On Mixed-Layer Modeling of the Stratocumulus-Topped Marine Boundary Layer

Howard P. Hanson Cooperative Institute for Research in Environmental Sciences, University of Colorado. Boulder, CO 80309

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Abstract

Research aircraft measurements of a well-developed marine stratocumulus cloud-topped boundary layer, made in June 1981 off the coast of California, are analyzed using the saturation point method developed by Betts. Estimates of the cloud-top entrainment rate made from the measurements permit construction of a mixing diagram in which the physics of the layer collapse to a single mixing line when diabatic processes and nonstationarity are accounted for. This is possible because the vertically-integrated (mixed-layer) budget equations balance to within measurement uncertainty.

The mixing diagram allows calculation of cloud-top entrainment from the geometry of surface, mixed-layer and above-inversion parcel saturation points (corrected for diabatic processes) and the cloud-top cooling rate. This method, basically an inversion of the thermodynamic budgets, can also be used to calculate surface fluxes. It should be adaptable to routine meteorological data.

While no insight is given into model parameterization of entrainment, it is concluded that, for stratocumulus layers such as the one measured in the data presented here, a mixed-layer model is likely to adequately represent the thermodynamic interactions. However, the data also indicate that the criterion for breakup of a stratocumulus deck used in such models has not been adequately developed.

Abstract

Research aircraft measurements of a well-developed marine stratocumulus cloud-topped boundary layer, made in June 1981 off the coast of California, are analyzed using the saturation point method developed by Betts. Estimates of the cloud-top entrainment rate made from the measurements permit construction of a mixing diagram in which the physics of the layer collapse to a single mixing line when diabatic processes and nonstationarity are accounted for. This is possible because the vertically-integrated (mixed-layer) budget equations balance to within measurement uncertainty.

The mixing diagram allows calculation of cloud-top entrainment from the geometry of surface, mixed-layer and above-inversion parcel saturation points (corrected for diabatic processes) and the cloud-top cooling rate. This method, basically an inversion of the thermodynamic budgets, can also be used to calculate surface fluxes. It should be adaptable to routine meteorological data.

While no insight is given into model parameterization of entrainment, it is concluded that, for stratocumulus layers such as the one measured in the data presented here, a mixed-layer model is likely to adequately represent the thermodynamic interactions. However, the data also indicate that the criterion for breakup of a stratocumulus deck used in such models has not been adequately developed.

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