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 Saturday, February 06, 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 contribute to hazardous air quality along Utah’s Wasatch Front
urban corridor and, through despoition onto the snowpack of the adjacent Wasatch Mountains,
regional hydroclimatic change. This study creates a climatology of these events using surfaceweather observations, GOES visible satellite imagery, and the North American Regional
Analysis. In hourly weather observations from the Salt Lake International Airport (KSLC), a
dust storm, blowing dust, and/or dust in suspension (i.e., dust haze) with a visibility 10 km (6 mi)
or less occurs an average of 4.3 days per water year (Oct–Sep), with considerable interannual
variability evident during the 1930–2010 study period. The monthly frequency of days with at
least one dust report is strongly bimodal with primary and secondary maxima in Apr and Sep,
respectively. Dust reports exhibit a strong diurnal modulation and are most common in the late
afternoon and evening.
Of the 33 recent (2001–2010) events, 16 were produced by transient cold-frontal troughs,
11 were produced by airmass convection and related outflow, 4 were produced by persistent
southwest flow ahead of a stationary trough, and 2 were generated by other mechanisms. During
the transient trough events, 25% of the dust reports occur with strong south–southwest winds in
the prefrontal environment, 100% during and/or within 3 h of a cold-frontal passage, and 13%
more than 3 h after cold-frontal passage. Synoptic composites and case studies further illustrate
the important parameters behind these mechanisms.
GOES satellite imagery and backtrajectories from the 33 recent events at KSLC, as well
as 61 additional events identified in the surrounding region, indicate that the primary dust
emission sources are clustered in the deserts and dry lake beds of southern Utah, the burn area of
the 2007 Milford Flat Fire, and the Carson Sink of Nevada. Efforts to reduce dust emissions in
2
these regions, especially those in southwest Utah, may help mitigate the frequency of severity of
hazardous air quality episodes along the Wasatch Front and dust loading in the adjacent Wasatch
Mountains.
1. Introduction
Despite the numerous studies on dust transport and origin in the Asian and African
Deserts, relatively few studies occurred in the western United States, and even fewer are specific
to Utah. 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). The most productive sources in these regions are the modern and ancient
depositional environments, such as dried lakebeds and runoff regions (Gillette, 1999). However,
the land surfaces in these regions are naturally resistant to wind erosion due to the presence of
physical, biological, and other vulnerable soil crusts (Gillette, 1980), but are easily disturbed.
Based on alpine lake sediments collected over the western United States, Neff et al. (2008) found
that dust loading increased by 500% following western expansion during the 19th century, a
likely consequence of livestock grazing and other activities that disturbed the natural crusts.
This post-western-settlement dust loading greatly exceeds the dust loading of the severe decadalscale mega-droughts that affected the southwest over the past 2000 years (Neff et al. 2008, p.
192).
Episodic dust events over the Wasatch Front produce hazardous air quality and contribute
to dust loading in the snowpack of the nearby Wasatch Mountains (Fig. 2), which provides water
for approximately 400,000 people (Salt Lake City Department of Public Utilities 1999). The
Wasatch Mountain snowpack is also vital for Utah's $1 billion winter sports industry,
internationally known for its “Greatest Snow on Earth” (Steenburgh and Alcott 2009). The
3
accumulation of dust in the seasonal mountain snowpack of western North America and other
regions of the world can change the regional climate and hydrologic cycle of those areas (Hansen
and Nazarenko 2004; Painter et al. 2007; Painter et al. 2010). Dust loading causes a decreased
snow albedo, which shortens the duration of snowcover by several weeks as seen in recent
studies of Colorado's San Juan Mountains (Painter et al. 2007). Dust is a multiple scatterer of
sunlight (in particular the visible wavelengths), which reduces snow’s albedo from ~0.9 down to
as low as ~0.33 (Skiles et al, in preparation) – [insert spectral albedo figure here – will send this
for Wasatch sites]. The impact in the visible wavelengths is powerful because half of the solar
radiation reaching the Earth’s surface lies in this wavelength range. Distributed hydrologic
modeling studies further suggest that the earlier snowmelt results in an advancement of peak
runoff and a decrease in annual runoff volume in the Upper Colorado River Basin (Painter et al.
2010).
Add something on air quality?????
Dust is transported in mesoscale and synoptic scale disturbances. Almost half of the dust
emission in the Sahara, the world’s largest Aeolian dust source (Swap et al., 1996), is due to the
winds associated with large mesoscale convective systems that propagate west into the North
Atlantic (Goudie, Middleton, 2001). These systems have associated dust plumes that travel in
the easterly flow for several days at a time (Carlson, 1979). In comparison, intense northeast
Asian dust events are typical in the spring and early summer in the postfrontal environment of
cold fronts associated with Mongolian Cyclones (Shoa, Wang, 2003; Qian et al., 2001).
