Dust Storm Over the Black Rock Desert: Larger‐Scale Dynamic Signatures John M

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 116, D06113, doi:10.1029/2010JD014784, 2011

Dust storm over the Black Rock Desert: Larger‐scale dynamic signatures John M. Lewis,1,2 Michael L. Kaplan,2 Ramesh Vellore,2 Robert M. Rabin,1,3 John Hallett,2 and Stephen A. Cohn4 Received 19 July 2010; revised 3 December 2010; accepted 7 January 2011; published 29 March 2011.

[1] A dust storm that originated over the Black Rock Desert (BRD) of northwestern Nevada is investigated. Our primary goal is to more clearly understand the sequence of dynamical processes that generate surface winds responsible for entraining dust from this desert. In addition to reliance on conventional surface and upper‐air observations, we make use of reanalysis data sets (NCAR/NCEP and NARR)—blends of primitive equation model forecasts and observations. From these data sets, we obtain the evolution of vertical motion patterns and ageostrophic motions associated with the event. In contrast to earlier studies that have emphasized the importance of indirect transverse circulations about an upper‐level jet streak, our results indicate that in this case the transition from an indirect to a direct circulation pattern across the exit region of upper‐level jet streak is central to creation of low‐level winds that ablate dust from the desert. It is further argued that the transition of vertical circulation patterns is in response to adjustments to geostrophic imbalance—an adjustment time scale of 6–9 h. Although unproven, we suggest that antecedent rainfall over the alkali desert 2 weeks prior to the event was instrumental in lowering the bulk density of sediments and thereby improved the chances for dust ablation by the atmospheric disturbance. We comprehensively compare/contrast our results with those of earlier investigators, and we present an alternative view of key dynamical signatures in atmospheric flow that portend the likelihood of dust storms over the western United States. Citation: Lewis, J. M., M. L. Kaplan, R. Vellore, R. M. Rabin, J. Hallett, and S. A. Cohn (2011), Dust storm over the Black Rock Desert: Larger‐scale dynamic signatures, J. Geophys. Res., 116, D06113, doi:10.1029/2010JD014784.

1. Introduction eterization of this widespread dust in the governing equa- tions for global circulation models. This is due in part to the [2] The impact of dust storms on human activity is often delicate interplay between reflection of sunlight by the dust dramatic as in the infamous San Joaquin Valley dust storms and the trapping of upwelling radiation by this same dust of 1977 [Wilshire et al., 1981] and 1991 (the “Interstate‐5 [Idso and Brazel, 1977; Miller and Tegan, 1998]. Despite Storm” [Pauley et al., 1996]) that caused a multitude of these uncertainties, the recent successful application of accidents and associated loss of life, and in the erosion of regional dynamical simulation to dust transport over the agricultural land so evident during the 1930s over the Gobi provides some encouragement [Liu et al., 2003]. Midwestern USA—the “Dust Bowl” years [Worster, 1979; [3] Prediction or analysis of a dust storm requires Schubert et al., 2004]. Further, the long‐term and wide- knowledge of the surface soil characteristics and the atmo- spread suspension of dust in the atmosphere that stems from spheric processes that give rise to the wind. Determination disturbances over the world’s gigantic deserts (such as the of soil characteristics—crustal roughness, bulk density, Gobi and Sahara) certainly impact climate [Idso, 1976; moisture content, etc.—and the relationship of these char- Washington et al., 2003; Goudie and Middleton, 2006; acteristics to the likelihood of dust storm generation is one Goudie, 2009]. Yet, significant uncertainty attends param- of the most challenging aspects of prediction. In part, this difficulty stems from reliance on remote observations from 1National Severe Storms Laboratory, NOAA, Norman, Oklahoma, space and the incompleteness and uncertainty of these USA. observations. But we are also ignorant of the micro- 2Division of Atmospheric Sciences, Desert Research Institute, Reno, meteorological processes that lead to lift of the sediments— Nevada, USA. 3 for example, sand/sediment entrainment and saltation [see Space Science and Engineering Center, Madison, Wisconsin, USA. Bagnold, 1973; Shao, 2000]. Laboratory work indicates that 4Earth Observing Laboratory, National Center for Atmospheric Research, Boulder, Colorado, USA. strong shear in the presence of unstable stratification, in essence a small Richardson number, is conducive to lift Copyright 2011 by the American Geophysical Union. [Richardson, 1920; Bagnold, 1973; Stull, 1988]. Unpublished 0148‐0227/11/2010JD014784

