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Rolan D. Christofferson and Dale A. Gillette

Abstract

A method is given to estimate the shape factor (K) of the Weibull distribution directly from wind observations, without iteration, plotting, or sorting of data. The estimate is of comparable accuracy as that given by the maximum-likelihood estimate but is obtained more economically in computer time. Estimation of the scale factor (C) is done by maximum likelihood.

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Dale A. Gillette and Irving H. Blifford Jr.

Abstract

A series of aerosol samples was collected using a three-stage impactor at altitudes to 10 km from locations near Scottsbluff, Neb., Death Valley, Calif., and over the Pacific Ocean about 250 km west of Santa Barbara, Calif. A few samples were obtained from other locations viz., the central Pacific, Venezuela, and Chicago, Ill. Chemical analyses were made by means of x-ray techniques. At the locations for which concentrations near the ground were measured, most average mass concentrations of the elements Cl, S, K, Ca and Ti decreased rapidly from the ground to about half or less the ground concentration at 1 km and then maintained more or less constant values up to 10 km. Low Cl concentrations (except near the ocean surface) suggest a small sea salt contribution at the higher altitudes. The soil component of the aerosol believed to Be indicated by uniform average mass ratios of Ti/SI, K/Si and Ca/Si was quite constant with respect to altitude and location. Ratios of these latter elements were found to be distributed virtually log normally. The distribution of the ratios Cl/Si and S/Si was not consistently log normal, presumably because of contributions from pollution sources of these elements. Averaged concentrations at all altitudes for the locations sampled ranged from 0.18–1.52 µg m−3 for Cl, 0.03–0.31 µg m−3 for S, 0.03–0.60 µg m−3 for K, 0.01–1.27 µg m−3 for Na, 0.04–0.76 µg m−3 for Si, 0.02–2.22 µg m−3 for Ca, and 0.004–0.13 µg m−3 for Ti. Mass median diameters ≲0.3 μm were indicated for most of the determined elements at the altitudes sampled.

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William M. Porch and Dale A. Gillette

Abstract

Fast-response light scattering measurements at two heights during a Texas dust storm are combined with horizontal and vertical wind data to derive and compare aerosol flux estimates using three techniques. The major result of this study is that a relative equivalence exists between the fine-particle (0.1 μm < radius < 1 μm) exchange coefficient and the eddy viscosity of the wind. The data also shed some light on the complex dependence of wind speed threshold for suspension and aerosol flux in high winds for different surface conditions and soil types. These results show the value of the experimental technique to studies of toxic particulate suspension and deposition by wind.

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Troy Leon Holcombe, Trevor Ley, and Dale A. Gillette

Abstract

A better understanding of the effects of precipitation and source area on blowing dust in the Sonoran Desert has been sought through the study of 1190 dust episodes occurring during the 1948–78 time period at Blythe, California, and Yuma, Arizona. Threshold mean hourly wind speeds (MHWSs) increase directly with prior precipitation in proportion to the inhibiting effect of the vegetation canopy, which blooms following periods of increased precipitation. Because of the time required for the vegetation canopy to fully develop and the persistence of the vegetation canopy once developed, correlation between the threshold MHWS and precipitation is highest for 4–6-month windows of total precipitation prior to each dust event at both stations. Many dust events associated with unusually low MHWSs are clustered in time, and these events can be correlated with interstate highway construction and soil preparation for new irrigation projects. Since threshold MHWSs for blowing dust lie well below the recorded MHWSs during most dust events at most times, it is possible to predict that mean annual precipitation could in the future increase to about 8 cm per 6 months without significantly reducing the occurrence of blowing dust episodes. On the other hand, increases in future mean annual precipitation to 10–12 cm per 6 months would raise the threshold MHWS to the point that conditions for blowing dust would be substantially reduced. Many of the infrequently occurring periods of elevated precipitation correlate in time with El Niño–Southern Oscillation events, which typically repeat every 2–8 yr. Average MHWSs, and threshold MHWSs for blowing dust, vary with wind direction at Blythe and Yuma. These variations can be related to variations in the susceptibility of upwind source areas in most instances, but in one or more instances this variation may be related to storm type.

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Dale A. Gillette, Irving H. Blifford Jr., and Charles R. Fenster

Abstract

To assess wind erosion as a source of atmospheric soil particles, vertical aerosol fluxes near the ground in an eroding field were computed by assuming a vertical transport mechanism similar to that for momentum. Aerosol gradients were measured by jet impactors located 1.5 and 6 m above the ground, and wind velocity gradients were measured by totalizing-three-cup anemometers located 1.5, 3 and 6 m above the ground.

Information on the aerosol size distributions and quantity in the size range 0.3 ≤ r ≤ 6 μm was obtained for a variety of erosive conditions in a field in rural Nebraska. In general, the size distributions in this range suggest the power law, dN/d(log r) ∝r −2 for 1 ≤ r ≤ 6 μm, and a flatter curve for 0.3 ≤ r ≤ 6 μm. The relation of the aerosol size distribution in the range 0.3 ≤ r ≤ 6 μm to the size distribution of soil was determined. Averaged soil size distributions characteristic of the sampling field, the area within a six-mile radius of the sampling field, and soil flowing (creeping) at ground level at the sampling site were found to be nearly identical. In addition, the size distribution curves for all three samples showed a general form in the ranges 0.3 ≤ r ≤ 1 μm and 1 ≤ r ≤ 6 μm which could be basically characterized by the same power laws as those suggested by the aerosol size distributions. Horizontal soil particle fluxes were measured and compared to the vertical fluxes. In addition, the measured erosion was compared to erosion predicted by an empirical formulation based on the parameters of soil moisture, field length, vegetative residue, mechanical stability, ridge roughness, and vertical momentum flux. These predictions agreed qualitatively with observed horizontal soil movement and with the observed vertical flux measurements. Measurements of aerosol size distributions from ground level to 11.9 km on two occasions when soil was very moist and no vertical flux of aerosol was observed showed the aerosol size distribution to be constant in shape and in concentration from the ground to the top of the mixing layer. Above the mixing layer, concentration decreased for all particle sizes above the mixing layer up to the base of the tropopause where an increase in concentration for the smallest particles and a decrease in concentration for the largest particles were observed.

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George E. Bowker, Dale A. Gillette, Gilles Bergametti, Béatrice Marticorena, and David K. Heist

Abstract

Within areas of the Chihuahuan Desert dominated by honey mesquite bushes (Prosopis glandulosa), soil erosion causes open eroded patches and the formation of large coppice dunes. The airflow patterns around the dunes and through the open areas are correlated with sand flux and erosion. This study uses wind velocity simulations from the Quick Urban and Industrial Complex (QUIC) model in combination with a sand flux parameterization to simulate sand fluxes for each of eight storms occurring in the springs of 2003 and 2004. Total sand fluxes based on the sum of all the sand collectors located within the study domain were usually within 50% of the measured values for each of the storms, with simulations for individual sand collectors also often within 50% of the measured values. Simulated fluxes based on two different sand flux parameterizations were generally within 10% of each other, differing substantially only when the sand flux was low (near the threshold velocity). Good agreement between the field observations with a Sensit instrument and QUIC simulations for the same location and time series suggests that QUIC could be used to predict the spatial and temporal variation of sand flux patterns for a domain.

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