Threshold wind speed for dust emission in east Asia and its seasonal variations

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 112,, doi: /2006jd007988, 2007 Threshold wind speed for dust emission in east Asia and its seasonal variations Y. Kurosaki 1,2 and M. Mikami 3 Received 1 September 2006; revised 19 April 2007; accepted 14 May 2007; published 6 September [1] We present maps of threshold wind speed for dust emission in east Asia, which are statistically evaluated from synoptic surface meteorological data. We define threshold wind speeds as u t5% and u t50%, which can be identified as threshold wind speeds at close to the most favorable land surface condition for dust emission and under normal land surface conditions, respectively. Spatial distributions of u t5% and u t50% are similar and roughly correspond to the land cover type. The threshold wind speed is low in desert regions such as the Taklimakan Desert (u t5% = 4.4 ± 0.6 m s 1 and u t50% = 6.7 ± 1.5 m s 1 ) and the Loess Plateau (u t5% = 6.9 ± 1.2 m s 1 and u t50% = 9.4 ± 1.6 m s 1 ). On the other hand, the highest is seen in northern Mongolia (u t5% = 9.8 ± 2.2 m s 1 and u t50% = 16.2 ± 2.5 m s 1 ), whose land cover type is grassland. One exception is the high threshold wind speed recorded in the Gobi Desert (u t5% = 8.9 ± 2.2 m s 1 and u t50% = 13.8 ± 2.0 m s 1 ). Seasonal variations in the threshold wind speed are narrow in desert regions such as the Taklimakan Desert, the Gobi Desert and Loess Plateau, but wide in grassland regions such as northern Mongolia. This suggests that land surface conditions are the similar throughout the year in desert regions, but seasonally variable in grassland regions. Citation: Kurosaki, Y., and M. Mikami (2007), Threshold wind speed for dust emission in east Asia and its seasonal variations, J. Geophys. Res., 112,, doi: /2006jd Introduction [2] Mineral dust is the dominant aerosol component in the troposphere. Dust aerosols have various impacts on Earth environments and human lives by causing climate change via direct and indirect radiation processes, changes in ice cap albedo, damage to agriculture and transportation, human health, ocean fertilization, neutralization of acid rain, etc. Global and regional models of the dust cycle (i.e., emission, airborne movement, and deposition processes) have been developed by numerous researchers and have played important roles in assessing the impact of dust [e.g., Tegen and Fung, 1994, 1995; Uno et al., 2001; Shao et al., 2003; Tanaka and Chiba, 2005]. However, the current models still lack accuracy, especially in the treatment of the emission process [Tegen et al., 2002]. Indeed, the magnitude of dust emission appreciably differs among the models [Zender et al., 2004; Uno et al., 2006]. [3] These inaccuracies in simulated dust emission derive from imperfections in dust emission schemes and the geographic information system (GIS) data set concerning land surface conditions. As indicated by Zender et al. 1 Center for Environmental Remote Sensing, Chiba University, Chiba, Japan. 2 Now at School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, Georgia, USA. 3 Meteorological Research Institute, Japan Meteorological Agency, Tsukuba, Japan. Copyright 2007 by the American Geophysical Union /07/2006JD [2003], dust models can be divided into two classes in terms of dust emission process. One is a simpler class and the other is a more complex class. In the simpler class, the amount of dust emission is parameterized by the third or fourth power of the wind speed or wind friction speed at a constant threshold wind speed, usually 6.5 m s 1 [e.g., Tegen and Fung, 1994, 1995; Takemura et al., 2000; Uno et al., 2001]. Although these models can simulate the outline of dust distribution, a constant threshold is the key difference with spatiotemporally variable threshold wind speeds in real situations. In the more complex classes, dust emission schemes represent detailed physical processes of soil particles, which are the threshold wind speed of wind erosion for different sizes, the horizontal flux of saltating sand particles, and the vertical flux of dust particles entrained into the atmosphere [Marticorena and Bergametti, 1995; Shao et al., 1996; Alfaro and Gomes, 2001; Shao, 2001]. However, these physically detailed models require a GIS data set that includes a variety of land surface conditions (e.g., soil particle size distribution, roughness length, soil wetness, vegetation cover, snow cover, land cover type), while no data set satisfies these requirements because of low spatiotemporal resolution and low certainty. [4] The threshold wind speed, a key parameter in the emission process, has been parameterized on the basis of wind tunnel experiments, and in situ observations [e.g., Gillette, 1978; Raupach et al., 1993; Mariticorena et al., 1997; Mikami et al., 2005]. However, the variation in land surface conditions where such experiments and observations can be carried out is finite in comparison with their immense 1of13

