Episodic Dust Events along Utah’s Wasatch Front
W. JAMES STEENBURGH AND JEFFREY D. MASSEY
Department of Atmospheric Sciences, University of Utah, Salt Lake City, UT
THOMAS H. PAINTER
Jet Propulsion Laboratory, Pasadena, CA
In preparation for submittal to
Journal of Applied Meteorology and Climatology
Draft of Tuesday, February 09, 2016
Corresponding author address: Dr. W. James Steenburgh, Department of Atmospheric Sciences,
University of Utah, 135 South 1460 East Room 819, Salt Lake City, UT, 84112.
E-mail: jim.steenburgh@utah.edu
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 Wasatch Front dust events based on surface-weather observations from the Salt Lake
City International Airport (KSLC), GOES satellite imagery, and the North American Regional
Reanalysis. Dust events at KSLC, defined as any day (MST) with at least one report of a dust
storm, blowing dust, and/or dust in suspension (i.e., dust haze) with a visibility of 10 km (6 mi)
or less, average 4.3 per water year (WY, Oct–Sep), with considerable interannual variability
from 1930–2010. The monthly frequency of dust-events is bimodal with primary and secondary
maxima in Apr and Sep, respectively. Dust reports are most common in the late afternoon and
evening.
An analysis of the 33 most recent (2001–2010 WY) events at KSLC indicates that 16
were associated with a cold front or baroclinic trough, 11 with airmass convection and related
outflow, 4 with persistent southwest flow ahead of a stationary trough or cyclone over Nevada,
and 2 with other synoptic patterns. GOES satellite imagery and backtrajectories from these 33
events, as well as 61 additional events from the surrounding region, illustrate that emissions
sources are mostly concentrated in the deserts of southern Utah and western Nevada, including
the Sevier dry lake bed, Escalente Desert, and Carson Sink. Efforts to reduce dust emissions in
these regions may help mitigate the frequency and severity of hazardous air-quality episodes
along the Wasatch Front and dust loading of the snowpack in the adjacent Wasatch Mountains.
2
1. Introduction
Dust storms impact air quality (Gebhart et al. 2001; Pope et al. 1995), precipitation
distribution (Goudie and Middleton, 2001), soil erosion (Gillette 1988; Zobeck 1989), the global
radiation budget (Ramanathan 2001), and regional climate (Nicholson 2000, Goudie and
Middleton 2001). Recent research regarding regional hydrologic and climatic change produced
by dust-radiative forcing of the mountain snowpack of western North America and other regions
of the world has initiated a newfound interest in dust research. (Hansen and Nazarenko 2004;
Painter et al. 2007; 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, thus
decreasing the duration of snow cover by several weeks (Painter et al. 2007). Modeling studies
further suggest that dust-radiative forcing results in an earlier runoff with less 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 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 northeast Asia, strong winds in the post-coldfrontal environment of Mongolian Cyclones drive much of the dust emissions (Yasunori and
Masao 2002; Shoa and Wang, 2003; Qian et al., 2001). The highest frequency of Asian dust
storms occurs over the Taklimakan and Gobi Deserts of northern China where dust is observed
200 d yr-1 (Qian et al., 2001). Fine dust from these regions can be transported to the United
3
States, producing aerosol concentrations above National Ambient Air Quality Standards (Husar
et al. 2001; Jaffe et al., 1999; Fairlie et al., 2007)
The Great Basin, Colorado Plateau, and Mojave and Sonoran Deserts produce most of the
dust emissions in North America (Tanaka and Chiba, 2006; see Fig. 1 for geographic and
topographic locations). Most land surfaces in these deserts are naturally resistant to wind erosion
due to the presence of physical, biological, and other crusts (Gillette et al., 1980). However,
these crusts are easily disturbed, leading to increased dust emissions, in some cases long after the
initial disturbance (e.g., Belnap et al. 2009). Based on alpine lake sediments collected over the
interior western United States, Neff et al. (2008) found that dust loading increased 500% during
the 19th century, a likely consequence of land-surface disturbance by livestock grazing, plowing
of agricultural soils, and other activities.
Several studies suggest that the synoptic and mesoscale weather systems that generate
dust emissions and transport over western North America vary geographically and seasonally. In
a dust climatology for the contiguous United States, Orgill and Sehmel (1976) proposed several
including convective systems, warm and cold fronts, cyclones, diurnal winds, and specifically for
the western United States, downslope (their katabatic) winds generated by flow-mountain
interactions. They identified a spring maximum in the frequency of suspended dust for the
contiguous United States as a whole, which they attributed to cyclonic and convective storm
activity, but found that several locations in the Pacific and Rocky Mountain regions have a fall
maximum. However, they made no effort to quantify the importance of the differing synoptic
and mesoscale systems.
