Sensitivity of extratropical cyclone mesoscale structure to the parametrization of ice microphysical processes

The ability of numerical weather prediction models to represent and forecast mesoscale structure accurately in extratropical cyclones depends on the resolution and dynamical formulation of the model and the way in which the subgrid-scale processes are parametrized. This study investigates the sensitivity of extratropical cyclone forecasts to aspects of the microphysical parametrization in a mesoscale version of the Met Office Unified Model. Case-studies of rapidly developing cyclones over the North Atlantic Ocean from the Fronts and Atlantic Storm-Track Experiment period are used to study the forecast sensitivity to the ice sublimation and deposition rate, and the ice terminal fall speed. Modifying these parameters not only changes the balance between ice content, liquid-water content and water vapour in the model but also affects the mesoscale dynamics of the frontal rain bands associated with the cyclone. In particular, the cooling due to sublimating ice beneath slantwise ascending frontal updraughts has a significant effect on the dynamics of the front and on the development of further post-frontal rain bands. The study highlights the need for a microphysical parametrization that produces an appropriate cooling profile from sublimating ice particles for the forecasting of mesoscale structure in extratropical cyclones.