Turbulence-driven saltation in the atmospheric surface layer

The relations between the turbulent structures that contribute to the Reynolds shear stress and the transport of saltating grains and between the wind speed and the saltation are studied. The saltation of sand particles and the turbulent flow field over a beach surface in the Western Baltic Sea are measured with a saltiphone, a sonic anemometer and cup anemometers. Continuous, synchronous measurements of horizontal sand transport at 0.1 m above ground and turbulent velocity fluctuations at 0.55 m were made with a sampling frequency of 1 Hz and 10 Hz, respectively. The stress is considered to be resulting from various turbulent structures including sweeps, ejections, inward interactions and outward interactions using the quadrant technique. The instantaneous horizontal drag force on grains, which initiates saltation, seems to be well represented by the instantaneous horizontal wind measured well above the surface. Results indicate an almost instantaneous response of transport to instantaneous horizontal wind velocity. A new method to establish the threshold friction velocity for intermittent aeolian sand transport is introduced. The observed threshold friction velocity varied between 0.28 ms(-1) to 0.36 ms(-1).