Dust storms and cyclone tracks over the arid regions in east Asia in spring

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 110,, doi: /2004jd004698, 2005 Dust storms and cyclone tracks over the arid regions in east Asia in spring Tetsuya Takemi Department of Environmental Science and Technology, Tokyo Institute of Technology, Tokyo, Japan Naoko Seino Meteorological Research Institute, Tsukuba, Japan Received 26 February 2004; revised 20 May 2004; accepted 4 June 2004; published 19 March [1] It has been argued that frequent dust storm developments in east Asia in spring are closely related to midlatitude synoptic-scale cyclone activity. This study investigates the relationship of springtime dust storms and other dust-related phenomena in east Asia to the tracks and locations of synoptic-scale cyclones by conducting statistical analyses of surface weather data, cyclone track data, and satellite data. Through these analyses, we discuss the role of cyclone activity on dust weather phenomena in east Asia. In the Gobi Desert and northeast China regions, strong cyclonic winds associated with strong cyclones are responsible for the dust weather developments, and the dust weather preferably occurs in the southwestern sector of the cyclone, where frontal activity and cold air action are significant. Despite the extremely dry climate, the formation of frontal cloud systems is evident particularly over the Gobi Desert, which will contribute to the higher frequency of severer dust weather. On the other hand, in the Taklamakan Desert severe dust weather (i.e., dust storm) is not so much affected by synoptic-scale cyclones, but weaker dust phenomena such as dust haze occur around the centers of cyclones that do not propagate farther eastward out of the Taklamakan region. Citation: Takemi, T., and N. Seino (2005), Dust storms and cyclone tracks over the arid regions in east Asia in spring, J. Geophys. Res., 110,, doi: /2004jd Introduction [2] Dust storms are one of the significant phenomena in desert regions, sometimes causing severe local disasters that affect human lives, agriculture, social infrastructures, transportation, and other industries. In addition, a large amount of dust particles transported in the free troposphere, which is classified as dust haze or suspended dust, could affect regional weather and climate in east Asia through the interaction with radiative energy transfer. These dust events caused by aeolian processes have recently been an important research issue from the viewpoint of not only transboundary air pollution but also the possible impact on climate [Goudie and Middleton, 1992; Qian and Zhu, 2001]. [3] In the arid regions of the continental east Asia, such as the Taklamakan and Gobi Deserts, dust storms frequently occur in spring [Littmann, 1991; Parungo et al., 1994], and these dust storms are a major source for dust transport in the atmosphere, and a major cause for the occurrence of yellow sand phenomena prevailing over east Asia in that season. Owing to the frequent cyclogenesis over northern China and Mongolia (between 45 and 50 N) in spring [Chen et al., 1991], higher winds are observed in those regions in spring Copyright 2005 by the American Geophysical Union /05/2004JD [Kurosaki and Mikami, 2003]; thus it is anticipated that the high frequency of dust storms in spring can be related to midlatitude cyclone activities. Qian et al. [2002] investigated the relationship between dust storms and cyclone activities by conducting the climatological analyses of 50-year weather data in China, and showed that a high positive correlation was found between the annual dust weather frequency and the annual cyclone frequency in Inner Mongolia, northeast China, and north China. They thus suggested that in northern China the cyclones over Mongolia are a major dynamical condition for dust weather occurrence, while in the Tarim Basin (i.e., the Taklamakan Desert) the dust weather occurrence has a low correlation with a cyclone activity. It is noted that their analysis was conducted on the basis of an annual mean climatological perspective. In addition to the climatological study of Qian et al. [2002], dynamical aspects of dust storms have been investigated through some case studies. For example, Takemi [1999] and Takemi and Satomura [2000] conducted a case study on the 5 May 1993 severe dust storm over the Gobi Desert through the analyses of mesoscale observational data and numerical simulations and showed that the dust storm was caused by a strong cold surface outflow associated with a frontal cloud system that developed in a synoptic-scale cyclone system. The front of the cold outflow had a gust front structure which is often found in other severe windstorm 1of11

