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Sedimentary characteristics of a haboob dust storm
Atmospheric Research 61 (2002) 75 – 85 www.elsevier.com/locate/atmos
Sedimentary characteristics of a haboob dust storm W. Chen a,*, D.W. Fryrear b a
Environmental Group, Alon USA Oil and Chemical Company, P.O. Box 1311, Big Spring, TX 79721, USA b Custom Products and Consultants, 7204 South Service Road, Big Spring, TX 79720, USA Received 23 April 2001; received in revised form 22 June 2001; accepted 22 June 2001
Abstract As a haboob dust storm passed through Big Spring, TX, 33 airborne dust samples were collected in a 1567-cm surface layer. Samples were dried, weighed, and then the size distributions were analyzed with an ultrasonic sieve and a vertical settling aerosol tube. The maximum measured dust concentration was 0.0013 kg m 3. The total horizontal particle flux was 84,960 kg km 2 h 1, within which the flux of particles 20 – 10 mm in diameter was 17,790 kg km 2 h 1. Vertical distributions of dust concentration decrease with height by a power function. The mean diameters of the dust particles were between 23 and 35 mm, whereas the 95th percentile diameters were from 8 to 14 mm. Particles smaller than 10 mm in diameter were from 1.5% to 10.76% of the total mass fluxes. Vertical distributions of the mean and the 95th percentile diameters showed a natural logarithmic function of height. The dust particles were unimodally distributed with a peak diameter of 40 mm in the 550 cm flow layer, whereas above 550 cm the peak diameter was 20 mm. The airborne dust was from moderately to poorly sorted and became less well sorted with the increasing height. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Haboob dust storm; Aerosol; Particle concentration; Mass flux; Grain-size distribution
1. Introduction Dust storms, as a main source of aerosol, frequently occur in the western United States (Udden, 1896; Kellogg, 1935; Sidwell, 1938; Langham et al., 1938; Swindford and Frye, 1945; Warn and Cox, 1951; Pe´we´, 1951; Chepil and Woodruff, 1957; Smith, 1970; Orgill and Sehmel, 1976; Fryrear, 1981; McCauley et al., 1981; Chen and Fryrear, 1996). However, the measured profiles of dust quantities are scarce because it is difficult to collect field data on natural occurring wind erosion events. It is even more difficult to collect data from a haboob type of dust storms. With detailed data, more precise models to *
Corresponding author. E-mail address: [email protected] (W. Chen).
0169-8095/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 8 0 9 5 ( 0 1 ) 0 0 0 9 2 - 8
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Fig. 1. A haboob dust-storm front approaches Big Spring, TX, from the north on 16 June 1997. The sampling tower was located at the center of the photograph.
predict dust profiles, soil loss, and dust transport in the atmosphere would become possible. A haboob type of dust storms approached Big Spring, TX, from the north on 16 June 1997 (Fig. 1). Detailed weather conditions were recorded. The system was active from 5:00 p.m. to 10:00 p.m. The front nose of the dust storm arrived at the sampling site at 6:00 p.m., and passed over at 7:15 p.m. The wind speeds, at the advancing dust-front nose, were 1.8 – 2 m s 1, then gradually increased to 2.5– 5.5 m s 1 (measured at 1000 cm above the ground). The wind speed at the rear of the system was 13 m s 1. Thirty-three airborne dust samples were collected in a 1567-cm profile. This report provides the data on the sedimentary characteristics of the dust storm. 2. Methods and procedures 2.1. Field sampling The observation site was at Big Spring, TX. Three-cup anemometers were installed on a tower at 60, 300 and 1000 cm above the ground, respectively. The wind direction was measured at 1000 cm above the surface. Five-minute average wind speeds, temperature, relative humidity, and the air pressure were recorded by a data-logger. Dust samples were collected by using the BSNE samplers (Fryrear, 1986). Vertical sampling intervals in the 0 –250 cm flow layer were 25 cm, whereas 50 cm intervals between 250 and 800 cm, and 100 cm intervals between 800 and 1567 cm.
