Episodic Dust Events of Utah’s Wasatch Front and Adjoining Region

1. Episodic Dust Events of Utah’s Wasatch Front and Adjoining Region Jeffrey D. Massey, W. J. Steenburgh Department of Atmospheric Sciences, University of Utah, Salt Lake City, Utah Thomas H. Painter Jet Propulsion Laboratory, Pasadena, California

2. Introduction ALTA • Recent studies from Colorado’s San Juan Mountains suggest dust on snow reduces the annual runoff in the upper Colorado River Basin (Painter et al. 2010) • Dust loading increases the snowpack’s absorption of solar radiation leading to an earlier meltout by several weeks (Painter et al. 2007) • Utah’s Wasatch Mountains experience episodic dust events with considerable interannual variability

3. Introduction • Alpine lake sediments from North America show dramatically larger dust deposition rates after the mid-nineteenth century (Neff et al. 2008 and Reynolds et al. 2010) • Farming, grazing, and recreation break the biological and physical crusts over desert surfaces Objectives • Establish a long term climatology of dust events for Salt Lake City (KSLC) • Identify the meteorological mechanisms responsible for dust generation and transport • Identify dust source regions over the Intermountain West

4. Long term Climatology • Hourly weather observations were taken from Salt Lake City International Airport (KSLC) from 1930 – 2010 • A dust event had at least one observer comment of blowing dust, dust in suspension, or dust storm with a visibility < 10 km during the day. • Dust concentrations are not quantified making the identification and classification of dust subjective

5. Considerable inter-annual variability • Bimodal monthly distribution • Afternoon peak

6. Wind Direction • Bimodal dust event wind directions with peaks at southerly and north-northwesterly. • Possibly representing pre and post frontal conditions • Intermountain cold front frequency is highly correlated to dust event frequency Monthly frequency of strong cold frontal passages (Schafer and Steenburgh, 2008)

7. Dust Event Classification (2001-2010) Recent dust events were classified subjectively using radar, NARR reanalysis, observer comments, and GOES imagery Dust Event Synoptic Conditions • Cold front or baroclinic trough (48%) • Airmass convection (33%) • Stationary or slowly moving fronts or baroclinic troughs (12%) • Other synoptic conditions (6%)

8. Dust detection algorithm Modified MODIS algorithm (Zhoa et al, 2010) • Albedo threshold screens for clouds • Infrared brightness temperature differences detect dust Limitations: 4km resolution Cannot detect shallow dust Cannot detect through clouds Cannot detect during low sun angles SLC

9. Baroclinic Trough or Cold Front Event 5/10/2004

10. Surface Conditions 3/30/2010

11. Plume Identification • Plumes had to be persistent for more than one frame and have a cloud free origin • For vast majority, southwestern most dust pixel taken as origin • Line approximates trajectory

12. Dust Source Regions: • Milford Valley • Sevier Desert • Escalente Desert • Carson Sink • Low elevation late Pleistocene to Holocene alluvial environments in southern and western Utah and southern and western Nevada

13. Conclusions • Dust events at KSLC occur with considerable inter-annual variability and have a bimodal distribution, with a peak in Apr and a secondary peak in Sep • Baroclinic troughs and cold fronts are the most common dust emitters for this region • Emission sources are concentrated in low elevation LatePleistocene to Holocene alluvial environments in southern and western Utah and southern and western Nevada