In Arizona, 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.
4
The frequency of dust emissions from fronts is highest from late Fall–Spring, thunderstorms
during the summer, and cutoff lows from May–Jun and Sep–Nov. Dust emissions produced by
tropical disturbances are infrequent, but are likely confined to Jun–Oct when tropical cyclone
remnants move across the southwest United States (Ritchie et al., 2011). For dust events in
nearby California and southern Nevada, Changery (1983) and Brazel and Nickling (1987) also
established linkages with frontal passages and cyclone activity, respectively.
In addition to
synoptic and mesoscale systems, these studies also cite the importance of land-surface conditions
(e.g. soil moisture, vegetation) for the seasonality and spatial distribution of dust events.
None of these studies, however, have specifically examined the Wasatch Front of
northern Utah, where episodic dust events produce hazardous air quality in the Salt Lake City
metropolitan area and contribute to dust loading of the snowpack in the nearby Wasatch
Mountains (Fig. 2). From 2002–2010, wind-blown dust events contributed to 13 exceedances of
the National Ambient Air Quality Standard for PM2.5 or PM10 in Utah (T. Cruickshank, Utah
Division of Air Quality, 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” (Salt Lake City Department of Public Utilities 1999; Steenburgh and Alcott 2008;
Gorrell, Salt lake tribune, 2011).
This paper examines the climatological characteristics and emissions sources during
Wasatch Front dust events. We find that Wasatch Front dust-events occur throughout the
historical (1930–2010 water year1) record, with considerable interannual variability. Events are
driven primarily by strong winds associated with cold fronts or airmass convection, with the
1
Hereafter, all years in this paper are water years (Oct–Sep).
5
deserts and dry lake beds of southern Utah, as well as the Carson Sink of Nevada, serving as
primary regional emission sources. Dust emission mitigation efforts in these regions may reduce
the frequency and severity of related hazardous air quality events along the Wasatch Front and
dust loading of the Wasatch Mountain snowpack.
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 1930–2010 when 97.9% of all possible
hourly observations are available.
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, NC (NCDC 2001, 2008). Studies of dust events frequently use similar
datasets (e.g., Nickling and Brazel 1984; Brazel and Nickling 1986; Brazel and Nickling 1987;
Brazel 1989; Hall 1981; Orgill and Sehmel 1976; Changery 1983; Qian 2002; Yasunori and
Masao 2002; Shao et al. 2003; Song et al. 2007; Shao and Wang 2003). Nevertheless, while
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.
6
These inconsistencies result in the misreporting of some events (e.g., dust erroneously reported
as haze) and preclude 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 blowing dust (category 7), 178 dust-in-suspension (category 6), 7 dust
storm (categories 9, 30–32, and 98) and one dust or sand whirl report (category 8) at KSLC.
There were no severe dust storm reports (categories 33–35). Amongst the blowing dust, dust-insuspension, and dust storm reports, there were 69 with a visibility > 6 statute miles (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; Federal Meteorological Handbook, 2005). Since these
events are weak, or may be erroneous, they were removed from the analysis. This includes all
but one of the 7 dust storm reports. The dust or sand whirl report was also removed since we are
interested in widespread events rather than localized dust whirl(s) (a.k.a. dust devils). The
resulting long-term dust-event climatology is based on the remaining 1033 reports. A dust event
is any day (MST) with at least one such dust report.
b. Characteristics of recent dust events
The analysis of the synoptic, meteorological, and land-surface conditions contributing to
Wasatch Front dust events concentrates on events at KSLC during most recent ten-year period
(2001–2010). This enables the use of modern satellite and reanalysis data, and limits the number
of events, making the synoptic analysis of each event tractable.
Resources used to synoptically classify dust events, composite events, and prepare case
studies include the North American Regional Reanalysis (NARR), GOES satellite imagery, Salt
7
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 based on the
National Centers for Environmental Predication (NCEP) Eta model and data assimilation system
(Mesinger et al. 2006). Compared to the ERA-Interim and NCEP-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 (see Jeglum et al. 2010). We obtained the
NARR data from the National Oceanic and Atmospheric Administration (NOAA) Operational
Model Archive Distribution System (NOMADS) at the National Climatic Data Center web site
(http://nomads.ncdc.noaa.gov/#narr_datasets), the level-II KMTX radar data from NCDC
(website: http://www.ncdc.noaa.gov/nexradinv/), and the GOES data from the NOAA
Comprehensive
Large
Array-Data
Stewardship
System
(CLASS,
http://www.class.ncdc.noaa.gov).
c. Dust emission sources
We identify dust emission sources during from 2001–2010 using a dust-retrieval
algorithm applied to GOES satellite 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 (Fig. 1), (2) the authors’ personal notes, 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.
