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