A double-moment parameterization for simulating autoconversion, accretion and selfcollection

A double-moment parameterization of microphysical processes in warm clouds is derived directly from the stochastic collection equation. Explicit rate equations for autoconversion, accretion and selfcollection are formulated using Long's piecewise polynomial collection kernel and universal functions following from a fundamental similarity relationship. These universal functions are estimated by numerically solving the stochastic collection equation. A comparison of results of the new parameterization and other double-moment parameterizations is given and the detailed spectral approach is used as a reference method. As an idealized test problem a one-dimensional rainshaft model is applied. The new parameterization is able to reproduce the results of the spectral reference model within a wide range of initial conditions, while other parameterizations show large errors when assuming continental clouds with small mean radii. (C) 2001 Elsevier Science B.V. All rights reserved.