Our dust-retrieval algorithm is a modified version of that described by Zhoa et al. (2010)
for MODIS. First, we substitute the GOES 10.7 μm channel for the MODIS 11.02 μm channel.
8
Then the two reflectance condition thresholds used to identify the presence of clouds from the
MODIS .47, .64, and .86 μm channels are replaced by a single threshold (.35) that uses the only
visible channel on GOES. Finally, the maximum threshold for the brightness temperature
difference between the 3.9 and 11 μm (11 and 12 μm) bands was changed from -.5 ºC to 0 ºC (25
ºC to 10 ºC). These adjustments enable the identification of visible dust over Utah using GOES
data, although uncertainties arise near cloud edges, when the sun angle is low, or when the dust
concentrations are low or near the surface. The algorithm is applied every 15 min during the
daylight hours (0700–1900 MST), with plume origin and orientation identified subjectively.
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, 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.
Based on 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 (5/8
statute mi) < visibility ≤ 10 km (6 statute mi)], a dust storm [0.5 km (5/16 statute mi) < visibility
≤ 1km (5/8 statute mi)], or a severe dust storm [visibility ≤ 0.5 km (5/16 statute mi)] in 95.40%,
9
2.59%, and 2.01% of the dust events, respectively (Fig. 4)2. Therefore, only a small fraction of
the dust events and observations meet dust storm or severe dust storm criteria.
To integrate the effects of event severity, frequency, and duration into an estimate of the
annual near-surface dust flux, we first estimate the dust concentration, C (µg m-3), for each dust
report following equations (6) and (7) presented in Shao et al. (2003):
C = 3802.29Dv-0.84
Dv< 3.5 km
C = exp(-0.11Dv + 7.62)
Dv ≥ 3.5 km
where Dv is the visibility. Multiplying by the wind speed and integrating across all observation
intervals yields an estimated mean annual near-surface dust flux of 399.4 g m-2, with a maximum
of 2810.2 g m-2 in 1935 (Fig. 5). Because it integrates event severity, frequency, and duration,
the annual near-surface dust flux provides a somewhat different perspective from the annual
number of dust events (cf. Figs. 3 and 5). For example, 1934 featured the most dust events, but
the greatest near-surface dust flux occurred in 1935. In 2010, there were only 2 dust events, but
also a pronounced decadal-scale maximum in annual near-surface dust flux.
The monthly distribution of dust events is bimodal, with primary and secondary peaks in
Apr and Sep, respectively (Fig. 6). Similar peaks are observed in the near-surface dust flux, but
with an additional peak in Jan (Fig. 7). This Jan peak is surprising, but careful examination of
the data revealed one unusually strong multiday event in January of 1943 that contributed to 83%
of the Jan monthly mean. In the summer, the dust flux minimum is distinctly lower compared to
the dust-event frequency (cf. Figs. 6 and 7), suggesting that summer dust events are shorter and
weaker. From Mar–May, which usually encompasses the climatological snowpack snow water
2
The visibility observations are taken and stored in statute miles, but approximate metric thresholds are
used hereafter.
10
equivalent maximum and beginning of the spring runoff, the mean three-month dust flux is 237 g
m-2, 59% of the annual flux.
Similar bimodal or modal distributions with a primary or single spring dust peak have
been identified in the Taklimakan desert of China, southern Great Plains of the United States,
Mexico City, and the Canadian Prairies (Yasunori and Masao 2002; Stout 2001; Jauregui 1989;
Wheaton and Chakravarti 1990). The spring peak appears to be the result of erodible landsurfaces combined with a high frequency of wind events driven by cyclones and fronts. In fact,
the bimodal distributions of dust-events and dust-flux at KSLC is very similar to that of
Intermountain cold fronts and cyclones, which are strongest and most frequent in the spring, and
have a secondary peak in the fall (e.g., Shafer and Steenburgh 2008; Jeglum et al. 2010). These
Intermountain cold fronts and cyclones produce persistently strong winds capable of generating
dust emissions and transport during favorable land-surface conditions. Interestingly, dust was
reported at KSLC within 3 h of the passage of 12 of the 25 strongest cold fronts identified by
Schafer and Steenburgh (2008).
The mean 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 m s-1 and 9.29 m s-1 found by Holcombe et al.
