The increase of soil mass flux with distance downwind, the fetch effect for wind erosion, has been observed and reported on since 1939. This model incorporates the following three mechanisms. (1) The 'avalanching' mechanism in which one particle moving downwind would dislodge one or more particles upon impact with the surface. The result of a chain of such events is an increase of mass flux with distance. (2) The 'aerodynamic feedback' effect, suggested by P. R. Owen, in which the aerodynamic roughness height is increased by saltation of particles; the resulting increased momentum flux increases saltation. These increases define a positive feedback loop with respect to distance downwind. (3) The 'soil resistance' mechanism, which is largely an expression of the change with distance of threshold velocity. Change of threshold velocities may be caused by inhomogeneities of the soil or progressive destruction of aggregates and crust in the direction of saltation fetch.An experiment was run in March 1993 at Owens Lake to test this model. Detailed measurements of wind profiles and mass fluxes were taken on a line parallel to the wind direction. These data support the proposed three-mechanism model. sublimation of snow particles decreases the transported snow (Pomeroy et al., 1993). Gregory and Borrelli (1986) expressed the increase of flux as an exponential increase using dimensional analysis to predict soil mass detached by airflow. Stout (1990) derived a similar semi-empirical expression for exponential increase of soil flux, where f is mass flux of soil particles at a given height z and downward distance x, f , , , is the maximum of that flux, and b is a function only of z.The relationship betweenf(x, z ) and q(x) iswhere H i s the top of the particle-containing layer. Stout derived an expression for b by rewriting the equation of mass conservation for sand by assuming that the first derivative divided by the second derivative of horizontal sand flux with respect to fetch distance is a function only of height. The variable b(z) was interpreted by Stout (1990) as an entrainment coefficient for loose saltation-size material. Since b has the units of length, it is also interpretable as the distance at which the flux reaches 63 per cent (1 -e-') of its maximum. The exponential form fitted rather well extensive data for the increase of sand flux with fetch for a circular sandy farm field in Big Spring, Texas; however, the values of b changed with height and with individual storms. The value of b was typically tens of metres to lOOm for homogeneous sand at Big Spring (J. E. Stout, U.S. Dept. of Agriculture, Agricultural Research Service, pers. comm., 1993). Shao and Raupach (1992) also found variation of q with downwind distance in a wind tunnel and successfully modelled it using the model of Anderson and Haff (1991). This model showed that a fetch of several metres was required for q to come to an equilibrium value. The scale of this effect for the wind tunnel work of Shao and Raupach was of the order of metres for homog...

Past research has shown that the most important areas for active sand movement in the northern part of the Chihuahuan Desert are mesquite-dominated desert ecosystems possessing sandy soil texture. The most active sand movement in the mesquite-dominated ecosystems has been shown to take place on elongated bare soil patches referred to as "streets". Aerodynamic properties of mesquite streets eroded by wind should be included in explaining how mesquite streets are more emissive sand sources than surrounding desert land. To understand the effects of wind properties, we measured them at two flat mesquite sites having highly similar soil textures but very different configurations of mesquite. The differences in wind properties at the two sites were caused by differences of size, orientation, and porosity of the mesquite, along with the presence of mesquite coppice dunes (sand dunes stabilized by mesquites growing in the dune and on its surface) found only at one of the two sites. Wind direction, u * (friction velocity), z 0 (aerodynamic roughness height) and D (zero plane displacement height) were estimated for 15-m tower and 3-m mast data. These aerodynamic data allowed us to distinguish five categories with differing potentials for sediment transport. Sediment transport for the five categories varied from unrestricted, free transport to virtually no transport caused by vegetation protection from wind forces. In addition, "steering" of winds below the level of the tops of mesquite bushes and coppice dunes allowed longer parallel wind durations and increased wind erosion for streets that aligned roughly SW-NE.

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The second Arctic Gas and Aerosol Sampling Program (AGASP-II) was conducted across the Alaskan and Canadian Arctic in April 1986, to study the in situ aerosol, and the chemical and optical properties of Arctic haze. The NOAA WP-3D aircraft, with special instrumentation added, made six flights during AGASP-II. Measurements of wind, pressure, temperature, ozone, water vapor, condensation nuclei (CN) concentration, and aerosol scattering extinction (b~,) were used to determine the location of significant haze layers. The measurements rhade on the ftrst three flights, over the Arctic Ocean north of Barrow and over the Beaufort Sea north of Barter Island, Alaska are discussed in detail in this report of the first phase of AGASP II. In the Alaskan Arctic the WP-3D detected a large and persistent region of haze between 960 and 750 mb, in a thermally stable layer, on 2, 8, and 9 April 1986. At its most dense, the haze contained CN concentrations >10,000 cm -3 and bsp of 80 × 10 -6 m -1 suggesting active SO2 to H2SO4 gas-to-particle conversion. Calculations based upon observed SO2 concentrations and ambient relative humidities suggest that 104-105 small H2SO4 droplets 18 G.A. HERBERT ET AL.could have been produced in the haze layers. High concentrations of sub-micron H2SO 4 droplets were collected in haze. Ozone concentrations were 5-10 ppb higher in the haze layers than in the surrounding troposphere.Outside the regions of haze, CN concentrations ranged from 100 to 400 cm -3 and bsp values were about (20-40) × 10 -6 m-1. Air mass trajectories were computed to depict the air flow upwind of regions in which haze was observed. In two cases the back trajectories and ground measurements suggested the source to be in central Europe.

Three long‐range trajectory models which have been applied to Arctic haze problems were compared. These models employ different parameterizations of vertical motions, and are based on different meteorological analyses. Median horizontal displacements between 5‐day trajectory endpoints were mostly in the 800–1000 km range. Model sensitivity to the meteorological data was shown to contribute a substantial component to the overall uncertainty. Based on these comparisons, we estimate that the usefulness of trajectory models after 5 days is limited to the identification of distant source regions with horizontal dimensions on the order of 1000 km.