.VI'MOSPHERIC RESEARCH ELSEV 1ER
Atmospheric Research 34 ( 1994 ) 169-176
The origin and evolution of dust clouds in Central Asia V.V. Smirnov a, D.A. Gillette b, G.S. Golitsyn c, D.J. M a c K i n n o n d alnst. Experimental Meteorology, Federal Hydrometeorological Service, Obninsk 249020, Russia bARS/ARL NOAA, Boulder, CO 80303, USA ~Inst. Atmospheric Physics, Russia Acad. of Sciences, Moscow, 10917, Russia aU.S. Geological Survey, Flagstaff AZ 86001, USA
(Received January 22, 1993; revised version accepted April 15, 1993)
Abstract Data from a high resolution radiometer AVHRR (580-680 nm optical lengthwaves) installed on the "NOAA-11" satellite as well as TV (500-700 nm) and IR ( 8000-12000 nm) equipment of the Russia satellite "Meteor-2/16" were used to study the evolution of dust storms for 1-30 September 1989 in Tajikistan, Uzbekistan, Turkmenistan and Afghanistan. These data help to validate the hypothesis, that long-term dusted boundary layer (duration of the order of a day or more), but of comparatively not high optical density (4-10 km meteorogical visibility range at the 20-50 km background), is formed after the northwest intrusions into a region of intensive cold fronts at the surface wind velocities of 7-15 m/s. Stability of dust clouds of vertical power to 3-3.5 km (up to an inversion level) is explained by an action of collective buoyancy factors at heating the dust particles of 2-4 /~m in mean diameter by solar radiation. The more intensive intrusions stimulate a formation of simultaneously dust and water clouds. The last partially reduce the solar radiation (by the calculations of the order of 30-50%) and decrease the role of buoyancy factors. Thus, initiated is the intensive but short-term dusted boundary layer at horizontal visibility of 50-200 m.
I. Introduction Space sounding m e a n s h a v e b e e n used for a long t i m e a n d successfully to detect dust storms, to observe the large-scale dust clouds a n d to estimate their density (e.g. G r i g o r ' y e v a n d Lipatov, 1974; Fraser, 1976 ). T h e N o r t h Africa a n d the Arab i a n Peninsula regions were properly studied with these means. 0169-8095/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDIO169-80950169-8095(93)EOI02-5
et al. I Atmospheric Research 34 (1994) 169-176
The paper seeks to study the space-time structure of severe dust storms in Central Asia in September 1989 during the USSR-US experiment. A complex utilization of the Tajikistan meteorological network data, aircraft sounding and space survey by the USSR (Meteor-2/12, TV channel of 500-700 nm, IR channel of 8000-12,000 n m ) and US (NOAA- l l, AVHRR radiometer) stations allowed to obtain information on the nature and initial conditions of dust storms in the region, on the character of interaction between water and dust clouds. A detailed information on the methods and results for two dust episodes ( 1516 September, when the storm was continuous but moderate in dust intensity, and 20 September, when the dust was intensive but comparatively short-lived) is presented in the collection book edited by G.S. Golitsyn, 1993.
2. Initial dust source
One of the factors of dust storm effects on soil and atmosphere is the physicalchemical and dispersion composition of dust particles. Then, the problem of identifying dust source and recognising soil and meteorological conditions of the process initiation arises. Note only some questions: The dust aerosol sources in the given region were not identified earlier. An analysis of dust chemical and mineral content in the storm of 20 September 1989 and soil of the Southern Tajikistan showed their significant difference (Smirnov, 1993 ). Significant differences were as well in the content of insoluble substances of the atmospheric aerosol samples taken at 500-6000 m heights under conditions
Fig. 1. Reconstruction of visible satellite image of studied region in Central Asia. (a) peculiar background, (b) long period dust storm l-water cloudiness; 2-mountain tops; 3-sand dusting soils for Fig. la, 3-sand cloud for Fig. lb; 4-direction movement of air masses in the region; 5-famous points (see text ); 6-state borders from 1.0 I. 1993.