Here we develop a climatology of dust events for the Wasatch Front and adjacent valleys
using hourly weather observations from the Salt Lake City International Airport (KSLC), hourly
weather observations from nearby stations, Geostationary Operational Environmental Satellite
4
(GOES) imagery, and the North American Regional Reanalysis (NARR). Dust-events occurred
throughout the study period (1930-2010 water years) and recent events are shown to be
associated with four different meteorological emission mechanisms.
Satellite imagery and
backtrajectories show that the Sevier Desert, Carson Sink, Escalente Dester, West Desert, and
Milford Flat area are the primary emission sources for many dust events, suggesting that
mitigation efforts in this region 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
Dust reports are identified in the Global Integrated Surface Hourly Database (DS-3505)
for KSLC, which was obtained from the National Climatic Data Center (NCDC). The analysis
covers the 1930–2010 water years (Oct–Sep) when the availability of hourly observations
exceeds XX% annually. Throughout this period the observing site was located at what is
presently known as the Salt Lake City International Airport, just east of downtown Salt Lake
City and the Wasatch Mountains (Fig. 1).
DS-3505 includes data collected and stored from multiple sources and it is decoded and
processed either at the operational weather centers or the Federal Climate Complex in Asheville,
NC (NCDC 2001, 2008). Similarly, Brazel (1989) used MF-10A surface weather observation
forms from the National Weather Service and the Local Climatological Data (LCD) publications,
which are also visual weather observations from an observer, to develop a dust climatology for
the southwestern United States. The identification and classification of dust using observer notes
is somewhat subjective and inconsistencies arise from observer bias, as well as changes in
instrumentation, reporting guidelines, and processing algorithms. These inconsistencies
5
complicate the interpretation of long-term trends and variability and likely result in the
misreporting of some events (e.g., dust erroneously reported as haze). Belnap et al. (2009) took
advantage of deposition traps in their analysis of sediment flux across the Colorado Plateau,
which offers a much more objective approach to dust detection. Nevertheless, our data offers a
sufficiently high reporting frequency to identify the general climatological characteristics and
synoptic environment of dust events.
Consistent with WMO guidelines (WMO 2009), the present weather record in DS-3505
includes 11 dust categories (Table 1). At KSLC, dust is most commonly reported as blowing
dust (category 7, 905 reports), followed by dust in suspension (category 6, 178 reports), several
categories of dust storm (categories 9, 30-32, and 98; 7 total reports), and dust or sand whirl(s)
(category 8, 2 reports). There were no severe dust storm reports (categories 33-35). Analysis of
the meteorological conditions in each category revealed 68 reports with a visibility exceeding 6
statute miles (10 km), which is near the thresholds used today by the WMO and national weather
agencies for reporting blowing dust or dust-in-suspension (Shao et al. 2003; FMH 2005). These
68 reports include all but one of the 7 total reports in the duststorm categories. Since these
events are weak, or may be erroneous, they were removed from the analysis. Reports in category
8 were also removed since we are interested in widespread events rather than localized dust
whirl(s) (a.k.a. dust devils). Thus, our analysis of dust events is based on the remaining 1023
reports of blowing dust, dust in suspension (i.e., dust haze) or dust storm with a visibility of 10
km or less. A dust day is any day (MST) with at least one such dust report.
b. Recent Events (2001-2010)
6
A recent climatology was compiled to take advantage of modern satellite and reanalysis
data. The recent ten-year climatology of dust events for KSLC was examined and each event
was given a synoptic type. Synoptic typing utilized GOES imagery, NARR data, and surface
observations and comments from the DS-3505 dataset. The NARR is a 32-km, 45-layer dataset
based on the eta model three-dimensional variational data assimilation (3DVAR) system
covering North America (Mesinger et al. 2006). This data was obtained from the National
Oceanic and Atmospheric Administration Operational Model Archive Distribution System
(NOMADS)
at
the
National
(http://nomads.ncdc.noaa.gov/#narr_datasets).
Climatic
Data
Center
web
site
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 (outlined in
Jeglum et al. 2010).
A synoptic classification scheme divided dust events into groups based off of the primary
synoptic mechanism for dust emission and transport. The three main mechanisms are airmass
convection, transient baroclinic troughs, and stationary baroclinic troughs.
The airmass
convection events usually had a thunderstorm, thunderstorm in the vicinity, or squall comment
within an hour of the dust observation(s), but events with visible convective towers on satellite
and a synoptic setup conducive to convection in the NARR reanalysis (i.e. back side of a warm
season upper level ridge) are also placed in this group. The transient baroclinic trough group had
a surface cold front passage within 24 hours of the dust observation, a distinct frontal band on
visible satellite, and evidence of a baroclinic trough or intermountain cyclone on NARR 700 hPa
temperature, and 850 hPa height fields (850 hPa heights used instead of MSLP by reasoning in
Jeglum et al. 2010) that is mobile through the research area. Events within this group are further
7
broken into prefrontal, frontal, and postfrontal. Prefrontal events have dust at least three hours
before frontal passage, frontal have dust within three hours of frontal passage, and postfrontal
have dust at least three hours after frontal passage. Events can belong to more than one of these
categories. The stationary baroclinic troughs are identified in the same manner as the transient
baroclinic troughs, but their axis never crosses KSLC in an organized manner within 24 hours of
when dust is reported.
