EFFECT OF TURBULENT-FLOW STRUCTURES ON SALTATION SAND TRANSPORT IN THE ATMOSPHERIC BOUNDARY-LAYER

The effect of turbulent flow structures on saltation sand transport wa s studied during two convective storms in Niger, West Africa. Continuo us, synchronous measurements of saltation fluxes and turbulent velocit y fluctuations were made with a sampling frequency of 1Hz, The shear s tress production was determined from the vertical and streamwise veloc ity fluctuations. The greatest stress-bearing events were classified a s turbulent structures, with sweep, ejection, inward interaction, and outward interaction described according to the quadrant technique. The classified turbulent structures accounted for 63.5 per cent of the av erage shear stress during the first storm, and 56.0 per cent during th e second storm. The percentage of active time was only 20.6 per cent a nd 15.8 per cent, respectively. High saltation fluxes were associated with sweeps and outward interactions. These two structures contribute positively (sweeps) and negatively (outward interactions) to the shear stress, but have in common that the streamwise velocity component is higher than average. Therefore, the horizontal drag force seems primar ily responsible for saltation sand transport, and not the shear stress . This was also reflected by the low correlation coefficients (r) betw een shear stress and saltation flux (0.12 and 0.14, respectively), whi le the correlation coefficients between the streamwise velocity compon ent and saltation flux were much higher (0.65 and 0.57, respectively). (C) 1998 John Wiley & Sons, Ltd.