2 over a desert area [Mitsuta et al., 1995]. Therefore a close connection between dust storm occurrence and cyclone activity has been anticipated by many scientists. [4] It is still not well known, however, where and how dust storms and other dust-related weather phenomena occur during the episodes of synoptic-scale cyclones in east Asia: the locations of dust weather occurrences with respect to cyclone tracks and their geographical characteristics in east Asia. As described earlier, there are some studies that address this issue through case analyses; however, to our knowledge, there are few studies that explicitly address the issue from a statistical point of view. [5] The purpose of the present study is to investigate the relationship of dust storms and other dust-related phenomena that occur in east Asia to the tracks and central locations of synoptic-scale cyclones and its geographical features. Since a peak frequency of cyclogenesis in northern China and Mongolia is found from April to May [Chen et al., 1991] and a maximum frequency of dust storms in those regions in April [Parungo et al., 1994], the present study deals with the dust weather phenomena in April. In section 2, the data used in this study and the analysis procedure are described. We use the data of the recent 4 years (i.e., ), which corresponds to the period of the ongoing Sino- Japanese joint research program Asian Dust Experiment on Climate Impact (ADEC). In section 3, the geographical features of dust storms and other dust-related phenomena and the connection between the dust weather phenomena and the cyclone tracks and locations are demonstrated. Frontal activities embedded in the cyclones during dust weather occurrences are also shown. By describing the relationship between dust storms and cyclone tracks in various locations in east Asia, the role of cyclone activity on the development of dust storms is discussed in section 4, followed by a summary in section Data and Analysis Method [6] All the data used in the present study are those obtained in the month of April from 2000 to As meteorological observations, we use 3-hourly records obtained at manned weather stations (SYNOP code in the observation categories defined by World Meteorological Organization (WMO)) in northern China and Mongolia. The locations of these weather stations are depicted in Figure 1. [7] Dust-related weather phenomena range from a severe hazardous windstorm (i.e., dust storm) which often takes a form of convective-scale gust front to turbulence-scale dust devil or dust whirl, and even to suspended dust where dust is entrained into the atmosphere at some upstream location [Warner, 2004, pp ]. The threshold between dust storm and other wind-driven dust event is a visibility: if the visibility is less than 1000 m, then a dust storm is reported. In order to identify dust storms and other dust-related phenomena in the present study, we followed the WMO observation code and used present weather records observed at the weather stations shown in Figure 1. These data seem to be less quantitative and might represent only a certain aspect of dust storms; however, we believe they are still useful for our analyses, since there is little quantitative data available such as visibility at all the weather stations. In Figure 1. Locations of the surface meteorological observations used in the present study (marked by points). Light shading indicates the elevations of higher than 2000 m. The characters at the top indicate the code names of the regions defined in the present analyses, demonstrated in section 3.3. addition, Kurosaki and Mikami [2003] were successful in describing the relationship between dust outbreaks and wind speeds with the use of the present weather reports at weather stations. [8] According to the observation manual prepared by the World Meteorological Organization [1995], present weather reports (denoted as ww) related to dust storms and other dust phenomena are as follows: (1) severe dust storm or sandstorm (ww = 33 35), (2) slight or moderate dust storm or sandstorm (ww = 30 32), (3) dust storm or sandstorm within sight (ww = 09), (4) thunderstorm combined with dust storms or sandstorms (ww = 98), (5) well-developed dust or sand whirl, but no dust storm or sandstorm (ww = 08), (6) dust or sand raised by wind, but no well-developed dust or sand whirl, and no dust storm or sandstorm seen (ww = 07), and (7) widespread dust in suspension in the air, not raised by wind (ww = 06). In our analyses these seven categories are organized into three classifications