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2.2. Mass quantities The samples were oven dried at 105 C for 24 h. The dried samples were weighed and the mass quantities were obtained. Gravimetric water contents of the measured dust samples were from 0.91% to 3.78%. The dried samples were weighed using an electronic scale with 0.0001 g precision to obtain total measured quantities, Qz, at each sampling height. Mass fluxes of dust at a height were calculated by Fm ðzÞ ¼
QðzÞ TA
ð1Þ
where Fm(z) is the horizontal mass flux (solid discharge) of dust at a height z. T is duration of sampling. A is the opening area of the samplers. Concentration of each size fraction of particles was obtained by CðdÞ ¼
QðdÞ Vp ðdÞTA
ð2Þ
where Q(d) is quantity of particles with a diameter d, which is the product of Q(z) multiplied by the probability density of particles with a diameter d, P(d). Vp(d) is traveling velocity of a particle with diameter d. 2.3. Particle sizes Two methods were used to measure the geometric and aerodynamic size distributions of the airborne samples. An ultrasonic sieve with 75 mm intervals for particles between 125 and 500 mm and 10 mm intervals for particles between 10 and 125 mm were used to determine the geometric diameter of the dust samples. A vertical settling aerosol tube (VSAT) method was used to measure the aerodynamic equivalent diameter (Chen and Fryrear, 2001). Grain-size parameters were calculated by the graphic method recommended in Folk (1966). The tested aerodynamic equivalent diameters of airborne particles showed a power function of geometric diameters at the 0.001 significance level, da ¼ 0:41dg0:6
R2 ¼ 0:996
ð3Þ
where da is the aerodynamic equivalent diameter, and dg is the geometric diameter.
3. Results 3.1. Weather conditions Weather conditions during the dust storm are shown in Fig. 2. The air pressure lowered from 1010.4 mb at 5:00 p.m. to 1009.7 mb at 7:00 p.m. Then, it rose continuously to 1013.4 mb at 11:00 p.m. (Fig. 2a). During the dust storm, air temperature rose from 30 C
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Fig. 2. Weather conditions during the dust storm: (a) the air pressure, which was lowest when the front passed over the sampling site; (b) temperature was high, whereas relative humidity was low, when the dust front passed by the sampling site; (c) wind direction veered from 200 to 350; and (d) wind velocities increased sharply when the dust front passed the sampling site.
before 5:00 p.m. to 33 C between 6:00 p.m. and 7:00 p.m. It dropped when the front passed over (Fig. 2b). The dust-laden air mass was drier. The relative humidity decreased from 13% to 14% before 5:00 p.m. to 11% from 6:00 to 7:00 p.m. It increased after 7:10 p.m. (Fig. 2b). Wind directions varied according to the movement of the low-pressure trough. When the front approached Big Spring, the wind direction was SW 200 –260, showing the center of the low pressure being north –east to Big Spring. Then the wind direction changed to NW 290 –320 as the front approached Big Spring. After 7:10 p.m., Table 1 Parameters of the vertical distribution equation of the horizontal mass flux F = a + bZ c
Total flux P20 – 10
a
b
c
R2
Fstat
0.666 0.193
3.14 0.465
1.100 0.992
0.96 0.75
438.46 28.12
F is mass flux of dust particles in the flow layer, g m 2 s 1; Z is height; a, b, and c are constants; R2 is the correlation coefficient; Fstat is the threshold value for F-test; P20 – 10 is the mass flux of particles 20 – 10 mm in diameter.
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Fig. 3. Vertical distributions of the particle concentrations of the total particles, the particles of 20 – 10 mm in diameter, and the particles finer than 10 mm in diameter.
the wind direction was NWN 320– 350 (Fig. 2c). Variations in wind speeds at 1000 cm above the ground are shown in Fig. 2d. It was calm before the dust front approached Big Spring. At the nose of the dust front, the wind speed was 2.4 F 0.22 m s 1 on average. From 6:00 p.m. to 7:00 p.m., the wind speed increased slightly to 3.5 F 1.34 m s 1. It was 11.5 F 2.03 m s 1 when the front passed. Then after the high pressure moved in and occupied the area, wind speeds slowed gradually. 3.2. Mass fluxes and particle concentrations The results of this investigation showed that the suspended dust in the flow layer above the surface decreased by a power function of height. The measured mass fluxes fit a power function at the 0.001 level (Table 1).
