1. Introduction
Dust storms have an impact on air quality (Pope et al. 1995; Gebhart et al. 2001), precipitation (Goudie and Middleton 2001), soil erosion (Gillette 1988; Zobeck et al. 1989), the global radiation budget (Ramanathan et al. 2001), and regional climate (Nicholson 2000; Goudie and Middleton 2001). Recent research examining dust-related radiative forcing of the mountain snowpack of western North America and other regions of the world has initiated a newfound interest in dust research (Painter et al. 2007; Flanner et al. 2009; Painter et al. 2010). For example, observations from Colorado’s San Juan Mountains indicate that dust loading increases the snowpack’s absorption of solar radiation, decreasing seasonal snow-cover duration by several weeks (Painter et al. 2007). Modeling studies further suggest that radiative forcing from increased dust deposition during the past 150 years results in an earlier runoff with reduced annual volume in the upper Colorado River Basin (Painter et al. 2010).
Synoptic and mesoscale weather systems are the primary drivers of global dust emissions and transport. Mesoscale convective systems that propagate eastward from Africa over the Atlantic Ocean produce one-half of the dust emissions from the Sahara Desert, the world’s largest aeolian dust source (Swap et al. 1996; 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 and ecological impacts across the tropical Atlantic and Caribbean Sea (Goudie and Middleton 2001; Prospero and Lamb 2003). In northeastern Asia, strong winds in the post-cold-frontal environment of Mongolian cyclones drive much of the dust emissions (Yasunori and Masao 2002; Shao and Wang 2003; Qian et al. 2002). The highest frequency of Asian dust storms occurs over the Taklimakan and Gobi Deserts of northern China, where dust is observed 200 days yr−1 (Qian et al. 2002). Fine dust from these regions can be transported to the United States, producing aerosol concentrations that are above National Ambient Air Quality Standards (Jaffe et al. 1999; Husar et al. 2001; VanCuren and Cahill 2002; Fairlie et al. 2007).
In North America, the Great Basin, Colorado Plateau, and Mojave and Sonoran Deserts produce most of the dust emissions (Reynolds et al. 2001; Tanaka and Chiba 2006; see Fig. 1 for geographic and topographic locations). Desert land surfaces are naturally resistant to wind erosion because of the presence of physical, biological, and other crusts (Gillette et al. 1980) but are easily disturbed, in some cases leading to increased dust emissions long after the initial disturbance (e.g., Belnap et al. 2009). From alpine lake sediments collected over the interior western United States, Neff et al. (2008) and Reynolds et al. (2010) find dramatically larger dust deposition rates since the mid–nineteenth century, a likely consequence of land surface disturbance by livestock grazing, plowing of agricultural soils, and other human activities.
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. [©2011 Google; imagery ©2011 TerraMetrics.]
Citation: Journal of Applied Meteorology and Climatology 51, 9; 10.1175/JAMC-D-12-07.1
Several studies suggest that the synoptic and mesoscale weather systems that generate dust emissions and transport over western North America vary geographically and seasonally. Orgill and Sehmel (1976) identified a spring maximum in suspended dust frequency over the contiguous United States as a whole, which they attributed to cyclonic and convective storm activity, but found that some locations in the Pacific Coast and Rocky Mountain regions have an autumn maximum. Brazel and Nickling (1986, 1987) found that fronts, thunderstorms, cutoff lows, and tropical disturbances (i.e., decaying tropical depressions and cyclones originating over the eastern Pacific Ocean) are the primary drivers of dust emissions in Arizona. The frequency of dust emissions from fronts is highest from late autumn to spring, that from thunderstorms is highest during the summer, and that from cutoff lows is highest from May to June and from September to November. Dust emissions produced by tropical disturbances are infrequent but are likely confined to June–October during which tropical cyclone remnants move across the southwestern United States (Ritchie et al. 2011). For dust events in nearby California and southern Nevada, Changery (1983) and Brazel and Nickling (1987) established linkages with frontal passages and cyclone activity, respectively, with land surface conditions (e.g., soil moisture and vegetation) affecting dust-event seasonality and spatial distribution. In northwestern Nevada, dust storms originating over the Black Rock Desert have been linked to strong winds associated with cold-frontal passages and geostrophic adjustment, with emissions being strongly dependent on antecedent rainfall and soil conditions (Lewis et al. 2011; Kaplan et al. 2011).
