6533b82dfe1ef96bd1291eb3

RESEARCH PRODUCT

Formulation and test of an ice aggregation scheme for two-moment bulk microphysics schemes

subject

description

A simple formulation of aggregation for 2-moment bulk microphysical models is de-rived. The solution involves the evaluation of a double integral of the collection kernelweighted with the crystal size (or mass) distribution. This quantity is to be inserted intothe differential equation for the crystal number concentration which has classical form. The double integrals are evaluated numerically for log-normal size distributions overa large range of geometric mean masses. A polynomial fit of the results is given thatyields good accuracy. Various tests of the new parameterization are described: aggre-gation as stand-alone process, in a box-model, and in 2-D simulations of a cirrostratuscloud. These tests suggest that aggregation can become important for warm cirrus, leading even to higher and longer-lasting in-cloud supersaturation. Cold cirrus cloudsare hardly affected by aggregation. The collection efficiency is taken from a parameter-ization that assumes a dependence on temperature, a situation that might be improvedwhen reliable measurements from cloud chambers suggests the necessary constraintsfor the choice of this parameter.