Table 2 Parameters in the power equation of concentration of suspended particles C = a + bZ c
Total particles Particles (20 – 10 mm) Particles ( < 10 mm)
a
b
c
R2
Fstat
0.126 0.035 0.010
1.061 0.170 0.027
1.207 1.041 1.009
0.99 0.88 0.51
917.04 66.67 9.94
C is the concentration of dust particles, g m 3; a, b, and c are statistical constants.
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The total horizontal mass flux through the observed 1567 cm surface layer can be obtained by integrating the vertical distribution of mass fluxes (Table 1). The total mass flux was 84,960 kg km 1 h 1 for the 1567 cm flow layer. The total mass flux of particles 20 –10 mm in diameter was 17,790 kg km 1 h 1 (Table 1). Vertical distributions of particle concentrations are shown in Fig. 3 and Table 2. Concentrations of total sediments, particles of 20 –10 mm, and particles finer than 10 mm in diameter decreased with the increase in height by a power law (Table 2). The exponent c in the power relation equation represents the slope of the fitted distribution curves of the
Fig. 4. Histograms of the size distributions of the dust particles. The numbers in the histograms are the sampling heights.
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Fig. 4. (continued ).
concentration of the suspended particles versus height. The finer the particles are, the smaller the slope is. The fine particles were more uniformly distributed in the whole surface layer, which agrees with the sediment diffusion theory (O’Brien, 1933). 3.3. Grain-size distribution Most of the samples showed an unimodal distribution (Fig. 4). The peak diameter was 40 mm in the flow layer below 550 cm; and 20 mm above 550 cm. Table 3 shows the grainsize distribution parameters of all samples. Both the mean and the 95th percentile diameters displayed a natural logarithmic function of height (Fig. 5). P95 ¼ 17:25 1:12 lnZ
R2 ¼ 0:61
ð4Þ
dm ¼ 41:61 2:22 lnZ
R2 ¼ 0:65
ð5Þ
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Table 3 Grain-size parameters of dust particles within a dust-storm on 16 June 1997 at Big Spring, TX Height
dp95 (mm)
dm (mm)
rI (j)
Sk (j)
Kg (j)
41 45 49 53 55 73 93 123 153 193 227 263 303 363 417 467 517 567 613 667 717 767 817 917 1017 1117 1567
12.8 13.7 11.8 12.7 12.4 13.1 12.1 11.1 10.0 12.0 10.3 13.0 13.1 11.5 10.7 11.3 11.7 10.3 10.1 10.1 9.7 8.7 9.2 8.7 8.0 8.8 8.2
33.4 34.7 32.8 33.9 30.7 33.3 30.8 30.5 27.0 28.1 27.9 31.5 31.1 30.2 30.8 29.7 29.3 27.3 27.4 28.9 27.4 23.7 23.4 24.8 24.4 24.1 23.0
0.72 1.08 0.85 0.75 0.62 1.13 0.72 0.76 0.76 0.69 0.78 0.71 0.74 0.78 0.92 0.95 0.83 0.66 0.97 0.81 0.93 0.85 0.80 1.02 1.08 1.46 1.19
0.298 0.104 0.269 0.281 0.102 0.065 0.326 0.150 0.310 0.048 0.080 0.096 0.074 0.087 0.033 0.029 0.027 0.241 0.013 0.007 0.055 0.006 0.106 0.056 0.093 0.332 0.355
0.879 2.144 1.369 1.030 1.127 1.770 1.023 0.933 0.853 0.801 0.974 0.950 1.056 1.074 1.162 1.190 1.139 1.232 1.404 1.496 1.137 1.268 1.558 1.746 1.373 1.244 1.478
dp95 is the 95th percentile particle diameter; dm is the mean diameter; rI is the inclusive graphic standard deviation; Sk is skewness; and Kg is graphic kurtosis.
