A general hydrodynamic theory for mixed-phase microphysics. Part III: Riming and aggregation

Bohm, J.P., 1992. A general hydrodynamic theory for mixed-phase microphysics. Part III: Riming and aggregation. Atmos. Res., 28: 103-123. A general semi-empirical solution is presented to the efficiencies and kernels for riming and aggregation. The present theory is based on a general parameterization for the shape of hydrometeors and on the theories of boundary-layers in axisymmetric and two-dimensional flow. The results of the present study are in fair agreement with numerical results on the collision efficiencies for riming. The rotatory evasive response of ice crystals is estimated theoretically and is found to have negligible effects on the collision efficiency. Furthermore a new stochastical criterion for the onset of riming is proposed. This onset typically occurs at a crystal size at which the collision efficiency is of the order of 0.1, a result that makes doubtful the common ad-hoc assumption of zero collision efficiency at the onset. The present theory is in fair agreement with field data on the onset of riming. At last theoretical collision efficiencies for aggregation are presented along with a new approach to the problem of the variation in fall speed. It is shown that not only the kernels considering or neglecting this variation are markedly different from each other but also the corresponding collision efficiencies.