Episodic dust events of Utah’s Wasatch Front and adjoining region produce hazardous air quality in the Salt Lake City, Utah, metropolitan area and dust loading of the snowpack in the Wasatch Mountains (Fig. 2). From 2002 to 2010 in Utah, wind-blown dust events contributed to 13 exceedances of the National Ambient Air Quality Standard for particulate matter of less than 2.5 (PM2.5) or 10 (PM10) μm in diameter (T. Cruickshank, Utah Division of Air Quality, 2011, personal communication). Dust loading in the Wasatch Mountains affects a snowpack that serves as the primary water resource for approximately 400 000 people and enables a $1.2 billion winter sports industry, known internationally for the “Greatest Snow on Earth” (Bear West Consulting Team 1999; Steenburgh and Alcott 2008; Gorrell 2011).
A snowpit from Alta, Utah, on 30 Apr 2009 that exhibits dust layers from episodic dust events.
Citation: Journal of Applied Meteorology and Climatology 51, 9; 10.1175/JAMC-D-12-07.1
This paper examines the climatological characteristics (or “climatology”) of episodic dust events of the Wasatch Front and adjoining region. The available meteorological data illustrate that dust events occur throughout the historical record and that they are associated primarily with synoptic cold fronts, baroclinic troughs (i.e., a pressure trough with a modest temperature gradient that is insufficiently strong to be called a front; Sanders 1999), and airmass convection. Emission sources are located primarily in low-elevation Late Pleistocene–Holocene alluvial environments in southern and western Utah and southern and western Nevada.
2. Data and methods
a. Long-term climatology
Our long-term dust-event climatology derives from hourly surface weather observations from the Salt Lake City International Airport (KSLC), which we obtained from the Global Integrated Surface Hourly Database (DS-3505) at the National Climatic Data Center (NCDC). KSLC is located in the Salt Lake Valley just west of downtown Salt Lake City and the Wasatch Mountains (Fig. 1) and provides the longest quasi-continuous record of hourly weather observations in northern Utah. The analysis covers the 1930–2010 water years (October–September) when 97.9% of all possible hourly observations are available.1
The hourly weather observations included in DS-3505 derive from multiple sources, with decoding and processing occurring at either operational weather centers or the Federal Climate Complex in Asheville, North Carolina (Lott et al. 2001; NCDC 2008). Studies of dust events frequently use similar datasets (e.g., Orgill and Sehmel 1976; Hall 1981; Changery 1983; Nickling and Brazel 1984; Brazel and Nickling 1986, 1987; Brazel 1989; Qian et al. 2002; Yasunori and Masao 2002; Shao and Wang 2003; Shao et al. 2003; Song et al. 2007). Nevertheless, although hourly weather observations are useful for examining the general climatological and meteorological characteristics of dust events, they do not quantify dust concentrations, making the identification and classification of dust somewhat subjective. Inconsistencies arise from observer biases, changes in instrumentation, reporting guidelines, and processing algorithms. These inconsistencies result in the misreporting of some events (e.g., dust erroneously reported as haze) and limit confident assessment and interpretation of long-term trends and variability.
Consistent with World Meteorological Organization (WMO) guidelines (WMO 2009), the present-weather record in DS-3505 includes 11 dust categories (Table 1). During the study period, there were 916 reports of blowing dust (category 7), 178 of dust in suspension (category 6), 7 of dust storm (categories 9, 30–32, and 98), and 1 of dust or sand whirl (category 8) at KSLC. There were no reports of severe dust storm (categories 33–35). Among the reports of blowing dust, dust in suspension, and dust storm, there were 69 with a visibility of greater than 6 statute mi (10 km), the threshold currently 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 or may be erroneous, they were removed from the analysis. They include all but one of the seven dust-storm reports. The report of dust or sand whirl was also removed because we are interested in widespread events rather than localized dust whirl(s) (also called “dust devils”). The resulting long-term dust-event climatology is based on the remaining 1033 reports. A dust event is any day [mountain standard time (MST)] with at least one such dust report.