where dm is the mean diameter. P95 is the 95th percentile diameter. The fitted relationships between particle diameters and height are significant at the 0.01 level. These results differ from other reports (Chepil and Woodruff, 1957; Gillette et al., 1974; Nickling, 1983; Chen and Fryrear, 1996), which show a power function of height. Grain-size distributions of the dust in the flow layer below 417 cm were positively skewed, except one negative sample at 73 cm. Above 417 cm grain-size distribution was negative, except for two positive skewed samples: one at 567 cm, and another at 717 cm (Table 3). Kurtosis values showed mesokurtic (0.9 – 1.1 j) to very leptokurtic (1.5 – 3.0j) distributions. There existed a flow layer from 93 to 263 cm where the grain-size distributed mesokurtically, whereas below and above which they were leptokurtic distribution (Table 3). Most of the dust samples collected from the flow layer below 817 cm were moderately sorted (j < 1), except 2 samples where j > 1. Dust samples collected above 817 cm were moderately to poorly sorted (j >1). Suspended dust becomes increasingly more poorly sorted with the increase in height. This is because all particles became
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Fig. 5. Vertical distributions of the mean and the P95 diameters. Both of them follow a natural logarithmic function of height.
relatively small and tended to be more uniformly mixed by the turbulent eddies. Besides, the development degree of a suspension process also influences the vertical distribution of sorting. The sorting process fluctuates and is affected by turbulent mixing and fluctuation of the air flow. This measurement shows the 0 – 1567 cm surface layer can be divided into three sub-layers. In the first flow layer from the ground surface to 100 cm, sorting became poorer with the increase in height. The second flow layer was 100 – 300 cm where the mean particle size was the smallest and the sorting was the most well sorted in the whole measured surface layer. In the third flow layer above 300 cm, the dust particles became increasingly more poorly sorted with the increase in height. If a higher flow layer could be sampled, it would be another flow layer with well-sorted finer particles.
4. Conclusions The total flux rate for the measured 1567 cm surface layer, in a haboob dust storm of 16 June 1997 over Big Spring, TX, was 84 960 kg km 2 h 1, within which the flux of particles 20 –10 mm in diameter was 17 790 kg km 2 h 1.
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Vertical distributions of dust concentration in a 1567-cm flow-layer decreased with the increase in height by a power function. The maximum dust concentration was 0.0013 kg m 3 at 100 cm above the surface. The fine particles distributed more uniformly in the measured 1567 cm flow layer, whereas the profile of the coarser particles showed a greater gradient. The mean diameters of the dust were between 23 and 35 mm. The P95 diameters were from 8 to 14 mm. Vertical distributions of the mean and the 95th percentile diameters showed a natural logarithmic function with height. Most of the dust samples are unimodal distributed with a peak diameter of 40 mm in the flow layer below 550 cm, above 550 cm the peak diameter decreased to 20 mm. The airborne dust was well to moderately sorted and became less well sorted with the increasing height.
Acknowledgements This work was performed while Weinan Chen held a Senior Resident Research Fellowship with the National Research Council-NOAA (ERL). We thank Dr. Dale A. Gillette, Wang Lu and Gao Shangyu for their help in the work.
References Chen, W., Fryrear, D.W., 1996. Grain-size distributions of wind eroded material above a bare flat soil. Physical Geography 17, 554 – 584. Chen, W., Fryrear, D.W., 2001. Aerodynamic and geometric diameters of airborne particles. Journal of Sedimentary Research 71, 365 – 371. Chepil, W.S., Woodruff, N.P., 1957. Sedimentary characteristics of dust storms: II. Visibility and dust concentration. American Journal of Science 255, 104 – 114. Folk, R.L., 1966. A review of grain size parameters. Sedimentology 6, 73 – 93. Fryrear, D.W., 1981. Dust storms in the southern Great Plains. Transactions of the ASAE 24, 991 – 994. Fryrear, D.W., 1986. A field dust sampler. Soil and Water Conservation 41, 117 – 120. Gillette, D.A., Blifford, I.H., Fryrear, D.W., 1974. The influence of wind velocity on the size distributions of aerosols generated by wind erosion of soils. Journal of Geophysical Research 9, 4068 – 4075. Kellogg, C.E., 1935. Soil blowing and dust storms. U.S. Department of Agriculture, vol 221, p. 11, Miscellaneous Publication. Langham, W.H., Foster, R.L., Daniel, H.A., 1938. The amount of dust in the air at plant height during wind storms at Goodwell, Oklahoma, in 1936 – 1937. Journal of the American Society of Agronomy 39, 139 – 144. McCauley, J.F., Breed, C.S., Grolier, M.J., Mackinnon, D.J., 1981. The U.S. dust storm of February 1977. Geological Society of America Special Paper, vol. 186, pp. 123 – 147. Nickling, W.G., 1983. Grain size characteristics of sediment transported during dust storms. Journal of Sedimentary Petrology 53, 1011 – 1024. O’Brien, M.P., 1933. Review of the theory of turbulent flow and its relation to sediment-transportation. Transactions, American Geophysical Union 14, 487 – 491. Orgill, M.M., Sehmel, G.A., 1976. Frequency and diurnal variation of dust storms in the Contiguous U.S.A. Atmospheric Environment 10, 813 – 825. Pe´we´, T.L., 1951. An observation on wind-blown silt. The Journal of Geology 59, 399 – 401. Sidwell, R., 1938. Sand and dust storms in vicinity of Lubbock, Texas. Economic Geography 14, 98 – 102. Smith, R.M., 1970. Dust deposition in relation to site, season, and climatic variables. Soil Science Society of America Proceedings 34, 112 – 117.