2 variety in dust emission regions worldwide. In east Asia, dust emission occurs from several kinds of land cover type: bare desert, semidesert shrubs, grassland, and cultivated land [Kurosaki and Mikami, 2005] (hereinafter referred to as KM2005). In addition to spatial variety, land surface conditions change seasonally and interannually in accordance with snow cover [Kurosaki and Mikami, 2004], and vegetation cover [Zou and Zhai, 2004; Piao et al., 2005], etc. To obtain threshold wind speeds for various land surface conditions, it is possible to use surface meteorological data observed in various regions and satellite data [Natsagdorj et al., 2003; Wang et al., 2003; Laurent et al., 2005]. [5] In this study, we present maps of the threshold wind speeds for dust emission that are statistically evaluated from surface meteorological data, and their correspondence with land cover type will be discussed. We define two kinds of threshold wind speed. One is u t5%, which is the threshold wind speed at close to the most favorable land surface for dust emission in a given region for a given period. The other is u t50%, which often represents the threshold wind speed at the normal land surface in a given region for a given period (section 4.2). Seasonal variations in the threshold wind speed will also be discussed. 2. Data 2.1. Surface Meteorological Data [6] The elements of surface meteorological data we employed were the present weather and the surface wind speed at a height of 10 m, which are included in SYNOP reports [World Meteorological Organization (WMO), 1995]. The locations of observatories, totally 306, in the analysis region are illustrated in Figure 1 by dots, circles, and squares (the difference of symbols will be shown in section 3). The period is from March 1988 to June The observation interval is 3 hours (i.e., 0000, 0300,..., and 2100 UTC) at most observatories, although there are some exceptions such as 6-hourly observations (i.e., 0000, 0600, 1200, and 1800 UTC), and irregular gaps in observations. [7] The present weather, as obtained from naked-eye observation, was used to detect dust emission events. All definitions of present weather (ww = 00 to 99) are shown by WMO [1995, Code Table 4677]. The present weathers, ww = 07, 08, 09, 30 35, and 98, are classified as dust emission (Table 1). This definition is the same as in KM2005. The wind speed in SYNOP reports is an average over the ten minutes immediately preceding the time of synoptic observation Land Cover Type Data [8] The land cover type data we used is the Global Ecosystems in Global Land Cover Characteristics Database Version 2.0 [Loveland et al., 2000], which is distributed by the U.S. Geological Survey. Although there are 96 kinds of land cover types in this data set, we classified these into nine types: bare desert, semidesert, semidesert shrubs, grassland, cultivated land, savanna, forest, tundra, and others. This classification is the same as in KM Snow Cover and NDVI Data [9] We used the snow cover and the normalized difference vegetation index (NDVI) data to discuss the seasonal variation in the threshold wind speed. The snow cover product we used is Near Real-Time SSM/I EASE-Grid Daily Global Ice Concentration and Snow Extent, which is distributed by the National Snow and Ice Data Center (NSIDC) [Nolin et al., 1998]. The data period was from 1995 to We define the snow cover fraction as the ratio of the number of snow cover days to the total days during the given month. The NDVI data we used is from the NOAA/NASA Pathfinder AVHRR Land (PAL) program. 3. Regions [10] Mountainous terrain is a geographical characteristic of east Asia such as the Tibetan Plateau, Tian Shan Mountains, Altai Mountains, etc., and they divide east Asia into several dust source regions. Another characteristic is the complex distribution of land cover type. While land cover types are distributed as east west belts in North Africa (not shown), they are distributed in the north south direction as well in east Asia (Figure 1b). [11] KM2005 defines a potential dust source as an observatory where the maximum monthly dust emission frequency is greater than 4% during the analysis period (March 1988 to May 2004). Observatories in a potential dust source, totally 144 observatories, are illustrated by a circle or a square in Figure 1. Referring to topography and