The DS-3505 dust-related present-weather categories, along with full and abbreviated descriptions (the latter are used in the text), total number of reports, and number of reports used in the analysis (in parentheses).
b. Synoptic classification of recent dust events
Our analysis of the synoptic conditions contributing to Wasatch Front dust events concentrates on events at KSLC during the most recent 10-yr period (2001–10). This enables the use of modern satellite and reanalysis data and limits the number of events, making the synoptic classification of each event feasible.
Resources used in our manual analysis to subjectively classify dust events and prepare case studies include the North American Regional Reanalysis (NARR), Geostationary Operational Environmental Satellite (GOES) imagery, Salt Lake City (KMTX) radar imagery, and hourly KSLC surface weather observations and remarks from DS-3505. The NARR is a 32-km, 45-layer reanalysis for North America that is based on the National Centers for Environmental Predication (NCEP) Eta Model and data assimilation system (Mesinger et al. 2006). Relative to the European Centre for Medium-Range Weather Forecasts Interim Re-Analysis (ERA-Interim) and NCEP–National Center for Atmospheric Research (NCAR) reanalysis, the NARR better resolves the complex terrain of the Intermountain West but still has a poor representation of the basin-and-range topography over Nevada (Jeglum et al. 2010). We obtained the NARR data from the National Oceanic and Atmospheric Administration (NOAA) Operational Model Archive Distribution System (NOMADS) at NCDC (online at http://nomads.ncdc.noaa.gov/#narr_datasets), the level-II KMTX radar data from NCDC (online at http://www.ncdc.noaa.gov/nexradinv/), and the GOES data from the NOAA Comprehensive Large Array–Data Stewardship System (CLASS; online at http://www.class.ncdc.noaa.gov).
c. Dust emission sources
We identify dust emission sources during 2001–10 using a dust-retrieval algorithm applied to GOES data. Because the algorithm only works in cloud-free areas and many dust events occur in conjunction with cloud cover, we expand the number of events to include those identified in 1) DS-3505 reports from stations in the surrounding region with at least 5 years of hourly data [Delta, Utah (KU24); Elko, Nevada (KEKO); and Pocatello, Idaho (KPIH); see Fig. 1]; 2) the authors’ personal notes, which derive from weather analysis over the past several years and include events identified visually in the Salt Lake Valley or using satellite imagery from the surrounding region; and 3) Utah Avalanche Center annual reports. This analysis is thus not specific to KSLC but does identify emissions sources that contribute to dust events in the region.
The dust-retrieval algorithm is a modified version of that used by Zhao et al. (2010, p. 2349) to detect dust over land with Moderate Resolution Imaging Spectroradiometer (MODIS) data, which uses brightness temperature Tb from three infrared channels (3.9, 11, and 12 μm) and reflectance from four visible channels (0.47, 0.64, 0.86, and 1.38 μm). GOES has three corresponding infrared channels (3.9, 10.7, and 12 μm) but only one visible channel (0.65 μm). Therefore, we use an albedo of 0.25 or greater in the 0.65-μm visible channel to screen for clouds. Then, we substitute the GOES 10.7-μm channel for the MODIS 11-μm channel and identify the existence of dust if Tb(3.9 μm) ≤ Tb(10.7 μm) and Tb(10.7 μm) − 10 K ≥ Tb (12 μm). These thresholds are slightly modified from those used by Zhao et al. (2010) and were selected through experimentation and comparison with dust detected visually and using the Zhao et al. (2010) technique applied to MODIS imagery from several events. Because uncertainties arise when the sun angle is low and when dust is near cloud edges, the algorithm is applied approximately every 15 min during the daylight hours (0700–1900 MST), with plume origin and orientation identified subjectively. Because the footprint of the GOES infrared channels is 4 km and the algorithm fails to identify shallow dust (Zhao et al. 2010), the plume origin is approximate.