D06113 1of22 D06113 LEWIS ET AL.: LARGER-SCALE DYNAMIC SIGNATURES D06113 laboratory work by coauthor J. Hallett also indicates that a is then viewed macroscopically with the aid of satellite vortex ring whose axis is perpendicular to a dusty surface imagery and surface‐based measurements of particulate causes the dust to expand outward and upward while matter. Upper‐air analysis follows with support from the remaining undisturbed at the stagnation point directly beneath 6 h updates of NCEP (National Center for Environ- the ring. And although we know that drought conditions favor mental Prediction)/NCAR (National Center for Atmospheric dust storms, it has now become apparent that antecedent Research and 3 h updates of NARR (North American rainfall or run‐off, occurring on the order of several weeks Regional Reanalysis). The NCEP/NCAR data set is a before a dust storm event, is a positive contributor. This synoptic‐scale analysis with 2.5° grid resolution [Kalnay influence stems from salt efflorescence (production of pow- et al., 1996], and the NARR data set is a subsynoptic‐ dery salts) and swelling of certain clays that serve to reduce scale analysis with a 32 km grid resolution [Mesinger et al., the bulk density of the sediment [Gillies et al., 1999; Bullard 2006]. These archived data sets permit analysis of dynam- et al., 2008; J. A. Gilles, personal communication, 2009]. ically consistent vertical motion fields. The secondary ver- [4] The initial source of dust for our study comes from the tical circulations are viewed in the context of dynamic Black Rock Desert (BRD) located in northwestern Nevada. imbalance and adjustments to imbalance. We end with The BRD and its neighboring desert, the Smoke Creek section 7, where we present: (1) a comparison/contrast of Desert (SCD), are dry lakebeds (playa) that lie within our results with those from earlier investigations, and (2) a the late Pleistocene Lake Lahontan. A topographical map summary and schematic diagram of the physical processes including the area covered by these deserts is found in germane to the dust storm over the BRD appended by a list Figure 1. A panoramic view of the BRD from a mountain of key signatures that portend the likelihood of dust storms location west of the desert, and an aerial view of the BRD over the western United States. and the SCD are found in Figure 2. These deserts are often referred to as “alkali deserts” where the dominant elements are those associated with alumino‐silicate minerals (e.g., Si, 2. Background Information on Dust Storms: Al, K, and Ti) [Gillies et al., 1999]. In line with statements General and Case Specific ‐ above regarding the influence of antecedent rainfall and run [7] Throughout this research paper we make frequent off from neighboring mountains, we examine precedent reference to various upper air stations in the U.S. West conditions over these deserts; but our primary goal focuses Coast and surface weather stations in Nevada. These stations on understanding the atmospheric processes that produce the along with topographical and geographical features in low‐level winds responsible for raising dust from these northwestern Nevada are shown in Figures 1 and 3. deserts. [5] As shown by Danielsen [1968, 1974a, 1974b] and 2.1. Dynamics Linked to Dust Storms Pauley et al. [1996], large‐scale ascent/descent couplets in [8] Two categories of dynamical processes have been the vicinity of the jet stream or jet streaks (maxima in wind linked to dust storms: (1) storms associated with cyclogen- speed along the axis of the jet stream) were central to esis where Danielsen’s paradigm of large‐scale descending atmospheric processes that led to dust storms. Reviews of trajectories is the central theme [Danielsen, 1968, 1974a, these vertical circulations around the jet stream are provided 1974b; Pauley et al., 1996] (also http://marrella.meteor. by Keyser and Shapiro [1986] and Carlson [1991]. Eliassen’s wisc.edu/Martin_2008.pdf, and J. E. Martin, personal com- several‐decadal studies of vertical circulations—secondary munication, 2010), and (2) secondary vertical circulations circulations—were more general and not restricted to cou- where geostrophic adjustments around jet streaks are central plets near the jet stream. He viewed these circulations as to the generation of surface winds [Karyampudi et al., responses to imbalance in a variety of dynamical systems— 1995]. In both cases, deep mixed layers adjoining the sur- in the context of quasi‐static vortex motion [Eliassen, 1952], face are often present. The dust storms that originate over quasi‐geostrophic dynamics in the vicinity of frontal systems the world’s gigantic deserts are typically associated with [Eliassen, 1962] (Sawyer‐Eliassen circulations – primarily cyclogenesis in springtime [Liu et al., 2003]. In these based on Sawyer [1956] and Eliassen [1962]), and in the storms, the transport is often on a continental/hemispheric more general primitive equation dynamics [Eliassen, 1983; space scale as found for the April 2001 storm that formed T. Iversen, personal communication, 2010]. And it is fair to over the Gobi Desert. This voluminous dust plume was say that stimulation for this line of research came from the tracked for 9 days (6–14 April) as it moved from the Gobi to pioneering geostrophic adjustment studies of Rossby [1937, the western USA [Szykman et al., 2003]. 1938] and Obukhov [1949] (English translation of Obukhov [1949] is available from the authors). Although these 2.2. Climatology of Dust Storms in North America pioneers relied on simplified dynamics (“shallow water” [9] Compared to the southwestern plains of the United dynamics), they provided insight into the temporal scales of States and the northeastern plains of Mexico—notably that adjustment for their system—a fast gravity wave mode and area on the eastern edge of the Rocky Mountains and the the slower inertial mode with periods of several hours to the Sierra Madre Oriental Mountains that runs from Monterrey half pendulum day (∼17 h at 45° latitude), respectively. A (Mexico) up through western Texas and along the border of masterful pedagogical review of these works is provided by Kansas and Colorado—the state of Nevada has relatively Blumen [1972]. few dust storms [Changery, 1983]. Based on the annual [6] To set the stage for our study, we review the average number of hours with visibility less than 3 miles dynamical processes that have been linked to dust storms (about 5 km) due to dust storms, the southwestern plains of and present a brief summary of dust storm climatology in the U. S. have a maximum of 45 h (centered near Lubbock, the USA. The particular dust storm event in February 2002 TX). Nevada, on the other hand, has a maximum of 20 h

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Figure 1. (left) A map showing the locations of the upper‐air stations and (right) topographic map of Nevada (source: 1 km resolution United States Geological Survey data) and the surface stations, used in the entire study (Black Rock Desert (BRD; elevation = 1193 m)); the Smoke Creek Desert (SCD) lies between the two stations GEL (Gerlach) and Smoke Creek (SMC). D06113 D06113 LEWIS ET AL.: LARGER-SCALE DYNAMIC SIGNATURES D06113

Figure 2. (a) A view of BRD from Calico Mountain (northwest of BRD) (courtesy of Stan White), and (b) an aerial view of BRD and SCD (source: Google Earth; http://earth.google.com). Alkali deserts appear white in Figure 2b.

(near Lovelock (LOL), NV). The frequency of dust storms 2.3. Precipitation Antecedent to the Dust Storm and associated visibility for several stations in Nevada are [10] The dust storm over the BRD was initiated at displayed in Table 1 (locations shown in Figure 1). In every approximately 2100 UTC on 28 February 2002. It occurred category of dust duration and associated poor visibility, in conjunction with an upper‐level jet streak that passed LOL ranks first. Since the primary dust emission sources are northeast of the BRD. We follow the evolution of this jet northwest of LOL, an area devoid of reporting stations in the streak and associated larger‐scale features over a 48 h Changery [1983] study, the maximum frequency of dust period, from 0000 UTC 27 February 2002 until 0000 UTC storms in Nevada likely lies northwest of this station. A brief 1 March 2002. Two weeks prior to the dust storm, wide- review of the literature relevant to dust storms in the con- spread precipitation occurred over NW Nevada. This pre- tiguous U. S. is provided by Orgill and Sehrnel [1976]. cipitation was associated with a surge of midtropospheric to

Figure 3. Geomorphology of Nevada. This map follows the style of Erwin Raisz, artist/illustrator of U.S. 40 Cross Section of the United States [Stewart, 1953, p. 234].

