Dust storms in the Mongolian People's Republic

Journalof Arid Environments (1991) 20: 287-297

Dust storms in the Mongolian People's Republic N.

J. Middleton

School ofGeography, Mansfield Road, University ofOxford, Oxford OX] 3TB, U.K. (Received 21 March 1990, accepted 10 May 1990) Mapping of the frequency and distribution of dust storms (visibility < 1 km) in the Mongolian People's Republic shows that most of the country is affected to some degree by such deflation events, with maximum activity occurring in the southern Gobi region. The seasonality of dust storm activity is closely related to other meteorological variables and the generating meteorological systems. Dust storms are primarily a spring-time, daylight-hour phenomenon. Human activity is adversely affected by dust storm events and in some cases has increased deflation.

Introduction Dust storms, the result of a strong, turbulent wind blowing over dry, sparsely vegetated terrain, are particularly common features of arid environments. Their importance in the world's dry lands is wide-ranging, affecting geomorphology, climatology, ecology and a number of human activities (see Middleton, 1989a, Table 1 for a comprehensive list). This paper is the latest in a series of regional geographical studies of the distribution, frequency, seasonality and controlling factors of dust storms (Middleton, 1984, 1985, 1986a, 1986b, 1989b). The study of aeolian dust from Mongolian arid zones has largely been confined to investigation of its deposition at a great distance from the source, such as in Chinese loess profiles (e.g. Derbyshire, 1983), Pacific Ocean sediments (e.g, Rea & Leinen, 1988) and islands (e.g. Shaw, 1980). However, basic knowledge of the source areas, and the processes driving their dust generation are at best limited, particularly in the English language literature. Given the importance of aeolian dust from the Mongolian People's Republic to studies of loess, ocean sedimentation, past and present climate and atmospheric circulation as well as the investigation of local geomorphology and human activity, it is the purpose of this paper to go some way towards providing this basic information on the frequency, distribution, seasonality and conditions for dust storm generation in the arid zones of the Mongolian People's Republic.

Structure and relief Mongolia is essentially a vast plateau with an average elevation of 1580 m above sea level. The country's highest point, Mount Khuiten, is 4374 m, while its lowest point, Kh chNuur is 560 m. The plateau is largely composed of granites, gneisses and crystalline schists of Archean and Primary age. Mongolia is bordered to the north-west by the more extensive mountain complex of the Altai, Tannu-ola and Sayan group and to the north-east by the Transbaikalian Highlands. To the south-east it is separated from the plain of northern 0140-1963/911030287

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China by the Inner Mongolian portion of the Gobi Desert and a belt of pronounced scarps that extends northward as the high faulted scarp of the Great Khingan that delimits the plateau on the Manchurian side. On the south-western border is the Chinese Ala Shan Desert and the Nan Shan range, the outermost part of the great mountain chains which enclose the Tibetan Plateau. Physiographically, Mongolia can be divided into four broad regions: the Altai mountains, the Hangai-Hentei mountain area, the East Mongolian region and the Gobi. The terrain of the mountainous regions is the result of a long geological history of tectonic movement that has created a mountain and basin topography in which a basin of several large lakes is today hemmed in by the Altai and Hangai ranges. The most important period of activity began in the late Pliocene and reached its greatest intensity at the beginning of the Quaternary (Davies, 1986). The intrusive Altai mountains change from an area of unprotracted uplift in the far western part of Mongolia, to become the series of NW-SE aligned fault blocks that are the Gobi-Altai mountains. The Gobi-Altai mountains are characterised by massive alluvial aprons, up to 40 km wide in places, particularly covering the northern slopes. They are also the centre of present-day seismic activity. The Hangai-Hentei range is essentially part of the Siberian Plateau. The Hangai mountains form a domed upland, the result of a complex uplift of long duration. The Hentei range of mountains with flat tops at a general elevation of 2000 m is also the result of block faulting. The eastern region is the flattest and lowest area of Mongolia, essentially steppe land, a gently undulating high planation surface of Palaeocene age, with undrained closed basins of tectonic origin and dry valleys. It cuts across crystalline and metamorphic rocks at an altitude of about 1450 m above sea level and is characterised by the occurrence of individual granite tors (Kotarba, 1980). In the extreme east of the country in the Dariganga area of Suhbaatur aimak there is a quiescent volcanic zone with some 220 extinct volcanoes (Sanders, 1968). The very eastern tip of the country is of Devonian age. The Gobi Desert is a structural plain with an average elevation of about 1000 m. Characteristically flat with stony and sand surfaces, the Gobi is a Tertiary-Cretaceous shield with a surface of arenaceous conglomerates with inselbergs, dry water courses (sairs) and many small saline closed depressions (solonchaks) (Petrov, 1976). Climate