3. Results
a. Long-term climatology
Dust events at KSLC occur throughout the historical record, with an average of 4.3 per water year (Fig. 3). Considerable interannual variability exists, with no events reported in seven years (1941, 1957, 1981, 1999, 2000, 2001, and 2007) and a maximum of 15 in 1934. No effort was made to quantify or assess long-term trends or interdecadal/interannual variability given the subjective nature of the reports and changes in observers, observing methods, and instrumentation during the study period. The general decline in dust-event frequency, however, is broadly consistent with a decrease in mass accumulation rates related to dust deposition in alpine lakes of western Colorado following the passage of the Taylor Grazing Act of 1934 (Neff et al. 2008).
Number of dust events at KSLC by water year.
Citation: Journal of Applied Meteorology and Climatology 51, 9; 10.1175/JAMC-D-12-07.1
On the basis of current weather-observing practices (Glickman 2000; Shao and Wang 2003), the minimum visibility when dust is reported meets the criteria for blowing dust [1 km (⅝ statute mi) < visibility ≤ 10 km (6 statute mi)], a dust storm [0.5 km (
Minimum visibility (km) during KSLC dust events.
Citation: Journal of Applied Meteorology and Climatology 51, 9; 10.1175/JAMC-D-12-07.1
Total dust flux during KSLC dust events by water year.
Citation: Journal of Applied Meteorology and Climatology 51, 9; 10.1175/JAMC-D-12-07.1
The monthly distribution of dust events is bimodal, with primary and secondary peaks in April and September, respectively (Fig. 6). Similar peaks are observed in the mean monthly total dust flux, but with an additional peak in January (Fig. 7). This January peak is surprising, but it results primarily from an unusually strong multiday event in January of 1943 that contributed to 83% of the January monthly mean. In the summer, the mean monthly near-surface minimum is distinctly lower relative to the dust-event frequency (cf. Figs. 6 and 7), suggesting that summer dust events are shorter and weaker. For March–May, which usually encompasses the climatological peak in snowpack snow water equivalent and the onset of the spring runoff, the mean monthly total dust flux is 237 g m−2, or 59% of the mean annual total dust flux.
Number of dust events at KSLC by month.
Citation: Journal of Applied Meteorology and Climatology 51, 9; 10.1175/JAMC-D-12-07.1
Mean monthly total dust flux during KSLC dust events.
Citation: Journal of Applied Meteorology and Climatology 51, 9; 10.1175/JAMC-D-12-07.1
Similar bimodal or modal distributions with a primary or single spring dust peak have been identified in the Taklimakan desert of China (Yasunori and Masao 2002), southern Great Plains of the United States (Stout 2001), Mexico City, Mexico (Jauregui 1989), and the Canadian prairies (Wheaton and Chakravarti 1990). The spring peak appears to be the result of a high frequency of wind events driven by cyclones and fronts passing over a recently dried, erodible land surface. Indeed, the bimodal distributions of dust events and mean monthly total dust flux at KSLC are very similar to that of cold fronts and cyclones in the Intermountain West, which are strongest and most frequent in the spring and have a secondary peak in the autumn (Shafer and Steenburgh 2008; Jeglum et al. 2010). These cold fronts and cyclones produce persistently strong winds that have been implicated in sand transport and dune morphology (Jewell and Nicoll 2011) and are capable of generating dust emissions and transport during favorable land surface conditions. In fact, dust was reported at KSLC within 3 h of the passage of 12 of the 25 strongest cold fronts identified by Shafer and Steenburgh (2008).
The mean sustained wind speed during dust reports at KSLC is 11.6 m s−1 (with a standard deviation of 4.0 m s−1), slightly higher than the 8.5 and 9.29 m s−1 found by Holcombe et al. (1997) for Yuma, Arizona, and Blythe, California, respectively. Therefore, we use 10 m s−1 as an approximate threshold velocity for dust emissions and transport assuming favorable boundary layer and land surface conditions. At KSLC, reports of sustained winds ≥ 10 m s−1 are most common in March and April, with additional, but weaker maxima in August and January (Fig. 8). The March and April peak resembles the springtime peak in dust events and mean monthly total dust flux, but the lack of an autumn secondary maximum and winter minimum suggests that other factors related to the spatial scale of the strong winds (e.g., convective vs synoptically driven), and seasonal changes to vegetation, soil conditions, and soil moisture (Gillette 1999; Neff et al. 2008; Belnap et al. 2009) contribute to the seasonality of dust events and total dust flux.