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Table 1. Dust Climatology for Cities in Nevadaa Visibility Visibility Visibility <11.2 km Visibility <1 km Visibility Visibility <11.2 km <1 km, (number of (number of <11.2 km (duration <1 km (duration City (hours/year) (hours/year) episodes/year) episodes/year) (hours)/episode) (hours)/episode) Lovelock (LOL) 43.4 12.30 19.7 8.5 2.2 1.5 Fallon (NFL) 27.2 2.70 19.6 5.0 1.4 0.5 Las Vegas (LSV) 18.9 1.20 13.2 3.6 1.4 0.3 Battle Mountain (BAM) 10.6 0.90 5.6 0.7 1.9 0.6 Winnemucca (WMC) 10.9 1.3 5.8 0.9 1.9 1.4 Elko (LKN) 6.1 0.10 3.1 0.3 2.0 0.3 Ely (ELY) 3.3 0.01 2.9 0.1 1.1 0.1 Tonopah (TPH) 2.3 0.1 2.0 0.2 1.2 0.7 Yucca Flats (UCC) 2.1 0.01 1.3 0.1 1.6 0.2 Reno (REV) 1.1 0.01 0.6 0.03 1.6 0.2 aStation abbreviations are given in parentheses, and their locations are shown in Figure 1. lower‐tropospheric moisture that traveled from the mid‐ 3.1. Evolution of the Vorticity Pattern Pacific to northern California and Nevada. The rainfall totals [14] The 500 hPa vorticity patterns displayed in this sec- (including the liquid equivalent of snow) for stations in NW tion are based on Bellamy’s graphical method [Bellamy, Nevada are listed in Table 2 (station locations are shown in 1949]. This method calculates relative vorticity at the cen- Figure 1). Although this was the only significant precipita- troids of adjoining triangles where triangle vertices are the tion event in NW Nevada during the month of February locations of the upper‐air wind observations. This approach 2002, the amount of precipitation exceeded the monthly obviates the need to interpolate wind observations to grid average at five of the seven stations. Gerlach (GEL on points. Such interpolation generally introduces interpolation Figure 1), at the southern end of the BRD, was left with noise—that is, it alters the observations and introduces – more than an inch of precipitation over a 3 4 day period. uncertainty into kinematic fields such as convergence/ divergence and vorticity. These kinematic fields are espe- 2.4. Satellite Imagery of the Dust Storm cially sensitive to incremental changes in the wind vectors. [11] Visible imagery from National Oceanic and Atmo- By choosing adjoining triangles of smallest area we analyze spheric Administration’s (NOAA’s) Geostationary Opera- vorticity on a spatial scale intrinsically tied to station sepa- tional Environmental Satellite (GOES‐8) indicated that dust ration. The structure of the field follows from scalar analysis was raised from the playa (NE corner of the BRD) at 2045 of the point values of vorticity at centroids of the triangles UTC 28 February 2002 (1300 LST 28 February 2002). The [Saucier, 1955]. visible image at this time and those at subsequent times are [15] At 1200 UTC (27 February), 36 h prior to the onset shown in Figure 4. Based on distinct features at the leading of the dust storm, an upper‐air disturbance was evident over edge of the dust storm, the southward‐directed propagation southwestern Yukon Territory (Figure 6a). The vorticity speed of the storm was estimated to be 10 m s−1. At 0102 UTC center at this time is located just south of Whitehorse (YXY 1 March 2002 (1700 LST 28 February 2002) (Figure 4d), in Figures 6a and 6b). The progression of this disturbance is the leading edge of the storm has just passed through Fallon shown in Figures 6c and 6d. The positive relative vorticity and is slightly north of Reno (REV in Figure 1). Dust striae center intensified and moved southward by 0000 UTC at the front edge of the dust extend backward toward the (28 February). At this time there is evidence of a jet streak NNE—an alignment close to the basin‐range orientation that runs from Annette Island, AK (PANT) down through shown in Figure 1. Quillayute, WA (UIL). Some wavering in subjective analyses of this vorticity center is evident over the 36 h period 2.5. PM10 Observations from 0000 UTC (28 February) to 1200 UTC (1 March), but [12] The particulate matter from the northern Nevada playa is generally classified as fine silt or clay with diameters Table 2. Precipitation in Northwestern Nevada During February in the range of 2–8 mm or less (see Udden‐Wentworth 2002a particle scale of Shao [2000, Table 5.2]). Evidence of the storm’s passage through Reno is found from observations of Total Percent Above (+) PM (particulate matter of aerodynamic diameter ≤ 10 mm). Precipitation or Below (−) Precipitation 10 Feb 2002 Normal, 15–20 Feb Observations of PM10 concentration are shown in Figure 5. Station (inches) Feb 2002 (inches) Based on this time series, the dust storm lasted about 8 h in Reno. Denio Jct (DJC) 0.91 +18 0.91 (18–20) Gerlach (GEL) 1.17 +60 1.06 (17–20) Lovelock (LOL) 0.39 −19 0.39 (19–20) Reno (REV) 0.24 −77 0.12 (15–17) 3. Salient Features of Upper‐Air Circulation Rye Patch (RYP) 0.71 +3 0.70 (17–20) Smoke Creek (SMC) 0.82 +3 0.78 (16–20) [13] In future reference to time designation, we numerically Winnemucca (WMC) 0.89 +44 0.87 (17–20) abbreviate as follows: hours and minutes UTC (month/day) aNumbers in parentheses in Precipitation 15–20 Feb refer to the days in i.e., 1200 UTC 27 February 2002 will be denoted as February when precipitation was recorded. Locations of the stations are 1200 UTC (27 February). shown in Figure 1.

