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DOI:10.2151/jmsj. 2018-023 J-STAGE Advance published date: January 26, 2018 The final manuscript after publication will replace the preliminary version at the above DOI once it is available. 1
2 Lidar Network Observation of Dust Layer Development
3 over the Gobi Desert in Association with a Cold Frontal
4 System on 22–23 May 2013
5
6 Kei KAWAI1, Kenji KAI
7
8 Graduate School of Environmental Studies 9 Nagoya University, Nagoya, Japan
10
11 Yoshitaka JIN, Nobuo SUGIMOTO
12
13 National Institute for Environmental Studies 14 Tsukuba, Japan
15
16 and
17
18 Dashdondog BATDORJ
19 National Agency for Meteorology and Environmental Monitoring 20 Ulaanbaatar, Mongolia 21 22 23 24 January 16, 2018 25 26 27 ------28 1) Corresponding author: Kei Kawai, Graduate School of Environmental Studies, Nagoya 29 University, Furo-cho, Chikusa-ku, Nagoya 464-8601, Japan. 30 Email: [email protected] 31 Tel: +81-52-788-6045 32 Fax: +81-52-789-4257
33
34 Abstract
35
36 The Gobi Desert is one of the major sources of Asian dust, which influences the
37 climate system both directly and indirectly through its long-range transport by the
38 westerlies. In this desert, three ground-based lidars are operated in Dalanzadgad,
39 Sainshand, and Zamyn-Uud, Mongolia. This study firstly combined these lidars into a
40 lidar network and shows the spatial development of a dust layer over the desert and the
41 long-range transport of the dust during 22–23 May 2013 via the lidar network. During this
42 dust event, a cold front accompanying an extratropical cyclone moved southeastward
43 across the desert and sequentially passed through Dalanzadgad, Sainshand, and
44 Zamyn-Uud. In Dalanzadgad, in the central part of the desert, a dust storm occurred
45 owing to the strong wind (6–10 m s-1) associated with the cold front and reached a top
46 height of 1.6 km. Some of the dust floated at a height of 0.9–1.6 km along the cold frontal
47 surface. In Sainshand and Zamyn-Uud, in the eastern part of the desert, the dust layer
48 extended from the atmospheric boundary layer (ABL) to the free troposphere in the
49 updraft region of warm air in the cold frontal system. Overall, while the dust layer was
1 50 moving across the desert with the cold frontal system, it was developing up to the free
51 troposphere. The mechanism of this development can be explained by the combination
52 of two processes as follows: (1) continuous emission of dust from the desert surface to
53 the ABL by the strong wind around the cold front and (2) continuous transport of the dust
54 from the ABL to the free troposphere by the updraft of the warm air in the cold frontal
55 system. This mechanism can contribute to the long-range transport of dust by the
56 westerlies in the free troposphere.
57
58 Keywords Asian dust; cold front; the Gobi Desert; lidar network observation;
59 long-range transport
60
2 61 1. Introduction
62 Asian dust originates in arid and semi-arid regions of East Asia such as the Gobi Desert,
63 Taklimakan Desert, and Loess Plateau (Sun et al. 2001; Kurosaki and Mikami 2005; Wu et
64 al. 2016). In the source regions, dust is emitted from the ground surface to the atmospheric
65 boundary layer (ABL) by strong wind. If the dust ascends from the ABL to the free
66 troposphere, it is transported over a long range toward the North Pacific by the
67 middle-latitude westerlies (Kai et al. 1988; Husar et al. 2001; Hara et al. 2009; Uno et al.
68 2009; Yumimoto et al. 2009). This transported dust influences the climate system both
69 directly and indirectly (Huang et al. 2014). Therefore, the spatial distribution of dust over the
70 source regions is a key factor in the long-range transport of dust.
71 A ground-based lidar is effective for observing vertical distribution of dust continuously. In
72 the Taklimakan Desert, a depolarization lidar was installed in 2002 (Tsunematsu et al. 2005;
73 Kai et al. 2008) through the Japan–China Joint Studies on Origin and Transport of Aeolian
74 Dust and its Impact on Climate (ADEC) (e.g., Mikami et al. 2005; Takemi 2005). In the
75 Loess Plateau, a group from Lanzhou University, China, conducts a lidar observation and
76 many other measurements (Huang et al. 2008). In the Gobi Desert, there are two lidars in
77 the eastern part (Sugimoto et al. 2008) and one in the central part (Kawai et al. 2015).
78 In the Gobi Desert, almost all dust events arise from cold frontal activity (Shao and Wang
79 2003; Takemi and Seino 2005). A cold front often passes through the desert in spring
80 (Hayasaki et al. 2006). This is related to frequent cyclogenesis in the lee of the
3 81 Altai-Hangayn-Sayan Mountains (Fig. 1) in spring (Chen et al. 1991; Adachi and Kimura
82 2007; Wang et al. 2009). Therefore, it is important to understand the relationship between
83 dust events and cold fronts. In previous studies, dust events associated with cold fronts
84 were analyzed via satellite images and numerical models (Husar et al. 2001; Adachi et al.
85 2007; Hara et al. 2009). However, the spatial relationship between dust events and cold
86 fronts, which involves the long-range transport of dust, is not yet shown in detail based on
87 the lidar observations in the source region.