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Swindford, A., Frye, J.C., 1945. A mechanical analysis of wind-blown dust compared with analyses of loess. American Journal of Science 234, 249 – 255. Udden, J.A., 1896. Dust and sand storms in the west. Popular Science Monthly 49, 655 – 664. Warn, G.F., Cox, W.H., 1951. A sedimentary study of dust storms in the vicinity of Lubbock, Texas. American Journal of Science 249, 553 – 568.
Sedimentary characteristics of a haboob dust storm W. Chen a,*, D.W. Fryrear b a
Environmental Group, Alon USA Oil and Chemical Company, P.O. Box 1311, Big Spring, TX 79721, USA b Custom Products and Consultants, 7204 South Service Road, Big Spring, TX 79720, USA Received 23 April 2001; received in revised form 22 June 2001; accepted 22 June 2001
Abstract As a haboob dust storm passed through Big Spring, TX, 33 airborne dust samples were collected in a 1567-cm surface layer. Samples were dried, weighed, and then the size distributions were analyzed with an ultrasonic sieve and a vertical settling aerosol tube. The maximum measured dust concentration was 0.0013 kg m 3. The total horizontal particle flux was 84,960 kg km 2 h 1, within which the flux of particles 20 – 10 mm in diameter was 17,790 kg km 2 h 1. Vertical distributions of dust concentration decrease with height by a power function. The mean diameters of the dust particles were between 23 and 35 mm, whereas the 95th percentile diameters were from 8 to 14 mm. Particles smaller than 10 mm in diameter were from 1.5% to 10.76% of the total mass fluxes. Vertical distributions of the mean and the 95th percentile diameters showed a natural logarithmic function of height. The dust particles were unimodally distributed with a peak diameter of 40 mm in the 550 cm flow layer, whereas above 550 cm the peak diameter was 20 mm. The airborne dust was from moderately to poorly sorted and became less well sorted with the increasing height. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Haboob dust storm; Aerosol; Particle concentration; Mass flux; Grain-size distribution
1. Introduction Dust storms, as a main source of aerosol, frequently occur in the western United States (Udden, 1896; Kellogg, 1935; Sidwell, 1938; Langham et al., 1938; Swindford and Frye, 1945; Warn and Cox, 1951; Pe´we´, 1951; Chepil and Woodruff, 1957; Smith, 1970; Orgill and Sehmel, 1976; Fryrear, 1981; McCauley et al., 1981; Chen and Fryrear, 1996). However, the measured profiles of dust quantities are scarce because it is difficult to collect field data on natural occurring wind erosion events. It is even more difficult to collect data from a haboob type of dust storms. With detailed data, more precise models to *
Corresponding author. E-mail address: [email protected] (W. Chen).