land cover type, we divide observatories possessing a potential of dust source into six regions, each of which is indicated by an alphabetic letter in a square: T (Taklimakan Desert; 9 observatories), G (Gobi Desert; 11 observatories), L (Loess Plateau; 15 observatories), M (northern Mongolia; 18 observatories), N (northeastern China; 16 observatories), and P (North China Plain; 13 observatories). [12] The Taklimakan Desert is located in the Tarim Basin, surrounded by high mountains comprising the Tibetan Plateau, Pamirs, and the Tian Shan Mountains. The predominant land cover type in this region is bare desert. The Gobi Desert extends from the south of Mongolia to the north of China, and its major land cover type is semidesert shrubs. In the Loess Plateau, the major land cover types are semidesert shrubs in the western part, cultivated land in the southeastern part, and grassland in the northeastern part. The major dust sources of east Asia are located in these regions: the Taklimakan Desert, the Gobi Desert, and the western part of the Loess Plateau (KM2005). Frequent dust emission events are observed in northern Mongolia as well, although the frequency is lower than these three major dust sources. The land cover type of northern Mongolia is grassland. The major land cover type in northeastern China and the North China Plain is cultivated land. Although dust emission events are infrequent in these two regions in normal years, they were frequent in the springs of , which are known as the frequent dust years of east Asia [Kurosaki and Mikami, 2003]. 4. Method 4.1. Frequency Distribution of Threshold Wind Speed [13] The bar chart in Figure 2a is an example of the frequency distribution of the surface wind speed in intervals of 1 m s 1 at a WMO synoptic observatory (52378) located in the southwest of the Gobi Desert, during March May 2of13

3 Figure 1. (a) Topography and (b) land cover types of east Asia. Dots, circles and squares indicate the distribution of surface meteorological observatories. Observatories represented by circles and squares are located in potential dust sources defined by Kurosaki and Mikami [2005]. An alphabetic letter in a square indicates the region name shown in section 3, while observatories in circles are not given any region name. Thin solid lines, thick solid lines, and white lines indicate every 1500 m A.S.L (i.e., 1500 m and 3000 m A.S.L.), national borders, and the Huang He River, respectively, in Figure 1a. Thin black lines, red lines, and blue lines in Figure 1b indicate every 1500 m A.S.L, national borders, and the Huang He River, respectively. 3of13

4 Table 1. Present Weathers in SYNOP Associated With Dust Weather [WMO, 1995] a ww Contents 06 widespread dust in suspension in the air, not raised by wind at or near the station at the time of observation 07 dust or sand raised by wind at or near the station at the time of observation, but no well-developed dust whirl(s) or sand whirl(s), and no dust storm or sandstorm seen; or, in the case of ships, blowing spray at the station 08 well-developed dust whirl(s) or sand whirl(s) seen at or near the station during the preceding hour or at the time of observation, but no dust storm or sandstorm 09 dust storm or sandstorm within sight at the time of observation, or at the station during the preceding hour 30 slight or moderate dust storm or sandstorm, has decreased during the preceding hour 31 slight or moderate dust storm or sandstorm, no appreciable change during the preceding hour 32 slight or moderate dust storm or sandstorm, has begun or has increased during the preceding hour 33 severe dust storm or sandstorm, has decreased during the preceding hour 34 severe dust storm or sandstorm, no appreciable change during the preceding hour 35 severe dust storm or sandstorm, has begun or has increased during the preceding hour 98 thunderstorm combined with dust storm or sandstorm at time of observation a These present weathers, except ww = 06, are classified as dust emission (i.e., ww = 07, 08, 09, 30 35, and 98). from 1988 to The hatched bars indicate the number of observations reporting dust emission at particular wind speed u (N emsju ), and the open bars indicate the number of observations when no dust emission was present at each wind speed u (N noemsju ). Each dot on the line graph indicates the dust emission frequency at each wind speed: f emsju ¼ N emsju =N allju 100 ð% Þ ð1þ where N allju = N emsju + N noemsju. Figure 2. (a) Frequency distribution of surface wind speed at a WMO synoptic observatory (52378, N, E) during March May from 1988 to Shaded and open bars indicate the frequency distributions of wind speed accompanied by dust emission and no dust emission, respectively. Each dot on the line graph indicates the dust emission frequency at each wind speed. (b) Frequency distribution of threshold wind speed. (c) Location of this observatory. 4of13

