ABSTRACT
After the sand grains have left the surface they are accelerated by the wind. This process has been modeled by several authors. Here we will use an approximate model of the grain trajectories that is explained in detail in Sørensen (1991) and which gives results that are comparable to those of numerical models (e.g. Anderson & Haff, 1991; McEwan & Willetts, 1991). In order to derive the explicit expression given below for the mean horizontal grain speed as a function of height, further simplifications are made. The details can be found in Rasmussen & Sørensen (2005, in prep.). Here we will just briefly mention that it is assumed that the vertical component of the launch velocity is exponentially distributed with mean value λ−1, and that the conditional expectation of the horizontal component of the launch velocity given that the vertical component has the value v0,2 equals κv0,2. Here κ= cot (θ), where θ is the mean launch angle of the grains. In accordance with results in White & Schulz (1977), Nalpanis (1985) & Sørensen (1985), θ is taken to be 50 degrees.