88 A dust event occurred in the Gobi Desert during the passage of a cold front on 22–23
89 May 2013. Our previous study showed the dust transport from the ABL to the free
90 troposphere by the cold frontal system during the dust event, based on the lidar in the
91 central part of the desert (Kawai et al. 2015). In the present study, we firstly combined the
92 three lidars in the desert into a lidar network and analyzed the spatial distribution of dust.
93 The purpose of the present study is to show the spatial development of a dust layer and the
94 long-range transport of the dust during the dust event by using the lidar network.
95
96 2. Observations and data
97 2.1 Gobi Desert lidar network
98 The lidar network in the Gobi Desert consists of three lidars: a ceilometer in the central
99 part and two AD-Net lidars in the eastern part. A ceilometer is a compact backscatter lidar
100 that is easy to operate and maintain. AD-Net is short for the Asian Dust and Aerosol Lidar
4 101 Observation Network, which is a widespread lidar network in East Asia (Sugimoto et al.
102 2008).
103
104 a. Ceilometer observation
105 The ceilometer is located in Dalanzadgad (43.58°N, 104.42°E, 1470 m above sea level
106 (ASL)) in southern Mongolia (Fig. 1). The ceilometer observation has been conducted since
107 the end of April 2013. It is part of the collaborative research between Nagoya University,
108 Japan, and the Information and Research Institute of Meteorology, Hydrology and
109 Environment (IRIMHE), Mongolia. The ceilometer is a Vaisala CL51. The laser wavelength
110 is 910 nm, the pulse repetition rate is 6.5 kHz, and the pulse energy is 3.0 μJ. A preliminary
111 ceilometer observation was carried out in Tsukuba, Japan, and the observational
112 performance of the ceilometer was verified via a sophisticated lidar (Jin et al. 2015).
113 The ceilometer outputs a vertical profile of attenuated backscatter coefficients up to a
114 height of 15.4 km with a height resolution of 10 m every 6 seconds. An attenuated
115 backscatter coefficient is a backscatter coefficient considering the attenuation of an emitted
116 laser pulse and the backscatter light in the atmosphere. The influence of water vapor on the
117 attenuated backscatter coefficients obtained from the ceilometer is probably small because
118 of low relative humidity in the Gobi Desert as discussed in Jin et al. (2015). The average of
119 the data for one minute was used in this study.
120 The major scatterers over the Gobi Desert in spring are dust, clouds, and precipitation.
5 121 Their detection and distinction are based on the magnitude, height, and distribution of
122 attenuated backscatter coefficients. During this dust event, the threshold of attenuated
123 backscatter coefficient for distinguishing dust and the others was about 9 × 10-3 km-1 sr-1. In
124 addition, relative humidity in a region of large attenuated backscatter coefficients supports
125 their distinction. Natsagdorj et al. (2003) showed that relative humidity was low in most dust
126 storms.
127
128 b. AD-Net lidar observations
129 The AD-Net lidars are located in Sainshand (44.87°N, 110.12°E, 937 m ASL) and
130 Zamyn-Uud (43.72°N, 111.90°E, 962 m ASL) in southeastern Mongolia (Fig. 1). These
131 lidars are operated by the National Institute for Environmental Studies (NIES), Japan, and
132 the National Agency for Meteorology and Environmental Monitoring (NAMEM), Mongolia.
133 The lidars are dual-wavelength polarization-sensitive backscatter lidars. The laser
134 wavelengths are 532 and 1064 nm, the pulse repetition rate is 10 Hz, and the pulse energy
135 is 50 mJ. The polarization of backscatter light is measured at a wavelength of 532 nm.
136 The observational data includes the vertical profiles of attenuated backscatter
137 coefficients at both wavelengths and volume depolarization ratio at a wavelength of 532 nm.
138 The time and height resolutions are 15 minutes and 30 m, respectively. The volume
139 depolarization ratio is an index of particle sphericity. This value is zero for a spherical
140 particle and increases with non-sphericity. We calculated volume color ratio, a relative index
6 141 of particle size, by dividing the 1064-nm attenuated backscatter coefficient by the 532-nm
142 one. This value increases as particle size increases. The 532-nm attenuated backscatter
143 coefficient near the ground in Zamyn-Uud was calibrated for an incomplete overlap function
144 by using data on 24 May 2013, when the atmosphere was clean following the dust event.
145
146 2.2 Meteorological data
147 Surface weather charts produced by the Japan Meteorological Agency (JMA) were used.
148 Surface meteorological observational data were obtained from 3-hourly SYNOP reports
149 acquired from the Integrated Surface Database produced by the National Oceanic and
150 Atmospheric Administration (NOAA), USA (Smith et al. 2011). Three-dimensional gridded
151 meteorological data produced by the National Centers for Environmental Prediction (NCEP)
152 of NOAA were used. The data are NCEP FNL (Final) Operational Global Analysis data of 1º
153 1º grids. This product is generated from the global meteorological data of the Global Data
154 Assimilation System (GDAS) using the same model as the NCEP global forecast system. In
155 addition, trajectory analyses were performed using the Hybrid Single-Particle Lagrangian
156 Integrated Trajectory (HYSPLIT) model provided by the NOAA Air Resources Laboratory
157 (Stein et al. 2015). The GDAS data of 1º 1º grids were selected as the input
158 meteorological data of the trajectory analyses. The local standard time (LST) of Mongolia,
159 which is 8 hours ahead of the coordinated universal time (UTC), is used in this paper.
