PBL Stratiform Cloud Inhomogeneities Thermally Induced by the Orography: A Parameterization for Climate Models

Abstract

This paper proposes a parameterization for use in climate models of the orographic variance and associated thermal circulation's impact on the macroscopic behavior of planetary boundary layer (PBL) stratiform clouds. Orographically induced inhomogeneities in the PBL cloud field can significantly alter the area-averaged cloud-radiative properties and consequently the PBL evolution. Current climate models do not include this effect. The design of the parameterization is based on a systematic set of experiments in which a cloud system resolving model (CSRM) explicitly simulates the diurnally varying interactions between PBL clouds and the orography for a variety of configurations. Analyses of the results suggest that the orographic effect on PBL clouds is parameterizable to a large extent due to a quasi equilibrium between cloud-radiative, turbulent, and mesoscale processes. Parameterizability holds in the range of parameters for which stratiform clouds play a dominant role in the PBL dynamics (cloud fraction ?0.5 or larger). A statistical parameterization of the liquid water path (LWP) spatial distribution in the PBL is formulated as a function of its mean state. The component of the LWP distribution not captured by the parameterization is mostly associated with a hysteresis effect in the PBL height and decreases with increasing orographic variance. The LWP distribution is characterized by an exponentially decaying function that describes the patchy clouds and a bell-shaped function that describes the solid cloud decks. Orographic variance impacts cloud fraction and the relative area cover of the patchy and solid clouds, mainly by reducing the incidence of the latter, while the characteristic horizontal scale of the orography has little influence on PBL cloudiness.