The climate of Mongolia is extremely continental, with generally low precipitation and great seasonal variations in temperature. According to Koppen's climatic classification much of the southern, western and eastern portions of the country have a cold desert climate (BWk), which is separated from the central northern section of cold snowy forest climate with cold winters (Dw) by a strip of cold steppe climate (BSk). The major synoptic pattern is the winter position of the Central Asian High which is typically located over the basins of Tannu Tuva or the Great Lakes Basin of western Mongolia. This system creates dry, stable conditions over most of Mongolia from October until April. It appears that western Mongolia acts as a centre for this high-pressure system no less than 91% of the time in January, and as much as 56% of the time even in July (Makhover, 1967). The winter cold air settles in the elevated intermontane basins and lodges there for months on end. Extreme, though often shallow (up to 1000 m), temperature inversions are typical across Mongolia and eastern Siberia during the winter months. Within the intermontane basins there are frequently two inversions: one at the surface induced by surface cooling, and another above this cold air dome produced by the warm sector air of cyclonic storms that are forced to override this cold air dome. During the spring months an intensification of the zonal circulation brings on a rapid progression of cyclones generally from west to east across the country. These low-pressure cells bring strong winds and often generate intense dust storms. This west to east

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circulation weakens as summer progresses and cyclonic activity strengthens as a diffuse low-pressure system engulfs much of the interior parts of Asia at the surface. Seasonal rainfall is brought to the country during this period by low-pressure cells that move generally WSW to ENE.

Data Data for this study were collected from the Institute of Hydrometeorology in Ulan Bator and the geographical locations of the 26 meteorological stations for which regular longterm data were available are shown in Fig. 1. For some stations, detailed daily records of dust storm events were available; in other cases data refer to 'dust storm days', which are defined as a day on which a dust storm is recorded. In all cases, where the term 'dust storm' is used it refers to a dust-raising event in which horizontal visibility is reduced to < 1000m.

Results

Frequency, distribution andseasonality Figure 2 shows the distribution of mean annual frequencies of dust storm days in the Mongolian People's Republic. The area of most frequent activity is in the southern region of the Gobi Desert where Zamiin Dud records the highest mean annual number of dust storm days: 34'4. The other area of high frequency is in the Great Lakes region to the north-west of the country. The level of dust storm activity puts the Mongolian People's Republic on a par with some ofthe dustiest locations in the world (see Middleton, 1989a, Table 12'4). The general distribution pattern is confirmed by Fig. 3 which shows the distribution of mean annual number of hours of dust storms. Again the area of highest frequency is centred on Zamiin Dud in the south-east with over 300 h year-I. The seasonality of dust raising is illustrated for a number of Mongolian stations by Table 1, which shows percentage frequency of dust storm days by month. The spring months of April and May are the time of greatest activity at all stations. The seasonality of activity can be explained by referring to Fig. 4 which shows mean monthly dust storm frequencies and wind speeds with mean monthly rainfall totals for four stations. The spring maximum of dust storm activity occurs at the time when average wind speeds reach a maximum, representing the onset of low-pressure frontal passage across the country. When these depressions bring annual precipitation in June, July and August, dust raising declines. A minor secondary peak in dust raising is discernible at most stations in the autumn (October), after the precipitation, but dust storm frequencies are low and continue at a low level throughout the winter months as wind speeds are generally lower on average. Perhaps more importantly, this is because low temperatures mean that ground surfaces are often frozen, if not covered in snow, and thus wind-erodible particles are not available for entrainment.

Diurnal variation The diurnal pattern of dust storm activity is indicated in Fig. 5 which shows the frequency per cent of dust storm occurrence during 3-h periods. All stations show a maximum during daylight hours when solar heating promotes turbulent airflow. Over most of the country dust-raising activity is at a peak in the afternoon from 1200 to 1500local time, but most of the stations east of Sainshand experience maxima in the morning hours, from 0900 to 1200.

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Meteorological systems The predominant meteorological system responsible for dust raising in the Mongolian People's Republic is the low-pressure cell. A succession of these frontal systems traverses the country in the spring months after the Siberian High breaks down at the end of winter and the air mass zone of preferential movement, located south of the country in winter, moves northwards to its summer position across Lake Baikal. The dust-raising winds are

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commonly associated with the cold front of these cells and thus operate over a wide area. Occasionally, however, dust raising occurs on the meso scale when dry downdraft winds from thunderstorms that form behind the cold front raise dust at the gust front. These haboob-type dust storms are more common in the early summer months. The characteristic frontal cyclone tracks for dust storm occurrence are shown in Fig. 6. The southern regions of the country are predominantly affected by south-westerly cyclones which are formed in the lee of the Altai mountains. These systems are not the ones that raise material from the Chinese desert of Taklamakan and the Loess Plateau, which usually pass to the south of the Mongolian People's Republic although do occasionally veer north-eastwards and cross the Mongolian Gobi. In the Great Lakes and northern regions of the Mongolian People's Republic the major dust-raising systems are depressions that move eastward and south-eastward.

