An Analysis of Cloud Drop Growth by Collection: Part IV. A New Parameterization

Abstract

A new parameterization of cloud particle growth combines the effects of accretion and self-collection, and frees the large-hydrometeor water mass and spectral moments to grow independently. The large-hydrometeor water mass grows through accretion in proportion to the cloud water mass. The spectral moments grow through self-collection in proportion to the large-hydrometeor water content. Accretion is also dependent upon the magnitude of the spectral moments and thus an important feedback loop occurs. The spreading or narrowing of the drop spectrum is dependent upon the collection kernel and the relative magnitudes of accretion and self-collection in the region of interest. These effects are included in the parameterization. In order for the spectrum to spread there must exist both the stochastic mode and a sufficiently rapid increase in the collection kernel with drop size.