Several events had stationary axis initially as flow around the Sierra-
Nevada created the boundary before up level support pushed the boundary through, but these
were still considered transient events as long as the boundary made it through KSLC within 24
hours. The last category is other, which is a catch all for the synoptically forced events that do
not fit the previous two categories. Other classifications schemes in the literature are Brazel and
Nickling’s (1986) five synoptic types for Arizona dust storm generation: (1) pre-frontal, (2) postfrontal, (3) thunderstorm/convective, (4) tropical disturbance, (5) upper level/cut-off low. Henx
and Woiceshyn (1980) created a hierarchy of weather-duststorm systems for the southern and
central Great Plains and their classifications were (1) dust devils, (2) Haboob, (3) Severe
Mountain Dust Storm, (4) Frontal, (5) Cyclogenic. The cyclogenic category was further broken
down into (a) low level jet, (b) upper level jet, (c) surface storm circulation, and (d) Severe
mountain downslope windstorm. Our categories are only specific to recent KSLC dust days.
Weather conditions during dust events are compared to a local ten-year (2001-2010)
monthly NARR climatology to describe the anomalous weather associated with dust events. The
climatology is specific to each of the 8 synoptic time steps (00Z, 03Z, 06Z, etc.) and is computed
over an area bounded by 37N, 42N, 112W, and 116W since many of the recent dust events
originate and are transported through this area (See Results section). This climatology was
compared to the area-averaged variables found at the onset of each dust event.
8
Surface, 700
hPa, and dynamic tropopause composites of recent events also used NARR data, which extends
from 1000 hPa to 100 hPa. Unfortunately our analysis of the 2 potential vorticity units (PVU)
dynamic tropopause is limited by the 100 hPa contraint so all higher levels of the dynamic
tropopause are forced to 100 hPa.
c. Dust Emission Sources and Transport
More stations across the northeastern intermountain west were added to the analysis in an
effort to locate all dust plumes over the recent ten year period. All stations with at least 5 years
of hourly data from 2000 to 2010 were examined within the bounds of 38N, 43N, 116W, and
109W. The dust days from these stations were added to our sample using the same criteria as
the KSLC dust days.
Personal notes from email correspondence, blog posts, and Utah
Avalanche Center annual reports also supplemented our dust days climatology for the Wasatch
Front.
A dust retrieval algorithm is applied to GOES data approximately every 15 minutes to the
daylight hours (14Z to 2Z) of each dust day going back to 2001 in an effort to track and discover
the origin of dust. GOES imager data is downloaded from the Comprehensive Large Array-data
Stewardship
System
(CLASS)
electronic
library
through
NOAA
(website:
http://www.class.ncdc.noaa.gov). Researchers have used other methods to track dust such as
visibility isoplething (Morales, 1979), taking photographs from satellite (Nakata et al., 1976),
aircraft measurements (Reid et al., 2000, Tanre et al., 2003) and, more recently, using satellite
data (Christopher et al., 2010). Christopher et al (2010) lists the Moderate Resolution Imaging
SpectroRadiometer (MODIS), Multi-angle Imaging SpectroRadiometer (MISR), Clouds and the
9
Earth's Radiant Energy System (CERES) scanner, Ozone Monitoring Instrument (OMI),
Polarization and Directionality of the Earth's Radiances (POLDER), Cloud Aerosol Lidar with
Orthogonal Polarization (CALIOP), Spinning Enhanced Visible and Infrared Imager (SEVIRI),
and Atmospheric Infrared Sounder (AIRS) as some of the satellite instruments able to observe
dust aerosols on regional to global scales. Currently, National Weather Service forecasters use a
GOES dust detection algorithm for their blowing dust product, which was created by
Cooperative Institute for Research in the Atmosphere (CIRA). GOES data has the advantage of a
large spatial and temporal scale. The basic characteristics of dust that make dust detection
possible are that dust becomes increasingly absorptive with decreasing visible wavelengths, dust
has a higher spectral absorption at 11 microns than 12 microns, and at 3.9 microns than 11
microns. Therefore the following algorithm is applied: if a pixel has a low reflectance (i.e. cloud
free), has a positive 12 and 11 micrometer brightness temperature difference, and has the 11
micrometer brightness temperature within 10 degrees Celsius of the 3.9 micrometer brightness
temperature then there is a high probability of dust in the pixel (modification of a MODIS
algorithm in Zhoa et al., 2010). Unfortunately the presence of clouds, a low sun angle, weak
dust concentrations, and low dust near the desert surface prevent dust retrieval and/or can lead to
false positives.