0169-8095/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 8 0 9 5 ( 0 1 ) 0 0 0 9 2 - 8
76
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Fig. 1. A haboob dust-storm front approaches Big Spring, TX, from the north on 16 June 1997. The sampling tower was located at the center of the photograph.
predict dust profiles, soil loss, and dust transport in the atmosphere would become possible. A haboob type of dust storms approached Big Spring, TX, from the north on 16 June 1997 (Fig. 1). Detailed weather conditions were recorded. The system was active from 5:00 p.m. to 10:00 p.m. The front nose of the dust storm arrived at the sampling site at 6:00 p.m., and passed over at 7:15 p.m. The wind speeds, at the advancing dust-front nose, were 1.8 – 2 m s 1, then gradually increased to 2.5– 5.5 m s 1 (measured at 1000 cm above the ground). The wind speed at the rear of the system was 13 m s 1. Thirty-three airborne dust samples were collected in a 1567-cm profile. This report provides the data on the sedimentary characteristics of the dust storm. 2. Methods and procedures 2.1. Field sampling The observation site was at Big Spring, TX. Three-cup anemometers were installed on a tower at 60, 300 and 1000 cm above the ground, respectively. The wind direction was measured at 1000 cm above the surface. Five-minute average wind speeds, temperature, relative humidity, and the air pressure were recorded by a data-logger. Dust samples were collected by using the BSNE samplers (Fryrear, 1986). Vertical sampling intervals in the 0 –250 cm flow layer were 25 cm, whereas 50 cm intervals between 250 and 800 cm, and 100 cm intervals between 800 and 1567 cm.
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2.2. Mass quantities The samples were oven dried at 105 C for 24 h. The dried samples were weighed and the mass quantities were obtained. Gravimetric water contents of the measured dust samples were from 0.91% to 3.78%. The dried samples were weighed using an electronic scale with 0.0001 g precision to obtain total measured quantities, Qz, at each sampling height. Mass fluxes of dust at a height were calculated by Fm ðzÞ ¼
QðzÞ TA
ð1Þ
where Fm(z) is the horizontal mass flux (solid discharge) of dust at a height z. T is duration of sampling. A is the opening area of the samplers. Concentration of each size fraction of particles was obtained by CðdÞ ¼
QðdÞ Vp ðdÞTA
ð2Þ
where Q(d) is quantity of particles with a diameter d, which is the product of Q(z) multiplied by the probability density of particles with a diameter d, P(d). Vp(d) is traveling velocity of a particle with diameter d. 2.3. Particle sizes Two methods were used to measure the geometric and aerodynamic size distributions of the airborne samples. An ultrasonic sieve with 75 mm intervals for particles between 125 and 500 mm and 10 mm intervals for particles between 10 and 125 mm were used to determine the geometric diameter of the dust samples. A vertical settling aerosol tube (VSAT) method was used to measure the aerodynamic equivalent diameter (Chen and Fryrear, 2001). Grain-size parameters were calculated by the graphic method recommended in Folk (1966). The tested aerodynamic equivalent diameters of airborne particles showed a power function of geometric diameters at the 0.001 significance level, da ¼ 0:41dg0:6
R2 ¼ 0:996
ð3Þ
where da is the aerodynamic equivalent diameter, and dg is the geometric diameter.
3. Results 3.1. Weather conditions Weather conditions during the dust storm are shown in Fig. 2. The air pressure lowered from 1010.4 mb at 5:00 p.m. to 1009.7 mb at 7:00 p.m. Then, it rose continuously to 1013.4 mb at 11:00 p.m. (Fig. 2a). During the dust storm, air temperature rose from 30 C
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Fig. 2. Weather conditions during the dust storm: (a) the air pressure, which was lowest when the front passed over the sampling site; (b) temperature was high, whereas relative humidity was low, when the dust front passed by the sampling site; (c) wind direction veered from 200 to 350; and (d) wind velocities increased sharply when the dust front passed the sampling site.
before 5:00 p.m. to 33 C between 6:00 p.m. and 7:00 p.m. It dropped when the front passed over (Fig. 2b). The dust-laden air mass was drier. The relative humidity decreased from 13% to 14% before 5:00 p.m. to 11% from 6:00 to 7:00 p.m. It increased after 7:10 p.m. (Fig. 2b). Wind directions varied according to the movement of the low-pressure trough. When the front approached Big Spring, the wind direction was SW 200 –260, showing the center of the low pressure being north –east to Big Spring. Then the wind direction changed to NW 290 –320 as the front approached Big Spring. After 7:10 p.m., Table 1 Parameters of the vertical distribution equation of the horizontal mass flux F = a + bZ c
Total flux P20 – 10
a
b
c
R2
Fstat
0.666 0.193
3.14 0.465
1.100 0.992
0.96 0.75
438.46 28.12
F is mass flux of dust particles in the flow layer, g m 2 s 1; Z is height; a, b, and c are constants; R2 is the correlation coefficient; Fstat is the threshold value for F-test; P20 – 10 is the mass flux of particles 20 – 10 mm in diameter.