Number of observations at KSLC with a sustained wind ≥ 10 m s−1 by month.
Citation: Journal of Applied Meteorology and Climatology 51, 9; 10.1175/JAMC-D-12-07.1
Dust reports exhibit a strong diurnal cycle and are most common in the late afternoon and evening hours (Fig. 9), as observed in other regions (Jauregui 1989; N’Tchayi Mbourou et al. 1997). The frequency of sustained winds ≥ 10 m s−1 at KSLC is about 3 times as high in the afternoon as in the morning (Fig. 10), which is consistent with the development of the daytime convective boundary layer. The peak for sustained winds ≥ 10 m s−1 occurs at 1400 MST, 4 h earlier than the peak in dust reports, a likely consequence of the time needed for dust to travel from its sources to KSLC.
Number of dust reports at KSLC by hour (MST).
Citation: Journal of Applied Meteorology and Climatology 51, 9; 10.1175/JAMC-D-12-07.1
Number of observations at KSLC with a sustained wind ≥ 10 m s−1 by hour (MST).
Citation: Journal of Applied Meteorology and Climatology 51, 9; 10.1175/JAMC-D-12-07.1
The frequency distribution of wind directions during dust events is bimodal, with peaks at southerly and north-northwesterly (Fig. 11). About 49% of the time, the wind is from the south-southwest through the south-southeast, and about 29% of the time the wind is northwesterly through northerly. Total dust flux is also greatest for winds from the south-southwest through south-southeast (Fig. 12).
Wind rose for KSLC dust reports.
Citation: Journal of Applied Meteorology and Climatology 51, 9; 10.1175/JAMC-D-12-07.1
Fraction (%) of total dust flux as a function of wind direction at KSLC.
Citation: Journal of Applied Meteorology and Climatology 51, 9; 10.1175/JAMC-D-12-07.1
b. Recent (2001–10) events
To classify dust events synoptically, we concentrate on 2001–10, which enables the use of modern reanalysis, satellite, and radar data. The monthly frequency distribution of the 33 dust events during this period resembles that of the long-term climatology except for a disproportionately high number of summer events (cf. Figs. 6 and 13).
Number of recent (2001–10) dust events at KSLC by month.
Citation: Journal of Applied Meteorology and Climatology 51, 9; 10.1175/JAMC-D-12-07.1
The 33 recent dust events were classified subjectively into one of four groups depending on the primary synoptic conditions responsible for the dust emissions and transport: 1) airmass convection, 2) a cold front or baroclinic trough entering Utah from the west or northwest, 3) a stationary or slowly moving front or baroclinic trough to the west or northwest of Utah, and 4) other synoptic conditions (Table 2). The 11 (33%) events generated by airmass convection featured a thunderstorm, thunderstorm in the vicinity, or squall comment in the DS-3505 reports within an hour of the dust observation, and/or nearby convection in satellite or radar imagery, but no significant large-scale temperature gradient at 700 hPa. These events tended to be short lived (usually less than 2 h) and all occurred between mid-May and mid-September. For example, at 1600 MST 19 May 2006, KSLC observed a 5 m s−1 southerly wind but KMTX radar imagery showed strong convection just to the south (Fig. 14a; KSLC observation not shown). The passage of a convective outflow boundary (i.e., gust front; Wakimoto 1982) at KSLC at 1607 MST was accompanied by south-southwest winds of 24 m s−1 with gusts to 28 m s−1, blowing dust, and a visibility of 6.4 km. By 1624 MST, blowing dust was no longer reported. A lack of strong flow and baroclinity at 700 hPa over northern Utah during this period further supports the classification of this event as airmass convection (Fig. 14b).
Date and primary synoptic conditions of recent (2001–10) dust events at KSLC. Abbreviations are AC (airmass convection), CF/BT (cold front or baroclinic trough entering Utah from the west or northwest), SF/BT (stationary or slowly moving front or baroclinic trough to the west or northwest of Utah), and O (other synoptic conditions).
