A Cloudiness Transition in a Marine Boundary Layer

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Abstract
In situ aircraft data and lidar data are used to analyze a transition in the boundary layer thermodynamic structure from a clear boundary layer through small cumulus and broken stratocumulus to a deck of solid stratocumulus. The data was collected in conjunction with a Landsat overpass on 7 July 1987 off the coast of southern California. A steady progression in mixing line stability is seen associated with the change in cloudiness. The (empirically based) stability threshold for the breakup of this stratocumulus is that the slope of the mixing line is 0.66 ± 0.04 of the slope of the wet virtual adiabat (the stability threshold for cloud-top entrainment instability). We propose a simple linear parameterization for cloud fraction in terms of mixing line stability. Surface flux measurements are consistent with bulk aerodynamic estimates. We present mean profiles for the four cloudiness regimes for further analysis.