GOES images for all the recent events were examined and if any dust plumes were
visible over the intermountain west then the image with the best depiction of the plume was
further examined to locate the starting location and orientation of the plume. The starting
positions of the visible plumes are all located based off of the location of the first dusty pixel of
the plume and the orientation is the line from the starting positions to the center of the end of the
plume. Plumes are only recorded if the starting location is visible and the plume is visible for
10
more than one frame to protect against false positive contamination. Since the GOES data only
has 4km resolution, and the dust retrieval algorithm is contaminated with false positives, these
locations and orientations are very approximate.
In order to avoid a bias towards cloud free events and daylight events a back trajectory
analysis offered an easy way to approximate dust sources. We used the Hybrid Single-Particle
Lagrangian Integrated Trajectory (HYSPLIT) model (Draxler and Rolph, 2011; Rolph, 2011) to
compute 6 hour backjectories ending at 1000 meters above KLSC. Gebhart et al. (2001) used the
ARL-ATAD model, which is an older lagrangian parcel model, to compute back trajectories of
fine particulates at Big Bend National Park. Our trajectories were computed using the 40km Eta
Data Assimilation System (EDAS), which only has data going back to 2004. The only dataset
available with a complete record for our recent 10 year recent climatology is the 2.5 degree
NCEP/NCAR Reanalysis, so we decided to sacrifice a full temporal extend for a higher spatial
resolution. 1000 meters is used as the ending height above the surface because it is below the
typical boundary layer height for this region, but high enough to avoid substantial interference
with the ground. The six hour time frame mimics the time for dust to reach KSLC after it is
emitted into the atmosphere, which is based off satellite animation of events with visible plumes.
Backtajectory analysis cannot pinpoint source regions, but it does provide a comprehensive
approach to determine the path dust takes.
d. Case Studies
NARR data and satellite imagery were also used for the three selected case studies, along
with
archived
Nexrad
level
2
KMTX
http://www.ncdc.noaa.gov/nexradinv/).
11
data
from
NCDC
(website:
3. Results
a. Long-term climatology
Dust events at KSLC occur throughout the historical record, with an average of 4.3 per
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 reported in 1934. No effort
was made to quantify or assess long-term trends or interdecadal variability given subjective
nature of the reports and changes in observers, observing methods, and instrumentation during
the study period.
Based on current weather observing practices (Shao and Wang 2003; Glossary of
Meteorology 2000), the minimum visibility during 95.40%, 2.59%, and 2.01% dust days meets
the criteria for blowing dust (>5/8 statute miles), a dust storm (5/16-5/8 statute miles), or a severe
dust storm (<5/16 statute miles), respectively (Fig. 4). By observation, 98.04%, 1.20%, and
0.76% fall into the aforementioned categories. Thus, only a small fraction of the dust events and
observations meet dust storm or severe dust storm criteria.
Given the variability in severity, frequency, and duration, we calculated the total annual
near-surface dust-mass transport using eq. 9.81 of Shao (2008, p. 334):
****EQUATION HERE****
where C is the dust concentration in g/m2, delta-t is the reporting interval, V is the mean
horizontal wind speed in m/s of the reporting interval, VIS is the mean visibility in meters of the
reporting interval, and N is the number of dust observations each year. For all dust flux
calculations dust is assumed to be present until an observation does not report dust. For example
if a 21Z observation had dust reported with a 5 km visibility, 5 m/s wind and a sequential 22Z
12
observation had no dust reported with a 10 km visibility and 10 m/s wind then V=7.5m/s,
VIS=7.5 miles, and N=1.
Using this approach the average annual near-surface dust-mass
transport is 399.4 g/m2, with a maximum of 2810.2 g/m2 in 1935 (Figure 5). The annual dust
flux (Figure 5) provides a somewhat different perspective from the annual dust day frequency
(Figure 3) through its integration of event frequency, duration, and severity. For example, 1934
featured the most dust events, but the greatest near-surface dust-mass transport occurs in 1935.
In 2010, there were only 2 events, but this year features a pronounced decadal-scale maximum in
near-surface dust-mass transport. Although observing system limitations preclude a confident
assessment of long-term trends and interdecadal variability, these results indicate dust emissions,
transport, and deposition occurred over northern Utah throughout the historical record, consistent
with sediment cores collected from alpine lakes in Colorado and Utah, which suggest an increase
in dust accumulation beginning in the mid-1800s (e.g., Neffs et al. 2008; Reynolds et al. 2010).
The monthly frequencies of dust days (figure 6) and estimated cumulative dust flux
(Figure 7) show a bimodal distribution, with primary and secondary peaks in April and
September, respectively. The two distributions are distinctly different during the summer and
winter months. The annual dust flux is lower in the summer compared to the annual frequency
suggesting summer dust events are characterized by shorter or weaker events than the rest of the
year.