and referred to as follows: (1) dust storm, which consists of the first four categories; (2) blowing dust event, which consists of the fifth and the sixth categories; and (3) dust haze, which consists of the last category. Similar to Qian et al. [2002], all these three categories are referred to as dust weather. [9] In addition to the surface weather data, hourly brightness-temperature data in the infrared wavelength of mm, obtained by the Japanese Geostationary Meteorological Satellite (GMS), are used in order to identify cloud activities related to synoptic-scale cyclones. The GMS data used here have a temperature resolution of 0.1 K and a spatial resolution of 0.25 by interpolating the original GMS data on a regular latitude-longitude grid. [10] For identifying the locations of cyclone centers and the cyclone tracks, we use storm track data set archived at The Climate Diagnostic Center of National Oceanic and Atmospheric Administration (NOAA)-Cooperative Institute for Research in Environmental Sciences (CIRES), University of Colorado. The storm tracks were analyzed from 6-hourly National Center for Environmental Prediction (NCEP) gridded data set (2.5 resolution) with the use of an algorithm developed by Serreze [1995]. This algorithm first detects cyclones from sea level pressure (SLP) analyses by testing whether a grid point SLP value is surrounded by grid point values at least 2 hpa higher than 2of11

3 Figure 2. Spatial distribution of the percentage frequency of (a) dust storms, (b) blowing dust events, and (c) dust hazes calculated in a 2.5 by 2.5 area in the month of April during , with contour lines of 1, 2, 4, 8, 12, and 24%. Light shading indicates the elevations of higher than 2000 m. in an area of E and N, which corresponds to the Gobi Desert, and the frequency of blowing dust event occurrence is high not only in the Gobi Desert region but also in the Tarim Basin region and in the northeastern part of China. On the other hand, the occurrence of dust haze is the most frequent in the Tarim Basin; its frequency exceeds 10% at some locations. [12] The occurrence of these dust weather phenomena has a strong correlation with the surface wind speed as many previous observational, theoretical, and numerical studies have shown [e.g., Shao, 2000; Kurosaki and Mikami, 2003]. In the present study, the averaged surface wind fields at the times of each dust weather occurrence at each weather station are shown in Figure 3. As expected, the magnitudes of winds are smaller in a weaker dust weather case. In the case of dust storms, the wind fields are generally westerly to northwesterly in the Gobi Desert region, and some easterly winds are found over the Taklamakan Desert. The mean winds in the blowing dust case have similar directions. A significant feature seen in Figure 3 is that the winds are much weaker in the dust haze case than in the other two cases. This is probably because dust haze is defined as a state of suspended dust in the atmosphere, which is not induced by locally strong winds but transported from an upstream environment. It it noted that the mean wind speeds in the Taklamakan Desert are generally weaker than those in the Gobi Desert area and the northeastern part of China, and this feature was demonstrated by Kurosaki and Mikami [2003]. We consider that the wind fields shown in Figure 3 are strongly controlled by synoptic-scale weather disturbances, the central point being tested, and then tracks the system by searching nearest neighbor locations between time steps with a maximum distance threshold. Detailed description of this algorithm can be found in Serreze [1995] and Serreze et al. [1997]. The data set includes the central pressure and position of each cyclone, the local Laplacian (which indicates the system intensity) and SLP tendency at each cyclone center. According to Serreze [1995], manual comparisons with weather charts show that the algorithm correctly identifies cyclone centers over 99.5% of the time and tracks cyclones over 98% of the time, which strongly indicates the usefulness of the data set. It is noted that the surface elevations in the present analysis area are mostly around m, and hence pressure analyses at the 850-hPa level might be more appropriate [e.g., Qian et al., 2002]. However, Chen et al. [1991] successfully elucidated cyclogenesis over east Asia from SLP maps except in the region of the Tibetan Plateau whose altitudes exceed more than 5000 m, and therefore we believe that the analyses from the SLP data in our study are reliable. 3. Results 3.1. Features of Dust Weather [11] General features of dust weather phenomena in April are given in the first place. Figure 2 shows the horizontal distribution of dust weather frequencies in April during The frequencies here are defined as a percentage rate of dust weather occurrence to the total observations in each 2.5 by 2.5 area. Dust storms occur most frequently Figure 3. Averaged wind fields at the times of the observations of (a) dust storms, (b) blowing dust events, and (c) dust hazes at each weather station. A unit wind vector (10 m/s) is indicated in the top left corner in each panel. Light shading indicates the elevations of higher than 2000 m. 3 of 11