Meteorological conditions during the 19 May 2006 dust event: (a) 1605 MST KMTX 0.5° radar reflectivity (dBZ; color scale at lower left) and topography (color filled with transitions at 1350, 1700, 2050, and 2400 m) and (b) 1700 MST NARR 700-hPa temperature (shaded; scale at bottom) and wind (full and half barb denote 2.5 and 5 m s−1, respectively).
Citation: Journal of Applied Meteorology and Climatology 51, 9; 10.1175/JAMC-D-12-07.1
The 16 (48%) recent events produced by a cold front or baroclinic trough from the west or northwest featured at least one dust report at KSLC within 3 h of the cold-frontal or baroclinic-trough passage and a distinct frontal cloud band in visible satellite images. Thirteen of these events accompanied a cyclone over the Great Basin or adjoining northwestern United States as based on the existence of a closed 850-hPa isohypse at 30-m intervals, although dust reports at KSLC are concentrated around the timing of the accompanying frontal or baroclinic-trough passage [Fig. 15; see West and Steenburgh (2010) for a detailed case study of one of these 13 events (15 Apr 2002)]. Of the 16 events, 4 reported dust more than 3 h before the frontal passage, 8 reported dust within the 3 h before frontal passage, 14 reported dust within the 3 h after the frontal passage, and 2 reported dust more than 3 h after the frontal passage. A representative example occurred on 10 May 2004, when strong southerly–southwesterly flow ahead of a cold front and concomitant pressure trough produced several dust plumes that extended from southwest Utah to the Wasatch Front (Figs. 16a–c). Hourly and special aviation routine weather reports (METAR) archived by the MesoWest cooperative networks (Horel et al. 2002) show that dust was first reported at KSLC at 1655 MST, just before the frontal passage, which occurred between the 1655 and 1710 MST observations. The visibility was 8 km, with sustained winds of 18 m s−1 and wind gusts to 22 m s−1. The dust-limited visibility dropped to 2.8 km following the frontal passage at 1710 MST, but by 1955 MST the visibility was greater than 10 km and dust was no longer reported. The entrainment of dust into the postfrontal air mass, combined with cold-frontal convergence, appeared to contribute to increased dust concentrations and decreased visibility during and immediately following frontal passage, as occurs in many events.
Number of dust reports relative to frontal passage during recent (2001–10) dust events at KSLC with a cold-frontal or baroclinic-trough passage from the west or northwest.
Citation: Journal of Applied Meteorology and Climatology 51, 9; 10.1175/JAMC-D-12-07.1
Meteorological conditions at 1700 MST during the 10 May 2004 dust event: (a) GOES visible satellite imagery with dust identified in red, (b) NARR 850-hPa geopotential height (m; shaded, with scale at bottom) and wind (full and half barb denote 2.5 and 5 m s−1, respectively), and (c) NARR 700-hPa temperature (°C; shaded, with scale at bottom) and wind [as in (b)].
Citation: Journal of Applied Meteorology and Climatology 51, 9; 10.1175/JAMC-D-12-07.1
Strong prefrontal southerly flow within a deep convective boundary layer contributes to dust emissions and transport during these 16 cold-frontal or baroclinic-trough events. In comparison with a 21-day weighted climatology centered on the event dates, the NARR 700-hPa wind speed at KSLC at the time of the initial dust report during these events is skewed to much higher values, with the distribution of flow directions from 160° to 260° (Figs. 17a,b). Maximum boundary layer depths on these dust-event days are skewed toward much higher values than climatological values, with a mode at 5000 m AGL (Fig. 17c). As shown by Shafer and Steenburgh (2008), strong winds within a deep convective boundary layer are common during strong Intermountain West cold-frontal events. As noted previously, of the 25 strong Intermountain West cold fronts identified at KSLC by Shafer and Steenburgh (2008, see their Table 1), 12 were accompanied by at least one dust report within 3 h of frontal passage. These results indicate that Intermountain West cold fronts and baroclinic troughs play an important role in regional dust emissions and transport. Further, the frequency of these cold-frontal and baroclinic-trough passages is greatest in the spring when dust-related radiative forcing can have its greatest impact on snowmelt (Painter et al. 2007).