(1997) for Yuma, AZ and Blythe, CA, respectively. Therefore, we use 10 m s-1 as an
approximate threshold velocity for dust emissions and transport. At KSLC winds ≥ 10 m s-1 are
most common in Mar and Apr, with an additional, but relatively weak, maxima in Aug and Jan
(Fig. 8). The Mar and Apr peak resembles the springtime peak in dust events and flux, but the
lack of a fall secondary maximum and winter minimum suggests other factors related to seasonal
changes to vegetation, soil conditions, and soil moisture (Neff et al., 2008, Belnap et al., 2009,
Gillette, 1999) contribute to the seasonality of dust events and fluxes.
11
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; Mbouro et al. 1997). The
frequency of winds ≥ 10 m s-1 at KSLC is about three times higher in the afternoon than morning
(Fig. 10), which is consistent with the development of the daytime convective boundary layer.
The peak for winds ≥ 10 m s-1 occurs at 14 MST, roughly four hours earlier than the peak in dust
reports; a likely consequence of the time needed for dust to travel from its source to KSLC.
The frequency distribution of wind directions during dust events is bimodal, with peaks at
southerly and north-northwesterly (Fig. 11). About 50% of the time, the wind is from the southsouthwest through south-southeast and about 28% of the time the wind is northwesterly through
northerly. Dust flux is also greatest for winds from the south-southwest through south-southeast
(Fig. 12), even more so than the frequency, suggesting events with southerly winds transport
more dust.
b.
Recent (2001–2010) events
To characterize and classify dust events, we concentrate on a subset of events from 2001–
2010 enabling the use of modern reanalysis, satellite, and radar data in an effort to synoptically
diagnosis each event. The monthly frequency distribution of these 33 recent dust events
resembles that of the long-term climatology except for a disproportionately high number of
summer events (cf. Figs. 6, 13).
Based on synoptic analysis, we classified these recent dust events into one of four groups
depending on their primary mechanism for dust emissions and transport: (1) airmass convection,
(2) cold fronts or baroclinic troughs, (3) upstream stationary fronts or baroclinic troughs, and (4)
other mechanisms (Table 2). Here we define baroclinic trough as a pressure trough, cyclonic
wind shift, and an insufficiently strong temperature gradient to be called a front. The 11 (33%)
12
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, or nearby
convection evident in satellite or radar imagery without strong nearby 700-hPa baroclinicity in
NARR analyses. These events tended to be shortlived, usually < 2 h, and all occurred between
the middle of May and the middle of September. An airmass convection event on May 19th,
2006 reduced visibilities to 6.4 km and lasted for only 17 minutes. At 16 MST KSLC reported a
5 m s-1 southerly wind, the strongest reported wind thus far that day, but at 16:07 MST, seven
minutes later, winds increased to 24.1 m s-1 with a gust to 27.7 m s-1 and dust was reported along
with a squall at or within sight of the station comment. This dramatic wind shift was associated
with an outflow boundary from a convective cell visible on radar imagery at 16:05 MST (Fig.
14a). The weak returns present over KSLC are likely associated with the leading edge of the
outflow boundary. Weak large scale forcing is present during this event, as is the case for most
airmass convection events. A 17 MST NARR dynamic tropopause analysis, which acts as a
proxy for the upper level flow, shows KSLC centered under an upper level ridge (Fig. 14b).
Unfortunately, NARR data only extends to 100 hPa so all levels of the two potential vorticity
unit (PVU) dynamic tropopause > 100 hPa are forced to 100 hPa. 700 hPa winds were also weak,
generally around 5 m s-1, and were associated with limited 700 hPa baroclinicity (Fig. 14c) and
850 hPa height depressions (Fig. 14d) signaling very weak lower tropospheric large scale
forcing. In fact, compared to climatology, all airmass convection events have only slightly higher
700 hPa winds (Fig. 17a) and a slight affinity for southerly wind directions (Fig. 17b). The
height of the Planetary Boundary Layer (PBL) is similar to climatology (Fig. 17c) preventing
higher winds at higher levels from being fully realized at the surface.
The climatological
variables occurred on the nearest NARR 3 h time step for all days within the month of the dust
13
event. Event and climatological variables were area-averaged from 37N to 41N and 112W to
114W, roughly corresponding to western Utah south of KSLC.