4 Figure 4. Tracks of synoptic-scale cyclones in the month of April in (a) 2000, (b) 2001, (c) 2002, and (d) Circles indicate the locations of cyclogenesis, and crosses the locations of cyclolysis. and we will discuss this point later by associating with cyclone tracks and activities Tracks of Synoptic-Scale Cyclones [13] In this study, we are primarily interested in the effects of synoptic-scale cyclones, not diurnally induced ones; hence we choose cyclones that have a lifetime of longer than or equal to one day, expecting to exclude diurnally induced systems such as heat lows. Hereinafter, we refer to the cyclones thus selected as a synoptic-scale cyclone. Accordingly, the tracks of the synoptic-scale cyclones as well as the locations of cyclogenesis and cyclolysis are illustrated in Figure 4. The primary routes of the cyclones in east Asia are seen in the regions of Mongolia and northern to northeastern China (40 50 N) and of higher latitudes at around 60 N, although there are some variabilities among the four years, such as less frequent passages of cyclones in the region of Mongolia and northern China in 2003 than in the other three years. The overall feature shown in Figure 4 agrees well with that found in the earlier study by Chen et al. [1991]. This agreement of the spatial distribution of the cyclones strongly suggests that the present analyses on the relationship between dust weather and cyclone track would give reliable statistics even with a relatively limited amount of the present data set as compared with the works by Chen et al. [1991] and Qian et al. [2002]. [14] As a common knowledge of meteorology, in the middle latitudes frontal cloud systems usually develop associated with a synoptic-scale cyclone. On the other hand, one might expect that little cloud development is found in arid regions owing to a limited amount of moisture. According to Itano [1998], frontal cloud systems indeed develop with the passage of cyclones over the arid region in northwest China in August when a relatively large amount of moisture is available for the cloud development. In contrast, precipitable water vapor at a weather station in the Gobi Desert region (i.e., Zhangye at 38.9 N and E) in spring varies around 2 to 10 mm which is about 1/3 of that in August [Itano, 1997]. The amount of moisture in the boundary layer is so small that deep convection seems not to develop quite often over the desert in spring [Takemi and Satomura, 2000]. Thus in the present study frontal cloud activities are investigated with GMS brightness temperature data in order to see how often cloud formation is found in spring. According to the frequent passage of cyclone between N shown in Figure 4, brightness temperatures are meridionally averaged in this latitudinal band, and the cold temperatures of lower than 253 K are demonstrated by a time-longitude diagram in Figure 5 along with the longitudinal tracks of the cyclones that pass the region of N. As clearly seen in Figure 5, there are a couple of cloud streaks in all the four years, and a close look at cloud images over the region at each time (not shown here) indicated that those cloud streaks correspond to frontal cloud bands. Comparing the cloud streaks and the cyclone tracks shown in Figure 5, cloud formation is actually seen along the cyclone tracks in most cases even in April when a moisture amount is significantly smaller than that in August [Itano, 1997] Dust Weather Occurrence in the Presence of Cyclones [15] In this section, the locations of dust weather developments relative to the cyclone centers and tracks, and the cloud patterns and meteorological changes associated with the cyclones at the times of dust weather observations are investigated from the statistics of the 4-year data. [16] The cyclones that pass the meridional range of N (shown in Figure 5) are chosen in the analyses, and five geographical regions are defined in the area of E and N: the west Taklamakan Desert region (referred to as WTAK, E); the east Taklamakan region (ETAK, E); the west Gobi Desert region (WGOBI, E); the east Gobi region (EGOBI, E); and the northeastern part of China (NECHINA, E) (see also Figure 1 for the region definition). First in each region the location of a cyclone center is identified; then within a 40 by 40 area centered at 4of11