K V. Smirnov et al./Atmospheric Research 34 (I 994) 169-176
Meteor-~ Ig, 0~-~2 1.T ,~0.09. ~9 4O° ~
35°~___~~ ~ 6~°
~,;q!~ a) 50
Meteor - 2 TV, og- ~0 I, T 20. Og. gg
Fig. 2. Reconstruction o f l R (a) and TV (b) satellite images of squally dust storm in Central Asia.
without and with long-term dust storm of 15-16 September 1989. At the same time by the data (Gillette, 1993) the site of Shaartuz in the Southern Tajikistan is simultaneously a potential source and collector of dust. Consider the corresponding satellite observations' data. Fig. 1 gives pictures obtained using radiometer on the NOAA- 11 satellite in the period of long-term dust storm of 16 September 1989 and in the period preceeding the storm. The ground sites were organized in the Kafirnigan river valley along 1 - 2 - 3 - 4 line where l-Shaartuz (37 °, 68°E), 2-Isanbai (38 °, 68°20'E), 3-Dushanbe ( 38 ° 36', 68 ° 49' E ), 4-Altyn-Mazar (the Fedchenko Glacier ). According to the satellite data in the periods of cold air mass intrusion in spring and autumn, the most probable is the situation when northern or north-western strong air flows firstly turn east along the Gindukush spurs. Then, rounding the
V.V. Smirnov et al. / Atmospheric Research 34 (1994) 169-176
Pamir mountain system from the west, they move to the north-east across the Kafirnigan and Vakhsh river valleys filling them with dust. As seen from Fig. 1b, it has been just this situation which was observed in the studied dust episodes of 15-16 September. The comparison of Fig. 1a and b shows that the desert sand plateaux in the northern Afghanistan corresponds to the dusty regions. The foothills are cut through by numerous drying valleys (vadi), in which the sediments are the mixture of fine sand particles and clay. They are poorly fastened by plants and eroded during high winds. Approximately in an analogous way a formation of intensive dust storm occured in the episode of 20 September 1989 (Fig. 2). The dusting source in Afghanistan was initiated in the night, what succeeded in obtaining, thanks to presence of it, on the "Meteor-2" satellite and scanning IR-radiometer (Fig. 2a). The dusting source is restricted by coordinates. At an early stage the dust cloud was between 65-69°E and 36-38°N. First, from the dusting points the streamlines were stretched to the north-east along the trajectory of powerful cyclonic formation. Some streams were broken passing of about 30-100 km, another crossed the zone of continuous cloudiness which occupied the area limited by 37-50°N and 65-77 ° E, the third were generated along the primary dust mass movement.
3. On the dust cloud evolution
In brief, we cite a summary of data and characteristics of dust clouds above Tajikistan using data of airborne surveys (Belan et al., 1993 ) and ground instruments (Smirnov, 1993b). ( 1 ) A function of dust size particles' distribution is conservative in ratio to a sounding height (up to 500 m) and presence of dust storm. In 0.5-10/~m interval it is an inverse power function with small mode in the range of 2-3/zm in diameter. A mean-cubic diameter is close to 3/tin. A particle concentration of more than 5/zm in diameter quickly flows down with the height. General counting concentration of particles up to an inversion height of 3400 m can be estimated as 100 cm-3, in the absence of storm ~ 1 cm-3. In the dusted and surface layer the mass particle concentration reached 400-500/~g/m 3 in the storm of 16-18.09.89 and even 4-6 m g / m 3 in the storm of 20.09.89. (2) Height profiles of light scattering coefficient were homogeneous up to an inversion layers' height and a mean estimation is equal ~= 0.8 _+0.2 k m - ~ in the storm of 16.09.89; under the background conditions a was increased from 0.1 k m - I on the ground level to 0.02 k m - I on the level of 3 kin. The higher the inversion level, tr is ~ 0.005 k m - ~in both situations. The optical dust cloud depth in 0.4-0.8/~m wavelength reached 0.7+_0.05 (De Luisi and Reddy, 1993) and values of 1 +_0.2 and greater in the period of intensive storm of 19-20.09.89. The presence of notable cloudiness in the studied region, as was found, is the
V. lL Smirnov et al. / Atmospheric Research 34 (1994) 169-176
important factor of dust storm evolution within the period of 19-20 September (Smirnov, 1993). According to the Meteor-2 TV channel data (Fig. 2b) by 0840 -LT the dust cloud was propagated to the foothills of Tajikistan and eastern Uzbekistan. The northern boundary of a dust cloud, however, was covered by clouds and according to the network data this boundary was not beyond the limits of the southern slopes of the Zeravshan mountain ridge. The southern boundary of a dust cloud passing along 36 ° 30' is properly identified in the TV images (in the positive-by white short lines, in Fig. 3b by circle spots). Their effect is explained by a high reflecting power of the dust cloud near the source (nearly the same negative blackening density when imaging the stratorain cloud). So, the dust cloud dimensions from south to north can be estimated as 250300 km and from east to west as 400-500 km. Nearly similar estimates were obtained from NOAA-II A V H R R radiometer (MacKinnon, 1993 ). The total area occupied by the dust cloud can be characterized as significant (about 105 km 2) even in weather-climate scales. The general weight of dust elevated into the air is estimated as 3.106 ton. It is interesting, that as opposed to the first dust episode the time of dust cloud existance in the whole observational zone was short (less than 6 hours), though in this case the velocity of a dust front (Fig. 2 ) was twice as high.
