Theoretical investigation of mixing in warm clouds - Part 2: Homogeneous mixing

Abstract

The evolution of monodisperse and polydisperse droplet size distributions (DSDs) during homogeneous mixing is analyzed. Time-dependent universal analytical relations of supersaturation and liquid water content, which depend on a sole non-dimensional parameter, are obtained for a monodisperse DSD. The evolution of moments and moment-relation functions in the course of the homogeneous evaporation of polydisperse DSDs is analyzed using a parcel model. It is shown that the classic conceptual scheme, according to which homogeneous mixing leads to a decrease in the droplet mass under constant droplet concentration, is valid only in cases of monodisperse or initially very narrow polydisperse DSDs. In cases of wide polydisperse DSDs, mixing and successive evaporation lead to a decrease of both mass and concentration such that the characteristic droplet sizes remain nearly constant. As this feature is typically associated with inhomogeneous mixing, we conclude that in cases of an initially wide DSD at cloud top, homogeneous mixing is nearly indistinguishable from inhomogeneous mixing.