Frequency of NARR (a) 700-hPa wind speed (m s−1) at initial dust report, (b) 700-hPa wind direction (°) at initial dust report, and (c) maximum boundary layer depth (m AGL) at KSLC during recent (2001–10) dust events associated with a cold front or baroclinic trough entering Utah from the west or northwest (solid) relative to a weighted climatology that is based on 21 days centered on each event date (dashed).
Citation: Journal of Applied Meteorology and Climatology 51, 9; 10.1175/JAMC-D-12-07.1
Closely related to the cold-frontal and baroclinic-trough events noted above are four (18%) additional events that were produced by stationary or slowly moving fronts or baroclinic troughs to the west or northwest that remained upstream of KSLC for at least 24 h after the initial dust observation. During these events, dust emissions and transport occur in the strong southerly or southwesterly flow ahead of the frontal or baroclinic trough, as discussed above. One event (30 Aug 2009) may be erroneous since observer comments and satellite imagery indicate that smoke, not dust, likely reduced visibilities.
Two (6%) events were associated with other synoptic conditions. On 16 September 2003, KSLC reported dust in intensifying northwesterly flow as a surface trough and cyclone developed to the south. On 13 March 2005, dust was produced by strong winds following the passage of a cold front from the north. The large-scale evolution of this event resembled that found to contribute to two dust storms originating over the Black Rock Desert of northwestern Nevada by Lewis et al. (2011) and Kaplan et al. (2011).
The fraction of the total dust flux by event type clearly shows the dominant contribution of cold and quasi-stationary fronts and baroclinic troughs (81%; Fig. 18). Although airmass convection produces 11 of the 33 recent events, it only generates 8% of the total dust flux.
Fraction (%) of total dust flux at KSLC by synoptic condition. Abbreviations are CF/BT (cold fronts and baroclinic troughs entering Utah from the west or northwest), SF/BT (stationary or slowly moving fronts and baroclinic troughs to the west or northwest of Utah), AC (airmass convection), and Other (other synoptic conditions).
Citation: Journal of Applied Meteorology and Climatology 51, 9; 10.1175/JAMC-D-12-07.1
c. Dust emission sources
As described in section 2, we use a dust-retrieval algorithm applied to GOES imagery to identify the origin and orientation of dust plumes during the recent dust events. Given that plumes are not identifiable in some events because of cloud cover and/or an insufficient solar zenith angle, we include in this analysis the 33 recent (2001–10) events described above, as well as 61 additional events observed in DS-3505 reports from three weather stations in the surrounding region (Delta, Elko, and Pocatello; see Fig. 1) or identified in the authors’ notes and annual Utah Avalanche Center reports. After applying the GOES dust-retrieval algorithm, 120 independent dust plumes were subjectively identified during 47 (50%) of the 94 dust events. Airmass convection and cold- or stationary-frontal or baroclinic-trough events with two or fewer dust observations most commonly were without visible plumes.
The origins of the 120 identifiable dust plumes are clustered primarily in low-elevation Late Pleistocene–Holocene alluvial environments in southern and western Utah and southern and western Nevada (Fig. 19). These include the Sevier Desert, Sevier Dry Lake Bed, Escalante Desert, Milford Valley, and West Desert of Utah and the Black Rock Desert, Carson Sink, and Great Basin and Mojave Deserts of Nevada. Since July of 2007, dust emissions from the Milford Valley likely include contributions from the Milford Flat fire scar (Miller et al. 2012). Plumes oriented toward KSLC, the Wasatch Front, and northern Utah originate primarily from southern and western Utah, consistent with what might be inferred from Figs. 11, 12, and 17 given the dominance of southerly flow. On average 2.6 plumes are identified on days with visible plumes, indicating that synoptic conditions that contribute to episodic dust events frequently activate multiple emissions sources. Because of obscuration, these results do not include dust plumes that form beneath existing plumes.