160
7 161 3. Results
162 3.1 Meteorological conditions in Mongolia
163 Surface weather charts for Mongolia on 22–23 May 2013 are shown in Fig. 2. An
164 extratropical cyclone covered central Mongolia at 14 LST on 22 May (Fig. 2a) and moved
165 northeastward for the next 18 hours. The center pressure of the cyclone was almost
166 constant at 988 hPa during this period. Warm and cold fronts accompanied the cyclone
167 from 20 LST on 22 May (Figs. 2b–2d). The cold front extended from the cyclone center to
168 southern Mongolia across the Gobi Desert at 20 LST on 22 May (Fig. 2b) and then moved
169 southeastward through the desert at a speed of about 30 km h-1. During the movement, it
170 passed through Dalanzadgad between 14 and 20 LST on 22 May, Sainshand between 20
171 LST on 22 May and 02 LST on 23 May, and Zamyn-Uud between 02 and 08 LST on 23
172 May.
173 Figure 3 shows the meteorological fields observed around Mongolia at the same time as
174 the second and third weather charts (Figs. 2b and 2c) obtained from the SYNOP report. At
175 20 LST on 22 May, the cold air behind the cold front covered central Mongolia (including
176 Dalanzadgad), and the warm air in front of the cold front covered eastern Mongolia (Fig. 3a).
177 At 02 LST on 23 May, the cold air expanded to eastern Mongolia (including Sainshand)
178 while the cold front moved southeastward (Fig. 3b). At both these times, the wind directions
179 were between north and northwest in the cold air and between south and west in the warm
180 air. These wind conditions show cold and warm air advections. The cold and warm airflows
8 181 converged along the cold front.
182 Figure 4 presents meteorological fields at 700 hPa at the same times as in Fig. 3
183 obtained from the NCEP FNL data. The height at 700 hPa is about 1.5–2.0 km above the
184 ground in Mongolia. At both times in Fig. 4, updrafts spread the warm air in along the
185 surface cold front. These updrafts were caused by the convergence of the cold and warm
186 air on the ground (Fig. 3). Therefore, the warm air on the ground ascended onto the cold air,
187 as shown in the general cold frontal system (e.g., Simpson 1997; Wallace and Hobbs
188 2006).
189
190 3.2 Results in Dalanzadgad, located in the central part of the desert
191 Figure 5 indicates the temporal variations in surface meteorological elements observed in
192 Dalanzadgad from 12 LST on 22 May to 09 LST on 23 May 2013. Figure 6 shows the
193 time-height cross sections of a ceilometer observation result and meteorological conditions
194 obtained from the NCEP FNL data in Dalanzadgad during the same period as in Fig. 5. In
195 Fig. 6, four characteristic regions were discerned and are indicated by the letters A–D. They
196 are discussed as follows.
197 Region A of medium to large attenuated backscatter coefficients (>1.0 × 10-3 km-1 sr-1,
198 light blue to red) shows a dust storm between the ground and a height of 1.6 km from 14
199 LST on 22 May to 02 LST on 23 May (Fig. 6a). Near the ground during the dust storm, the
200 relative humidity was 11–14% (Fig. 5c), and the wind speed was 6–10 m s-1 (Fig. 5d). Thus,
9 201 the strong wind raised the dust from the desert surface. In the dust storm at 20 LST on 22
202 May, the potential temperature was constant at 312 K with height, and the wind speed was
203 12–14 m s-1 (region A in Fig. 6b). The atmospheric neutral stability and the strong wind
204 show that vertical and horizontal mixing was active because of turbulence in the dust storm.
205 The sea level pressure increased by 23.3 hPa from 17 LST on 22 May to 08 LST on 23
206 May (Fig. 5a). The temperature decreased from 28.9 °C at 17 LST on 22 May to 9.4 °C at
207 08 LST on 23 May (Fig. 5b). Hence, the cold front (Fig. 2) passed between 17 and 20 LST
208 on 22 May, and the cold air (Fig. 3) started to advect on the ground during the dust storm.
209 Judging from the potential temperature and wind conditions, the vertical structure of the
210 cold air is shown by region B under the dotted line in Fig. 6b. The sloping top height
211 between 20 LST on 22 May and 01 LST on 23 May was the cold frontal surface. The top
212 height of the cold air is assumed to be about 1.4 km at 02 and 08 LST on 23 May. In
213 particular, the top height at 08 LST corresponds to the cloud base height observed by the
214 ceilometer (Fig. 6a). The potential temperature was 301 K at 02 LST and 294–298 K at 08
215 LST on 23 May (Fig. 6b). The wind directions in and above the cold air were northeast and
216 west, respectively. Most of the cold air indicates small attenuated backscatter coefficients
217 (<0.5 × 10-3 km-1 sr-1, light purple) from 2230 LST on 22 May (Fig. 6a), where little dust was
218 floating. The leading edge of the cold air was occupied by the dust storm.