5 Similarly, when dust emission does not occur at a given wind speed u, the threshold wind speed u t is higher than u (i.e., u t > u), and this means that the number of threshold wind speeds exceeding u (i.e., N ut>u ) is the same as the number of no dust emission at a wind speed of u (i.e., N noemsju ): N ut>u ¼ N noemsju : ð4þ From equations (1) (4), we obtain: F utu ¼ N utu =N allju 100 ð% Þ ¼ N emsju =N allju 100 ð% Þ ¼ f emsju ð% Þ: ð5þ [17] The frequency of the threshold wind speed between u and u + Du, f utjuut<u+du (%), can be expressed as follows: f utjuut<uþdu ¼ F utuþdu F utu : ð6þ Substituting equation (5) into equation (6) gives the following formula: f utjuut<uþdu ¼ f emsjuþdu f emsju : ð7þ Figure 3. Same as Figure 2a but assuming a constant threshold wind speed of 6.5 m s 1. [14] The threshold wind speed is assumed to be a constant value of 6.5 m s 1 in several models [e.g., Tegen and Fung, 1994, 1995; Takemura et al., 2000; Uno et al., 2001]. When we apply this assumption in Figure 2a, we obtain Figure 3. No dust emission occurs at any wind speed below 6.5 m s 1, but dust emission always occurs at wind speeds exceeding 6.5 m s 1. In other words, the dust emission frequency jumps from 0% to 100% at 6.5 m s 1. In the real data (Figure 2a), however, there are many cases where dust emission does not occur even if the wind speed exceeds 6.5 m s 1. The dust emission frequency gradually increases with the wind speed. Figure 2a demonstrates that the threshold wind speed has varying values in the field, and the frequency distribution of the threshold wind speed shown in Figure 2b is obtained by the method described below. [15] In N allju times of wind speed u during the analysis period, we assume that we had N utu observations of threshold wind speed less than or equal to u, and N ut>u observations of threshold wind speed exceeding u (i.e., N allju = N utu + N ut>u ). The frequency of threshold wind speed less than or equal to u is: F utu ¼ N utu =N allju 100 ð% Þ: ð2þ [16] When dust emission occurs at a given wind speed u, the threshold wind speed u t is less than or equal to u (i.e., u t u). This means that the number of threshold wind speed less than or equal to u (i.e., N utu ) is the same as the number of dust emission at a wind speed of u (i.e., N emsju ): N utu ¼ N emsju : ð3þ Equation (7) shows that the frequency of the threshold wind speed (the bars in Figure 2b) can be obtained from the difference between the dust emission frequency at the wind speeds u and u + Du (the dots in the line graph in Figure 2a). [18] In the case of observatory 52378, because the threshold wind speed was always less than the maximum wind speed during the analysis period, the maximum dust emission frequency is 100% (Figure 2a), and we can illustrate the whole frequency distribution of the threshold wind speed (Figure 2b). However, at most observatories, because the threshold wind speed frequently exceeds the maximum wind speed during the analysis period, the maximum dust emission frequency is lower than 100%. In the case of observatory 51828, because the maximum dust emission frequency is 76.9% at a wind speed of 8 m s 1 (Figure 4a), we cannot obtain the upper 23.1% of the frequency distribution of the threshold wind speed, which is at wind speeds exceeding 8 m s 1 (Figure 4b) Definitions of Threshold Wind Speed (u t5% and u t50% ) [19] Here, we define two kinds of threshold wind speed. They are u t5% and u t50%, which are the wind speeds for which the dust emission frequencies are 5% and 50%, respectively. The value of u t5% represents the threshold wind speed when the land surface conditions are close to the most favorable for dust emission at a given observatory during a given period. The value of u t50% often represents the threshold wind speed for the normal land surface condition at a given observatory during a given period. In practice, these threshold wind speeds are obtained from a linear interpolation. The arrows in Figure 2a show the examples at observatory In this case, u t5% and u t50% are 7.6 m s 1 and 11.7 m s 1, respectively. [20] We cannot obtain the whole frequency distribution at most observatories as indicated in section 4.1. For this 5of13

