A new boundary layer mixing scheme. Part II: tests in climate and mesoscale models

A new turbulent mixing scheme, described in Part I of this paper, is tested in the climate and mesoscale configurations of the U.K. Met. Office's Unif ied Model (UM). In climate configuration, the scheme is implemented along w ith increased vertical resolution below 700 hPa (the same as that in the me soscale model), in order to allow the different boundary layer types and pr ocesses to be identified and treated properly. In both configurations, the new boundary layer (PBL-N) mixing scheme produces some improvement over the current boundary layer (PBL-C) scheme. The PBL-N scheme is able to diagnos e different boundary layer types that appear to be consistent with the obse rved conditions, and the boundary layer structure is improved in comparison with observations. In the climate model, the boundary layer and cloud stru cture in the semipermanent stratocumulus regions of the eastern subtropical oceans are noticeably improved with the PBL-N scheme. The deepening and de coupling of the boundary layer toward the trade cumulus regime is also simu lated more realistically. However, the cloud amounts in the stratocumulus r egions, which were underestimated with the PBL-C scheme, are reduced furthe r when the PBL-N scheme is included. Tests of the PBL-N scheme in the UM si ngle-column model and in a development version of the UM, where the dynamic s, time stepping, and vertical grid are different from the standard version . both show that realistic stratocumulus cloud amounts can be achieved. Thu s, it is thought that the performance of the PBL-N scheme in the standard U M may be bring Limited by other aspects of that model. In the mesoscale mod el, improvements in the simulation of a convective case are achieved with t he PBL-N scheme through reductions in layer cloud amount, while the simulat ion of a stratocumulus case is improved through better representation of th e cloud and boundary layer structure. Other mesoscale model case studies sh ow that there is a consistent improvement in fog probabilities and forecast s of cloud-base height. The root-mean-square errors in screen-level tempera ture are also reduced slightly. The weak daytime bias in wind strength is i mproved greatly through a systematic increase In the IO-m wind speed in uns table conditions. As a result of these trials, the scheme has been implemen ted operationally in the mesoscale model.