RELATION BETWEEN MEAN BOUNDARY-LAYER STRUCTURE AND CLOUDINESS AT THE R V VALDIVIA DURING ASTEX/

The relationship between boundary-layer thermodynamic structure and cl oud fields and their diurnal variation are explored using seven days o f data from the R/V Valdivia during the Atlantic Stratocumulus Transit ion Experiment. Cloudiness is at a maximum before dawn, when the bound ary layer (BL) has the thermodynamic structure of a partially mixed, c onditionally unstable stratocumulus layer, which is dose to mean satur ation below the inversion. Cloudiness falls during the daytime, and in the late afternoon the BL has two distinct layers: a warmer, drier cl oud layer (characteristic of trade cumulus) above a more well-mixed su bcloud layer. The observed mean profiles are consistent with an earlie r suggestion that there is a cloud-cover transition once the BL mixing -line slope exceeds half that of the moist adiabat. In contrast, the B L structure in the ECMWF model for the same week has a much drier, war mer, more stable ''cloud'' layer than the observations.