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Figure 4. GOES‐8 satellite imagery of cloud and dust valid at (a) 2045 UTC, (b) 2215 UTC, (c) 2315 UTC (28 February), and (d) 0102 UTC (1 March) (1700 LST 28 February 2002). Locations of BRD, SCD, Reno, and Fallon are indicated. Arrows point to the area covered by dust storm.

Figure 5. Observed PM10 (particulate matter of aerodynamic diameter ≤ 10 mm) concentration (units in mgm−3) at Reno, Nevada.

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Figure 6

7of22 D06113 LEWIS ET AL.: LARGER-SCALE DYNAMIC SIGNATURES D06113 this fluctuation is due in part to the limited resolution of NE wind and the temperature gradient, cold air advection the rawinsonde network (∼400 km station spacing and amounts to 0.58°C h−1. One can assume that the other pri- 12‐hourly balloon launch cycle). What is beyond doubt is mary contribution to cooling in the absence of high the large‐amplitude positive vorticity center in the triangular humidity/precipitation stems from adiabatic cooling, i.e., region between Boise (BOI), Elko (LKN), and Reno (REV) adiabatic ascent in a stably stratified atmosphere. Judging at 0000 UTC (1 March). The rapid amplification of this from the vertical motion pattern displayed in Figure 7, vorticity center between 1200 UTC (27 February) and there is weak lifting at 500 hPa over BOI at 1200 UTC 0000 UTC (1 March) was followed by rapid decay. It is (28 February). This gives way to descending motion by appropriate to mention that the evolution of vorticity as well 0000 UTC (1 March). To achieve adiabatic cooling the as amplitude that came from the Bellamy approach was in order of 0.5°C h−1 at BOI, ascent of 2–3cms−1 is required. close agreement to the NARR reanalysis. The reanalysis failed in this context. Thus, the vertical motion fields from the NCEP reanalysis are inconsistent 3.2. Transverse Circulation with the observed cooling at BOI. This inconsistency is discussed further in section 6. [16] We rely on the reanalysis to provide information on the evolution of the synoptic‐scale vertical motion. The [19] Cooling occurred at REV, delayed relative to the analysis of vertical motion greatly benefits from these cooling at BOI. In Figure 8, we display the Reno soundings dynamically consistent analyses that blend forecasts with at 2324 UTC (28 February) and 1116 UTC (1 March). The observations. The 500 hPa NCEP/NCAR reanalyzed verti- soundings were taken by NCAR personnel as part of the special field program for graduate students at the University cal motion between 0000 UTC (28 February) and 0000 UTC ‐ (1 March) is shown in Figure 7. At 0000 UTC (28 February), of Nevada Reno [Cohn et al., 2004, 2006]. The tropopause the ascending branch of the circulation pattern resides over is close to the 250 hPa level based on the isothermal structure of the atmosphere above this level. Comparison of the left side (NE side) of the exit region with weaker descent – to the right (SW side) of the exit region. This pattern is soundings in Figure 8b indicates a 12 h cooling of 10° 20°C similar to the four‐quadrant model of a jet streak as discussed below 700 hPa and 5°C atop the inversion. by Carlson [1991, section 14.1]—dynamically forced ascent (descent) in the left (right) exit region and right (left) entrance region (entrance regions not shown in Figure 7). 4. Surface Analysis [17] In this case, the vertical motion couplet is consistent 4.1. Evolution of the Gust Front with an indirect circulation pattern, i.e., a pattern where the [20] We have amassed reports from first‐order stations cold air northeast of the exit region is rising and the warm (manned National Weather Service stations at airports) and air to the southwest of the exit region is sinking. Over the stations in the Regional Automated Weather Station (RAWS) next 24 h period, the jet streak moves southeastward and the ’ network. The locations of these stations are displayed in ascending branch migrates southwestward across the jet s Figures 1 and 3, and the observations are shown in Figure 9. exit region until it covers most of northern and central Visibilities are not recorded at the RAWS stations and we Nevada at 0000 UTC (1 March). At this time, the ascent/ only show visibilities less than 10 miles (16 km) at the first‐ descent couplet is principally oriented along the jet streak as order stations. opposed to across the streak. The development of curvature [21] As one macroscopically views the surface reports in in the geopotential field—creation of a smaller‐scale trough — Figure 9, the feature that is most intimately tied to the dust from BOI toward REV shown at 0000 UTC (3/1) is an storm is the gust‐front line, the line that defines the shift in important component in analyzing the vertical motion wind direction at the leading edge of the cold front. The [Eliassen, 1984]. That is, the propagation of this trough wind gusts vary from 20 to 40 mph. The gust front along the stream contributes to lift ahead of the trough and passed Winnemucca (WMC) and Catnip Mountain (CTP) at sinking behind it. 1800 UTC (28 February). It then proceeded past Bluewing Mountain (BLM) and Lovelock (LOL) 2–4 h later (see also 3.3. Cooling Figure 1 for station locations). Based on these observations, [18] As shown in Figure 6, dramatic cooling takes place the gust front is oriented NW–SE and propagates toward the over Boise, ID (BOI) between 1200 UTC (28 February) and SW. A temperature drop of approximately 10°C follows the 0000 UTC (1 March). At 500 hPa, the temperature drops gust front passage (delayed by approximately 6 h). This from −24°C to −38°C over this 12 h period —slightly delay was in response to significant surface heating during greater than 1°C h−1. There is minimal cold air advection in the late morning and afternoon hours—especially evident at the vicinity of BOI at 1200 UTC (28 February). By 0000 UTC LOL, DYL, NFL, and REV as displayed in Figure 9. The (1 March), however, the 500 hPa vorticity pattern has lowest visibility was reported at Fallon (NFL): 1/8 mile intensified at a location just south of BOI, and the 500 hPa (0.2 km) in blowing dust (symbol: $) at 0200 UTC (1 March). wind shifts from NW to NE at this location. Based on this As a complement to the time series at surface stations, we

Figure 6. The 500 hPa relative vorticity (solid line; contour interval = 2 × 10−5 s−1) and geopotential heights (dashed line; contour interval = 6 dm), air temperature (T; °C), air temperature minus dew point temperature (T ‐ Td; °C) and winds (kt; 10 kt = 5 m s−1) from the upper air soundings valid at (a) 1200 UTC (27 February), (b) 0000 UTC (28 February), (c) 1200 UTC (28 February), and (d) 0000 UTC (1 March). Note that the analysis area shifts further southward in Figures 6c and 6d. X in Figure 6d marks the location of the Black Rock Desert.