219 Region C of medium to large attenuated backscatter coefficients (>1.0 × 10-3 km-1 sr-1,
220 red) shows clouds over a height of 1.6 km between 02 and 07 LST on 23 May (Fig. 6a). It is
10 221 likely that the clouds were generated by the updraft of the warm air (Fig. 4), as a general
222 cold frontal system (e.g., Simpson 1997; Wallace and Hobbs 2006). Precipitation is shown
223 by the region of middle to large attenuated backscatter coefficients (>1.0 × 10-3 km-1 sr-1,
224 light blue to red) under the clouds between 05 and 07 LST on 23 May. The precipitation led
225 to the increase in relative humidity at 08 LST on 23 May (Fig. 5c).
226 Region D, which is indicated by medium attenuated backscatter coefficients (1.0–1.4 ×
227 10-3 km-1 sr-1, light blue) at a height of 0.9−1.6 km between 22 LST on 22 May and 00 LST
228 on 23 May, shows a dust layer (Fig. 6a). The thickness of the dust layer ranged from 0.2 to
229 0.5 km. Because it was located over the cold frontal surface, the dust was probably
230 transported from the dust storm by the updraft of the warm air (Fig. 4). The subsequent
231 movement of the dust layer will be discussed in Section 4 by using trajectory analysis.
232
233 3.3 Results in Sainshand, located in the eastern part of the desert
234 Figure 7 presents the temporal fluctuations in surface meteorological elements observed
235 in Sainshand in the eastern part of the Gobi Desert from 18 LST on 22 May to 15 LST on 23
236 May 2013. Figures 8 and 9 show the time-height cross sections of lidar observation results
237 and meteorological conditions obtained from the NCEP FNL data, respectively, for
238 Sainshand during the same period as in Fig. 7.
239 Between 22 LST on 22 May and 01 LST on 23 May, a dust storm is shown by region E of
240 large attenuated backscatter coefficients (>5.5 × 10-3 km-1 sr-1, yellow to red) on the ground
11 241 (Fig. 8a). At 23 LST on 22 May, during the dust storm, the wind speed was 11 m s-1 (Fig. 7d),
242 and the relative humidity was 19% (Fig. 7c). In the dust storm, the volume depolarization
243 ratio was 0.2–0.3 (Fig. 8b), and the volume color ratio was about 2.0 (Fig. 8c).
244 The sea level pressure increased by 21.3 hPa from 20 LST on 22 May to 08 LST on 23
245 May (Fig. 7a). The temperature dropped from 35.4 °C at 17 LST on 22 May to 13.3 °C at 08
246 LST on 23 May (Fig. 7b). Between 20 and 23 LST on 22 May, the wind direction changed
247 from south to northwest, and the wind speed increased from 6 to 11 m s-1 (Fig. 7d).
248 According to these features, the cold front shown in Fig. 2 passed through between 20 and
249 23 LST on 22 May, and the cold air replaced the warm air, as shown in Fig. 3.
250 Judging from the potential temperature and wind conditions, the vertical structure of the
251 cold air is indicated in region F below the dotted line in Fig. 9a. The top height was 1.1 km
252 at 02 LST and 1.3 km at 08 and 14 LST on 23 May. The potential temperature was 303–307
253 K at 02 LST and 295 K at 08 and 14 LST on 23 May. At 08 and 14 LST on 23 May, there
254 was an inversion layer between 1.1 and 1.5 km in height. The wind directions in and above
255 the cold air were north and west, respectively. The attenuated backscatter coefficients were
256 small (<2.0 × 10-3 km-1 sr-1, blue) in the cold air after the dust storm (region F in Fig. 8a),
257 indicating little floating dust.
258 Region G of medium to large attenuated backscatter coefficients (>2.0 × 10-3 km-1 sr-1,
259 red) shows clouds, which were located around a height of 2–5 km (Fig. 8a). Most of the
260 clouds indicate volume depolarization ratios of more than 0.2 (Fig. 8b), indicating the
12 261 presence of ice crystals (Shimizu et al. 2004). The temperatures around the clouds were
262 between 0 and -10 °C according to the NCEP FNL data (not shown). Therefore, it is likely
263 that the dust particles mentioned later worked as ice nuclei, as suggested by Sakai et al.
264 (2003). The volume color ratio in the clouds was 2.0–2.4 (Fig. 8c). The region of medium
265 attenuated backscatter coefficients (2.0–6.0 × 10-3 km-1 sr-1, light blue to yellow) under the
266 clouds around 11 LST on 23 May shows precipitation (Fig. 8a). In the precipitation, the
267 volume depolarization ratio was about 0.1 (Fig. 8b), which is consistent with that reported
268 by Sassen (2006). The volume color ratio of the precipitation was 2.0–3.0 (Fig. 8c).
269 A dust layer is shown by the medium attenuated backscatter coefficients (2.0–6.0 × 10-3
270 km-1 sr-1, light blue to yellow) in region H (Fig. 8a). It extended from the dust layer (region E)
271 to the clouds (region G) over the cold air (region F). It seems that the dust layer was mixed
272 with the clouds. The top height of the dust layer increased from 1.6 km at 22 LST on 22 May
273 to at least 4.0 km at 04 LST on 23 May. The presence of the dust layer above a height of
274 4.0 km was unclear because of the clouds. The thickness of the dust layer was between 1.6
275 and 2.5 km. In the dust layer, the volume depolarization ratio was 0.1–0.3 (Fig. 8b), and the
276 volume color ratio was 1.0–2.0 (Fig. 8c). According to the volume color ratio, the dust
277 particles were smaller than the precipitation particles and the ice crystals of the clouds.
