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Effect of frozen ground on dust outbreaks in spring on the eastern Mongolian Plateau
Geomorphology 129 (2011) 412–416
Contents lists available at ScienceDirect
Geomorphology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / g e o m o r p h
Effect of frozen ground on dust outbreaks in spring on the eastern Mongolian Plateau Lijian Han ⁎, Atsushi Tsunekawa, Mitsuru Tsubo Arid Land Research Center, Tottori University, 1390 Hamasaka, Tottori 680–0001, Japan
a r t i c l e
i n f o
Article history: Received 8 September 2010 Received in revised form 22 February 2011 Accepted 10 March 2011 Available online 16 March 2011 Keywords: Dryland Dust outbreak QuikSCAT Freeze–thaw cycles
a b s t r a c t Very little is known about the effect of frozen ground on dust outbreaks in East Asia. We therefore examined the influence of springtime frozen ground and thawed ground and non-frozen ground before the first rainfall event on dust outbreaks on the eastern Mongolian Plateau, the principal contributor to the long-range transport of dust to the North Pacific, during the springs of 2000–2007. Radar backscatter data, MODIS data and meteorological records were used to investigate the relationship between frozen ground and dust events. We found that dust events occurred in association with strong wind on frozen ground (15.7 m/s) and thawed ground (12.6 m/s), compared with non-frozen ground (10.1 m/s). The number of dust events that occurred before the thaw correlated positively (R2 = 0.82, P b 0.01) with the proportion of non-frozen ground. The number of dust events after the thaw, when water content was high, correlated negatively (R2 = 0.88, P b 0.01) with the proportion of frozen ground. Our results indicate that dust outbreaks in our study area in spring are fewer in years when greater areas of ground are frozen during winter. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Dust outbreaks during spring in East Asia are controlled mainly by surface wind speed and the condition of the land surface. Kurosaki and Mikami (2003; 2007) studied threshold wind speeds for dust outbreaks in East Asia and found that they varied for different land surface conditions (e.g., vegetation cover and soil moisture). Understanding the effects of land surface conditions is important in studies of Asian dust events, and they have been studied at both regional and global scales (McKenna-Neuman and Nickling, 1989; Tegen et al., 2002; Engelstaedter et al., 2003; Ishizuka et al., 2005; Kimura et al., 2009). For example, vegetation cover can suppress dust outbreaks on the Loess Plateau of China (Kimura et al., 2009) and threshold wind speeds increase with increasing soil moisture in the Taklimakan Desert (Ishizuka et al., 2005). Kurosaki and Mikami (2004) showed that snow cover markedly increases the threshold wind speed for dust outbreaks in the midlatitudes, implying that frozen ground might reduce dust outbreaks. Little attention has been paid to frozen ground in the mid-latitudes (e.g., Kimball et al., 2004; Bartsch et al., 2007; Han et al., 2010), so the contribution of frozen ground to reducing dust emissions there remains unclear. Because most areas where dust outbreaks occur are too dry for the ground to freeze, there have been few studies of the relationship of dust events to soil freeze–thaw cycles. In previous studies of Asian dust events, the effect of snow cover on threshold wind velocity is the only aspect of frozen ground
⁎ Corresponding author. Tel.: + 81 857 23 3411; fax: +81 857 21 7214. E-mail addresses: [email protected], [email protected] (L. Han). 0169-555X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.geomorph.2011.03.005
condition that has been analyzed (Kurosaki and Mikami, 2004). The objective of our study therefore was to understand the impact of springtime frozen ground on dust outbreaks in middle latitude regions where soil water content is relatively high. We selected the lowlands of the eastern Mongolian Plateau (Fig. 1) for our study because this region is the principal contributor to long-range transport of dust to the North Pacific (Zhang et al., 2008), and because the ground is commonly frozen in early spring. We used radar backscatter data and meteorological records to investigate the relationship of the area of frozen ground in spring to the frequency of spring dust events in our study area. 2. Materials and methods We processed Ku-band backscatter time-series data (QuikSCAT level 2A data from the NASA Jet Propulsion Laboratory) for the first 180 days of each year from 2000 to 2007. We used the outer-beam record for this study because it has a wider swath and better temporal coverage than the inner-beam record. Averaged daily backscatter measurements were interpolated onto a 25 × 25 km grid. Soil thaw events were detected from the following backscatter signatures (Han et al., in press) that are typical of middle latitudes: 1) the spring soil thaw is indicated by a decreasing trend of the backscatter time series, whereas the lack of a thaw event is indicated by an increasing trend; and 2) during the freeze–thaw transition period, the radar backscatter time series changes markedly and repeatedly in response to changes of the dielectric constant of water caused by recurrent soil freeze–thaw events. The geographical boundaries of thaw events and primary thaw dates (defined as the middle day of the short period dominated by changes of soil water from solid to liquid phase) were
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Fig. 1. Map of northeastern China and Mongolia showing the study area (red rectangle). Dots indicate the location of meteorological stations from which meteorological data were used.