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Fig. 3. Vertical distributions of the particle concentrations of the total particles, the particles of 20 – 10 mm in diameter, and the particles finer than 10 mm in diameter.
the wind direction was NWN 320– 350 (Fig. 2c). Variations in wind speeds at 1000 cm above the ground are shown in Fig. 2d. It was calm before the dust front approached Big Spring. At the nose of the dust front, the wind speed was 2.4 F 0.22 m s 1 on average. From 6:00 p.m. to 7:00 p.m., the wind speed increased slightly to 3.5 F 1.34 m s 1. It was 11.5 F 2.03 m s 1 when the front passed. Then after the high pressure moved in and occupied the area, wind speeds slowed gradually. 3.2. Mass fluxes and particle concentrations The results of this investigation showed that the suspended dust in the flow layer above the surface decreased by a power function of height. The measured mass fluxes fit a power function at the 0.001 level (Table 1).
Table 2 Parameters in the power equation of concentration of suspended particles C = a + bZ c
Total particles Particles (20 – 10 mm) Particles ( < 10 mm)
a
b
c
R2
Fstat
0.126 0.035 0.010
1.061 0.170 0.027
1.207 1.041 1.009
0.99 0.88 0.51
917.04 66.67 9.94
C is the concentration of dust particles, g m 3; a, b, and c are statistical constants.
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The total horizontal mass flux through the observed 1567 cm surface layer can be obtained by integrating the vertical distribution of mass fluxes (Table 1). The total mass flux was 84,960 kg km 1 h 1 for the 1567 cm flow layer. The total mass flux of particles 20 –10 mm in diameter was 17,790 kg km 1 h 1 (Table 1). Vertical distributions of particle concentrations are shown in Fig. 3 and Table 2. Concentrations of total sediments, particles of 20 –10 mm, and particles finer than 10 mm in diameter decreased with the increase in height by a power law (Table 2). The exponent c in the power relation equation represents the slope of the fitted distribution curves of the
Fig. 4. Histograms of the size distributions of the dust particles. The numbers in the histograms are the sampling heights.
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Fig. 4. (continued ).
concentration of the suspended particles versus height. The finer the particles are, the smaller the slope is. The fine particles were more uniformly distributed in the whole surface layer, which agrees with the sediment diffusion theory (O’Brien, 1933). 3.3. Grain-size distribution Most of the samples showed an unimodal distribution (Fig. 4). The peak diameter was 40 mm in the flow layer below 550 cm; and 20 mm above 550 cm. Table 3 shows the grainsize distribution parameters of all samples. Both the mean and the 95th percentile diameters displayed a natural logarithmic function of height (Fig. 5). P95 ¼ 17:25 1:12 lnZ
R2 ¼ 0:61
ð4Þ
dm ¼ 41:61 2:22 lnZ
R2 ¼ 0:65
ð5Þ
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Table 3 Grain-size parameters of dust particles within a dust-storm on 16 June 1997 at Big Spring, TX Height
dp95 (mm)
dm (mm)
rI (j)
Sk (j)
Kg (j)
41 45 49 53 55 73 93 123 153 193 227 263 303 363 417 467 517 567 613 667 717 767 817 917 1017 1117 1567
12.8 13.7 11.8 12.7 12.4 13.1 12.1 11.1 10.0 12.0 10.3 13.0 13.1 11.5 10.7 11.3 11.7 10.3 10.1 10.1 9.7 8.7 9.2 8.7 8.0 8.8 8.2
33.4 34.7 32.8 33.9 30.7 33.3 30.8 30.5 27.0 28.1 27.9 31.5 31.1 30.2 30.8 29.7 29.3 27.3 27.4 28.9 27.4 23.7 23.4 24.8 24.4 24.1 23.0
0.72 1.08 0.85 0.75 0.62 1.13 0.72 0.76 0.76 0.69 0.78 0.71 0.74 0.78 0.92 0.95 0.83 0.66 0.97 0.81 0.93 0.85 0.80 1.02 1.08 1.46 1.19
0.298 0.104 0.269 0.281 0.102 0.065 0.326 0.150 0.310 0.048 0.080 0.096 0.074 0.087 0.033 0.029 0.027 0.241 0.013 0.007 0.055 0.006 0.106 0.056 0.093 0.332 0.355
0.879 2.144 1.369 1.030 1.127 1.770 1.023 0.933 0.853 0.801 0.974 0.950 1.056 1.074 1.162 1.190 1.139 1.232 1.404 1.496 1.137 1.268 1.558 1.746 1.373 1.244 1.478
dp95 is the 95th percentile particle diameter; dm is the mean diameter; rI is the inclusive graphic standard deviation; Sk is skewness; and Kg is graphic kurtosis.
