A Mass-Flux/K-Diffusion approach to the parameterization of the convective boundary layer: global model results

A more integrated and unified approach is needed to parameterize the convective boundary layer in large scale models. The traditional way of parameterizing turbulent mixing is by using a K-diffusion approach. The shallow convection mixing, on the other hand, is parameterized by using a mass-flux approach. Siebesma and Teixeira (see companion abstract) suggest a way of unifying the parameterization of boundary layer mixing by combining the k-diffusion and the mass-flux approaches.

Behind this approach is the idea that in the convective boundary layer there are two main mechanisms of mixing: "local" mixing that can be parameterized by a diffusion approach and "non-local" mixing performed by the strong thermals that can be parameterized by a mass-flux scheme. These thermals have a relatively small updraft area but can be responsible for a substantial part of the sub-grid transport.

This scheme has the same structure for all types of convective boundary layers and should be able to represent the buoyancy production mechanisms: surface generated buoyancy in the dry convective boundary layer, release of latent heat in shallow convection and cloud top cooling in stratocumulus.

This type of parameterization has been tested in the ECMWF global model. Results produced by the ECMWF model, with the new scheme, are compared with observations for dry convective situations and demonstrate the strength of this parameterization. The approach has also been tested in the Navy's global model (NOGAPS). As a result, the sub-tropical boundary layer structure is simulated in a more realistic way when the new approach is used.