detected from QuikSCAT backscatter data to illustrate the springtime frozen ground. Meteorological data (daily average, maximum, and minimum air temperature; and daily maximum wind speed at 10 m height) were obtained from the Global Summary of the Day from the National Climate Data Center of the U.S. Department of Commerce. Meteorological data were available from 14 stations within the study area (Fig. 1). In addition to the daily maximum wind speed, fraction velocity for dust emission (u*) was estimated using a roughness of 0.0058 in a similar area of Mongolia (Kimura and Shinoda, 2010). We used the dust events identified from Moderate Resolution Imaging Spectroradiometer (MODIS) data by Zhang et al. (2008) for each year from 2000 to 2006. We also used their method to identify dust events in 2007. 3. Results and discussion 3.1. Spring dust events Dust outbreaks in the study area were analyzed in relation to daily average temperature, daily maximum wind speed, precipitation, and springtime primary soil thaw date from the 60th to 150th day of each year from 2000 to 2007 (Fig. 2). Primary thaw events occurred mostly between the 80th and 90th day, which was when the average air temperature rose above zero. Most of the dust events were observed after the primary thaw dates: 23% of the dust events occurred before the primary thaw, and 77% after the primary thaw (22% after the first rainfall and 55% before the first rainfall). Frequent dust events occurred in 2000, 2001, 2002, and 2006; these accounted for more than 80% of the spring dust events during 2000–2007. Fewer dust events were observed in 2004, 2005, and 2007, and in 2003 there were no dust events. Dust events before the primary thaw were always associated with strong winds, suggesting that frozen ground might decrease the occurrence of dust outbreaks, whereas the number of dust events decreased when soil moisture content was higher after the first rainfall. 3.2. Wind speed for dust outbreaks We compared daily maximum wind speeds for dust outbreaks with the different ground conditions (Fig. 3). The wind speeds were higher on frozen ground (15.7 m/s, u* = 0.84 m/s) and thawed ground (12.6 m/s, u* = 0.68 m/s) than non-frozen ground (10.2 m/s, u* = 0.54 m/s). As a consequence of low air and soil temperatures,
there was almost no vegetation in the study area during early spring (SPOT-VGT NDVI b 0.2). Therefore, frozen ground before the thaw and thawed ground after the thaw were major factors that influenced the occurrence of dust events together with the wind speed for dust emission. Pore water in near-surface soil is a key factor that affects the occurrence of dust events in spring in cold semi-arid areas (McKennaNeuman, 1993). When temperature goes below freezing point, pore water turns into pore ice expanding by about 9% in volume (Setzer, 2004), and then pore ice fills the pore volume to a greater extent than before and increases the contact between soil particles. Therefore, as the binding strength increases, the possibility of dust outbreak decreases. Furthermore, the recurrent freeze/thaw process can increase the possibility of dust outbreak by breaking down the relatively stable or non-erodible soil aggregates. For instance, more dust events in years where the temperature hovers around the freezing point for a relatively long period of time (e.g. 2000, 2002 and 2006) might fit this scenario. The reason for thawed ground having a higher wind speed is attributed to the fact that the surface layer has relatively high water content after the thaw. 3.3. Extent of frozen ground in spring Freezing and thawing of soil water has a marked effect on radar backscatter data, which responds to the dielectric constant of the land surface (Henderson and Lewis, 1998). Because liquid water has a high dielectric constant (around 80), whereas ice and most natural soil materials have lower values (between 3 and 8), radar-derived soil freeze–thaw events can be used to determine the spatio-temporal changes of the extent of frozen ground. Primary thaw dates derived from QuikSCAT Ku-band radar backscatter data (Fig. 4) show that in 2000, 2002, and 2006, only a small part of the study area was frozen. However, in 2001, 2003, 2004, 2005, and 2007, large areas were frozen. Consequently, thawed ground needs higher wind speed to generate dust outbreak, so the likelihood of dust emissions was lower in those years, initially because of the large areas frozen, and subsequently because of the considerable increases of soil moisture content after the thaw. Thus, the extent of frozen ground is an important factor in spring dust outbreaks. 3.4. Relationship between the extent of frozen ground and dust outbreaks We considered three categories of spring dust events: 1) those before the primary thaw date (Period 1), 2) those after the primary
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Fig. 2. The relationships of springtime dust events (×) indentified from MODIS data in the study area from 2000 to 2007 to average air temperature (red line), maximum wind speed (black line), and the rainfall events (+). Vertical bars indicate standard deviations for average air temperature and maximum wind speed. The dashed vertical lines indicate primary thaw dates and the areas shaded blue represent their standard deviations.