5 Figure 5. Time-longitude diagram of brightness temperature (meridionally averaged between N) and cyclone centers (which pass within the range of N) in the month of April in (a) 2000, (b) 2001, (c) 2002, and (d) Brightness temperatures of lower than 253 K are indicated by light shading, and cyclone tracks are indicated by thick solid lines. the cyclone center the locations that observed dust weather at the same time with or three hours earlier with that cyclone are chosen; and the observed data are analyzed in a coordinate system relative to the cyclone center. [17] Figure 6 shows the frequency distribution of dust storm occurrence in the relative coordinate in each region. The frequency here is defined as a percentage rate of dust storm occurrence in each unit area of 2.5 by 2.5 to the total number of its occurrence in the 40 by 40 domain. In addition, we calculated mean winds at the weather stations that observed dust storms in each 2.5 by 2.5 unit area in the relative coordinate. In the Taklamakan regions (Figures 6a and 6b), high frequencies appear to be located in a scattered fashion around the cyclone center, and no clear tendency is found. It it noted that an anticyclonic flow can be seen in the northeastern quadrant of the ETAK region; this airflow is clearly not associated with the cyclones located in that region, and suggests that the occurrence of dust storms in the Taklamakan Desert might not always be associated with the cyclones observed in the desert. This point will be further discussed later in section 4. [18] On the other hand, in the Gobi Desert and northeast China regions (Figures 6c, 6d, and 6e) the locations of high frequencies are found around the center and in the southwestern quadrant of the regions. The associated wind fields also show a definite tendency of strong cyclonic flow. These features can also be seen for the case of blowing dust events (Figure 7), with a slight difference in the WGOBI region (Figure 7c) in which the highest-frequency area is seen in the northeastern sector. [19] In contrast, the frequency distributions for the dust haze case (Figure 8) seem to be quite different from those found in the cases of dust storms and blowing dust events. In the Taklamakan regions the areas of high frequency appear to be more organized than those seen in Figures 6 and 7, and hence it is suggested that cyclones located in the Taklamakan Desert would play a certain role in causing dust haze in that region; a cyclonic flow field, however, is not seen in this case. In the Gobi regions the frequency maps appear to be scattered, but a high-frequency area is seen basically in the southwestern sector of the cyclones. [20] In Figure 5 we have shown a significant tie between cyclone activity and cloud formation, and from the results shown in Figures 6 and 7 it can be said that strong winds associated with synoptic-scale cyclones are responsible for the dust storm and blowing dust occurrence in the Gobi and northeastern China regions. These results lead us to examine cloud activities associated with the cyclones in the WGOBI, EGOBI, and NECHINA regions. We have derived the 5of11

6 Figure 6. Percentage frequency of dust storm occurrence in a coordinate system relative to the cyclone centers that are located in the regions of (a) WTAK, (b) ETAK, (c) WGOBI, (d) EGOBI, and (e) NECHINA, which are shown by contour lines of 1, 2, 4, and 8%. Vectors indicate mean winds calculated in each unit area of 2.5 by 2.5 with the unit vector given in the top left corners. spatial distributions of composite average brightness temperatures by GMS in the cyclone-relative coordinate system at the times of observations of both dust storms and blowing dust events. The distributions are demonstrated in Figure 9. In all the regions, cold top clouds can be seen around the cyclone centers, and the brightness temperatures become colder from the southeastern sector to the northwestern sector. It appears that there is a sign of frontal cloud band extending from the cyclone center into the southwestern quadrant in the WGOBI region (Figure 9a), and that the southern half of the region becomes warmer as the region goes eastward (Figures 9b and 9c). [21] The examination of the variability (i.e., standard deviation from the average seen in Figure 9) of the brightness temperatures, demonstrated in Figure 10, indicates that a region of a large variability is seen to extend from the