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− Figure 7. NCEP/NCAR reanalyzed vertical motion (contour interval = 1 cm s 1; solid line, upward; dashed line, downward) − at 500 hPa overlain on the 500 hPa jet position (isotachs; m s 1; gray) valid at (a) 0000 UTC (28 February), (b) 1200 UTC − (28 February), and (c) 0000 UTC (1 March), relative to the jet streak position at 500 hPa (isotachs plotted in m s 1). Loca- tion of BOI is shown. D06113 D06113 LEWIS ET AL.: LARGER-SCALE DYNAMIC SIGNATURES D06113

Figure 8. Rawinsonde soundings at Reno (REV), Nevada valid at (a) 2324 UTC (28 February) and (b) 1116 UTC (1 March) plotted on a Tephigram. Wind speeds (kt; 10 kt = 5 m s−1). Point values of temperature at 2324 UTC (28 February) and 0516 UTC (1 March) are overlain on the temperature profile at 1116 UTC (1 March). also include the 850 hPa temperature and winds valid at where u (v) is the east–west (north–south) component of the 0000 UTC (1 March). As can be seen, there is no organized mean wind. Ri is a ratio of buoyant production of turbulent extratropical cyclone development over the area of interest. kinetic energy (which is the energy required to vertically Rather, there is a frontal push of cold air (Figure 10). displace the eddies) to the mechanical production (which is the supply of energy derived from the mean wind). Thus, 4.2. Mixed Layer turbulent energy increases when Ri < 1. In practice, it has been found that Ri < 0.25 is a more meaningful criterion for [22] In Richardson’s classic paper [Richardson, 1920] the onset of turbulence [Wallace and Hobbs, 1977; Stull, that extended Osborne Reynolds’ criterion for turbulence 1988, Galperin et al., 2007; Zilitinkevich et al., 2008]. [Reynolds, 1893], he showed that turbulence would be This might be expected in light of Richardson’s assumption generated in the atmosphere when the supply of energy to that turbulence developed about a state of rest which is eddies exceeds the losses [Richardson, 1920, section 4]. rarely the case in practice. Using the conservation of energy principle, he found the [23] From upper‐air observations near the BRD (REV, criterion to be LKN, and BOI), we have calculated Ri at 0000 UTC 2 @U g @ (1 March) (1600 LST (28 February)) and the results are > ; ð1Þ shown in Table 3. Under essentially clear skies at these @z @z stations, a superadiabatic layer of about 50 hPa adjoined the where U is the horizontal velocity, z is the geometric height, ground. Above this unstable layer, a slightly stable layer is the potential temperature and g is the acceleration of extended to 700 hPa. Using boundary values of potential gravity. A nondimensional number indicating the dynamic temperature (at the surface and 700 hPa) and the corre- instability of the flow known as the Richardson number (Ri) sponding shears, this criterion indicates that turbulent stemmed from this work and is given by energy in this layer increased at REV and BOI whereas it decreased at LKN. g @ @ ¼ "#z ð2Þ 5. Adjustment to Thermal Wind Imbalance Ri 2 2 @u @v þ [24] At 1200 UTC (28 February), the momentum associ- @ @ z z ated with the jet streak is out of balance with the mass field (pressure gradient force). Analyses of wind and geopotential

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Figure 9. Time series of surface observations at stations in northwestern Nevada, Doyle (California), and Boise (Idaho) from 1200 UTC (28 February) to 0600 UTC (1 March). Station identifiers are shown in Figure 1 (temperatures (°C), winds (full barb = 10 kt; 10 kt = 5 m s−1), wind gust (m s−1), and visibility (km; indicated in bold)).

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Figure 10. NARR temperature (contour interval = 2.5°C; dashed line), geopotential height (contour interval = 30 m; solid line), and wind vectors at 850 hPa valid at 0000 UTC (1 March). height at 300 hPa indicate a subgeostrophic flow regime at jointly act to oppose the pressure gradient force. In short, 1200 UTC (28 February) (Figure 11). This is also evident by the dynamic imbalance in the nearly rectilinear flow at examination of the geopotential thickness pattern for the 1200 UTC (28 February) can be ameliorated through 700/300 hPa layer shown in Figure 12. The thermal wind is development of cyclonic curvature. The previous work by greater than the observed shear at most of the upper‐air Moore and VanKnowe [1992] lends support to the impor- stations. The thickness pattern in Figure 12a exhibits only tance of curvature as a mechanism for restoration of dynamic slight curvature. Under this condition, the difference balance in synoptic‐scale flow regimes. between the thermal wind and the observed shear is a [26] To calculate the gradient winds at 700 and 300 hPa measure of the geostrophic imbalance. A substantial change (and the associated shear), we assume that the trajectory takes place in the 700/300 hPa thickness pattern and asso- curvature (denoted by K) is approximated by streamline ciated wind shears by 0000 UTC (1 March) as shown in curvature on level surfaces. Realizing this assumption is Figure 12b. Most notable is the presence of cyclonic cur- subject to error in the presence of vertical motion and vature in the vicinity of the jet’s exit region. Further, the geopotential thicknesses have precipitously dropped—most Table 3. Richardson Numbers (Ri) Valid at 0000 UTC 1 March noticeable along the line of stations BOI–LKN–REV (cutting 2002a across the exit region). @ @ 2 g @ −1 U −5 −2 @z [25] In this case of curved flow, the difference between (°K km ) (10 s ) Ri = 2 @z @z @U the thermal wind and the observed shear is not an accurate Station Layer (mbar) @z measure of imbalance. A more appropriate measure of REV 842/700 −1.20 5.65 −0.69 balance/imbalance is the vector difference between gradi- LKN 835/700 1.47 2.24 2.25 − − ent wind shear and observed shear [Forsythe, 1945]. BOI 916/700 0.24 1.43 0.58 With cyclonic curvature, the centrifugal and Coriolis forces aU is the horizontal velocity vector and is the potential temperature.