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thaw date but before the first rainfall (Period 2), and 3) those after the first rainfall (Period 3). During Period 3, soil moisture increases immediately after rainfall events; analysis of this period is beyond the scope of our study. Thus, we considered how Periods 1 and 2 were related to the extent of frozen ground within the study area (Fig. 5). During Period 1, the threshold wind speed for dust uplift was higher for areas of frozen ground, reducing the frequency of dust outbreaks from these areas. Thus, before the primary thaw date, nonfrozen ground was the primary contributor to dust events. A positive relationship (R2 = 0.82, P b 0.01) was found between the proportion of non-frozen ground and the number of dust events before the primary thaw. During Period 2, soil moisture in thawed ground was high and reduced the likelihood of dust outbreaks. A negative relationship (R2 = 0.88, P b 0.01) was found between the proportion of thawed ground and frequency of dust occurrences in spring. More dust events
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Fig. 4. Maps showing the progression (Julian days) of primary thaw dates in the study area during 2000–2007. No spring soil thaw events were detected in the areas shaded gray.
occurred before the primary thaw when more than 60% of the area was not frozen, whereas fewer dust events occurred after the primary thaw when more than 40% of the area was thawed ground. 4. Conclusions This study is the first to investigate the influence of the springtime frozen ground on dust outbreaks on the eastern Mongolia Plateau. By examining the wind speeds for dust events for frozen, thawed and non-frozen grounds, and considering the relationship of the areas of frozen and thawed ground in spring to the frequency of dust events, we reached the following conclusions. Dust events in the study area occur mainly after the primary soil thaw date. Strong wind speeds are needed to generate dust events on both frozen ground (15.7 m/s) and thawed ground (12.6 m/s) but not on non-frozen ground (10.1 m/s). Non-frozen ground in spring increases the likelihood of dust outbreaks before the primary thaw.
After the primary thaw, dust outbreaks on thawed ground are suppressed by the high soil water content of the previously frozen soil. The results of our study contribute to understanding the influence of frozen ground on the mechanisms of dust outbreaks in spring, and to the forecasting of future dust events in the study area. In addition, further field observations are still needed to understand the dust outbreak mechanism in detail. Acknowledgments This research was supported by the Japanese Society for Promotion of Science Core University Program and the Global Center of Excellence Program for Dryland Science of the Japanese Ministry of Education, Culture, Sports, Science and Technology. We thank the following data providers: NASA Jet Propulsion Laboratory for QuikSCAT Level 2A data; NOAA National Climate Data Center for
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References
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Dust events (number of days) after primary thaw date Fig. 5. Relationships during 2000–2007 of the frequency of dust events with (A) the coverage of frozen ground before the spring thaw (Period 1) and (B) the coverage of thawed ground after the spring thaw and before the first rainfall (Period 2).
meteorological data; and Dr. Baolin Zhang of the Inner Mongolia Normal University for dust outbreak data. We also thank Profs. Tao Wang and Xinrong Li of the Cold and Arid Regions Environmental and Engineering Research Institute of the Chinese Academy of Sciences for their assistance with field investigations. Special thanks go to the two reviewers for their professional comments.