6 Figure 4. (a c) Same as Figure 2 but in a WMO synoptic observatory (51828, N, E). Because the maximum dust emission frequency is 76.9% at the wind speed 8 m s 1 (Figure 4a), the upper 23.1% of the frequency distribution of the threshold wind speed, which is at wind speeds exceeding 8ms 1, is not illustrated (Figure 4b). reason, we cannot estimate the variance and standard deviation of the threshold wind speed at most observatories. As an index of dispersion of threshold wind speed, we introduce the difference between u t5% and u t50%. Hereafter, this difference will be indicated as Du t (i.e., u t50% u t5% ). The frequency distributions of threshold wind speed at observatories (Figure 2b) and (Figure 4b) show that the dispersion of the threshold wind speed is much wider at observatory than at From Figures 2a and 4a, we can see that Du t acts as an index of dispersion of the threshold wind speed. 5. Results 5.1. Spatial Distribution of u t5% [21] Figure 5 shows a map of u t5% in the spring in east Asia. They were calculated from data obtained in March, April and May from 1988 to Numbers in circles indicate u t5% values, while a dot indicates that u t5% cannot be evaluated because of few strong winds exceeding threshold wind speeds. Figure 6 indicates the frequency distribution of u t5% in each region. [22] There are three regions connected with the desert (i.e., bare desert and semidesert shrubs). These are the Taklimakan Desert, the Gobi Desert, and the Loess Plateau. In terms of the Loess Plateau, although the western part is desert, the northeastern and the southeastern parts are covered by grassland and cultivated land, respectively. The solid and shaded bars in Figure 6c indicate the results from observatories in desert and nondesert, respectively. While all observatories in the Gobi Desert are located in desert areas, the shaded bars in Figure 6b indicate the results from observatories neighboring grassland. [23] The u t5% of the Taklimakan Desert is the lowest in east Asia, at 4.4 ± 0.6 m s 1, where these values are the average and the standard deviation. The u t5% is especially low from the central to the southwestern part. It is also low in the Loess Plateau, at 6.9 ± 1.2 m s 1. As shown in Figure 6c, the u t5% values in desert regions are especially low at 6.1 ± 1.0 m s 1. Although the u t5% is low in the above two desert regions, u t5% in the Gobi Desert is high and its spatial difference is also high at 8.9 ± 2.2 m s 1.Itis especially high at the four observatories neighboring grassland (Figure 6b). However, even if we exclude these observatories, the u t5% is not low, at 7.8 ± 2.0 m s 1.Itis the highest in the desert regions. [24] In northern Mongolia, whose land cover type is grassland, the u t5% is 9.8 ± 2.2 m s 1. It is the highest in 6of13

7 Figure 5. Map of u t5% in spring in east Asia. The threshold wind speed at each observatory was calculated from data during March, April and May from 1988 to east Asia, and its spatial difference is also wide. Several observatories, whose u t5% is relatively low, at about 6 to 9ms 1, are distributed in the western part. On the other hand, numerous observatories of high u t5%, at about 9 to 15 m s 1, are located in the eastern part. [25] The major land cover type of northeastern China and the North China Plain is the same, which is cultivated land. However, their u t5% values are significantly different. They are 9.0 ± 1.5 m s 1 and 6.6 ± 1.3 m s 1 in northeastern China and the North China Plain, respectively Spatial Distribution of u t50% [26] Figure 7 shows a map of u t50% for east Asia during March, April and May from 1988 to Figure 8 indicates the frequency distribution of u t50% in each region. [27] The regional differences in u t50% are basically similar to those of u t5% : (1) The u t50% is the lowest at 6.7 ± 1.5 m s 1, in the Taklimakan Desert, and it is especially low from the central to the southwestern part; (2) the u t50% is also low, at 9.4 ± 1.6 m s 1, in the desert of the Loess Plateau; (3) the u t50% is higher, at 13.8 ± 2.0 m s 1, in the Gobi Desert than in other deserts; (4) the u t50% is the highest, at 16.2 ± 2.5 m s 1, in northern Mongolia, whose land cover type is grassland, and it is relatively low in the western part and high in the eastern part; and (5) the u t50% is significantly different between northeastern China and the North China Plain, although the land cover type is cultivated land in both regions, and it is high, at 12.3 ± 0.9 m s 1, in northeastern China and low, at 9.7 ± 1.4 m s 1,inthe North China Plain. [28] A different point concerning the result of u t5% is that the u t50% can be evaluated at very few observatories in the Loess Plateau, northern Mongolia, northeastern China, and the North China Plain. A nonhatched bar at 21 m s 1 on the abscissa indicates the number of observatories where the u t50% cannot be evaluated because the threshold wind speeds frequently exceed the maximum wind speed during the