The local maximum in dust flux during January was rather surprising, but careful
examination of the data revealed a particularly strong two-day event in January of 1943 that
contributed to 83% of the January monthly mean. For the spring season of March, April, and
May an average of 237 g/m2 of dust is transported through KSLC, which roughly corresponds to
peak snowpack in the nearby Wasatch Range. Interestingly, Yasunori and Masao (2002) found a
similar bimodal monthly distribution for dust events originating in the Taklimakan desert of
13
China, which is on a similar latitude circle as Utah. The bimodal distribution at KSLC is also
very similar to the monthly frequency of strong Intermountain cold fronts Schafer et al (2008)
found, and to the intermountain cyclone frequency Jeglum et al. produced (Figure 8). Strong
intermountain cold fronts are associated with intermountain cyclones and produce persistently
strong winds that can lead to dust emission so the correlation between dust, cold front, and
cyclone frequency is expected. Also, 12 out of the top 25 strongest cold fronts identified by
Schafer et al (2008) to impact KSLC had dust reported within 24 hours of its passage.
Strong winds are needed to entrain and transport dust and the mean wind speed during
dust events at KSLC is 11.6 m/s with a sigma of 4.0 m/s. A threshold velocity for dust emission
was set to 10 m/s based on the mean and Holcombe et al. (1996) distribution of mean hourly
wind speeds for 1190 dust episodes over the Sonoran Desert being between roughly 5 and 15
m/s. Threshold velocities are contingent upon many factors such as particle size, vegetation,
biological crusts, and soil moisture (Neff et al., 2008, Belnap et al., 2009, Gillette, 1999), but the
Sonoran Desert has a similar basin and range characteristic as Utah and Nevada, and similar
alluvial and evaporite deposits that emit most of the dust. The 10 m/s threshold is obtained or
surpassed most frequently in the spring months of March and April (Figure 9) and the peak in
March is 50% higher than the annual mean occurrences and are fairly constant for the remainder
of the year with only a weak minimum in July, which is 23% below the mean. Winds alone
cannot explain the bimodality of the monthly dust distribution. Land surface processes appear to
play a major role since threshold velocities are rather high in the winter months, but dust event
frequency is at its minimum.
Dust reports also exhibit a strong diurnal cycle and are most common in the late
afternoon and evening hours (Figure 10). Mbouro et al. (1997) found a similar diurnal cycle for
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North African dust days with visibilities less than 5 km, except the diurnal cycle was not as
pronounced. Strong surface winds are needed to entrain dust and the maximum number of
occurrences with winds greater than our threshold velocity of 10 m/s occurs in the afternoon
(Figure 11), which is consistent with the timing of a well mixed boundary layer. Winds are three
times more likely to reach our threshold velocity in the afternoon than the early morning. The
peak in threshold wind occurrence (14 MST) precedes the peak in dust event start times (18
MST) by four hours and this discrepancy is likely due to the travel time needed for the dust to
travel from its source to KSLC after it becomes entrained.
The wind direction at the onset of
each event has a bimodal distribution (Figure 12). About 50 percent of the wind directions at the
onset of when dust is reported were from the south-southwest, south-southeast, or south and
another 28 percent were from the northwest, north-northwest, and north. These two modes
probably correspond to prefrontal and postfrontal conditions, respectively.
b. Recent (2001-2010) events
22 (67%) of recent dust events at KSLC are synoptically forced and 11 (33%) are forced
by mesoscale airmass convection. 16 (73%, 48% of total) of the synoptically forced events are
forced by a transient cold-frontal trough, 4 (18%, 12% of total) are stationary troughs upstream
of KSLC, and 2 (9%, 6% of total) fall into the other synoptic category. Out of the 16 transient
cold-frontal trough events, 4 (25%) reported dust in the prefrontal environment, 16 (100%)
reported dust as the surface front passed, and 2 (13%) reported dust in the postfrontal
environment. A list of all the recent events and their respective typing is presented in Table 2.
The September 16, 2003 event is placed in the other category because a trough and surface front
15
formed downstream of KSLC, placing KSLC in the postfrontal environment.
A distinct
boundary never moved through KSLC. The March 13, 2005 event is in the other category
because an arctic front coming from the north forced the dust event instead of the more typical
Intermountain Cold front that comes from the northwest.
The August 30, 2009 event is
questionable since all the other observations with low visibility after the one dust observation
had smoke comments, and smoke plumes are visible on satellite images, but this event is
included in all analyses since the dust observation cannot be disproven. Three of the airmass
convection events did not have a thunderstorm reported within an hour of the dust observation,
but satellite imagery showed convective towers in the vicinity of KSLC during the time dust was
reported and NARR analysis showed a distinct monsoonal setup.