Bartsch, A., Kidd, R.A., Wagner, W., Bartalis, Z., 2007. Temporal and spatial variability of beginning and end of daily spring freeze/thaw cycles derived from scatterometer data. Remote Sensing of Environment 106, 360–374. Engelstaedter, S., Kohfeld, K.E., Tegen, I., Harrison, S.P., 2003. Controls of dust emissions by vegetation and topographic depressions: an evaluation using dust storm frequency data. Geophysical Research Letters 30, 1294. Han, L., Tsunekawa, A., Tsubo, M., 2010. Monitoring near-surface soil freeze–thaw cycles in northern China and Mongolia from 1998 to 2007. International Journal of Applied Earth Observation and Geoinformation 12, 375–384. Han, L., Tsunekawa, A., Tsubo, M., in press. Radar remote sensing of springtime nearsurface soil thaw events at mid-latitudes. International Journal of Remote Sensing. doi:10.1080/01431161.2010.542203. Henderson, F.M., Lewis, A.J., 1998. Principles and application of image radar. John Wiley & Sons, New York, pp. 161–162. Ishizuka, M., Mikami, M., Yamada, Y., Zeng, F., Gao, W., 2005. An observational study of soil moisture effects on wind erosion at a gobi site in the Taklimakan Desert. Journal of Geophysical Research 110, D18S03. Kimball, J.S., McDonald, K.C., Frolking, S., Running, S.W., 2004. Radar remote sensing of the spring thaw transition across a boreal landscape. Remote Sensing of Environment 89, 163–175. Kimura, R., Shinoda, M., 2010. Spatial distribution of threshold wind speeds for dust outbreak in northeast Asia. Geomorphology 114, 319–325. Kimura, R., Bai, L., Wang, J., 2009. Relationships among dust outbreak, vegetation cover, and surface soil water content on the Loess Plateau of China, 1999–2000. Catena 77, 292–296. Kurosaki, Y., Mikami, M., 2003. Recent frequent dust events and their relation to surface wind in Asia. Geophysical Research Letters 30, 1736. Kurosaki, Y., Mikami, M., 2004. Effect of snow cover on threshold wind velocity of dust outbreak. Geophysical Research Letters 31, D03106. Kurosaki, Y., Mikami, M., 2007. Threshold wind speed for dust emission in East Asia and its seasonal variations. Journal of Geophysical Research 112, D17202. McKenna-Neuman, C., 1993. A review of aeolian transport processes in cold environments. Progress in Physical Geography 17, 137–155. McKenna-Neuman, C., Nickling, W.G., 1989. A theoretical and wind tunnel investigation of the effect of capillarity water on the entrainment of sediment by wind. Canadian Journal of Soil Science 69, 79–96. Setzer, M., 2004. Modeling and testing the freeze–thaw attack by micro-ice-lens model and CDF/CIF-test. International RILEM Symposium on Concrete Science and Engineering: A Tribute to Arnon Bentur. RILEM Publications SARL, Illinois, U.S, pp. 17–27. Tegen, I., Harrison, S.P., Kohfeld, K., Prentice, I.C., Coe, M., Heimann, M., 2002. Impact of vegetation and preferential source areas on global dust aerosol: results from a model study. Journal of Geophysical Research 107, 4576. Zhang, B., Tsunekawa, A., Tsubo, M., 2008. Contributions of sandy lands and stony deserts to long-distance dust emission in China and Mongolia during 2000–2006. Global Planet Change 60, 487–504.