analysis period. Such observatories tend to be located in grassland and cultivated land, which are the land cover types of northern Mongolia, northeastern China, and the North China Plain. While no u t50% can be evaluated at nine observatories in the Loess Plateau, eight are located in grassland and cultivated land. In deserts, on the other hand, the u t50% can be evaluated at most observatories where u t5% can be evaluated Spatial Distribution of Du t [29] Figure 9 shows a map of dispersion of threshold wind speed (Du t = u t50% u t5% ) in spring (March, April and May) of east Asia from 1988 to Figure 10 indicates the frequency distribution of Du t in each region. A nonhatched bar around 21 m s 1 on the abscissa indicates the number of observatories where Du t cannot be evaluated because the u t50% cannot be evaluated. [30] The largest Du t is distributed around northern Mongolia, and is especially high in its eastern part. The Du t 7of13

8 Figure 6. Frequency distribution of u t5% in each region: (a) Taklimakan Desert, (b) Gobi Desert, (c) Loess Plateau, (d) northern Mongolia, (e) northeastern China, and (f) North China Plain. Solid and shaded bars in Figure 6c indicate results in desert (i.e., semidesert shrubs and bare desert) and nondesert areas, respectively. Shaded bars in Figure 6b indicate the results from observatories neighboring grassland, while all observatories of the Gobi Desert are located in the desert itself. The numbers in each panel indicate the average and standard deviation of t5% in each region. The average and standard deviation of solid bars in Figures 6b and 6c are indicated in parentheses. Figure 7. Same as Figure 5 but showing u t50%. 8of13

9 Figure 8. Same as Figure 6 but showing u t50%. An open bar at 21 m s 1 on the abscissa indicates the number of observatories where u t50% cannot be estimated because of few strong winds exceeding threshold wind speeds. value is also high in the Gobi Desert. On the other hand, Du t is the lowest in the Taklimakan Desert, and it is especially low, m s 1, from the central to the southwestern part. In the other regions, the Loess Plateau, northeastern China, and the North China Plain, Du t possesses similar values with averages of m/s Seasonal Variation [31] Figure 11 shows seasonal variations in u t5%, u t10%, u t25%, and u t50% in each region. In the same way of u t5% and u t50% (section 4.2), u t10% and u t25% are estimated as the wind speeds at which the dust emission frequencies are 10% and 25%, respectively. In the estimation of these, we used the total number of dust emission (N emsju ) and the total number of observation (N allju ) of all observatories in each region in equation (1). [32] We can see less seasonal variation in u t5% in the desert regions, especially the Taklimakan Desert, the Gobi Desert, and the Loess Plateau. However, obvious seasonal variations can be seen in u t25%, and u t50% in the Gobi Desert (Figure 11b), and u t10% and u t25% in the Loess Plateau (Figure 11c). They are the lowest in spring in every region, but the seasons of the maximum differ according to region. [33] A clear seasonal variation of threshold wind speed can be seen in northern Mongolia (Figure 11d), whose land cover type is grassland, although u t50% are shown only in March and April. The threshold wind speeds are low in spring and autumn, and high in summer and winter. Similar seasonal variations can be seen in northeastern China (Figure 11e), whose land cover type is cultivated land. Although not every kind of threshold wind speed can be evaluated in summer and winter, it is likely that they are higher than those in spring and autumn. In the North China Plain (Figure 11f), whose land cover type is cultivated land, very few threshold wind speeds can be evaluated because the threshold wind speeds almost always exceed the maximum wind speed during the analysis period. 6. Discussion 6.1. Spatial Distributions of ut 5% and ut 50% [34] Spatial distributions of u t5% and u t50% in spring (March, April, and May) are similar (sections 5.1 and 5.2). Threshold wind speeds are low in desert regions such as the Taklimakan Desert and the Loess Plateau. On the other hand, they are the highest in northern Mongolia, whose land cover type is grassland. These results indicate that vegetation cover and vegetation type are likely to be a major factor affecting threshold wind speed. However, we have exceptions. Threshold wind speeds are various even in the same land cover type like in the Gobi Desert (semidesert shrubs) and northern Mongolia (grassland). Moreover, threshold wind speeds, especially u t50%, are much higher in the Gobi Desert than the western part of Loess Plateau whose