16 (48%) recent events were produced by cold fronts or baroclinic troughs and they
featured a cold front or baroclinic trough passage at KSLC within 24 h of the dust observation, a
distinct frontal cloud band in visible satellite images, and evidence of a mobile pressure trough
and/or baroclinic zone in lower-tropospheric NARR analyses. Dust is usually reported right
around frontal passage. In fact, 48% and 77% of recent dust reports reported dust within an hour
and 3 hrs of frontal passage, respectively. Figure 15 shows that most reports (35%) occurred
during the hour following frontal passage, no reports occurred 3 hrs post frontal passage, and
dust was reported as early as 10 hrs before frontal passage. 4 (25%) out of the 16 events reported
dust in the prefrontal environment, 16 (100%) during frontal passage, and two (13%) in the
postfrontal environment, where prefrontal and postfrontal describe the conditions three hours
before and after surface frontal passage, respectively. On 17 MST May 10, 2004 dust reduced
visibilities to 8 km during a frontal passage that dropped temperatures 12.8 C in an hour, and
produced consistent southerly surface winds > 13.9 m s-1 during the 6 h period prior to frontal
passage. Dust plumes initiated in the prefrontal environment over the Escalente Desert and
Milford area a few hours earlier and the eastern edge of these plumes reached KSLC and
extended into Idaho at the time dust was first reported (Fig. 16a). Large scale forcing was
impressive during this time. The 17 MST dynamic tropopause analysis shows an upper level
trough positioned over Nevada with an embedded 30 m s-1 southwesterly jet on that surface
directly over KSLC (Fig. 16b). 700 hPa temperature and wind analysis shows confluent winds
along a strong baroclinic zone just upstream of KSLC (Fig. 16c), with the leading edge of the
baroclinic zone collocated with an 850 hPa height trough directly over KSLC (Fig. 16d). This
14
case is similar to other baroclinic trough or cold front events, which are often associated with
upper level troughing to the west of KSLC coupled with a low level pressure minimum, strong
baroclinicity, and strong lower tropospheric winds. In fact, the initial 700 hPa wind speeds for
all cold front or baroclinic trough events is centered around 15 ms-1 (Figure 17d), which occurs
only 4% of the time climatologically, and wind directions are only in the southwest quadrant of
the compass (Fig. 17e) meaning dust is only initially transported in southwesterly flow for these
events, The height of the planetary boundary layer is also much higher than climatology during
dust events (Fig. 17f) allowing strong winds aloft to be better realized at the surface.
4 (18%) events were forced by upstream stationary fronts or baroclinic troughs. The
major difference between this group and baroclinic troughs or cold fronts is the trough axis
remains quasi-stationary and to the west of KSLC within 24-hrs of the initial dust observation.
The event on April 1, 2003 was forced by a stationary trough and produced eight observations of
dust between 10 MST and 24 MST that dropped visibilities to as low as 8 km. Winds remained
southerly throughout the day, night, and following morning and averaged 9.8 m s-1 for all the
dust observations. Unfortunately, clouds were present over most of the region during the event,
limiting chances of observing plumes, but Figure 18a shows a small dust plume coming off the
Escalente Desert at 13:45 MST. Upper level analysis at 10 MST reveals a strong southwesterly
jet centered over northwestern Nevada ahead of an upper level trough offshore of California
(Fig. 18b). The leading edge of a 700 hPa baroclinic zone over Nevada has weakly confluent
flow that is mainly parallel to the isotherms (Fig. 18c), and an associated 850 hPa trough (Fig.
18d), suggesting the presence of a stationary front or baroclinic trough. To summarize, a cold
front or baroclinic trough is present over Nevada, but the lack of upper level support and cross
isotherm flow prevents transient motion leaving Utah positioned under persistent large scale
15
southerly flow capable of emitting dust. August 30, 2009 is listed as a stationary baroclinic
trough or cold front event, but this event may be erroneous because observer comments and
satellite imagery indicate smoke, not dust, likely reduced visibilities.
Two (6%) events were associated with other mechanisms. On September 16, 2003
KSLC reported dust in northwesterly flow when a surface front developed downstream of the
area putting KSLC in the post-frontal environment.
This event did not fit with the other
categories because there was no surface frontal passage or stationary boundary. The other event
occurred on March 13, 2005 and was forced by an equatorward travelling arctic front. Although
there was frontal passage at KSLC, the nature and evolution of an arctic front is very different
from intermountain cold fronts and baroclinic troughs so it would be inappropriate to classify it
similarly.
Past studies suggest soil moisture has a capillary effect on soil grains, which increases the
friction velocity of the soil making it less erodible (Saleh and Fryrear 1995; Bisal and Hsieh
1966; Chepil and Woodruff, 1963; McKenna-Neuman and Nickling, 1989). However, Gillette
(1999) observed wind erosion 10-30 minutes after a soaking rain because the eroding layer needs
only to be a millimeter thick and strong winds can dry a layer that thin very quickly. The NARR
soil moisture content is calculated from the soil surface down to 200 cm so it may be an
inappropriate proxy for surface dryness.
c.