Contents lists available at ScienceDirect
Geomorphology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / g e o m o r p h
Effect of frozen ground on dust outbreaks in spring on the eastern Mongolian Plateau Lijian Han ⁎, Atsushi Tsunekawa, Mitsuru Tsubo Arid Land Research Center, Tottori University, 1390 Hamasaka, Tottori 680–0001, Japan
a r t i c l e
i n f o
Article history: Received 8 September 2010 Received in revised form 22 February 2011 Accepted 10 March 2011 Available online 16 March 2011 Keywords: Dryland Dust outbreak QuikSCAT Freeze–thaw cycles
a b s t r a c t Very little is known about the effect of frozen ground on dust outbreaks in East Asia. We therefore examined the influence of springtime frozen ground and thawed ground and non-frozen ground before the first rainfall event on dust outbreaks on the eastern Mongolian Plateau, the principal contributor to the long-range transport of dust to the North Pacific, during the springs of 2000–2007. Radar backscatter data, MODIS data and meteorological records were used to investigate the relationship between frozen ground and dust events. We found that dust events occurred in association with strong wind on frozen ground (15.7 m/s) and thawed ground (12.6 m/s), compared with non-frozen ground (10.1 m/s). The number of dust events that occurred before the thaw correlated positively (R2 = 0.82, P b 0.01) with the proportion of non-frozen ground. The number of dust events after the thaw, when water content was high, correlated negatively (R2 = 0.88, P b 0.01) with the proportion of frozen ground. Our results indicate that dust outbreaks in our study area in spring are fewer in years when greater areas of ground are frozen during winter. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Dust outbreaks during spring in East Asia are controlled mainly by surface wind speed and the condition of the land surface. Kurosaki and Mikami (2003; 2007) studied threshold wind speeds for dust outbreaks in East Asia and found that they varied for different land surface conditions (e.g., vegetation cover and soil moisture). Understanding the effects of land surface conditions is important in studies of Asian dust events, and they have been studied at both regional and global scales (McKenna-Neuman and Nickling, 1989; Tegen et al., 2002; Engelstaedter et al., 2003; Ishizuka et al., 2005; Kimura et al., 2009). For example, vegetation cover can suppress dust outbreaks on the Loess Plateau of China (Kimura et al., 2009) and threshold wind speeds increase with increasing soil moisture in the Taklimakan Desert (Ishizuka et al., 2005). Kurosaki and Mikami (2004) showed that snow cover markedly increases the threshold wind speed for dust outbreaks in the midlatitudes, implying that frozen ground might reduce dust outbreaks. Little attention has been paid to frozen ground in the mid-latitudes (e.g., Kimball et al., 2004; Bartsch et al., 2007; Han et al., 2010), so the contribution of frozen ground to reducing dust emissions there remains unclear. Because most areas where dust outbreaks occur are too dry for the ground to freeze, there have been few studies of the relationship of dust events to soil freeze–thaw cycles. In previous studies of Asian dust events, the effect of snow cover on threshold wind velocity is the only aspect of frozen ground
⁎ Corresponding author. Tel.: + 81 857 23 3411; fax: +81 857 21 7214. E-mail addresses: [email protected], [email protected] (L. Han). 0169-555X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.geomorph.2011.03.005
condition that has been analyzed (Kurosaki and Mikami, 2004). The objective of our study therefore was to understand the impact of springtime frozen ground on dust outbreaks in middle latitude regions where soil water content is relatively high. We selected the lowlands of the eastern Mongolian Plateau (Fig. 1) for our study because this region is the principal contributor to long-range transport of dust to the North Pacific (Zhang et al., 2008), and because the ground is commonly frozen in early spring. We used radar backscatter data and meteorological records to investigate the relationship of the area of frozen ground in spring to the frequency of spring dust events in our study area. 2. Materials and methods We processed Ku-band backscatter time-series data (QuikSCAT level 2A data from the NASA Jet Propulsion Laboratory) for the first 180 days of each year from 2000 to 2007. We used the outer-beam record for this study because it has a wider swath and better temporal coverage than the inner-beam record. Averaged daily backscatter measurements were interpolated onto a 25 × 25 km grid. Soil thaw events were detected from the following backscatter signatures (Han et al., in press) that are typical of middle latitudes: 1) the spring soil thaw is indicated by a decreasing trend of the backscatter time series, whereas the lack of a thaw event is indicated by an increasing trend; and 2) during the freeze–thaw transition period, the radar backscatter time series changes markedly and repeatedly in response to changes of the dielectric constant of water caused by recurrent soil freeze–thaw events. The geographical boundaries of thaw events and primary thaw dates (defined as the middle day of the short period dominated by changes of soil water from solid to liquid phase) were
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Fig. 1. Map of northeastern China and Mongolia showing the study area (red rectangle). Dots indicate the location of meteorological stations from which meteorological data were used.