land cover type is also the semidesert shrubs. These suggest that threshold wind speeds cannot be determined by land cover type alone; instead, the effect of other land surface conditions such as soil particle size distribution, roughness length, soil wetness, vegetation cover, snow cover, and so on need to be taken into consideration. [35] Laurent et al. [2005] presents a map of threshold wind speed in east Asia, which is evaluated using an aerodynamic roughness length derived from satellite data, bidirectional reflectance distribution function (BRDF) of POLDER/ADEOS-1. According to their map, the threshold wind speed can be read as about 7 m s 1 in the Taklimakan Desert, about 7 14 m s 1 in the western part of the Loess Plateau, about m s 1 in the Gobi Desert in China, and about m s 1 in the Gobi Desert in Mongolia. This spatial distribution of threshold wind speed corresponds closely to our spatial distribution of u t50%. This 9of13

10 Figure 9. Map of dispersion of threshold wind speed (Du t ) during March, April and May from 1988 to 2005 in east Asia. suggests that the reason for high threshold wind speeds in the Gobi Desert might be due to the nonerodible roughness length elements, such as gravel, pebbles etc. [36] A constant threshold wind speed, 6.5 m s 1, is often used in dust models. However, the map of u t50% reveals that threshold wind speeds are much higher than 6.5 m s 1 in east Asia, except in the Taklimakan Desert. The maps of u t5%, u t50%, and Du t also show that threshold wind speeds are significantly variable in space and time. Figure 10. Same as Figure 6 but showing dispersion of threshold wind speed (Du t ). The open bar at 21 m s 1 on the abscissa is the same as in Figure of 13

11 Figure 11. Seasonal variation in u t5% (solid dots), u t10% (open triangles), u t25% (open squares), and u t50% (solid inverted triangles) in (a) the Taklimakan Desert, (b) the Gobi Desert, (c) the Loess Plateau, (d) northern Mongolia, (e) northeastern China, and (f) the North China Plain Seasonal Variation [37] The amplitude of seasonal variation is wide in northern Mongolia (grassland) and narrow in the Taklimakan Desert, the Gobi Desert, and the Loess Plateau (desert) (section 5.4). [38] In desert regions, we see less seasonal variation in u t5%. This suggests that land surface conditions remain constant in these regions, and that the land surface is close to the most favorable conditions for dust emission (e.g., the driest year). In u t10%, u t25%, and u t50%, however, we can see clear seasonal variations. This suggests that the land surface condition undergoes a clear seasonal variation in a normal year, even in desert regions. [39] On the other hand, we can see a clear seasonal variation in u t5% in northern Mongolia. Figure 12 shows the dust emission frequency, snow cover fraction and NDVI in east Asia in January (Figure 12a), April (Figure 12b), July (Figure 12c), and October (Figure 12d). This figure for northern Mongolia shows snow cover in January, less snow cover and little vegetation activity in April, strong vegetation activity in July, and less snow cover and little vegetation activity in October. The clear seasonal variation in the threshold wind speed might be derived from these seasonal variations in land surface Dispersion of Threshold Wind Speed (Du t ) [40] Since the value of Du t (i.e., u t50% u t5% ) is an index of dispersion of the threshold wind speed during a given period, the results in section 5.3 suggest the dispersion in land surface conditions from March to May and interannual variation from 1988 to The Du t in northern Mongolia is the largest in east Asia; this might be a result of snow cover in early spring and vegetation activity in the late spring. [41] On the other hand, the Du t is the lowest from the central to the southwestern part of the Taklimakan Desert, ranging between 1.0 and 2.0 m s 1. In situ observations of threshold wind speed were conducted in the southern part of the Taklimakan Desert from 2002 to 2004 by the Japan-Sino joint project, Aeolian Dust Experiment on Climate impact (ADEC) [Mikami et al., 2006]. From these observations, Ishizuka et al. [2005] indicates that the mean threshold wind speed was 7.5 m s 1 on 14 April 2002, when the land surface was dry (dry condition). However, it was 9.5 m s 1 in 15 March 2003, when the land surface was rather wet and was in the process of drying after the snowfall at the beginning of March (wet condition). Ishizuka et al. [2005, Table 3] indicate that soil moisture is rapidly approaching to the dry condition. A comparison of their results with our threshold wind speeds at observatory (Figure 4), the nearest to the ADEC site, shows that the threshold wind speeds at their site is higher than those at observatory 51828: Their threshold wind speeds in dry and wet conditions (7.5 m s 1 and 9.5 m s 1 ) are about 3.6 m s 1 larger than u t5% and u t50% (4.1 m s 1 and 5.7 m s 1 ), respectively. This difference will be due to differences of observation conditions. For example, the saltation flux is measured with Sand Particle Counter (SPC) at ADEC site located in 11 of 13