Dust emission sources and transport
Additional dust days were added to our recent dust day climatology in an effort to locate
as many source regions as possible. Hourly dust observations have been reported in accordance
16
with our 10 km visibility and present weather (e.g. no dust whirls) constraints at four different
stations across the Intermountain West (IMW) since 2001 (33 at KSLC, 30 at Delta, UT, 18 at
Pocatello, ID, 6 at Elko, NV for 87 total, but 79 individual events due to overlap). The dust days
from these stations were further supplemented with personal observations of dust along the
Wasatch Front, and from Utah Avalanche Center annual reports, bringing the total number of
dust days to 94. The characteristics of these events are consistent with the long-term climatology
of KSLC in terms of annual, monthly, and diurnal distribution (not shown). Our GOES satellite
dust retrieval algorithm is applied to all 94 dust days in an effort to locate all visible dust plumes
originating in the IMW. For the 94 dust days, 120 independent plumes were identified during 47
(50%) dust days. The remaining 47 (50%) dust days may not have had any observable plumes
because clouds blocked the dust from the satellite detection, the dust occurred at night or during
a low sun angle, or the dust concentration was too weak for the detection algorithm to pick it up.
Airmass convection events and baroclinic trough or cold front events with two or fewer dust
observations were the most common types of dust events without any visible plumes.
GOES data indicates the low topographical ancient depositional environments (e.g.,
ancient lake beds) in southwest Utah and Western Nevada are the primary dust plume emission
sources for the IMW. Figure 18 shows the approximate plume origins are mostly clustered in
certain lowland regions, most notably the Sevier Desert, Milford Flat area, Escalente Desert, and
West Desert in southwest Utah and the Carson Sink in Nevada. Gillette (1999) calls small areas
of frequent dust production “hot spots”, which are depositional environments in transitional arid
regions that have had their biological and physical crust disturbed. The aforementioned areas
receive heavy recreation and agricultural use so the crusts in these areas are likely disturbed.
The plumes are mostly oriented from the SSW and SW, indicating that the plumes are mainly
17
transported in southwesterly flow. Only 8.3% of the plumes were directed towards the south and
they all originated in Nevada. The length of the plume lines only represents the length of the
plume at one particular time step and the lines are not related to plume strength or to the distance
the plume traveled. 40 (33%) plumes occurred on Delta, UT dust days, 29 (24%) on KSLC dust
days, 10 (8%) on personal notes and UAC annual report dust days, 9 (8%) on Pocatello, ID dust
days, 6 (5%) on Elko dust days, and 26 (22%) on dust days reported at multiple stations.
Interestingly, many observed plumes are not oriented towards their respective station, and days
with visible plumes have an average of 2.55 plumes, meaning there are multiple dust sources
throughout the IMW on any given dust day. Not all of the dust we observed on satellite started
as a point source. There are 11 cases when large areas of dust showed up on the satellite with no
clear origin. The majority of these cases occurred over western Nevada and moved southeast
during the day, but a couple of these cases also occurred over central Utah, and one over the
Snake River Plain of Idaho.
It is important to note that the postfrontal environment of an intermountain cold front is
usually cloudy, which blocks satellite detection of dust plumes. In an effort to avoid a bias
towards southwest transport we computed backtrajectories for all KSLC dust events since 2004.
Using the web version of the NOAA Hybrid Single-Particle Lagrangian Integrated Trajectory
model
(HYSPLIT,
http://www.arl.noaa.gov/ready/hysplit4.html;
http://www.arl.noaa.gov/ready/hysplit4.html), we computed 6 hr backtrajectories ending at 1000
m above KSLC using 40km Eta Data Assimilation System (EDAS) data. Previous studies have
used lagrangian transport models to trace atmospheric aerosols back to their source (Haller et al.,
2011; Gebhart et al., 2001), but they have employed different durations, ending heights, and used
ensembles. The 25 KSLC backtrajectories computed since 2004 reveal the majority of dust
18
comes from the south-southwest and southwest (Fig. 23) making the starting locations found
from GOES imagery a good proxy for the primary emission sources.
Only the airmass
convection event on July 26, 2006 and the arctic frontal passage on March 13, 2005 have
backtrajectories starting north of KSLC. The airmass convection event did not have a visible
dust plume, but the arctic front did have a diffuse area of dust show up over the Snake River
Plain in Idaho and move south towards KSLC. Since this was a diffuse area and not a plume it
was not recorded on the plume plot (Fig. 22). The rest of the events all point towards source
regions identified by GOES imagery.
4. Conclusions
Dust events at KSLC occurred throughout the historical record, with considerable interannual
variability. The vast majority of these events have visibilities above dust storm or severe dust
storm criteria and blowing dust is the most common observer comment.
Dust events have a
bimodal monthly distribution with a primary peak in the spring and a secondary peak in the fall.
