Evaluation of a new PBL parameterization with emphasis in surface fluxes

Parameterization of PBL processes is of crucial importance for the simulation of air-sea interaction with coupled Atmosphere-Ocean system models. Fluxes of latent and sensible heat as well as that of momentum at the sea surface are among those processes. Also, it has been recognized that PBL cloudiness strongly influences the surface radiative fluxes and hence the predicted SSTs in coupled Atmosphere-Ocean simulations.

In this paper, the physical processes within the PBL are parameterized following a hybrid approach. To represent the effects of convectively active large eddies we use a bulk formulation, which predicts the turbulence kinetic energy vertically averaged over the PBL (TKE). The surface fluxes are determined from an aerodynamic formula, in which both the square root of the bulk TKE and the mean large-scale PBL velocity are used to determine the velocity scale. With this formulation, the surface fluxes are expected to be better estimated compared to the traditional methods, since the mean wind can be weak while the convective mixing is strong. The PBL-top mass entrainment is explicitly computed with a formulation that also uses the bulk TKE. To represent the effects of diffusive small eddies, on the other hand, we use a K-closure formulation, in which the square root of the bulk TKE is considered in determining the K coefficient.

PBL processes formulated as above have been implemented into a version of the UCLA AGCM. As in other versions of the UCLA AGCM, the sigma-type vertical coordinate defines a coordinate surface at the PBL-top. This framework facilitates the explicit representation of the processes concentrated near the PBL top, which is crucial for predicting PBL clouds. In the new parameterization, we introduce multiple layers between the PBL top and the earth surface, thus allowing for explicit prediction of the vertical profiles of potential temperature, water mixing ratio and horizontal wind vectors.

Simulated results with the UCLA AGCM using the new PBL parameterization described above will be presented at the conference. Preliminary results show realistic simulations of PBL cloudiness and surface fluxes. A discussion of the AGCM performance when it is coupled to the POP Ocean GCM will also be presented.