Relation between Mean Boundary-Layer Structure and Cloudiness at the R/V Valdivia during ASTEX

Abstract

The relationship between boundary-layer thermodynamic structure and cloud fields and their diurnal variation are explored using seven days of data from dw R/V Valdivia during the Atlantic Stratocumulus Transition Experiment. Cloudiness is at a maximum before dawn, when the boundary layer (BL) has the thermodynamic structure of a partially mixed, conditionally unstable stratocumulus layer, which is close to mean saturation below the inversion. Cloudiness falls during the daytime, and in the late afternoon the BL has two distinct layers: a warmer, drier cloud layer (characteristic of trade cumulus) above a more well-mixed subcloud layer. The observed mean profiles are consistent with an earlier suggestion that there is a cloud-cover transition once the BL mixing-line slope exceeds half that of the moist adiabat. In contrast, the BL structure in the ECMWF model for the same week has a much drier, warmer, more stable ?cloud? layer than the observations.