northeastern into the southeastern, and to the southwestern sector of the cyclone system. These regions of large variability are considered to correspond to the activities of frontal cloud bands, because these frontal cloud bands are composed of deep convective cloud cells which are, by nature, short timescale phenomena and thus may lose their signature in a time-averaged sense but still maintain their feature in a variability viewpoint. In addition, these regions of high brightness-temperature variability correspond well with the locations of high frequency of dust storms and blowing dust events seen in Figures 6 and 7. [22] Cyclone activity and its associated frontal cloud development induce unique surface weather changes such as sharp increases in wind speed and pressure and a decrease in air temperature. We examine here the characteristics of surface weather changes associated with dust weather in the presence of cyclones. Figures 11 and 12 show the distribution of surface pressure change and temperature change respectively, from the 3-hour earlier observations at the stations where dust storms and blowing dust events were observed, by making composites in the cyclone-relative coordinate system in the same way with shown in Figure 6. The results obtained in the Gobi Desert and northeastern China regions are shown in Figures 11 and 12. For pressure change (Figure 11), a definite feature in all the regions is that pressure rise is seen in the western half of the regions and pressure drop in the eastern half. The variation in the WGOBI region is the most pronounced (Figure 11a), and the location of pressure rise appears to correspond to the locations of the low brightness-temperature band found in the southwestern sector in Figure 9a and of the large cloud temperature variability found in Figure 10a, which is consistent with the view that wind-induced dust weather is a consequence of frontal activity. From the distribution of surface temperature 6of11

7 Figure 7. Same as Figure 6 except for the blowing dust case. change (Figure 12), pronounced temperature drop in the western half and temperature rise in the eastern half are seen in the WGOBI region, while in the EGOBI and NECHINA regions the area of temperature drop prevails in the whole area with some patchy areas of temperature increase. [23] These results of the surface changes and the features found from the brightness temperature analyses shown earlier in this section (see Figures 9 and 10) strongly suggest that in the Gobi area frontal cloud activity and its associated surface changes plays an important role in spawning dust storms and blowing dust events, and that in the northeast China region cold air action (without frontal clouds) may play a role. A close look at weather maps issued by Japan Meteorological Agency indicates (not shown) that the situations in northeast China are in most cases affected by occluded cyclones, which means that cold air outbreaks are a major cause for dust weather development in that region. The mean wind field in northeast China seen in Figure 3 seems to be consistent with the effect of occluded cyclones. [24] In addition to the averaged field, the frequency distributions of surface pressure and temperature changes for the dust storm and blowing dust cases are shown in Figure 13. The frequency is calculated with 1-hPa (1-K) interval in the five regions from the Taklamakan Desert to northeastern China. As can be seen, the variability of the surface changes is quite large; but still we see some tendency of pressure rise and temperature drop, particularly in the WGOBI, EGOBI, and NECHINA regions. Thus the effects of the frontal activity can also be identified in this statistics. 4. Role of Cyclone Activity on Dust Storms in East Asia [25] From the statistics of various types of meteorological data, we have shown that wind-induced dust outbreaks (i.e., dust storms and blowing dust events) in the Gobi Desert and northeastern China regions are strongly tied with strong winds associated with synoptic-scale cyclone activities. The occurrence of the dust storms and blowing dust events can be seen primarily in the southwestern quadrant of the synoptic-scale cyclones that pass over the Mongolia and northern to northeastern China regions. The examination of brightness temperature distribution and variability as well as surface pressure and temperature changes strongly suggests the role of frontal activity on the development of dust storms and blowing dust events in those regions. Despite the dry climate which appears to be detrimental for cloud development, the formation of frontal cloud systems is very active particularly in the Gobi Desert area; this will contribute to the high frequency of the severer dust weather case (i.e., dust storms) over that desert. [26] There has been a number of case studies which pointed out that dust storms were resulted from cyclone activities. The previous case studies, however, basically focused on most severe cases [Takemi, 1999], and these cases are in fact rare: a few times in a decade [Mitsuta et al., 7of11