4. A qualitative model of dust storms in the region The primary cause of dust storms in Central Asia is the intrusion of cold air from north-west and north accompanied by surface winds of similar directions. In the narrow passages between mountain systems (Fig. 1 ) the wind velocity increases up to 15-20 m / s at the vane level that is quite enough for local soil dusting. The Southern Tajikistan regions are not the source of dust and the local name of dust storms "afghanets" is fully justified. Two types of dust storms are possible: long-term and squally.
4.1. The scheme of long-term dust storms They originate with intruding cold fronts of low vertical thickness (of about 1 km or less). In this case the cold air flowing below warm masses lifts them (Fig. 3a). The developed so-called prefrontal air flows stimulate the surface soil erosion. The lighter particles are entrained upward by a warm flow and reach the height of 3-4 km, where they can exist for several days. The effect is caused by the fact that due to the dust shield the arrival of solar radiation to the Earth surface sharply decreases. The cold lower air is slightly heated and the warm upper air is continuously being heated and does not loose its buoyancy. The increase of dust content in the surface layer under these conditions is related to particle sedimentation from the upper layers. Since the mean air velocities values in the zone
V.V. Smirnov et al. / Atmospheric Research 34 (1994) 169- ! 76
soi ,. ,',,d,,,tio. l
\" \ ' %
" " ~ ~"
Fig. 3. The scheme of long-term (a) and squall (b) dust storms. Table 1 Model parameters of water-drop (1-3) and dust (4) clouds, used in calculations n/n
No (cm -3)
1 2 3
4.0 4.0 6.0
6.0 4.3 10.5
100 100 100
o9 ( g / m 3) 6.2.10 -2 3-10 -2 0.3 10 -2
6 8 4
1 3 1
10 - 2
Cloud analog cumulus stratus cumulonimbus dust storm
(km - t ) 0.3 0.1 7.10 -2 2.1.10 -2 (2.3.10 -2 )
of dust storm were comparatively not large, the principal mass of dust particles seems to be the result of colic dust wind erosion, first of all the finest loess dust (Smirnov, 1993b).