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Figure 11. Geostrophic wind (kt; denoted by the shorter‐shafted vectors), observed wind (kt; denoted by the long‐shafted vectors) and geopotential heights (contour interval = 6 dm) on 300 hPa valid at 1200 UTC (28 February).

Figure 12. Observed wind shear (kt; denoted by the long‐shafted vectors), thermal wind (kt; denoted by the shorter‐shafted vectors) and layer thickness (contour interval = 6 dm) in 700–300 hPa layer valid at (a) 1200 UTC (28 February) and (b) 0000 UTC (1 March).

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Table 4. Geostrophic Wind Shear (Thermal Wind), Gradient available at 3 h intervals. Table 5 displays results for BOI at Wind Shear, and Observed Wind Shear Along the Line of Stations 500 hPa and for BRD at 700 hPa. This choice was dictated Southwest of the Exit Region of the Jet Stream at 0000 UTC by our desire to show changes in midtroposphere at BOI 1 March in the 700/300 hPa Layera where the cold front aloft moved past this station and in the lower troposphere (at BRD where the cold gust front moved Thermal Gradient Observed ∼ ∼ Station Wind Wind Shear Wind Shear past the station). The cooling at BOI was 12°C and 9°C at BRD. At the earlier times during the 12 h period, cooling BOI 36/315 20/325 18/300 due to adiabatic ascent dominated cooling due to advection. LKN 46/295 34/285 39/290 REV 39/320 24/315 23/330 During the later part of the period, the cold air advection dominated. From the data, it is difficult to completely a −1 Values given in format: m s /standard meteorological convention for understand the concerted action of adiabatic cooling and direction. The gradient wind shear is the mean of three calculations with ± 20% uncertainty in the radius of curvature. cold air advection in explaining the temperature drop. That is, averaging the cooling due to both sources does not account for the analyzed drop in temperature. transient flow [Saucier, 1955], we include an uncertainty of [29] Further examination of the vertical motion in a cross ±20% in the radius of curvature (R) estimate section extending from north of BOI down through the BRD – 1 @ (SW NE line of cross section is shown in Figure 13) ¼ ¼ ; ð3Þ adds important information to the examination of the cool- K @ R s ing process. These vertical cross sections at 1200 UTC a (28 February) and 0000 UTC (1 March) are shown in where (in radians) denotes wind direction and s denotes Figure 14. Between 0600 and 1200 UTC, the upward distance along the streamline. K is positive (negative) for – ‐ motion increases just north northeast of BOI (figure not cyclonic (anticyclonic) curvature associated with the upper shown). At 1200 UTC, there is significant ascent (4–6cms−1) level troughs (ridges). Under these conditions, Table 4 dis- in the 700–400 hPa layer above BOI. At this time, the jet plays the gradient wind shear in the 700/300 hPa layer at streak was right above BOI. Thereafter, the upward vertical BOI, LKN, and REV alongside the associated thermal wind motion pattern rapidly moves to the southwest and across and observed shear. Even in the presence of curvature the exit region (see also Figure 13). Although this transition uncertainty, the results shown in Table 4 indicate near bal- is consistent with a flow pattern where curvature signifi- ance between momentum and mass when curvature effects cantly increases with time [Moore and VanKnowe, 1992], are included. Based on the 3 h data sets from NARR dis- the time scale of the change in the vertical motion at BOI cussed in section 6, we know that the adjustment period is and neighboring locations takes place over a period less than 6–9 h, considerably smaller than the half pendulum day at ∼ 12 h The isentropic analysis at 0000 UTC (1 March) also 45° latitude ( 17 h). captures the cold front aloft that gradually descends to the surface with a pronounced well‐mixed boundary layer with a depth of about 2 km AGL (NARR planetary boundary 6. NARR Analysis layer depths ∼2500 m AGL at 2100 UTC (28 February) [27] As a complement to the large‐scale depiction of in northwestern Nevada). This is also seen in the Reno ascent/descent in the vicinity of the jet (Figure 7), we have sounding at 0516 UTC (see Figure 8). accessed vertical motion fields from the NARR. This archive contains a high‐resolution (32 km) climate data set 6.2. Isallobaric Wind for the North American continent (including oceanic areas [30] The isallobaric wind Visal is the ageostrophic wind bordering the landmass). Postprocessed forecasts from the component that forms in response to local pressure changes— operational NCEP’s Eta model, augmented by a variety quantitatively proportional to the gradient of pressure ten- of observations, are the basis for the reanalysis [Mesinger dencies and directed opposite to this gradient [Saucier, et al., 1988]. 1955; Bluestein, 1992; Martin, 2006; Rochette and Market, 2006]. On a constant pressure surface, it is expressed as 6.1. Vertical Motion follows: [28] The NARR vertical motion serves to deliver a finer‐ scale space and time complement to the synoptic‐scale lifting/ 1 @F Visal ¼ r ; ð4Þ descent from NCEP/NCAR discussed earlier. In particular, f 2 p @t we have chosen to examine the changes in the NARR ascent/descent pattern over an 18 h period, 0600 UTC where f is the Coriolis parameter and F is the geopotential. (28 February) to 0000 UTC (1 March), shown in Figure 13. On a constant height surface, pressure tendency replaces Consistent with the earlier analyses, the exit region of the height tendency. As expected, a rapidly moving pressure/ jet streak moves southeastward over the 18 h period. At weather system or a short‐wave trough as found in our case 0600 UTC (28 February), there is pronounced ascent to the will lead to significant pressure rise/fall couplets and asso- left (NE) of the exit region and weaker descent to the right ciated isallobaric winds. This is the case for the rapid (SW) of the exit region. As time proceeds, the ascending pressure/height changes along the line of stations BOI‐ motion intensifies and moves across the exit region. We LKN‐REV in our study. The boundary layer isallobaric examine the cooling at two locations BOI and BRD between winds (winds in the surface to 700 hPa layer for our study) 1200 UTC (28 February) and 0000 UTC (1 March). The are northeasterly/northerly along this line during the after- examination is made using the NARR where analyses are noon of dust storm initiation (Figure 15). The isallobaric

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Figure 13. NARR vertical velocity fields (contour interval = 1 cm s−1) at 500 hPa overlain on the 500 hPa jet position (isotachs; m s−1) valid at (a) 0600 UTC, (b) 1200 UTC, (c) 1800 UTC (28 February), and (d) 0000 UTC (1 March). Locations of BOI, LKN, REV, and SLC are shown.

