Numerical models of diffusion and rainout of stratospheric radioactive materials

Abstract

A numerical model in two dimensions capable of simulating diffusion, transport, particle fall velocity and tropospheric rainout is described. The model incorporates a general anisotropic diffusion process. Rainout is simulated by fractional removal of debris at selected rain “formation” levels; the fraction removed at any latitude is proportional to the observed rainfall at that latitude. In the present paper the transport terms are neglected and an attempt is made to reproduce available data with a diffusion—rainout—settling velocity model. The parameters of the model are varied in an attempt to reproduce quantitatively the known behavior of tungsten-185 injections in equatorial regions and, at least qualitatively, the behavior of high latitude stratospheric injections. Three types of models are described; they are characterized respectively by the principal diffusion axis being (a) along a horizontal surface, (b) along surfaces of constant potential temperature, and (c) along surfaces of constant potential vorticity. All three types can reproduce the latitudinal variation of rainout in a satisfactory manner, but only the third can reproduce the observed pattern of concentration in the stratosphere. The third type yields satisfactory results using lines of constant potential vorticity (only tentatively established by Hering and Borden) or using lines parallel to the tropopause as the principal axis of diffusion.