4.2. The scheme of squall dust storms The squall "afghanets", appearing with the passage of a cold intrusion front of height vertical depth (up to 3-5 km ) and having the surface wind velocity of 2030 km/h, raises a lot of large soil particles. However, the zone of convective cloudiness is formed simultaneously. Because of dust particle shield from solar radiation, the life time of a dust cloud decreases and it was observed in the second dust episode. Quantitative constructions will be the subject of future studies. We cite some qualitative pictures by meaning an estimation of "screening" degree by cloud of water droplets in radius r and dust cloud particle concentration n(r). For simplicity we will use model gamma-spectra in sizes of drops and dust particles
n(r) = A . r ~ e x p [ - o f f T " (r/rm) ~] ,
where rm is model radius; A, 7 and o~ are parameters, the values of which are
K V. Smirnov et al./Atmospheric Research 34 (1994) 169-176
presented in Table 1. As for other designations in the Table: N and co are average concentration values of drops and liquid-water content, req is equivalent radius of aerosol particle,
req= i n(r)r3dr/ i n(r)r2dr. o
Model parameters of droplet clouds 1, 2, 3 are taken from (Bazhenov et al., 1992). Model of dust particles 4 accounts for airplane sounding data (Belan et al., 1993). Values of mass concentration and aerosol radius req in used approximations of spectrum determine a radiation attenuation index ~ in the visible wavelength interval through the familiar relationship e(km-l),,~ 3 =~[o)/ (p.req) ]
Transfer to the optical depth of clouds is possible using a formula r=~-H,
where H is a cloud power. For a droplet cloud H = 0.5 km, for dust cloud ~ 5 km. The calculation results by models 1-4 using Eqs. (2) and (3) are given in Table 1. For a model of dust cloud the mean experimental value e is given in brackets, it confirms the expedience of simplifications being made. We can obtain calculating estimates of solar radiation flux, having passed the droplet cloud, using an expression:
S=I-N(1-e -~) ,
Where N is the relative cloud amount ( N = 0 . . . I ) , the optical depth r is determined from Eq. (3). The Eq. (4) describes a case, when the sun is in zenith, what is specific for the situation under investigation. The calculations within the presented model show that solar radiation flux's decrease in 30-50% on the upper edge of dust cloud takes place, if a cloudiness of 1 km power is stratus or cumulus. This way, the nature of short-lived (squally) but intensive dust storms can be explained by development of effect of particulate dust cloud optical screening by water clouds, simultaneously being formed by powerful fronts. When this, the troposphere layer temperature, is reduced (Golitsyn, 1993 ), the intensity of turbulent pulsations is decreased and the velocity of dust particles' fallout into-a soil is increased. References Bazhenov, O.E., Kasiyanov, E.I. and Komashov, D.N., 1992. Influence of microphysical parameters of clouds on radiation characteristics of broken clouds. Opt. Atmos. Ocean, 5 (N3): 232-238 (in Russian ).
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Belan, D.D., Kabanov, D.M., Panchenko, M.B. et al., 1993. Airborne sounding of atmospheric parameters in dust experiment. Joint Soviet-American Experiment on Arid Aerozol. Hydrometeoizdat, St. Petersburg, pp. 43-54. De Luisi, J.J. and Reddy, P., 1993. Dust Optical Depth Measurements and Estimation of Column Mass in the Tajikistan Desert Dust Project. ibidem, pp. 75-82. Fraser, K.S., 1976. Satellite measurement of mass of Sahara dust in the atmosphere. Appl. Opt., 15 ( 10 ): 2471-2479. Gillette, D.A., 1993. Model of Transport/Deposition of Desert Dust in the Kafirnigan River Valley from Shaartuz to Dushanbe. ibidem, pp. 50-55. Gillette, D.A., Golitsyn, J.S., MacKinnon, D.J. and Smirnov, V.V., 1992. Satellite observation of interaction between droplet and dust clouds in Central Asia. Proc. 1 lth Int. Conf. Clouds Precipitation. Montreal ( 17-21 Aug. 1992 ) Elsevier, pp. 1156-1158. Gillette, D.A., Gomes, L. and Smirnov, V.V., 1992. A generalized model on spectrum of arid aerosol. In: N. Fukuta and P. Wagner (Editors), Nucleation and Atmospheric Aerosols. Deepak Publ., Hampton, pp. 461-464. Golitsyn, G.S., 1993. Complex Soviet-American Dust Experiment. ibidem, pp. 3-6. Grigoryev, A.A. and Lipatov, S.V., 1974. Dust storms based on the space investigation data. L. Gidrometeoizdat, 31 pp. (in Russian). MacKinnon, D., 1993. NOAA AVHRR Satellite Imagery of Tajikistan Storms of 16 and 20 September 1989. ibidem, pp. 26-27. MacKinnon, D.J. and Chavez, P.S., Jr., 1993. Dust storms. Earth, 2(3): 60-65. Smirnov, V.V., 1993a. Genesis of Dust Storms in the Southern Tajikistan based on the "Meteor-2"" Satellite and Surface Meteoanalysis data. ibidem, pp. 28-35. Smirnov, V.V., 1993b. Morphological, Structural and Chemical Peculiarities of Dust Storm Aerosols in Tajikistan. ibidem, pp. 127-134.