Anisotropic diffusion coefficients and the global spread of volcanic dust

Abstract

The global spread of volcanic dust from the Mt. Agung eruption of March 17, 1963, is assessed on the basis of the decrement in direct solar radiation. The data suggest the establishment of an equatorial reservoir at height of about 22 km, followed by poleward transport into each hemisphere during winter. The poleward-moving waves of dust appeared to have constant amplitude in any one season, with a poleward progression rate of 40 cm sec−1 between 30° and 90° latitude. Analytic solutions are found for the diffusion coefficients Kyy and Kyz for the following cases: (1) 〈ν〉 = 0, i.e., no mean motion. (2) 〈ν〉 = 20 sin 6ϕ, corresponding to a three-cell pattern for the meridional circulation. (3) Same as (2) but with assumption of also latitude attenuation, in an attempt to allow for a stratospheric source of dust particles from gaseous products of the volcano. The results for Kyy are in general agreement with findings of other workers, but the values of Kyz appear to be too high, particularly in the region 30°–60° latitude. It is speculated that better agreement could be found if the diffusion coefficients were chosen to vary seasonally.