where dm is the mean diameter. P95 is the 95th percentile diameter. The fitted relationships between particle diameters and height are significant at the 0.01 level. These results differ from other reports (Chepil and Woodruff, 1957; Gillette et al., 1974; Nickling, 1983; Chen and Fryrear, 1996), which show a power function of height. Grain-size distributions of the dust in the flow layer below 417 cm were positively skewed, except one negative sample at 73 cm. Above 417 cm grain-size distribution was negative, except for two positive skewed samples: one at 567 cm, and another at 717 cm (Table 3). Kurtosis values showed mesokurtic (0.9 – 1.1 j) to very leptokurtic (1.5 – 3.0j) distributions. There existed a flow layer from 93 to 263 cm where the grain-size distributed mesokurtically, whereas below and above which they were leptokurtic distribution (Table 3). Most of the dust samples collected from the flow layer below 817 cm were moderately sorted (j < 1), except 2 samples where j > 1. Dust samples collected above 817 cm were moderately to poorly sorted (j >1). Suspended dust becomes increasingly more poorly sorted with the increase in height. This is because all particles became
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Fig. 5. Vertical distributions of the mean and the P95 diameters. Both of them follow a natural logarithmic function of height.
relatively small and tended to be more uniformly mixed by the turbulent eddies. Besides, the development degree of a suspension process also influences the vertical distribution of sorting. The sorting process fluctuates and is affected by turbulent mixing and fluctuation of the air flow. This measurement shows the 0 – 1567 cm surface layer can be divided into three sub-layers. In the first flow layer from the ground surface to 100 cm, sorting became poorer with the increase in height. The second flow layer was 100 – 300 cm where the mean particle size was the smallest and the sorting was the most well sorted in the whole measured surface layer. In the third flow layer above 300 cm, the dust particles became increasingly more poorly sorted with the increase in height. If a higher flow layer could be sampled, it would be another flow layer with well-sorted finer particles.
4. Conclusions The total flux rate for the measured 1567 cm surface layer, in a haboob dust storm of 16 June 1997 over Big Spring, TX, was 84 960 kg km 2 h 1, within which the flux of particles 20 –10 mm in diameter was 17 790 kg km 2 h 1.
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Vertical distributions of dust concentration in a 1567-cm flow-layer decreased with the increase in height by a power function. The maximum dust concentration was 0.0013 kg m 3 at 100 cm above the surface. The fine particles distributed more uniformly in the measured 1567 cm flow layer, whereas the profile of the coarser particles showed a greater gradient. The mean diameters of the dust were between 23 and 35 mm. The P95 diameters were from 8 to 14 mm. Vertical distributions of the mean and the 95th percentile diameters showed a natural logarithmic function with height. Most of the dust samples are unimodal distributed with a peak diameter of 40 mm in the flow layer below 550 cm, above 550 cm the peak diameter decreased to 20 mm. The airborne dust was well to moderately sorted and became less well sorted with the increasing height.
Acknowledgements This work was performed while Weinan Chen held a Senior Resident Research Fellowship with the National Research Council-NOAA (ERL). We thank Dr. Dale A. Gillette, Wang Lu and Gao Shangyu for their help in the work.
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