detected from QuikSCAT backscatter data to illustrate the springtime frozen ground. Meteorological data (daily average, maximum, and minimum air temperature; and daily maximum wind speed at 10 m height) were obtained from the Global Summary of the Day from the National Climate Data Center of the U.S. Department of Commerce. Meteorological data were available from 14 stations within the study area (Fig. 1). In addition to the daily maximum wind speed, fraction velocity for dust emission (u*) was estimated using a roughness of 0.0058 in a similar area of Mongolia (Kimura and Shinoda, 2010). We used the dust events identified from Moderate Resolution Imaging Spectroradiometer (MODIS) data by Zhang et al. (2008) for each year from 2000 to 2006. We also used their method to identify dust events in 2007. 3. Results and discussion 3.1. Spring dust events Dust outbreaks in the study area were analyzed in relation to daily average temperature, daily maximum wind speed, precipitation, and springtime primary soil thaw date from the 60th to 150th day of each year from 2000 to 2007 (Fig. 2). Primary thaw events occurred mostly between the 80th and 90th day, which was when the average air temperature rose above zero. Most of the dust events were observed after the primary thaw dates: 23% of the dust events occurred before the primary thaw, and 77% after the primary thaw (22% after the first rainfall and 55% before the first rainfall). Frequent dust events occurred in 2000, 2001, 2002, and 2006; these accounted for more than 80% of the spring dust events during 2000–2007. Fewer dust events were observed in 2004, 2005, and 2007, and in 2003 there were no dust events. Dust events before the primary thaw were always associated with strong winds, suggesting that frozen ground might decrease the occurrence of dust outbreaks, whereas the number of dust events decreased when soil moisture content was higher after the first rainfall. 3.2. Wind speed for dust outbreaks We compared daily maximum wind speeds for dust outbreaks with the different ground conditions (Fig. 3). The wind speeds were higher on frozen ground (15.7 m/s, u* = 0.84 m/s) and thawed ground (12.6 m/s, u* = 0.68 m/s) than non-frozen ground (10.2 m/s, u* = 0.54 m/s). As a consequence of low air and soil temperatures,
there was almost no vegetation in the study area during early spring (SPOT-VGT NDVI b 0.2). Therefore, frozen ground before the thaw and thawed ground after the thaw were major factors that influenced the occurrence of dust events together with the wind speed for dust emission. Pore water in near-surface soil is a key factor that affects the occurrence of dust events in spring in cold semi-arid areas (McKennaNeuman, 1993). When temperature goes below freezing point, pore water turns into pore ice expanding by about 9% in volume (Setzer, 2004), and then pore ice fills the pore volume to a greater extent than before and increases the contact between soil particles. Therefore, as the binding strength increases, the possibility of dust outbreak decreases. Furthermore, the recurrent freeze/thaw process can increase the possibility of dust outbreak by breaking down the relatively stable or non-erodible soil aggregates. For instance, more dust events in years where the temperature hovers around the freezing point for a relatively long period of time (e.g. 2000, 2002 and 2006) might fit this scenario. The reason for thawed ground having a higher wind speed is attributed to the fact that the surface layer has relatively high water content after the thaw. 3.3. Extent of frozen ground in spring Freezing and thawing of soil water has a marked effect on radar backscatter data, which responds to the dielectric constant of the land surface (Henderson and Lewis, 1998). Because liquid water has a high dielectric constant (around 80), whereas ice and most natural soil materials have lower values (between 3 and 8), radar-derived soil freeze–thaw events can be used to determine the spatio-temporal changes of the extent of frozen ground. Primary thaw dates derived from QuikSCAT Ku-band radar backscatter data (Fig. 4) show that in 2000, 2002, and 2006, only a small part of the study area was frozen. However, in 2001, 2003, 2004, 2005, and 2007, large areas were frozen. Consequently, thawed ground needs higher wind speed to generate dust outbreak, so the likelihood of dust emissions was lower in those years, initially because of the large areas frozen, and subsequently because of the considerable increases of soil moisture content after the thaw. Thus, the extent of frozen ground is an important factor in spring dust outbreaks. 3.4. Relationship between the extent of frozen ground and dust outbreaks We considered three categories of spring dust events: 1) those before the primary thaw date (Period 1), 2) those after the primary
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Fig. 2. The relationships of springtime dust events (×) indentified from MODIS data in the study area from 2000 to 2007 to average air temperature (red line), maximum wind speed (black line), and the rainfall events (+). Vertical bars indicate standard deviations for average air temperature and maximum wind speed. The dashed vertical lines indicate primary thaw dates and the areas shaded blue represent their standard deviations.
