alluvial environments in southern and western Utah and southern and western Nevada. 1. Introduction the snowpack’s absorption of solar radiation, decreasing seasonal snow-cover duration by several weeks (Painter Dust storms have an impact on air quality (Pope et al. et al. 2007). Modeling studies further suggest that ra- 1995; Gebhart et al. 2001), precipitation (Goudie and diative forcing from increased dust deposition during Middleton 2001), soil erosion (Gillette 1988; Zobeck the past 150 years results in an earlier runoff with re- et al. 1989), the global radiation budget (Ramanathan duced annual volume in the upper Colorado River et al. 2001), and regional climate (Nicholson 2000; Basin (Painter et al. 2010). Goudie and Middleton 2001). Recent research exam- Synoptic and mesoscale weather systems are the pri- ining dust-related radiative forcing of the mountain mary drivers of global dust emissions and transport. snowpack of western North America and other regions Mesoscale convective systems that propagate eastward of the world has initiated a newfound interest in dust from Africa over the Atlantic Ocean produce one-half research (Painter et al. 2007; Flanner et al. 2009; Painter of the dust emissions from the Sahara Desert, the et al. 2010). For example, observations from Colorado’s world’s largest aeolian dust source (Swap et al. 1996; San Juan Mountains indicate that dust loading increases Goudie and Middleton 2001). Dust plumes generated by these systems travel for several days in the large- scale easterly flow (Carlson 1979), with human health Corresponding author address: Dr. W. James Steenburgh, Dept. of Atmospheric Sciences, University of Utah, 135 South 1460 East, and ecological impacts across the tropical Atlantic and Rm. 819, Salt Lake City, UT 84112. Caribbean Sea (Goudie and Middleton 2001; Prospero E-mail: [email protected] and Lamb 2003). In northeastern Asia, strong winds in DOI: 10.1175/JAMC-D-12-07.1 Ó 2012 American Meteorological Society SEPTEMBER 2012 S T E E N B U R G H E T A L . 1655 FIG. 1. Google Earth image of the Intermountain West with geographic features that are discussed in the text annotated. The inset box in the left panel shows the location of the right panel, which encompasses the Sevier Desert, Sevier Dry Lake Bed, Escalante Desert, and Milford Valley region. [Ó2011 Google; imagery Ó2011 TerraMetrics.] the post-cold-frontal environment of Mongolian cyclones transport over western North America vary geo- drive much of the dust emissions (Yasunori and Masao graphically and seasonally. Orgill and Sehmel (1976) 2002; Shao and Wang 2003; Qian et al. 2002). The identified a spring maximum in suspended dust fre- highest frequency of Asian dust storms occurs over the quency over the contiguous United States as a whole, Taklimakan and Gobi Deserts of northern China, where which they attributed to cyclonic and convective storm 2 dust is observed 200 days yr 1 (Qian et al. 2002). Fine activity, but found that some locations in the Pacific dust from these regions can be transported to the United Coast and Rocky Mountain regions have an autumn States, producing aerosol concentrations that are above maximum. Brazel and Nickling (1986, 1987) found that National Ambient Air Quality Standards (Jaffe et al. fronts, thunderstorms, cutoff lows, and tropical distur- 1999; Husar et al. 2001; VanCuren and Cahill 2002; bances (i.e., decaying tropical depressions and cyclones Fairlie et al. 2007). originating over the eastern Pacific Ocean) are the pri- In North America, the Great Basin, Colorado Pla- mary drivers of dust emissions in Arizona. The fre- teau, and Mojave and Sonoran Deserts produce most of quency of dust emissions from fronts is highest from late the dust emissions (Reynolds et al. 2001; Tanaka and autumn to spring, that from thunderstorms is highest Chiba 2006; see Fig. 1 for geographic and topographic during the summer, and that from cutoff lows is highest locations). Desert land surfaces are naturally resistant to from May to June and from September to November. wind erosion because of the presence of physical, bio- Dust emissions produced by tropical disturbances are logical, and other crusts (Gillette et al. 1980) but are infrequent but are likely confined to June–October easily disturbed, in some cases leading to increased dust during which tropical cyclone remnants move across the emissions long after the initial disturbance (e.g., Belnap southwestern United States (Ritchie et al. 2011). For et al. 2009). From alpine lake sediments collected over dust events in nearby California and southern Nevada, the interior western United States, Neff et al. (2008) and Changery (1983) and Brazel and Nickling (1987) es- Reynolds et al. (2010) find dramatically larger dust de- tablished linkages with frontal passages and cyclone position rates since the mid–nineteenth century, a likely activity, respectively, with land surface conditions (e.g., consequence of land surface disturbance by livestock soil moisture and vegetation) affecting dust-event grazing, plowing of agricultural soils, and other human seasonality and spatial distribution. In northwestern activities. Nevada, dust storms originating over the Black Rock Several studies suggest that the synoptic and meso- Desert have been linked to strong winds associated with scale weather systems that generate dust emissions and cold-frontal passages and geostrophic adjustment, with 1656 JOURNAL OF APPLIED METEOROLOGY AND CLIMATOLOGY VOLUME 51 FIG. 2. A snowpit from Alta, Utah, on 30 Apr 2009 that exhibits dust layers from episodic dust events. emissions being strongly dependent on antecedent enables a $1.2 billion winter sports industry, known in- rainfall and soil conditions (Lewis et al. 2011; Kaplan ternationally for the ‘‘Greatest Snow on Earth’’ (Bear et al. 2011). West Consulting Team 1999; Steenburgh and Alcott Episodic dust events of Utah’s Wasatch Front and 2008; Gorrell 2011). adjoining region produce hazardous air quality in the This paper examines the climatological characteris- Salt Lake City, Utah, metropolitan area and dust load- tics (or ‘‘climatology’’) of episodic dust events of the ing of the snowpack in the Wasatch Mountains (Fig. 2). Wasatch Front and adjoining region. The available From 2002 to 2010 in Utah, wind-blown dust events meteorological data illustrate that dust events occur contributed to 13 exceedances of the National Ambi- throughout the historical record and that they are as- ent Air Quality Standard for particulate matter of sociated primarily with synoptic cold fronts, baroclinic less than 2.5 (PM2.5) or 10 (PM10) mmindiameter troughs (i.e., a pressure trough with a modest temper- (T. Cruickshank, Utah Division of Air Quality, 2011, ature gradient that is insufficiently strong to be called personal communication). Dust loading in the Wasatch a front; Sanders 1999), and airmass convection. Emis- Mountains affects a snowpack that serves as the primary sion sources are located primarily in low-elevation water resource for approximately 400 000 people and Late Pleistocene–Holocene alluvial environments in SEPTEMBER 2012 S T E E N B U R G H E T A L . 1657 southern and western Utah and southern and western of greater than 6 statute mi (10 km), the threshold cur- Nevada. rently used by the WMO and national weather agencies for reporting blowing dust or dust in suspension (Shao et al. 2003; OFCM 2005). Because these events are weak 2. Data and methods or may be erroneous, they were removed from the anal- a. Long-term climatology ysis. They include all but one of the seven dust-storm re- ports. The report of dust or sand whirl was also removed Our long-term dust-event climatology derives from because we are interested in widespread events rather hourly surface weather observations from the Salt Lake than localized dust whirl(s) (also called ‘‘dust devils’’). The City International Airport (KSLC), which we obtained resulting long-term dust-event climatology is based on the from the Global Integrated Surface Hourly Database remaining 1033 reports. A dust event is any day [mountain (DS-3505) at the National Climatic Data Center (NCDC).