Abstract
Finite rate diffusion theory is developed to allow for a sedimentation component in the diffusion equations. Theory shows that the condition in which the wave front of the upper boundary of a dispersing cloud moves horizontally is consistent with the ratio v s u ∗ = b (b ⋍ 1.3) in which v s is the settling velocity and u ∗ the friction velocity in the adiabatic surface layer. For the case v s u ∗ > b the plume is forced downwards due to the dominance of sedimentation. In this situation, the distance x max over which the airborne fraction persists is given by x max ⋍ h ln (h/(ez 0))/(k( v s u ∗ −b)) where h is the release height, z 0 the roughness length and k von Karman's constant. Theoretical predictions are shown to be broadly consistent with experimental data for droplets with diameters in the size range of 10–240 μm.
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