25 20 15
15.7 m/s 12.6 m/s
10
10.1 m/s
5 0
Frozen
Thawed
Non-frozen
Ground condition Fig. 3. Daily maximum wind speed for dust outbreaks from frozen, thawed and nonfrozen ground. Black dots represent average daily maximum wind speed with vertical bars showing their standard deviations.
thaw date but before the first rainfall (Period 2), and 3) those after the first rainfall (Period 3). During Period 3, soil moisture increases immediately after rainfall events; analysis of this period is beyond the scope of our study. Thus, we considered how Periods 1 and 2 were related to the extent of frozen ground within the study area (Fig. 5). During Period 1, the threshold wind speed for dust uplift was higher for areas of frozen ground, reducing the frequency of dust outbreaks from these areas. Thus, before the primary thaw date, nonfrozen ground was the primary contributor to dust events. A positive relationship (R2 = 0.82, P b 0.01) was found between the proportion of non-frozen ground and the number of dust events before the primary thaw. During Period 2, soil moisture in thawed ground was high and reduced the likelihood of dust outbreaks. A negative relationship (R2 = 0.88, P b 0.01) was found between the proportion of thawed ground and frequency of dust occurrences in spring. More dust events
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Fig. 4. Maps showing the progression (Julian days) of primary thaw dates in the study area during 2000–2007. No spring soil thaw events were detected in the areas shaded gray.
occurred before the primary thaw when more than 60% of the area was not frozen, whereas fewer dust events occurred after the primary thaw when more than 40% of the area was thawed ground. 4. Conclusions This study is the first to investigate the influence of the springtime frozen ground on dust outbreaks on the eastern Mongolia Plateau. By examining the wind speeds for dust events for frozen, thawed and non-frozen grounds, and considering the relationship of the areas of frozen and thawed ground in spring to the frequency of dust events, we reached the following conclusions. Dust events in the study area occur mainly after the primary soil thaw date. Strong wind speeds are needed to generate dust events on both frozen ground (15.7 m/s) and thawed ground (12.6 m/s) but not on non-frozen ground (10.1 m/s). Non-frozen ground in spring increases the likelihood of dust outbreaks before the primary thaw.
After the primary thaw, dust outbreaks on thawed ground are suppressed by the high soil water content of the previously frozen soil. The results of our study contribute to understanding the influence of frozen ground on the mechanisms of dust outbreaks in spring, and to the forecasting of future dust events in the study area. In addition, further field observations are still needed to understand the dust outbreak mechanism in detail. Acknowledgments This research was supported by the Japanese Society for Promotion of Science Core University Program and the Global Center of Excellence Program for Dryland Science of the Japanese Ministry of Education, Culture, Sports, Science and Technology. We thank the following data providers: NASA Jet Propulsion Laboratory for QuikSCAT Level 2A data; NOAA National Climate Data Center for
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Coverage of non-frozen ground
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References
100%
A Period 1
2006
2002 2000
80% 2004
60%
2001
2005 2007 2003
40%
y = 0.08x + 0.49 R² = 0.82, P < 0.01
20% 0%
0
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5
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Dust events (number of days) before primary thaw date 100%
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80% y = -0.04x + 0.58 R² = 0.88, P < 0.01
2003
60%
2007
2005
40%
2001 2004
20% 0%
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0
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4
6
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8
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Dust events (number of days) after primary thaw date Fig. 5. Relationships during 2000–2007 of the frequency of dust events with (A) the coverage of frozen ground before the spring thaw (Period 1) and (B) the coverage of thawed ground after the spring thaw and before the first rainfall (Period 2).
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