Observations of the solar radiant energy incident upon clouds and emerging from them indicate that clouds absorb more radiation than predicted by scattering theory. A possible explanation of this discrepancy, called anomalous cloud absorption, is that the theory neglects the effects of electrical charges on the surface of the cloud drops. Most drops are in fact charged, the effect of which is to introduce a surface current that is neglected in conventional Mie scattering theory. There is no available theory to account for the effect of the surface current, although approximate calculations suggest that charged drops should absorb more light than electrically neutral drops. This project consists of an experimental and theoretical determination of the extent to which the presence of charge affects the scattering and absorbing properties of drops. Initially the scattering phase function of charged drops will be measured and compared with that of uncharged drops to determine the extent and character of the effect. The dependence on wavelength, drop size, and charge will be assessed. Next, a theoretical analysis will determine empirically the functional form of the surface current needed to explain the observed phase functions. This information will then be used to modify the Mie scattering theory to include a surface current term and enable calculation of the absorbing properties of charged drops. The importance of this work is that it could both provide a partial explanation of anomalous cloud absorption and improve the parameterization of radiative transfer in atmospheric computer models.