Google Earth image with GOES-derived dust-plume origins and orientations on days during which dust is reported at KSLC, at KSLC and other stations, in the authors’ notes or Utah avalanche center reports, at KU24, at KEKO, at KPIH, or at multiple stations other than KSLC. The rectangles in the upper-left panel show the locations of the areas shown in the other two panels. [©2011 Google; imagery ©2011 TerraMetrics.]
Citation: Journal of Applied Meteorology and Climatology 51, 9; 10.1175/JAMC-D-12-07.1
Not all of the dust identified in satellite imagery could be traced to a clear origin as there were 11 examples of broad areal dust emissions. The dust in a majority of these examples originated over western Nevada and moved to the southeast, but there was one event with areal dust emissions over central Utah and another with areal dust emissions over the Snake River Plain.
4. Conclusions
Episodic dust events contribute to hazardous air quality and dust loading of the snowpack along Utah’s Wasatch Front and adjoining region. Surface weather observations from the Salt Lake City International Airport show that these dust events occur throughout the 1930–2010 study period, with considerable interannual variability. The annual dust-event frequency and total dust flux exhibit a general decline during the study period that is broadly consistent with decreased mass sedimentation rates related to dust deposition in alpine lakes of western Colorado that followed passage of the 1934 Taylor Grazing Act (e.g., Neff et al. 2008).
The distributions of monthly dust-event frequency and mean total dust flux are bimodal, with a primary peak in spring (April) and a secondary peak in autumn (September) that closely resemble the monthly frequency of strong Intermountain West cold fronts and Intermountain West cyclones (Shafer and Steenburgh 2008; Jeglum et al. 2010). The total dust flux is greatest during periods of strong southerly winds, with a weaker secondary maximum associated with flow from the northwest.
An analysis of 33 recent (2001–10) events shows 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 stationary or slowly moving front or baroclinic trough west of Utah, and 2 were associated with other synoptic patterns. The fraction of total dust flux observed at KSLC is strongly dominated by cold- and quasi-stationary-frontal or baroclinic-trough events, many of which feature strong southerly winds in a deep convective boundary layer.
Subjective analysis of dust plumes identified using GOES imagery indicates that regional dust emission sources during these episodic dust events are clustered primarily in low-elevation Late Pleistocene–Holocene alluvial environments in southern and western Utah and southern and western Nevada. Areas with the greatest concentration of emission sources include the Sevier Desert, Sevier Dry Lake Bed, Escalante Desert, Milford Valley, and West Desert of Utah and the Black Rock Desert, Carson Sink, and Great Basin and Mojave Deserts of Nevada.
These findings are based on the analysis of episodic dust events identified in conventional meteorological observations. Dust emissions, transport, and deposition during other periods may also influence snowpack, soil, and lake-sediment composition over the region. In addition, an important aspect of episodic dust events not investigated here is land surface variability and its contribution to enhanced dust fluxes under climate change (Munson et al. 2011; Okin et al. 2011). Improved understanding of soil moisture, vegetation, and anthropogenic disturbance (e.g., Neff et al. 2005; Reynolds et al. 2007; Belnap et al. 2009; Miller et al. 2012) may help to improve the prediction of these events. Moreover, mitigation efforts in the areas of frequent emissions identified above, especially those in southern and western Utah, may reduce the frequency and severity of episodic dust events over the Wasatch Front and adjoining region. Such mitigation efforts are currently being investigated by regional and federal land and water management agencies.
Acknowledgments
We thank Trevor Alcott for his contributions to the KSLC dust-event climatology and Paul Jewell for assisting with the identification of dust emission sources. John Lewis and two anonymous referees provided reviews that helped to improve the manuscript. We gratefully acknowledge the provision of datasets, software, and/or computer time and services by NCEP, NCAR, Unidata, the Center for Ocean–Land–Atmosphere Studies, and the University of Utah Center for High Performance Computing. This research was supported by National Science Foundation Grant AGS-0627937, NASA Grant NNX10AO97G, and USDA Forest Service Agreement 10-PA-11041914-045. Any opinions, findings, and conclusions or recommendations expressed in this paper are those of the authors and do not necessarily reflect the views of the National Science Foundation, NASA, or the USDA Forest Service.
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