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Dustsources Although the geomorphology and pedology of the Mongolian People's Republic are not well known, some useful initial comments can be made on possible sources of dust and the processes leading to the production and accumulation of dust-sized particles. Dorzhgotov (pers. comm.) suggests that the stony surface of the Gobi is not usually a significant source ofair-borne dust because of the protection offered by pebbles commonly 2-3 em in diameter. These surfaces usually contain large quantities of silt and clay-sized

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particles beneath the pebble layer, often rich in gypsum and carbonates (Dorzhgotov & Kowalkowski, 1981), and this may be deflated as dust from localised areas that have been disturbed by animals, tracks or construction activities. Other possible sources of dust on the Gobi surface include wadis that dissect the hamada, and the saline terminal depressions that are characteristic of the area. These depressions fill with fine material as a result of periodic flash flooding, and further breakdown of coarser grains may occur by in situ salt weathering. The conditions of low annual precipitation, combined with long periods of sub-zero temperatures, promote low rates of chemical weathering and clay formation in soils -(Nogina & Dorzhgotov, 1982). Frost and insolation are the most widespread weathering agents in the semi-arid regions of the country according to Lamborincen & Cagaan (1979),

DUST STORMS

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but the action of salt weathering must also be significant in the saline soils of the Gobi region. Scanning electron microscopy of quartz grains from permafrost solonchaks in the central arid steppe zone have shown cryohydrothermal block and granular disintegration and cryothermal and cryomechanical fracture and chipping (Kowalkowski & MycielskaDowgiallo, 1985).

Dust raising and human activities Wind erosion and the action of dust storms are typical features of the Mongolian climate. Atmospheric dust is a perennial problem for animal herders and breeders, particularly in the Gobi region where the name ugalz (meaning spiral or whirl) is used locally for dust storms. Dust storm warnings are occasionally issued by the meteorological services, particularly to agriculturalists and the transport industry who are adversely affected by loss of visibility. Eroded walls and frosted glass on buildings in Sainshand bear witness to the regularity of north-westerly winds laden with sand and dust. In Ulan Bator, respiratory ailments are rife and represent the second biggest cause of child mortality. A number of examples of human actions increasing dust storm occurrence locally can be documented. In parts of the Great Lakes region between the Altai and Hangai Mountains the cutting of sparse vegetation for fuel wood has exacerbated the problems of deflation and dust storms which adversely affect breeders and herders of cattle, sheep, horses, goats and camels in the region. Problems have also been experienced in northern regions after a 'virgin lands' scheme was initiated in 1959 to plough up steppe land for cultivation. Three main areas were targeted: the Orhon-Selenge River; the Kherlen River; and the Khalh River. jambaajamts (pers, comm.) suggests that 10-12 ern of topsoil has been lost to wind erosion in these three areas during the last 20-25 years. Sanders (1982) cites the example of the Atar ('Virgin Land') state farm, west of Ulan Bator, where 7000 ha of soil has been severely eroded since its inception in 1977. In the capital, Ulan Bator, a trend of increasing dust storms from the mid-1970s to the early 1980s can be clearly discerned from the meteorological records [Fig. 7(a)]. While there is a concomitant decline in precipitation during this period, and the fall-off of dustraising activity in the later 1980sis during a period of increased precipitation, observers at the station also suggest that human action may influence this pattern. The most important activities are (i) increasing vehicle movement on unpaved roads and tracks both outside the city centre and in and around the pallisades of traditional felt tents that surround the modern centre (ii) off-road vehicle movement on the outskirts of the city where increasing numbers of vehicles have driven on grass beside the tracks, thus expanding the unvegetated area and exposing more sediment to aeolian action (iii) construction work which involves clearing of vegetation and the movement of heavy lorries on dirt roads. This process probably has a significant bearing on the data in Fig. 7(a) since building of apartment blocks reached within 200 m of the station in the early 1980s (iv) pollution from power stations and factories at the western end of the valley in which the city is situated may also have a bearing on visibility data.

Duststorm frequency over time Figure 7(b) and (c) shows the annual variation in dust storm events with annual precipitation totals for Zamiin-Uud and Choibalsan over periods of 31 and 41 years respectively. No particular trends are discernible from either station's record, although the effect of precipitation on dust storm frequency is clear, particularly at Zamiin-Uud.

296

N.