Climatological winds have a local maximum during the spring months as well, but are fairly
consistent for the remainder of the year demonstrating that winds alone cannot explain the dust
day monthly frequency. Annual and monthly dust flux calculations offer a different perspective
than the frequency distributions, but results are very similar with the exception of a more
dramatic local minimum during the summer in the monthly dust flux distribution.
This
difference is attributed to the shorter and less intense airmass convection events common during
the summer. Winds were southerly and southwesterly at the onset of the majority of dust events
and these directions also transported most of the dust. The timing of events was skewed heavily
towards afternoon hours coinciding with the climatological diurnal cycle.
19
Using only events from 2001 – 2010, we categorized each event by the primary mechanism
for dust emissions and transport. These categories were: (1) airmass convection, (2) cold fronts
or baroclinic troughs, (3) upstream stationary fronts or baroclinic troughs, and (4) other
mechanisms.
Airmass convection events had weak upper and lower tropospheric synoptic
forcing, were shortlived, and were initiated by outflow from nearby convection. Transient and
stationary baroclinic troughs and cold fronts made up three quarters of recent dust events. They
have much higher lower tropospheric winds than climatology, a nearby baroclinic zone with
associated 850 hPa height trough, and primarily occur in the spring and fall, but stationary
baroclinic trough or cold front events have weaker upper level support and frontogenetical flow
than transient events. All baroclinic trough or cold front events reported dust within 3 h of
frontal passage.
There were only two other mechanism events that had synoptic patterns
different from the typical baroclinic trough or cold front. All categories produce stong winds
capable of emitting dust, but winds alone cannot explain the frequency of dust emission. Land
surface factors, such as soil moisture and erobability, are also important, but are beyond the
scope of this research.
GOES dust retrieval data and HYSPLIT back trajectories indicate the ancient
depositional environments (e.g., ancient lake beds) in southwest Utah and Western Nevada are
the primary dust emission sources for the Intermountain West. Specifically Sevier Desert,
Carson Sink, Escalente Desert, and the Milford Flat area are common emitters. These areas
experience high agricultural and recreational use, which are dust disturbing practices that lead to
increased dust emission. Mitigating crust disturbing practices in these areas will help decrease
dust flux over the IMW, which will improve air quality and decrease dust loading in the
mountain snowpack
20
5. Acknowledgments
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27
Table 1: DS-3505 dust-related present-weather categories, including full and abbreviated (i.e.,
used in the text) descriptions and the number of total and used reports at Salt Lake City.
Category
Full Description
Abbreviated Description
Reports
06
Widespread dust in suspension in the air, not raised by wind at or near
Dust in suspension
178 (155)
Blowing dust
905 (867)
Dust whirl(s)
1 (0)
Duststorm
2(1)
Duststorm
1 (0)
Duststorm
1 (0)
Duststorm
1 (0)
the station at the time of observation
07
Dust or sand raised by wind at or near the station at the time of
observation, but no well-developed dust whirl(s) or sand whirl(s), and
no duststorm or sandstorm seen
08
Well developed dust whirl(s) or sand whirl(s) seen at or near the station
during the preceding hour or at the time of observation, but no
duststorm or sandstorm
09
Duststorm or sandstorm within sight at the time of observation, or at the
station during the preceding hour
30
Slight or moderate duststorm or sandstorm has decreased during the
preceding hour
31
Slight or moderate duststorm or sandstorm no appreciable change
during the preceding hour
32
Slight or moderate duststorm or sandstorm has begun or has increased
during the preceding hour
33
Severe duststorm or sandstorm has decreased during the preceding hour
Duststorm
0 (0)
34
Severe duststorm or sandstorm no appreciable change during the
Duststorm
0 (0)
Duststorm
0 (0)
preceding hour
35