The recent event climatology resembles the long term climatology except for a
disproportionate number of summer events. 2001 and 2007 water years do not have any reported
dust days and 2009 has a maximum of 7 days, but there is no observable recent trend in the
annual number of dust days (Figure 13). The monthly frequency (Figure 14) shows the primary
springtime peak and secondary fall peak, much like the long term climatology, but July and
August also have a local maximum. Since airmass convection dust events typically occur in the
summer months, the recent percentage of these events may be unrepresentative of the long term
mean.
NARR analysis of these events suggests upper level troughing upstream of KSLC
coupled with a low level pressure minimum is the most common setup for dust emission. The
dynamic tropopause composite (Figure 15) shows a mean trough to the west and an embedded
southwesterly jet over Utah. The 700 hPa temperature and wind analysis (Figure 16) further
shows a tight temperature gradient over Nevada being advected towards Utah and strong
16
southwest flow out ahead of the baroclinic zone. 700 hPa wind speeds associated with dust
events are considerably higher than climatology (Figure 17a) and skewed to more southwesterly
compared to climatology (Figure 18a). These features are coupled with a surface trough over
northern Utah and Eastern Nevada, which is evident on the 850 hPa height analysis (Figure 19).
The height of the planetary boundary layer is also much higher than climatology for dust events
(Figure 20a) so the strong winds above can be better realized at the surface. Interestingly, soil
moisture during dust events does not vary significantly from climatology (Figure 21). Past
studies suggest soil moisture increases the friction velocity of a soil making it harder for the soil
to become entrained in the atmosphere (Saleh and Fryrear 1995, Bisal and Hsieh 1966, Chepil
and Woodruff, 1963). McKenna-Neuman and Nickling (1989) showed how the capillary effect
of soil moisture held sand grains together thus decreasing erodibility. 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 in a very short time frame.
The NARR soil moisture content is calculated from the soil surface down to 200 cm.
Transient cold-frontal trough events and airmass convection events were compared
against climatology and 700 hPa wind speeds were drastically different comparatively for these
two groups. The maximum for the transient troughs is centered around 15m/s (Figure 17b),
which occurs only 4% of the time climatologically. The airmass convection events roughly
following climatology and are only skewed slightly towards higher values (Figure 17c). Wind
directions are only in the southwest quadrant of the compass for transient trough events (Figure
18b), whereas airmass convection events have some more northerly directions (Figure 18c).
Planetary boundary layers are larger than climatology for both groups (Figures 20b and 20c), but
soil moisture does not appear to be any different from climatology (Figures 21b and 21c).
17
b. Dust emission sources and transport
Dust is reported in the hourly observations at four different stations across the IMW since
2001 (33 at KSLC, 30 at Delta, 18 at Pocatello, 6 at Elko 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 bringing the total number of dust days to 94 (15
from personal notes). The characteristics of these events are consistent with the long-term
climatology of KSLC in terms of annual, monthly, and diurnal distribution. A 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. Out of the 94 dust days 47 (50%) had visible plumes. 120 plumes were
identified so each dust day with visible dust had an average of 2.55 plumes. The remaining 50%
of dust days did not have any observable plumes due to at least one of the following reasons: (1)
clouds blocked the dust from the satellite, (2) the dust occurred at night or during a low sun
angle, or (3) the dust concentration was too weak for the detection algorithm to pick it up.
Airmass convection events and frontal passages with only a couple of dust observations were the
most common types of dust events without any visible plumes.
GOES data indicates the 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 22 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 (Figure 22a) and the Carson Sink in Nevada (Figure 22b). 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.
18
The
aforementioned areas receive heavy recreation and agricultural use so the crusts in these areas
are likely disturbed. The plumes are mostly directed to the NNE and NE indicting they are
transported in SSW and SW flow. The extent 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.
Only (8.3%) of the plumes had a southerly component and they all
originated in Nevada. These plumes all formed in a postfrontal environment after a frontal band
on satellite moved over the region. It is important to note that the postfrontal environment of an
intermountain cold front is usually cloudy, which effectively blocks satellite detection of dust
plumes. 40 (33%) of the plumes coincide to Delta, UT dust days, 29 (24%) to KSLC, 10 (8%) to
personal notes, 9 (8%) to Pocatello, ID, 6 (5%) to Elko, and 26 (22%) coincide to dust days
reported at multiple stations. Interestingly, many of the dust days do not have plumes directed
towards their respective stations. There are several possible explanations for this, including
plumes directed towards the station are blocked by clouds, new plumes form near the station
after sunset, or the dust over the station is too diffuse to be detected by satellite. 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 and then were transported with the flow. 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 of the Snake River Plain of Idaho.
Back trajectory analysis for all events indicates the plume starting locations from the
GOES imagery are a good proxy for the primary emission sources for all KSLC dust events.