J. MIDDLETON Conclusion

Most of the area of the Mongolian People's Republic is subject to dust storm activity. Highest frequencies are recorded in the Gobi Desert, with Zamiin-Uud experiencing an annual average of 34'4 dust storm days. Wind erosion activity is largely confined to the spring months when the dominating Central Asian high-pressure cell breaks down and a succession of generally eastward-moving frontal cyclones traverses the country. Deflation thus occurs before the brief summer wet season when monthly wind speeds are at their peak. Dust storms are typical of daylight hours, particularly around midday. Given the importance of aeolian dust from the Mongolian People's Republic to studies outside the region, particularly in the fields of ocean sedimentation, loess profiles and palaeoclimates, the present study should be considered as a first stage in the study of Mongolian dust, its origins and processes behind its formation and transport. I would like to thank the Government of the Mongolian People's Republic and the British Council who jointly funded my research trip to Mongoliaas part of the Anglo-MongolianCultural Exchange Programme. Special thanks are due to Mr Dembereldorj and Mrs Tsetseg of the Institute of Hydrometeorology, Ulan Bator, and Mr Sod and Mr Chimbat of the University, Ulan Bator. The Cartography Department at the School of Geography, University of Oxford, drew the figures.

References Davies, J. (1986). Lost lakes of the Gobi. Geographical Magazine, 58: 350-355. Derbyshire, E. (1983). Origin and characteristics of some Chinese loessat two locationsin China. In: Brookfield, M. E. & Ahlbrandt, T. S. (Eds), Eolian Sediments and Processes. pp. 69-90. Amsterdam: Elsevier. Dorzhgotov, D. & Kowalkowski, A. (1981). Fundamental regularities in the geographical zonality of soils in Mongolia. Quaestiones Geographicae, 7: 21-34. Kotarba, A. (1980). Splash erosion in the steppe zone of Mongolia. Zeitschrift fur Geomorphologie N.F. Supplemunt-Bund, 35: 92-102. Kowalkowski, A. & Mycielska-Dowgiallo, E. (1985). Weathering of quartz grains in the liquefied horizon of permafrost solonchaks in the arid steppe zone, central Mongolia. CATENA, 14: 179190. Lamborincen, R. & Cagaan, C. (1979). Pecularities and varieties involved in rock decay in the semiarid climate of the Mongolian People's Republic. Petermanns Geographische Mitteilungen, 123: 79-82 (in German). Makhover, Z. M. (1967). On the causes for the stable position of the centre of the Asiatic Anticyclone over Mongolia. Trans. Nauchnoizzled. Inst. Aeroklimatol., 38: 54-59. Middleton, N. J. (1984). Dust storms in Australia: frequency, distribution and seasonality. Search, 15: 46-47. Middleton, N. J. (1985). Effect of drought on dust production in the Sahel. Nature, 316: 431-434. Middleton, N. J. (1986a). Dust storms in the Middle East. Journal ofAridEnvironments, 10: 83-96. Middleton, N. J. (1986b). A geography of dust storms in South-west Asia. Journal of Climatology, 6: 183-196. Middleton, N. J. (1989a). Desert dust. In: Thomas, D. S. G. (Ed.), Arid-zone Geomorphology. pp. 262-283. London: Belhaven Press. Middleton, N. J. (1989b). Climatic controls on the frequency, magnitude and distribution of dust storms: examples from India/Pakistan, Mauritania and Mongolia. In: Leinen, M. & Sarnthein, M. (Eds), Paleoclimatology andPaleometeorology: Modern andPastPatterns of Global Atmospheric Transport. pp. 97-132. NATO ASI Series C, Vol. 282. Dordrecht: Kluwer Academic Publishers. Nogina, N. A. & Dozhgotov, D. (1982). Soil-geographiczoning of Mongolia. SovietSoil Sciences, 14: 19-26. Petrov, M. P. (1976). Deserts of theWorld. New York: J. Wiley & Sons. 447 pp. Rea, D. K. & Leinen, M. (1988). Asian aridity and the zonal westerlies: late Pleistocene and Holocene record of eolian deposition in the northwest PacificOcean. Palaeogeography, Palaeoclimatology, Palaeoecology, 66: 1-8.

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Sanders, A. J. K. (1968). The People's Republic of Mongolia: A General Reference Guide. London: Oxford University Press. 245 pp. Sanders, A. J. K. (1982). Golden herds and 130 production targets. Far Eastern Economic Review, 19 November: 40-42. Shaw, G. E. (1980). Transport of Asian desert aerosol to the Hawaiian Islands. Journal of Applied Meteorology, 19: 1254-1259. Tuvdendorzh, D. (1974). The dust storms of the territory of the Mongolian People's Republic. Unpubl. Ph.D. Thesis, Moscow State University. 190 pp. (in Russian).