8 Figure 8. Same as Figure 6 except for the dust haze case. 1995]. To our knowledge, there have been few studies that explicitly address the issue of the relationship between dust storms, cyclone tracks, and frontal activities from a statistical point of view. The analyses by Qian et al. [2002] may be one of the statistical studies, and, on the basis of over 40-year data, indicated the positive correlation between dust weather frequency and cyclone frequency in northern China and suggested cyclone forcing is important for the dust weather occurrence. In addition, they mentioned that cold air activity in eastern China may play an important role on the dust weather formation. Their results gave a general picture of the dust weather in China; however, their statistics were focused on long-term climatic variability and were based on the annual mean values of some relevant parameters and the correlation analysis from long-term variation. Hence a more direct connection between dust Figure 9. Composite average brightness temperature distribution in the both dust storm and blowing dust cases in a relative coordinate system centered at cyclone centers in (a) the WGOBI region, (b) the EGOBI region, and (c) the NECHINA region. The legend of the temperature is given in the bottom in each panel. Mean wind fields are indicated by vectors with the unit given in the top left corners. 8of11

9 Figure 10. Same as in Figure 9 except for variability (standard deviation from the average shown in Figure 9) of brightness temperature and without wind vectors. weather development and cyclone activity and its geographical characteristics in east Asia have not been addressed previously. [27] Compared with the characteristics found in the Gobi Desert and northeast China regions, the relationship between dust weather formation and cyclone activity in the Taklamakan Desert region is less clear. Although there is a slight tendency of frontal activity or cold air action in the surface meteorological changes (i.e., pressure rise and temperature drop) for the dust weather cases in the Taklamakan region (Figures 13a and 13c), the wind fields and the locations of dust weather outbreaks indicate no significant influence of synoptic-scale cyclones. In other words, the development of the dust weather in the Taklamakan region may not be due to the direct effect of the synoptic-scale cyclone. [28] Qian et al. [2002] argued that the reason why the correlation between the frequency of dust weather and cyclone in the Tarim Basin is low is due to the few number of strong cyclones. Thus we compare in Figure 14 the monthly mean number of all the synoptic-scale cyclone occurrence in each unit area of 2.5 by 2.5, as well as the stronger ones that have central SLP of less than 1005 hpa. As clearly seen, the frequency of the stronger cyclone is quite low in the Tarim Basin, while the frequency in the Gobi Desert and northeast China regions seems not to largely change between Figures 14a and 14b; these features are consistent with the analysis by Qian et al. [2002]. Therefore the cyclones over the Gobi Desert and northeast China are basically strong, whereas those over the Taklamakan Desert are weaker. In addition, most of the cyclones in the Taklamakan region seen in Figure 5 do not propagate farther eastward, which suggests that these cyclones may not be an ordinary midlatitude cyclone but a locally induced one. This may be one evidence that the dust weather in the Taklamakan region is not directly affected by synoptic-scale cyclones. [29] N. Seino is currently performing numerical simulations of some of the dust weather events observed in the Tarim Basin in the spring of 2002 during the ADEC Figure 11. Spatial distribution of composite average pressure change at the times of both dust storms and blowing dust events in a cyclone-relative coordinate in (a) the WGOBI region, (b) the EGOBI region, and (c) the NECHINA region, indicated by contours with the 2-hPa interval. Positive values are contoured by solid lines, and negative values are contoured by dashed lines, with the zero contours indicated by thick long-dashed lines. Vectors indicate mean winds with the unit vector given in the top left corners. 9of11

