Numerical modelling of mixed-phase frontal clouds observed during the CWVC project

The Met Office's cloud-resolving model was used to model mixed-phase frontal Clouds observed as part of the Clouds, Water Vapour and Climate programme between 1999 and 2001. The clouds were studied using the Met Office's C130 aircraft and the Chilbolton radar facility. In the model, precipitation forms in updraughts then falls, giving rise to regions of high radar reflectivity similar to those observed. The vertical motions were driven by conditional instability or shear instability. The sensitivity of the model results to the microphysics was examined by varying the number of primary ice nuclei and switching off the Hallett-Mossop process. It is demonstrated that the ice nucleation processes, both primary nucleation and secondary ice particle production, are key to the precipitation production in the cloud. This occurs both through the detail of cloud microphysics and the release of latent heat of fusion in the cloud. This is very sensitive to the details of the microphysics and can markedly change the cloud dynamics. In both cases it seems that the cloud dynamics can be simulated locally without considering the large-scale forcing.