12 Figure 12. Dust emission frequency, snow cover fraction, and NDVI in east Asia in (a) January, (b) April, (c) July, and (d) October. The dust emission frequency, snow cover fraction, and NDVI are indicated by solid circles, white-gray hatching, and green-brown hatching, respectively. absolute bare desert. On the other hand, the present weather is observed by naked eyes at observatory located in an oasis. However, the difference between dry and wet conditions (2.0 m s 1 ) is comparable with the difference between u t5% and u t50% (Du t =1.6ms 1 ), and this suggests our results are reasonable. 7. Conclusions [42] We have indicated a new method for statistically evaluating the threshold wind speed for dust emission using synoptic surface meteorological data. Using this method, we presented maps of threshold wind speed in east Asia. The defined threshold wind speeds are u t5% and u t50%, which are wind speeds for which the dust emission frequencies are 5% and 50%, respectively. We can identify u t5% as the threshold wind speed at close to the most favorable land surface conditions for dust emission and u t50% as that for normal land surface conditions. The values of u t10% and the u t25% are defined in the same way, and we discussed the seasonal variation of threshold wind speeds from u t5%, u t10%, u t25%, and u t50%. [43] From the maps of u t5% and u t50% in spring (March May), we obtained results showing that the spatial distributions of u t5% and u t50% are similar. The spatial distribution of the threshold wind speed generally corresponds to that of the land cover type. The threshold wind speed is low in desert regions such as the Taklimakan Desert (u t5% = 4.4 ± 0.6 m s 1 and u t50% = 6.7 ± 1.5 m s 1 ) and the Loess Plateau (u t5% = 6.9 ± 1.2 m s 1 and u t50% = 9.4 ± 1.6 m s 1 ). The lowest is found from the central to the western part of the Taklimakan Desert. On the other hand, it is the highest in northern Mongolia (u t5% = 9.8 ± 2.2 m s 1 and u t50% = 16.2 ± 2.5 m s 1 ), where the land cover type is grassland. However, there are some exceptions, one being the higher threshold wind speed in the Gobi Desert (u t5% = 8.9 ± 2.2 m s 1 and u t50% = 13.8 ± 2.0 m s 1 ) than seen in other desert regions. [44] We compared the map of u t50% with the map of threshold wind speed by Laurent et al. [2005], which is evaluated using an aerodynamic roughness length derived from POLDER/ADEOS-1 satellite data. The maps of threshold wind speed are similar between theirs and ours. In their map as well, the threshold wind speed of the Gobi Desert is higher than in other desert regions. This result suggests the major reason for the large threshold wind speed in the Gobi Desert to be the roughness length. [45] Although many dust models set a threshold wind speed for dust emission as a constant 6.5 m s 1,theu t50% is higher than 6.5 m s 1 in most regions of east Asia, except in the Taklimakan Desert. The maps of threshold wind speed also show that threshold wind speeds are significantly variable in space and time. [46] The index of dispersion of threshold wind speed, Du t (i.e., u t50% u t5% ), is distributed similarly to u t5% and u t50%. The value of Du t is the lowest from the central to the 12 of 13

13 western part of the Taklimakan Desert and the highest in northern Mongolia. This high Du t in northern Mongolia might be due to the snow cover in early spring and the vegetation activity in late spring. [47] Seasonal variations in the threshold wind speed are wide in grassland regions such as northern Mongolia and narrow in desert regions such as the Taklimakan Desert, the Loess Plateau and the Gobi Desert. In desert regions, less seasonal variation is seen in u t5% although clear variations are seen in u t10%, u t25%, and u t50%. This suggests that land surface conditions are similar throughout the year in desert regions, when land surface remains close to the most favorable conditions for dust emission (e.g., the driest year). On the other hand, even the u t5% clearly varies with seasonal changes in northern Mongolia. [48] Acknowledgments. We are grateful to Kremena Darmenova and Anton Darmenov; their suggestions improved the descriptions in the section of method. 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