Studies of stratocumulus-topped boundary layers using a numerical weather prediction model

Sammanfattning: Persistent stratocumulus clouds are commonly observed in many areas on Earth. In general, stratocumulus clouds cool the Earth-atmosphere system due to their influence on the radiation budget. It is therefore essential to describe such clouds accurately in climate and numerical weather prediction models.This thesis deals with the parameterization of stratocumulus as well as other processes and conditions that control the evolution of such clouds. A parameterization of the in-cloud turbulent fluxes is developed and tested on a simplified stratocumulus case. Typical features of a stratocumulus-topped boundary layer driven by radiative cooling are reproduced; a maximum buoyance flux at the cloud top and a quasi-linear total water flux. It is concluded that the suggested parameterization perfoms well.A numerical weather prediction model, with the suggested parameterization, is used to simulate the ASTEX first Lagrangian. The model performs quite well with improved initial conditions but the deepening of the boundary layer beginning at the middle of the trajectory is not simulated. This is due to negative synoptic-scale vertical velocities that oscillated. These were therefore investigated with a Fast Fourier Technique. It was found that inertia-gravity waves influence the vertical velocities up to a period of 15 h. Frequences less than 15 h were filtered out and the synoptic-scale vertical velocity at the MBL top was estimated to approximately -0.2 cm s-1. It is suggested that non-local production of entrainment included in the parameterization may improve the simulation of the deepening of the boundary layer.An Arctic stratocumulus case is simulated with the same numerical prediction model. Focus of this study was on the surface heat budget and the net long-wave radiation. Two model versions with different turbulence closures, the suggested parameterization and one without cloud-turbulence interaction were run. It was found that the cloud-turbulence interaction is important to simulate the net long-wave radiation. The best simulation of the surface heat budget was accomplished by setting the surface albedo to the observed value (0.8).