An Observational Study of Cloud-Topped Mixed Layers

Abstract

The turbulence and mean structure of oceanic stratocumulus was studied using aircraft data collected during the summer of 1976 off the coast of California. Three cloud-topped mixed layers were studied in detail. They consisted of 1) a thin cloud capped by an inversion at a height of ?1000 m, 2) a relatively thick but broken cloud layer capped by a weak inversion at ?600 m and 3) a solid cloud capped by a strong inversion at ?600 m. The mean temperature, moisture, liquid water and radiative characteristics obtained for these three cases were compared. Heat and moisture fluxes were also calculated and compared. Although there was considerable variation in the characteristics of the three cloud-topped mixed layers studied, all indicated the validity of the general approach used in simple mixed-layer models of stratocumulus. But for these models to be useful, they should be generalized to allow clouds other than solid clouds to be modeled. The measurements indicate that cloud microphysics may be important in regulating the structure of stratocumulus clouds and that the cloud structure is important in regulating the distribution of radiative cooling. The thin cloud case satisfied conditions for cloud-top entrainment instability, but no evidence of enhanced entrainment was observed. In the analysis it is shown that this cloud may have had insufficient liquid water available to drive the instability. Restrictions on cloud-top entrainment instability criteria are discussed, and it is shown that the instability may be neither a necessary nor a sufficient condition for the breakup of stratocumulus.