Archived six hour back trajectory for the 25 KSLC dust days since 2004 reveal the majority of
dust comes from the south-southwest and southwest (Figure 23). Only two out of the 25 back
trajectories have a starting point north of KSLC; an airmass convection event on July 26, 2006
19
and the “other mechanism” event on March 13, 2005, which is an arctic frontal passage. 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 (Figure 22). The
rest of the events all point towards source regions identified by GOES imagery.
c. Case Studies
a. 5/10/2004
May 5, 2004 has a common setup for a transient trough induced dust event. The day
starts with a baroclinic zone over western Nevada out ahead of a landfalling Pacific trough. By
15Z a weak surface trough takes shape and 700 hPa winds reach 25 m/s downstream in the
prefrontal environment (Figure 24a). The frontal band is seen clearly over northwestern Nevada
on visible satellite, but dust is not detected (Figure 25a), and winds are only 4.9 m/s from the
south at KSLC. By 18Z the trough has become better organized and has shifted to the east
(Figure 24b) and satellite imagery shows a similar shift to the frontal band and the initiation of
dust near the frontal boundary in Nevada and over western Utah (Figure 25b). At 21Z the
baroclinic zone nears northern Utah and 850 hPa heights continue to fall indicting a deepening
intermountain cyclone (Figure 24c). The frontal band is now more organized and further east
with pre and post frontal dust visible over Nevada, and a very distinct prefrontal dust plume
originating from the Escalente Desert and Milford area and extending to near the Utah/Idaho
border (Figure 25c). Winds at KSLC during this time are at 16.5 m/s from the south-southeast,
but dust is not yet reported. The surface trough nears KSLC at 00Z on May 11, 2004 with
apparent deformation frontogenesis (Figure 24d) and the prefrontal dust plume reached KSLC
20
and extended well into Idaho (Figure 25d). Visibilites at KSLC dropped to 8 km at 23Z with a
haze comment and dust was reported at 0Z with winds at 15.6 m/s from the south. By 1Z the
wind shifted to the north-northwest, the temperature dropped nearly 13 degrees celcius signaling
frontal passage, but visibility remained at 8 km with dust observed.
c. 10/17/2004
The event on 0ctober 17, 2004 is a stationary trough example that produced one
observation of dust at 05Z and dropped visibilities to 10 km. At 18Z on the 16th, a zonal
baroclinic zone set up over the Pacific Northwest (Figure 26a) out ahead of an approaching
trough. Six hours later, at 0Z on the 17th, a weak surface trough formed over central Nevada and
700 hPa southwesterly winds intensified downstream (Figure 26b). Unlike the May 10th event,
the baroclinic zone remained weak over Nevada and the upper level potential vorticity support is
much less (not shown). One hour after the dust observation, at 6Z, a surface trough is well
established and strong southwesterly winds are evident over Utah.
Isotherms are oriented
meridionally over western Nevada, but the gradient is weak (Figure 26c). During this time
KSLC has winds at 12.1 m/s from 160 degrees and visibility at 13 km. Unfortunately the sun
had already set so satellite imagery could not resolve any dust plumes. Six hours later, at 12Z,
the surface trough and baroclinic zone over Nevada remained fairly stationary and weakened, but
a new surface trough formed over western Idaho (Figure 26d).
This feature eventually
strengthens downstream of KSLC so KSLC never experiences a frontal passage. All the while a
longwave upper level trough with its PV support sits off the west of coast of the United States
preventing transient cold-frontal trough event from happening.
21
b. 5/19/2006
The 11 airmass convection events over the recent period of record are characterized by
very similar parameters. All of these events occurred between the middle of May and the middle
of September in a monsoonal surge scenario. May 19th, 2006 is the earliest in the year of these
events, but still demonstrates the typical setup. Figure 27 shows the surface convective available
potential energy (CAPE), 700 hPa winds, and the atmospheric column precipitable water (PW).
Both the CAPE and PW have a local maximum over KSLC. Although CAPE values around 400
J/kg, and PW values around 20 mm are not that impressive compared to other regions in the
United States, it is substantial enough for convection in Utah. As mentioned earlier, one of the
distinguishing factors between airmass convection dust events and synoptic dust events is the
700 hPa wind speed, which is much lower for airmass convection. These low wind speeds mean
even with the 700 hPa winds mixing down to the surface they would still be insufficient in
entraining dust. Strong winds must originate in smaller mesoscale processes within convection.
At 23Z on May 19th, 2006 winds at KSLC are only 4.5 m/s from the south and this is the
strongest they have been all day. Then, at 23:07 winds go to 24.1 m/s and gust to 27.7 m/s and
dust is reported along with a squall at or within sight of the station comment. Radar imagery just
two minutes before reveals a convective cell very close to the station with returns greater than 60
dBZs (Figure 28). By 01Z on the 20th winds are back down to 4 m/s since the storm is fully
passed.