278 Vertical velocity represents updraft in the dust layer at 02 LST on 23 May (region H in Fig.
279 9b). Therefore, the dust layer was distributed in the updraft region of the warm air in the
280 cold frontal system.
13 281
282 3.4 Results in Zamyn-Uud, located in the eastern part of the desert
283 The time series of surface meteorological elements observed in Zamyn-Uud on 22–23
284 May 2013 are indicated in Fig. 10. Figures 11 and 12 show the time-height cross sections of
285 the lidar observation results and meteorological conditions obtained from the NCEP FNL
286 data, respectively, for Zamyn-Uud during the same period as in Fig. 10. Three characteristic
287 regions were found and are denoted with the letters I–K. As with Sainshand, a dust layer
288 (region I) was located over the cold air (region J) and probably mixed with a cloud (region
289 K).
290 The dust layer over the cold air is shown by medium attenuated backscatter coefficients
291 (1.6–4.0 × 10-3 km-1 sr-1, light blue to green) up to a height of about 4 km until 17 LST on 23
292 May (region I in Fig. 11a). The top height increased from 1.4 km at 03 LST to 3.8 km at 15
293 LST on 23 May. The thickness ranged from 0.3 to 1.7 km. In the dust layer, the volume
294 depolarization ratio was 0.2–0.3 (Fig. 11b), and the volume color ratio was 1.0–2.4 (Fig.
295 11c). As with Sainshand, the dust layer was located in the updraft region (region I in Fig.
296 12b).
297 The sea level pressure increased by 13.4 hPa from 17 LST on 22 May to 11 LST on 23
298 May (Fig. 10a). The temperature decreased from 32.5 °C at 17 LST on 22 May to 18.4 °C at
299 05 LST on 23 May (Fig. 10b). The wind direction changed from southwest to northwest
300 between 02 and 05 LST on 23 May (Fig. 10d). These characteristics show that the cold
14 301 front (Fig. 2) passed through between 02 and 05 LST on 23 May, and the cold air replaced
302 the warm air (Fig. 3).
303 Judging from the potential temperature and wind conditions, the cold air is indicated by
304 region J below the dotted line in Fig. 12a. The top height of the cold air was about 1.0 km at
305 08 LST and 1.3 km at 14 and 20 LST on 23 May. The potential temperature in the cold air
306 was 297–306 K. The wind directions rotated from north in the cold air to west above the
307 cold air. This counterclockwise rotation is consistent with the cold air advection. The
308 attenuated backscatter coefficients were small (<1.6 × 10-3 km-1 sr-1, blue) in the upper part
309 of the cold air (region J in Fig. 11a).
310 Region K, which is indicated by medium to large attenuated backscatter coefficients
311 (>1.6 × 10-3 km-1 sr-1, red) over a height of 2.2 km between 1700 and 1930 LST on 23 May,
312 shows a cloud (Fig. 11a). It seems that the cloud was mixed with the dust layer. In the cloud,
313 the volume depolarization ratio was mostly greater than 0.3 (Fig. 11b), and the volume color
314 ratio was more than 3.0 (Fig. 11c). As with Sainshand, it is likely that the dust particles
315 worked as ice nuclei. The ice crystals of the cloud were larger than the dust particles based
316 on their color ratios.
317
318 4. Discussion
319 4.1 Dust layer development
320 Figure 13 illustrates the observational models at the lidar observation sites during the
15 321 dust event. The characteristics of the dust layers over the cold air are summarized in Table
322 1. In Dalanzadgad, a dust storm occurred around the cold front and reached a top height of
323 1.6 km (Fig. 13a). A dust layer was distributed at a height of 0.9–1.6 km over the cold air,
324 with a thickness that ranged from 0.2 to 0.5 km. Forward trajectories for the dust layer are
325 presented in Fig. 14. The trajectories extended eastward, reaching the eastern part of the
326 Gobi Desert (Fig. 14a), and ascended to a height of more than 2.0 km for the first 13–15
327 hours (Fig. 14b). The relative humidity along the trajectories was about 50% until 08 LST on
328 23 May (Fig. 14c). In contrast, the dust layers observed in Sainshand and Zamyn-Uud
329 reached a height of about 4 km (Figs. 13b and 13c) and were 1.6–2.5 and 0.3–1.7 km thick,
330 respectively. Therefore, the dust layer produced during this dust event was developing
331 upward from the ABL to the free troposphere while moving eastward through the Gobi
332 Desert with the cold frontal system.
333 It is suggested that the development of the dust layer was based on the combination of
334 the following two processes: (1) the continuous emission of dust from the desert surface to
335 the ABL and (2) the continuous transport of the dust from the ABL up to the free
336 troposphere.
337 Process (1): Dust was raised from the ground to the ABL over the Gobi Desert by the
338 strong wind around the cold front. It is likely that the dust layer was supplied continuously
339 with new dust from the desert surface while moving through the desert.
340 Process (2): The dust in the ABL was transported to the free troposphere by the updraft
16 341 of the warm air in the cold frontal system, as suggested by Kawai et al. (2015). The updraft
342 is shown in Figs. 9b, 12b, and 14b. This transport must have continued while the dust layer
343 was moving through the desert with the cold frontal system.
