Development and implementation of a turbulence-based parametrization of moist convection

The Met Office Unified Model(MetUM), as with the majority of GCMs, currently uses a mass flux scheme to represent sub-gridscale moist convective processes. The scheme was originally developed by Gregory and Rowntree and has subsequently received a number of enhancements to improve performance. In parallel with the ongoing incremental improvements to the mass flux scheme, we have been developing an alternative approach to convection parametrization using a turbulence-based method, using ideas similar to those commonly seen in the parametrization of the atmospheric boundary layer. In essence this approach represents the subgrid fluxes of heat and moisture by a local diffusive down-gradient term and a non-local component.

The work has followed two main strands; a parametrization which represents deep convection and one which represents warm non-glaciated convective clouds ranging from non-precipitating shallow clouds to deeper warm congestus clouds. While there is much in common between these strands, there are obvious differences in the microphysical processes which need to be represented and also, to a certain extent, the philosophical issues surrounding the basic approach and its application.

Both the deep and warm components of the scheme have been implemented in the MetUM and, along with a summary of the theoretical aspects of this work, examples of the application of these schemes will be presented in the context of both climate and NWP simulations.