Severe duststorm or sandstorm has begun or has increased during the
preceding hour
28
98
Thunderstorm combined with duststorm or sandstorm at time of
Duststorm
observation, thunderstorm at time of observation
Table 2
Date
Airmass
Stationary baroclinic
Convection
trough or cold front
Baroclinic trough or cold
front
Prefrontal
Frontal
Postfrontal
Prefrontal
Frontal
Postfrontal
Prefrontal
Frontal
Postfrontal
Prefrontal
Frontal
Postfrontal
Prefrontal
Frontal
Postfrontal
Prefrontal
Frontal
Postfrontal
Prefrontal
Frontal
Postfrontal
Prefrontal
Frontal
Postfrontal
Prefrontal
Frontal
Postfrontal
Prefrontal
Frontal
Postfrontal
3/23/2002
4/15/2002
6/1/2002
X
9/16/2002
X
2/1/2003
4/1/2003
X
4/2/2003
9/16/2003
4/28/2004
5/10/2004
29
Other Synoptic
Patterns
X
X
X
X
X
X
X
X
2 (0)
7/9/2004
Prefrontal
Frontal
Postfrontal
Prefrontal
Frontal
Postfrontal
Prefrontal
Frontal
Postfrontal
Prefrontal
Frontal
Postfrontal
Prefrontal
Frontal
Postfrontal
Prefrontal
Frontal
Postfrontal
Prefrontal
Frontal
Postfrontal
Prefrontal
Frontal
Postfrontal
Prefrontal
Frontal
Postfrontal
Prefrontal
Frontal
Postfrontal
Prefrontal
Frontal
Postfrontal
Prefrontal
Frontal
Postfrontal
Prefrontal
Frontal
Postfrontal
Prefrontal
Frontal
Postfrontal
Prefrontal
Frontal
Postfrontal
Prefrontal
Frontal
Postfrontal
Prefrontal
X
10/17/2004
X
3/13/2005
4/13/2005
5/16/2005
7/22/2005
X
7/30/2005
X
5/19/2006
X
7/19/2006
X
7/26/2006
X
4/29/2008
5/20/2008
7/27/2008
X
8/31/2008
3/4/2009
3/21/2009
X
30
X
X
X
X
X
X
X
X
6/30/2009
X
8/5/2009
X
Frontal
Postfrontal
Prefrontal
Frontal
Postfrontal
Prefrontal
Frontal
Postfrontal
Prefrontal
Frontal
Postfrontal
Prefrontal
Frontal
Postfrontal
Prefrontal
Frontal
Postfrontal
Prefrontal
Frontal
8/6/2009
8/30/2009
X
9/30/2009
3/30/2010
X
X
X
X
X
X
X
X
4/27/2010
Postfrontal
Figure Captions
Fig. 1: Topography and geography of the study region.
Fig. 2: Examples of dust layering in the late-season Wasatch Mountain snowpack. (a) Ben
Lomond Peak (XXXX m), April 2005. (b) Alta 2009, (b) Alta 2010.
Fig. 3: Annual number of dust events reported at KSLC (1930 – 2010 water year)
Fig. 4: Histogram of minimum reported visibility (km) of each dust event.
Fig. 5: Histogram of annual near surface dust flux (g m-2) at KSLC
31
Fig. 6: Number of dust events by month
Fig. 7: Histogram of monthly near surface dust flux (g m-2) at KSLC
Fig. 8: Histogram of monthly KSLC observations above our arbitrary threshold velocity of 10 m
s-1
Fig. 9: Number of dust events by hour (MST)
Fig. 10: Histogram of hourly (MST) KSLC observations above our arbitrary threshold velocity
of 10 m s-1
Fig. 11: Wind Rose of initial surface wind directions for each dust event broken down by wind
speed (m s-1)
Fig. 12: Directional frequency (%) of near surface dust flux
Fig. 13: Number of dust events by month for recent dust events (2001 – 2010 water year)
Fig. 14: Analysis of the 19 May 2006 mesoscale convective event at 16 MST. (a) NEXRAD .5
degree tilt radar reflectivityat 16:05 MST, (b) NARR dynamic tropopause (2 PVU) pressure
(shaded every 50 hPa), dynamic tropopause wind barbs [full (half) barbs denote 5 (2.5) m s-1],
32
and isotachs (contoured every 15 m s-1). (c) 700 hPa temperature (every 2 C) and wind barbs ([as
in (a)]. (d) 850 hPa geopotential heights (every 40 m) and wind barbs [as in (a)].
Fig. 15: Number of dust reports relative to approximate time of frontal passage
Fig. 16: Analysis of the 10 May 2004 baroclinic trough or cold front event at 17 MST. (a)
Visible satellite image with dust highlighted in red. (b) – (d) same as in Fig. 14
Fig. 17: (a) Frequency of 700 hPa wind speed (m s-1), (b) direction, and (c) height of the
planetary boundary layer (m) for airmass convection events and [(d)-(f)] baroclinic trough or
cold front events (solid black line) compared to the NARR climatological values (dashed gray
line).
Fig. 18: Same as Fig. 16 but for stationary baroclinic trough or cold front event on 10 MST 1
Apr 2003
Fig. 19: Plot of plume origin (black cross) and orientation for KSLC (blue line), Delta (orange
line), Elko (cyan line), and Pocatello (magenta) dust events, dust events reported at multiple
stations (red), and dust events from personal notes (purple). 1 km Terrain (m) shaded.
Fig. 20: HYSPLIT 6 h backtrajectories for each KSLC dust event since 2004 ending 1000 m
above KSLC
33