Coupled simulations obtained by the UCLA AGCM with a new PBL parameterization and the MIT global OGCM

Parameterization of PBL processes is of crucial importance for the simulation of air-sea interaction in coupled Atmosphere-Ocean models. Especially processes that control fluxes of latent and sensible heat as well as that of momentum at the sea surface are among the most crucial. Also, it has been recognized that PBL cloudiness strongly influences the surface radiative fluxes and hence the predicted SSTs in coupled Atmosphere-Ocean simulations. This work shows coupled simulations with the UCLA AGCM that has a new PBL parameterization and the MIT global OGCM. As in other versions of the UCLA AGCM, the sigma-type vertical coordinate used in the PBL shares a coordinate surface with the free atmosphere at the PBL top. This framework facilitates an explicit representation of the processes concentrated near the PBL top, which is crucial for predicting PBL clouds. In the new PBL 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 winds. Additionally, the physical processes within the PBL are parameterized following a hybrid approach, the effects of convectively active large eddies are represented by a bulk formulation, predicting the bulk turbulence kinetic energy (TKE), and the effects of diffusive small eddies are represented by a K-closure formulation. The surface fluxes are determined from an aerodynamic formula, in which a combination of the square root of the bulk TKE and the grid-scale surface wind are used to represent the velocity scale. With this formulation, the surface fluxes over oceans are expected to be better estimated compared to the traditional methods, since the grid-scale 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. A 25-year simulation is performed. The results show reasonably realistic SST features. For example, the SST difference between western and eastern equatorial Pacific is approximately 4oC, which is typically 5oC in observations, the eastern equatorial Pacific has an asymmetric “cold tongue” with colder temperatures at the South. At the equatorial Atlantic, the East-West SST difference has correct sign, although its magnitude is underestimated. In our simulations a weak double ITCZ is apparent, nevertheless the ITCZ display an asymmetry with respect to the equator. A similar feature is also found in the equatorial Atlantic. The atmospheric general circulation as displayed by the sea level pressure (SLP) and surface wind stress shows realistic features in terms of global pattern and seasonality, although Southern Hemisphere anticyclones and wind stress over the oceans are somewhat weak. The simulated stratus cloud incidence at the eastern oceans is also realistic, and, partially due to that, shortwave heat flux compares reasonably well to those obtained from observational analyses. Interannual variability at the equatorial Pacific viewed in a Hovmuller diagram shows ENSO like anomalies. As in the observations, the maximum and minimum of the anomalies appear at the eastern Pacific during Northern Hemisphere winter. The simulated ENSO like signal in the 25 years run has a period between 3 to 7 years and amplitude of 2 to 3 oC.