10 Figure 12. Same as in Figure 11 except for temperature changes indicated by contours with the 2-K interval. intensive observation period. From the preliminary results, it was found that dust weather in the basin was affected by cold air intrusion as a part of a synoptic-scale circulation system that developed far behind synoptic-scale cyclone center. This initial finding may correspond to the surface changes of temperature drop and pressure rise seen in Figures 13a and 13c and the easterly winds in the Taklamakan Desert seen in Figures 3a and 3b, and seems to agree well with the low frequency of strong cyclones in the desert. We consider that the finding is a plausible mechanism for the dust weather development in the Taklamakan region. However, the present analyses also indicate that the dust weather develops around the cyclone center, particularly in the case of dust haze; this issue should be further investigated in a future study. 5. Concluding Remarks [30] This study reveals the role of synoptic-scale cyclone and frontal activity on dust storms and the relevant dust events in east Asia in spring from statistical analyses of the recent 4-year data, and the geographical characteristics of the effect of the cyclones on dust weather phenomena are investigated. In the Gobi Desert and northeast China, strong cyclonic wind fields associated with cyclones are responsible for the dust weather developments, and the high Figure 13. Percentage frequency distributions of changes in meteorological parameters for the both dust storm and blowing dust cases: pressure changes in (a) the Taklamakan Desert regions and (b) the Gobi Desert and northeast China regions; and temperature changes in (c) the Taklamakan Desert regions and (d) the Gobi Desert and northeast China regions. 10 of 11

11 frequency of the dust outbreaks is found preferentially in the southwestern sector of the cyclone, where frontal activity is pronounced. The frontal effect is particularly significant when cyclones are located in the Gobi Desert region, which contributes to the high frequency of dust weather over the Gobi Desert. [31] We examined the data obtained in April when cyclone activity is the most active and dust storm frequency is the highest throughout the year. It might be argued that a large seasonality of the relationship between dust weather occurrence and cyclone activity is expected owing to the difference in other relevant factors such as land surface characteristics (e.g., snow cover, vegetation, sand type and size). Even in spring there might be a large difference in the relationship among March, April, and May. For instance, Kurosaki and Mikami [2004] investigated the effect of snow cover on the threshold wind speed for dust outbreaks, and demonstrated a difference of this effect in the months between March and April because of different surface conditions. On the other hand, in May strong surface heating promotes the development of deep convective clouds that will induce severe dust storms [Takemi, 1999]. These effects of surface conditions and convective cloud activity, which have a local nature, are also critical in diagnosing and forecasting dust weather, and will add some variety to the results of the present study. The role of mesoscale convective cloud systems on dust storm development has been investigated in detail by Takemi and Seino [2005]. In order to accurately describe the mechanisms of dust weather development, total effects induced by not only mesoscales to synoptic scales but also microscale and local characteristics should be comprehensively addressed. [32] Acknowledgments. We would like to thank Y. Kurosaki of Meteorological Research Institute (MRI) for providing the SYNOP data archived at MRI. The GMS data were obtained from the Web site Weather Home at Department of Information Science, Kochi University. The cyclone track data were obtained from the Web site of The Climate Diagnostic Center/NOAA-CIRES. This research was supported by the Special Coordination Fund for Promoting Science and Technology from the Ministry of Education (MEXT) of Japan. Figure 14. Distribution of mean frequency of (a) all the synoptic-scale cyclones and (b) strong cyclones (which have a central sea level pressure of less than 1005 hpa) per 2.5 by 2.5 area in April. The contour lines are 1, 2, 4, and 8%. References Chen, S. J., Y. H. Kuo, P. Z. Zhang, and Q. F. Bai (1991), Synoptic climatology of cyclogenesis over east Asia, , Mon. Weather Rev., 119, Goudie, A., and N. J. Middleton (1992), The changing frequency of dust storms through time, Clim. Change, 20, Itano, T. 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