A Model of Marine Boundary-Layer Cloudiness for Mesoscale Applications

Abstract

A one-dimensional version of a multilevel mesoscale model is used to represent the cloud-topped boundary layer (CTBL). Turbulent exchanges are parameterized with a prognostic equation for the turbulent kinetic energy and an improved length-scale formulation. Furthermore, the scheme is extended to give a statistical description of the subgrid-scale condensation with a one-and-a-half-order closure.Several observed reference cases are simulated in order to test the model against observational data and results obtained with a higher-order turbulence model. The latter one is used as a powerful approach for testing the closure of the second-order moments involved in the partial cloudiness scheme. Two of the reference cases are extracted from stratocumulus (Sc) observations off the coast of the United Kingdom with a purely buoyancy-driven and a purely shear-driven CTBL, respectively. The third experiment tries to reproduce a case of Californian Sc clouds where both turbulent effects are important. Finally, the last numerical experiment concentrates on a cloudiness transition case observed during FIRE (First International Satellite Cloud Climatology Project Regional Experiment) with a series of soundings documenting the cloudiness transition from a solid Sc cloud deck over a partly covered region to a clear-sky region.The model results are shown to be in reasonable agreement with both observational data and numerical outputs from the higher-order turbulence model. Finally, it is shown that the partial cloudiness scheme does not only produce more realistic cloudiness and cloud water content than a simple “all or nothing” condensation scheme, but that it also assures model stability by producing a smooth onset of condensation.