Table 5. The 500 hPa Cooling at BOI and 700 hPa Cooling at BRD Based on NARR Analyses Between 1200 UTC (28 February) and 0000 UTC (1 March) Cooling Rate due to Pressure Air Temperature w‐Vertical Horizontal Temperature Adiabatic Cooling Time Station Level (hPa) (°C) Velocity (cm s−1) Advection (°C h−1) Rate (°C h−1) (28 Feb) 1200 UTC BOI 500 −24.0 +3.93 +0.21 −0.64 (28 Feb) 1800 UTC BOI 500 −29.7 +0.01 −0.45 +0.00 (1 Mar) 0000 UTC BOI 500 −35.6 −3.60 −0.60 +0.70 (28 Feb) 1200 UTC BRD 700 −2.5 +2.75 −0.01 −0.36 (28 Feb) 1800 UTC BRD 700 −5.1 −1.51 −0.36 +0.34 (1 Mar) 0000 UTC BRD 700 −11.2 −2.32 −1.12 +0.11

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− Figure 14. NARR X‐P cross sections of isentropes (contour interval = 2.5 K), horizontal winds (wind barbs; 10 kt = 5 m s 1), − isotachs (contour interval = 5 m s 1), and vertical velocity (shaded; highest and lowest magnitudes are indicated in boxes) along the cross section shown in Figure 13 at (left) 1200 UTC (28 February), and (right) 0000 UTC (1 March). Locations of REV, BRD, and BOI are shown. D06113 D06113 LEWIS ET AL.: LARGER-SCALE DYNAMIC SIGNATURES D06113

Figure 15. NARR surface pressure tendency (mbar (6 h)−1) valid during 1800 UTC (28 February) to 0000 UTC (1 March) and the isallobaric wind vectors at 700 hPa are shown by the bold arrows. Locations of BOI, LKN, and REV are shown. wind has physical meaning and is especially relevant in our 6.3. Profiler Winds Versus NARR Wind Profiles case study since its magnitude and direction (as shown in [33] As part of the academic field exercise conducted by Figure 15) is approximately equal to the ageostrophic wind NCAR/University of Nevada‐Reno, a wind profiler was vector. For example, from the NARR analyses at 700 hPa deployed that made measurements at the time of dust storm over the BRD, the total wind speed and ageostrophic wind ‐ −1 passage through Reno [Cohn et al., 2004, 2006]. The upper speed at 0000 UTC (1 March) are 11.2 and 8.6 m s , air wind structure in the lowest 2–3 km at the time of storm respectively, while the isallobaric wind speed over the passage (∼0130 UTC (1 March)) as well as several hours period 1800 UTC (28 February) to 0000 UTV (1 March) is −1 before and after passage is shown in Figure 16. Beside the 6.1 m s . The directions of the ageostrophic and isallobaric profiler data we have plotted the profiles of wind at REV winds are NNE. from the NARR data set. A most notable feature of wind [31] There is also some evidence of confluence in the profiler data is the presence of a marked discontinuity in isallobaric winds along this line of stations that contributes wind speed at about 1.5 km (AGL) at the time of storm to strengthening the cold front and its attendant turbulence passage. This would appear to be indicative of a low‐level ’ kinetic energy (TKE). The NARR analysis indicates TKE s wedge (a 1.5 km deep wedge) of northerly wind similar to – −1 the order of 2 3Jkg at 825 hPa (not shown). the outflow from a thunderstorm. There is an absence of [32] The isallobaric winds have formed in response to the such a discontinuity in the NARR profile. Another feature of mass adjustments above the boundary layer. The time scale importance is evidence of an abrupt shift in wind direction at of Visal in the boundary layer is linked to the time scale of the lowest levels of the profiler data just prior to dust storm — −1 mass adjustments above this boundary layer adjustments passage—a shift from weak westerly wind (∼1ms )toa ‐ − associated with vertical motions/divergence convergence stronger northerly wind (∼14 m s 1) at 0145 UTC. Again, patterns. this feature is absent in the NARR profiles. This shift is

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Figure 16. A comparison of (a) wind profiler measurements and (b) NARR wind profiles at Reno valid from 1200 UTC (28 February) to 1200 UTC (1 March) (in knots; 10 kt = 5 m s−1). indicative of a confluence zone near the frontal boundary at BOI, LKN, and REV during the period 1200 UTC and is likely correlated with a zone of strong convergence (27 February) to 0000 UTC (1 March) are 276 hPa (9.7 km), and associated strong lift. 243 hPa (10.6 km), and 228 hPa (11.0 km), respectively. [35] Another primary difference between our results and 7. Discussion and Conclusions those of EFD and PBB is the nature of the transverse or secondary circulation. Although EFD refrained from dis- Danielsen 7.1. Comparison/Contrast With [1974b] cussing vertical circulation in baroclinic zones, it is clear Pauley et al. and [1996] that the wide sweep of descending air in his case was [34] In their studies of dust storms over the western consistent with the indirect circulation about the jet streak’s United States, Danielsen [1974b] (author’s full name is exit region— descending air on the anticyclonic side of the abbreviated in the following discussion as EFD) and Pauley jet. PBB’s arguments rest firmly on the indirect circulation et al. [1996] (abbreviated in the following discussion as that straddled the jet streak’s exit region. In essence, these PBB) indicated that the high‐momentum air impacting the authors argue that the descending branch of the indirect surface layer came from the stratosphere. In this case study, circulation is responsible for the vertical transport of high‐ the air that impacts the BRD is not associated with a momentum air to the top of the mixed layer—a mixed layer stratospheric intrusion. This is borne out by: (1) structure that was present in their cases as well as ours. In our case, of isentropic surfaces along the line of stations from synoptic analysis indicates a transition from indirect to St. George, BC (ZXS) to BOI at 0000 UTC (1 March) direct circulation about the jet streak’s exit region—a cir- (Figure 17), and (2) the absence of ozone intrusions from the culation that leads to cooling and creation of a cold front that stratosphere into the troposphere (satellite imagery not tracked from the midtroposphere to the surface. The cold shown). In Figure 17 the isentropes tend to parallel the front moved into and likely enhanced the zone of deep tropopause as opposed to intersecting the troposphere‐ mixed layer over the BRD and ablated dust that mixed to stratosphere boundary. The mean tropopause pressures