4. Conclusions
Dust events at KSLC occur throughout the historical record, with considerable interannual
variability. The vast majority of these events have visibilities above dust storm or severe dust
22
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 do not have the same distribution, but do have a local maximum during the
spring months. 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 from shorter and less intense airmass
convection events primarily occurring in the summer. A third of all recent events at KSLC are
forced by mesoscale airmass convection and the rest are synoptically forced. Almost three
quarters of synoptically forced events are transient cold-frontal troughs with associated
intermountain cyclones and cold fronts. Dust is reported within 3 hours of frontal passage in
each of these events and in the prefrontal environment 25% of the time. Only two of these
events had dust three hours after frontal passage in the postfrontal environment. The remaining
synoptically forced events are stationary surface troughs upstream of KSLC and events that do
not fit these guidelines.
NARR analysis suggests upper level troughing upstream of KSLC coupled with strong
southwest 700 hPa winds and a low level pressure minimum are commonly associated with
KSLC dust events. These composites are skewed towards the synoptic events since airmass
convection 700 hPa wind speed and direction are closer to climatology. Planetary boundary
layer heights are higher than climatology for all dust events. Surprisingly, soil moisture in the
NARR analysis does not appear to effect dust emission.
GOES data and HYSPLIT back trajectories indicate the ancient depositional
environments (e.g., ancient lake beds) in southwest Utah and Western Nevada are the primary
23
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 intermountain west, which will improve air quality and decrease dust loading in the mountain
snowpack
5. Acknowledgments
6. References
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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.
28
Fig. 3. Blowing dust climatology at Delta, UT. (a) Blowing dust reports by year. (b) Blowing
dust reports by month. (c) Blowing dust reports by time of day (may need to normalize
by report frequency given Delta’s frequent lack of reporting). (d) Wind roses of all
blowing dust events. (e) Wind roses of blowing dust events with visby less than 1 mile.
Fig. 4. Same as Fig. 2 except for KSLC.
Fig. 5. Dust layer climatology derived from UAC reports. (a) Number of events by year. (b)
Number of events by month.
Fig. 6. Satelite (dust product), MesoWest, and manual surface analysis during selected events.
(a) 2002 Tax Day storm, (b-?) other events. Maybe pick four.
Fig. 7. Camera images from selected events, especially the nasty 5 April (date?) 2010 event with
the haboob.
Fig. 8. Meteograms for (a) Delta and (b) KSLC during the 10-12 April 2010 event.
Fig. 9. PM2.5 or PM10 for sites in the Salt Lake Valley during the 10-12 April 2010 event.
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
06
Full Description
Widespread dust in suspension in the air, not raised by wind at or near
the station at the time of observation
29
Abbreviated Description
Dust in suspension
Reports
178 (155)
07
08
09
30
31
32
33
34
35
98
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
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
Duststorm or sandstorm within sight at the time of observation, or at
the station during the preceding hour
Slight or moderate duststorm or sandstorm has decreased during the
preceding hour
Slight or moderate duststorm or sandstorm no appreciable change
during the preceding hour
Slight or moderate duststorm or sandstorm has begun or has increased
during the preceding hour
Severe duststorm or sandstorm has decreased during the preceding
hour
Severe duststorm or sandstorm no appreciable change during the
preceding hour
Severe duststorm or sandstorm has begun or has increased during the
preceding hour
Thunderstorm combined with duststorm or sandstorm at time of
observation, thunderstorm at time of observation
Blowing dust
905 (867)
Dust whirl(s)
1 (0)
Duststorm
2(1)
Duststorm
1 (0)
Duststorm
1 (0)
Duststorm
1 (0)
Duststorm
0 (0)
Duststorm
0 (0)
Duststorm
0 (0)
Duststorm
2 (0)
Table 2
Date
Airmass
Convection
Persistent SW
Flow
Transient trough
Prefrontal
Frontal
Postfrontal
Prefrontal
Frontal
Postfrontal
Prefrontal
Frontal
Postfrontal
Prefrontal
Frontal
Postfrontal
Prefrontal
Frontal
Postfrontal
Prefrontal
Frontal
Postfrontal
Prefrontal
3/23/2002
4/15/2002
6/1/2002
X
9/16/2002
X
2/1/2003
4/1/2003
X
30
Other
X
X
X
X
Notes
4/2/2003
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
Frontal
9/16/2003
4/28/2004
5/10/2004
7/9/2004
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
31
X
Trough forms to the south
X
X
X
Arctic front
X
X
X
X
X
X
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
8/31/2008
3/4/2009
3/21/2009
X
6/30/2009
X
8/5/2009
X
8/6/2009
8/30/2009
X
9/30/2009
3/30/2010
4/27/2010
32
Stationary trough at first
X
Stationary trough at first
X
X
X
Many Smoke Obs
X
X
X
X
X
X