344 In conclusion, the cold frontal system induced the dust layer development from the ABL
345 up to the free troposphere by the combination of the two processes. This mechanism
346 should have contributed to the long-range transport of the dust by the middle-latitude
347 westerlies. It will be discussed below (Section 4.2) through trajectory analyses.
348
349 4.2 Long-range transport of the dust
350 Forward trajectories for the dust layers observed in Sainshand and Zamyn-Uud are
351 shown in Figs. 15 and 16, respectively. The trajectories extended northeastward or
352 eastward and reached northeastern China and Russian Far East (Figs. 15a and 16a).
353 These directions are consistent with the westerly winds over the cold air (Figs. 9a and 12a).
354 Most of the trajectories extended upward and exceeded a height of 4 km (Figs. 15b and
355 16b). They were located in the free troposphere, where the middle-latitude westerlies
356 prevail. The height increase of these trajectories agrees with the updrafts in the dust layers
357 (Figs. 9b and 12b). Hence, it was caused by the updraft of warm air in the cold frontal
358 system. The relative humidity along the trajectories increased and, at times, exceeded 80%
359 (Figs. 15c and 16c). The high relative humidity indicates the presence of clouds. It is likely
360 that the clouds were generated by the updraft of warm air in the cold frontal system. In
17 361 conclusion, it is suggested that the dust produced during this dust event was transported
362 over a long range by the westerlies, as reported in previous studies (e.g., Kai et al. 1988;
363 Husar et al. 2001; Hara et al. 2009; Uno et al. 2009; Yumimoto et al. 2009).
364
365 5. Conclusions
366 The Gobi Desert lidar network observed the spatial distributions of dust during the dust
367 event that occurred over the desert on 22–23 May 2013. In this study, the dust event was
368 analyzed by using the lidar network and various meteorological data. The main findings are
369 summarized below.
370 (1) During the dust event, a cold front moved southeastward across the desert.
371 (2) In Dalanzadgad, in the central part of the desert, a dust storm was observed around
372 the cold front from the ground to a height of 1.6 km. A dust layer was distributed at a height
373 of 0.9–1.6 km over the cold frontal surface. The updraft of warm air in the cold frontal
374 system should have transported the dust emitted from the dust storm.
375 (3) In Sainshand and Zamyn-Uud, in the eastern part of the desert, a dust layer ascended
376 from the ABL to the free troposphere over the cold air and reached a top height of about 4
377 km. The dust layer was located in the updraft region of warm air.
378 (4) The overall dust layer was developing from the ABL up to the free troposphere while
379 moving across the desert with the cold front. It is suggested that this development was
380 caused by the combination of the continuous emission and transport of dust by the cold
18 381 frontal system.
382 This is the first study showing the spatial development of a dust layer in association with
383 a cold frontal system by using the Gobi Desert lidar network. It is important to maintain and
384 expand the lidar network and analyze many dust events. This effort will help us better
385 understand the mechanisms of the emission and transport of Asian dust.
386
387 Acknowledgments
388 The authors gratefully acknowledge the Dalanzadgad Meteorological Observatory for
389 supporting the ceilometer observation, Dr. Atsushi Shimizu of NIES for teaching the
390 calibration method of the AD-Net lidar data, JMA for the provision of weather charts
391 (http://www.jma.go.jp/jma/), NOAA’s National Climatic Data Center for the provision of ISD
392 data (https://www.ncei.noaa.gov/isd), NCEP for the provision of FNL data
393 (https://rda.ucar.edu/datasets/ds083.2/), and NOAA’s Air Resources Laboratory for the
394 provision of the HYSPLIT transport and dispersion model and the READY website
395 (http://www.ready.noaa.gov). This study was supported by Grants-in-Aid for Scientific
396 Research from JSPS (Nos. 24340111 and 16H02703) and by the JSPS Core-to-Core
397 Program (B. Asia-Africa Science Platforms).
398
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488 models. Atmos. Chem. Phys., 9, 8545–8558.
489
490 List of Figures
491
492 Fig. 1 Topographic map of Mongolia and surrounding areas.
493
494 Fig. 2 Surface weather charts for Mongolia at (a) 14 LST and (b) 20 LST on 22 May and at
495 (c) 02 LST and (d) 08 LST on 23 May 2013 provided by JMA. The red dots show the
496 locations of Dalanzadgad (D), Sainshand (S), and Zamyn-Uud (Z). The sparse and
497 dense red-shaded areas indicate altitudes of more than 1500 and 3000 m above sea
498 level, respectively.
499
500 Fig. 3 Potential temperature (red contours; every 3 K) and wind (barbs) observed at (a) 20
24 501 LST on 22 May and (b) 02 LST on 23 May 2013 obtained from the SYNOP report. The
502 half and full barbs represent the wind speeds of 2 and 4 m s-1, respectively. The blue
503 lines indicate the cold fronts transcribed from the weather charts (Figs. 2b and 2c). The
504 blue circles with the letters D, S, and Z show the locations of Dalanzadgad, Sainshand,
505 and Zamyn-Uud, respectively.
506
507 Fig. 4 Temperature (contours; every 3 K) and vertical velocity (color) at 700 hPa at (a) 20
508 LST on 22 May and (b) 02 LST on 23 May 2013 obtained from the NCEP FNL data. For
509 the vertical velocity, a negative value (red) indicates an updraft, whereas a positive value
510 (blue) indicates a downdraft. The green lines indicate the surface cold fronts transcribed
511 from the weather charts (Figs. 2b and 2c). The black dots with D, S, and Z show the
512 locations of Dalanzadgad, Sainshand, and Zamyn-Uud, respectively.