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Figure 17. X‐Z vertical cross section of potential temperature (°K), geopotential height (dm), and wind barbs (full barb = 10 kt, 10 kt = 5 m s−1) valid at 0000 UTC (1 March).

1–2 km. This sequence stands in contrast to the one described 7.2. Succinct Summary of Dust Storm Dynamics by EFD: [37] Based on the surface and upper‐air analyses and arguments related to the adjustment, we develop a schematic During its [momentum] descent, the jet decelerates in speed but wind speeds from 60–80 kt do reach the adiabatic layer and then rapidly view of the dust storm that is shown in Figure 18. It is mix to the ground as strong gusts. These strong wind gusts sandblast difficult to place a single time on the schematic. In essence, the arid soil, disaggregating soil particles, creating small soil aerosols we try to depict processes at work over the period of which then mix vertically through the adiabatic volume into the stable adjustment —the 6 h period between 1800 (28 February) … transition layer above . Danielsen [1974a, p. 171] and 0000 UTC (1 March). [38] The schematic indicates the movement of lifting from [36] The issue of large‐scale dynamic imbalance and the cold‐air side (left side when looking downstream) of the restoration of balance was central to our theme. In our case, jet streak’s exit region to the warm air side (right side) the development of large‐scale cyclonic curvature in the (Refer to NARR vertical motion patterns, Figure 13). A flow field gives rise to centrifugal force, that when paired corresponding descending motion moves into the region with Coriolis force, counterbalances the pressure gradient force and serves to force the system back toward equilibrium. northwest of BOI. In conjunction with the movement of the upward motion pattern, the vorticity increases in response to Complete understanding of the subsynoptic‐ and synoptic‐ tilting and stretching the vortex tube. Consistent with the scale interactions that result in an adjustment time scale less than the half pendulum day is beyond the scope of this vorticity pattern, a northeasterly wind develops at BOI and this is associated with cold air advection (a cold front aloft) investigation. Nevertheless, realizing that this is a crucial where cooling is also linked to adiabatic ascent. Below the issue, we further explore these interactions in a companion region of lifting, the surface pressure increases (net mass paper (M. L. Kaplan et al., Dust storm over the Black Rock flux convergence in the column) and leads to an ageos- Desert: Subsynoptic analyses of unbalanced circulations trophic (essentially isallobaric) wind from the northeast, across the jet streak, manuscript in preparation, 2011) where which organizes a cold gust front that moves over the BRD the Advanced Weather Research Forecast Model [Skamarock and ablates dust. et al., 2008] is used.

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Figure 18. Schematic diagram depicting the dynamical processes associated with the dust storm during 1800 UTC (28 February) to 0000 UTC (1 March).

[39] Despite some difference in synoptic regimes and in physical processes that lead to the planetary boundary layer the modes of adjustment for the cases we have compared structures found in the profiler data from Reno. To gain and contrasted, there is sufficient overlap to identify key more insight into these processes, high‐resolution simula- signatures in the analyses that portend the likelihood of dust tions with a new generation mesoscale model such as WRF storms in the western U.S. The key features are the fol- will be required [Skamarock et al., 2008]. In particular, we lowing: (1) the presence of a pronounced/well‐defined jet will employ WRF in a successor study to determine how the streak in the midlevel to upper level of the troposphere, ascending motion on the right side of the jet becomes (2) evidence of vertical or secondary circulation in response stronger (Figure 14) as a result of unbalanced mass/ to dynamic imbalance, (3) development of substantial momentum adjustments resulting in strengthening the low‐ curved flow aloft (increasing with time), (4) lower tropo- level cold front and PBL turbulence. Preliminary work with spheric isallobaric component of the ageostrophic wind WRF is underway. signal orthogonal to the jet’s exit region, and (5) a cold front, initially aloft, that descends to the surface and impacts a dust emission source overlain by a well‐mixed layer where [41] Acknowledgments. We are especially grateful to the graduate students and National Center for Atmospheric Research (NCAR) technical the bulk density of the sediments is low. staff who took part in the NCAR/UNR (University of Nevada‐Reno) Field [40] Although knowledge of these signatures offers fore- Program in the Washoe Valley during February and March 2002. This field casting guidance, critical questions remain unanswered. program was a major component of the UNR graduate level course in Atmo- spheric Instrumentation and Observations that was taught by coauthors Among them are the following: what are the consequences John Hallett and Steve Cohn. The enthusiastic attitude and the diligent of adjustments in the presence of supergeostrophy as effort of the students and technical support staff to collect data were essen- opposed to the subgeostrophic state we investigated?, and tial to the project’s success. One of the authors, Ramesh Vellore, now a what is the relative frequency of dust storm genesis that postdoctoral scholar at Desert Research Institute, was a student in that class, and two others, Claudio Mazzoleni and Lynn Reinhart Mazzoleni, now pro- follows the scenario we investigated compared to the fessors of physics and chemistry at Michigan Technical University, respec- Danielsen [1974b]/Pauley et al. [1996] scenario? Beyond tively, supplied us with the PM10 data. Trond Iversen, a protégé of Arnt these questions, it is important to understand the interplay of Eliassen, is credited with supplying us with an insightful view of Eliassen’s

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