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Howard P. Hanson

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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Howard P. Hanson

Abstract

Mixing processes associated with tropical marine boundary-layer cumuli are examined with a one-dimensional, steady-state model of the cloud and inversion layers which explicitly differentiates between cloud-scale and subcloud-scale motions. Physically, the clouds are modeled as an ensemble average of well-mixed turbulent bursts into an otherwise quiescent, subsiding environment. The cloud-base vertical velocity distribution is found from a new closure based on a simplified vertical momentum budget. The clouds are described by two free parameters, fractional area and a detrainment time scale.

Faster detrainment rates are shown to be associated with undiluted clouds, in the sense that the cloud entrainment rate becomes zero. For this case, the cloud-scale fluxes dominate the boundary-layer mixing. Conversely, slower detrainment and large entrainment imply a diluted cloud, with the boundary-layer mixing dominated by the subcloud-scale turbulence.

Simple qualitative arguments show that the vertically averaged temperature perturbation of the clouds is very small and that increases in cloud-base moist static energy fluxes lead to enhancement of the cloud-top entrainment instability and vertical mixing associated with increasingly larger scales.

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Howard P. Hanson

Abstract

The entrainment closure for the turbulence energy budgetcommonly used in unsaturated mixed layer models is re-derivedfrom the perspective of potential energy changes, and it isconcluded that the correct physical interpretation of theenergetics based on experimental evidence is that the turbulenceform entrainment at about the 20% efficiency level(not the 4% level). In the case of a cloud-toppedmixed layer, this physical interpretation leads to are-examination of the condition for stratocumulus cloud-topentrainment instability and the conclusion that recentcorrections for liquid water loading may yet be toorestrictive.

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Howard P. Hanson

Abstract

The differential absorption and emission of radiation with height inside clouds creates sources and sinks of buoyancy and thus can be an important factor in the turbulence-maintaining and dissipating processes of the clouds. This paper is concerned with the roles that solar and infrared radiation play in the turbulence budget of layer clouds, with primary emphasis on marine stratocumulus and inferential discussion of other layer cloud systems.

Physically realistic parameterizations of solar and infrared (IR) fluxes are used to show how the turbulence generation by cloud-top IR cooling can be more than offset by stabilization due to absorption of sunlight, and how the role of cloud-base IR warming depends crucially on the height of the cloud base. In the context of a mixed-layer model, these effects can be cast entirely in terms of the height of the layer's center of mass relative to the net heating and/or cooling due to the radiative transfer. Implications for the diurnal cycle and for a thin-cloud instability are discussed.

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Howard P. Hanson
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Patricia L. Gruber

Abstract

The mixed-layer stratocumulus model first developed by Lilly is extended to include liquid-water-dependent solar optical properties and infrared radiative fluxes. The ocean-surface heat budget under these clouds is discussed as a function of ocean temperature, wind speed and large-scale divergence.

Comparison of diurnally-varying solar forcing with daily-averaged forcing indicates the importance of the nonlinear effect of the clouds becoming thin during mid-day, when the sun is strongest. Absorption of solar energy by the cloud is responsible for this: it tends to cut off turbulent entrainment, and the cloud top becomes lower; it heats the layer, and the cloud base rises.

The ocean-surface heat budget is generally negative (oceanic heating) under these clouds, and tends to become positive as the ocean temperature is raised. The climatic implications of this negative feedback, and a similar feedback at the cloud-top level are discussed.

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