513
514 Fig. 5 Time series of the (a) sea level pressure (SLP), (b) temperature (T), (c) relative
515 humidity (RH), and (d) wind observed in Dalanzadgad from 12 LST on 22 May to 09 LST
516 on 23 May 2013 (after Kawai et al. 2015). The half and full barbs and the pennants
517 represent the wind speeds of 1, 2, and 10 m s-1, respectively.
518
519 Fig. 6 Time-height cross sections of the (a) 910-nm attenuated backscatter coefficient
520 (ABC) observed by the ceilometer (after Kawai et al. 2015) and (b) potential temperature
25 521 (PT) and horizontal wind obtained from the NCEP FNL data for Dalanzadgad during the
522 same period as in Fig. 5. The half and full barbs and pennants represent the wind speeds
523 of 2, 4, and 20 m s-1, respectively. Regions A–D correspond to a dust storm, cold air,
524 clouds, and a dust layer, respectively. The dotted line represents the top of the cold air.
525
526 Fig. 7 Same as Fig. 5, except that the location is Sainshand, and the period is from 18
527 LST on 22 May to 15 LST on 23 May 2013.
528
529 Fig. 8 Time-height cross sections of the (a) 532-nm attenuated backscatter coefficient
530 (ABC), (b) 532-nm volume depolarization ratio (VDR), and (c) 1064/532-nm volume color
531 ratio (VCR) at a height of 0–5 km observed by the AD-Net lidar in Sainshand during the
532 same period as in Fig. 7. Regions E–H correspond to a dust storm, cold air, clouds, and a
533 dust layer, respectively.
534
535 Fig. 9 Time-height cross sections of the (a) potential temperature (PT) and horizontal wind
536 and (b) vertical velocity (VV) and horizontal wind obtained from the NCEP FNL data for
537 Sainshand during the same period as in Figs. 7 and 8. The half and full barbs and
538 pennants represent wind speeds of 2, 4, and 20 m s-1, respectively. Regions F and H are
539 the same as in Fig. 8.
540
26 541 Fig. 10 Same as Fig. 5, except that the location is Zamyn-Uud, and the period is from 00
542 to 21 LST on 23 May 2013.
543
544 Fig. 11 Same as Fig. 8, except that the location is Zamyn-Uud, the height range is 0–4 km,
545 and the period is from 00 to 21 LST on 23 May 2013 (same as in Fig. 10). Regions I–K
546 correspond to a dust layer, cold air, and a cloud, respectively.
547
548 Fig. 12 Same as Fig. 9, except that the location is Zamyn-Uud, and the period is from 00
549 to 21 LST on 23 May 2013 (same as Fig. 10). Regions I and J are the same as in Fig. 11.
550
551 Fig. 13 Observational models for (a) Dalanzadgad, (b) Sainshand, and (c) Zamyn-Uud at
552 a height of 0–4 km during the dust event. The distance was calculated by using the
553 moving speed of the cold front (30 km h-1). The distance of 0 km indicates the leading
554 edge of the cold air (i.e., the cold front on the ground) at each observation site. The right
555 side is the moving direction of the cold front (southeast). The letter P means precipitation.
556
557 Fig. 14 (a) The locations, (b) the height above ground level (AGL), and (c) the relative
558 humidity (RH) of 24-hour forward trajectories of air parcels corresponding to the dust
559 layer that was observed over cold air in Dalanzadgad on 22–23 May 2013. In panel (a),
560 the dots on the trajectories indicate their location every 6 hours, and the numerical values
27 561 of 00 and 12 near the trajectories represent the hours of 00 and 12 LST, respectively.
562
563 Fig. 15 Same as Fig. 14, except that the location is Sainshand.
564
565 Fig. 16 Same as Fig. 14, except that the location is Zamyn-Uud, and the initial time of the
566 forward trajectories is 23 May 2013.
567
28 568
569
570
571 Fig. 1 Topographic map of Mongolia and surrounding areas.
572
29 573
574 575
576 Fig. 2 Surface weather charts for Mongolia at (a) 14 LST and (b) 20 LST on 22 May and at
577 (c) 02 LST and (d) 08 LST on 23 May 2013 provided by JMA. The red dots show the
578 locations of Dalanzadgad (D), Sainshand (S), and Zamyn-Uud (Z). The sparse and
579 dense red-shaded areas indicate altitudes of more than 1500 and 3000 m above sea
580 level, respectively.
581
30 582
583 584
585 Fig. 3 Potential temperature (red contours; every 3 K) and wind (barbs) observed at (a) 20
586 LST on 22 May and (b) 02 LST on 23 May 2013 obtained from the SYNOP report. The
587 half and full barbs represent the wind speeds of 2 and 4 m s-1, respectively. The blue
588 lines indicate the cold fronts transcribed from the weather charts (Figs. 2b and 2c). The
589 blue circles with the letters D, S, and Z show the locations of Dalanzadgad, Sainshand,
590 and Zamyn-Uud, respectively.
591
31 592
593 594
595 Fig. 4 Temperature (contours; every 3 K) and vertical velocity (color) at 700 hPa at (a) 20
596 LST on 22 May and (b) 02 LST on 23 May 2013 obtained from the NCEP FNL data. For
597 the vertical velocity, a negative value (red) indicates an updraft, whereas a positive value
598 (blue) indicates a downdraft. The green lines indicate the surface cold fronts transcribed
599 from the weather charts (Figs. 2b and 2c). The black dots with D, S, and Z show the
600 locations of Dalanzadgad, Sainshand, and Zamyn-Uud, respectively.
601
32 602
603 604
605 Fig. 5 Time series of the (a) sea level pressure (SLP), (b) temperature (T), (c) relative
606 humidity (RH), and (d) wind observed in Dalanzadgad from 12 LST on 22 May to 09 LST
607 on 23 May 2013 (after Kawai et al. 2015). The half and full barbs and the pennants
608 represent the wind speeds of 1, 2, and 10 m s-1, respectively.
609
33 610
611 612
613 Fig. 6 Time-height cross sections of the (a) 910-nm attenuated backscatter coefficient
614 (ABC) observed by the ceilometer (after Kawai et al. 2015) and (b) potential temperature
615 (PT) and horizontal wind obtained from the NCEP FNL data for Dalanzadgad during the
616 same period as in Fig. 5. The half and full barbs and pennants represent the wind speeds
617 of 2, 4, and 20 m s-1, respectively. Regions A–D correspond to a dust storm, cold air,
618 clouds, and a dust layer, respectively. The dotted line represents the top of the cold air.
619
34 620
621 622
623 Fig. 7 Same as Fig. 5, except that the location is Sainshand, and the period is from 18
624 LST on 22 May to 15 LST on 23 May 2013.
625
35 626
627
628
629 Fig. 8 Time-height cross sections of the (a) 532-nm attenuated backscatter coefficient
630 (ABC), (b) 532-nm volume depolarization ratio (VDR), and (c) 1064/532-nm volume color
631 ratio (VCR) at a height of 0–5 km observed by the AD-Net lidar in Sainshand during the
632 same period as in Fig. 7. Regions E–H correspond to a dust storm, cold air, clouds, and a
633 dust layer, respectively.
634
36 635
636 637
638 Fig. 9 Time-height cross sections of the (a) potential temperature (PT) and horizontal wind
639 and (b) vertical velocity (VV) and horizontal wind obtained from the NCEP FNL data for
640 Sainshand during the same period as in Figs. 7 and 8. The half and full barbs and
641 pennants represent wind speeds of 2, 4, and 20 m s-1, respectively. Regions F and H are
642 the same as in Fig. 8.
643
37 644
645 646
647 Fig. 10 Same as Fig. 5, except that the location is Zamyn-Uud, and the period is from 00
648 to 21 LST on 23 May 2013.
649
38 650
651
652
653 Fig. 11 Same as Fig. 8, except that the location is Zamyn-Uud, the height range is 0–4 km,
654 and the period is from 00 to 21 LST on 23 May 2013 (same as in Fig. 10). Regions I–K
655 correspond to a dust layer, cold air, and a cloud, respectively.
656
39 657
658 659
660 Fig. 12 Same as Fig. 9, except that the location is Zamyn-Uud, and the period is from 00
661 to 21 LST on 23 May 2013 (same as Fig. 10). Regions I and J are the same as in Fig. 11.
662
40 663
664 665
666 Fig. 13 Observational models for (a) Dalanzadgad, (b) Sainshand, and (c) Zamyn-Uud at
667 a height of 0–4 km during the dust event. The distance was calculated by using the
668 moving speed of the cold front (30 km h-1). The distance of 0 km indicates the leading
669 edge of the cold air (i.e., the cold front on the ground) at each observation site. The right
670 side is the moving direction of the cold front (southeast). The letter P means precipitation.
671
41 672
673 674
675 Fig. 14 (a) The locations, (b) the height above ground level (AGL), and (c) the relative
676 humidity (RH) of 24-hour forward trajectories of air parcels corresponding to the dust
677 layer that was observed over cold air in Dalanzadgad on 22–23 May 2013. In panel (a),
678 the dots on the trajectories indicate their location every 6 hours, and the numerical values
679 of 00 and 12 near the trajectories represent the hours of 00 and 12 LST, respectively.
680
42 681
682 683
684 Fig. 15 Same as Fig. 14, except that the location is Sainshand.
685
43 686
687 688
689 Fig. 16 Same as Fig. 14, except that the location is Zamyn-Uud, and the initial time of the
690 forward trajectories is 23 May 2013.
691
44 692
693 List of Tables
694
695 Table 1 Summary of the maximum top height and thickness range of the dust layer over
696 cold air observed in each site on 22–23 May 2013. In Sainshand, the presence of the
697 dust layer above a height of 4.0 km was unclear because of clouds.
698
699
45 700 Table 1 Summary of the maximum top height and thickness range of the dust layer over
701 cold air observed in each site on 22–23 May 2013. In Sainshand, the presence of the
702 dust layer above a height of 4.0 km was unclear because of clouds.
703
Dust layer Dalanzadgad Sainshand Zamyn-Uud
Maximum top height 1.6 km (4.0 km) 3.8 km
Minimum thickness 0.2 km 1.6 km 0.3 km
Maximum thickness 0.5 km 2.5 km 1.7 km
704
46