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Elements of the Microphysical Structure of Numerically Simulated Nonprecipitating Stratocumulus

Bjorn StevensDepartment of Atmospheric Science, Colorado State University, Fort Collins, Colorado

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Graham FeingoldCooperative Institute for Research in the Atmosphere, Colorado State University, Fort Collins, Colorado

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William R. CottonDepartment of Atmospheric Science, Colorado State University, Fort Collins, Colorado

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Robert L. WalkoDepartment of Atmospheric Science, Colorado State University, Fort Collins, Colorado

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Abstract

A set of 500 simulated trajectories and a simple parcel model are used to (i) evaluate the performance of a large eddy simulation model coupled to a detailed representation of the droplet spectrum (the LES-BM model) and (ii) gain insight into the microphysical structure of numerically simulated nonprecipitating stratocumulus. The LES-BM model reasonably reproduces many observed features of stratocumulus. The largest sources of error appear to be associated with limited vertical resolution, the neglect of gas kinetic effects and the inability of the model to properly represent mixing across cloud interfacial boundaries. The first two problems have simple remedies; for instance, a condensation–nucleation scheme is derived that includes gas–kinetic effects thus obviating the second problem. The third source of error poses a more vexing, and as yet unsolved, problem for models of the class described herein.

Trajectories timescales are analyzed and in-cloud residence times are found to be, in the mean, on the order of the large eddy turnover time. In addition, it is shown that the length of time trajectories spend near cloud top may be an important factor in the droplet growth equation for a certain favored subset of trajectories. An analysis of the adiabatic trajectory data also indicates that (i) values of diameter dispersion are a factor of 2 to 5 smaller than commonly observed; (ii) simulated values of the dispersion in number concentration are found to be explainable solely on the basis of trajectories having different updraft velocities; (iii) diameter dispersions are not found to be equal to a third of number dispersions, nor did they relate simply to the dispersion in the cloud-base updraft velocity.

Problems with coupling one- and two-dimensional models to detailed representations of the droplet spectrum are discussed. In the case of the former, the lack of an explicit representation of turbulent eddies requires that the coupling between the microphysics and the dynamics be parameterized. In the case of the latter, boundary layer eddies are represented, thus allowing for a more reasonable coupling between turbulence and microphysics. However, the resolved eddies have a different structure than their three-dimensional counterparts, one consequence of which is that timescales of in-cloud circulations are found to be shorter and have less variability.

Abstract

A set of 500 simulated trajectories and a simple parcel model are used to (i) evaluate the performance of a large eddy simulation model coupled to a detailed representation of the droplet spectrum (the LES-BM model) and (ii) gain insight into the microphysical structure of numerically simulated nonprecipitating stratocumulus. The LES-BM model reasonably reproduces many observed features of stratocumulus. The largest sources of error appear to be associated with limited vertical resolution, the neglect of gas kinetic effects and the inability of the model to properly represent mixing across cloud interfacial boundaries. The first two problems have simple remedies; for instance, a condensation–nucleation scheme is derived that includes gas–kinetic effects thus obviating the second problem. The third source of error poses a more vexing, and as yet unsolved, problem for models of the class described herein.

Trajectories timescales are analyzed and in-cloud residence times are found to be, in the mean, on the order of the large eddy turnover time. In addition, it is shown that the length of time trajectories spend near cloud top may be an important factor in the droplet growth equation for a certain favored subset of trajectories. An analysis of the adiabatic trajectory data also indicates that (i) values of diameter dispersion are a factor of 2 to 5 smaller than commonly observed; (ii) simulated values of the dispersion in number concentration are found to be explainable solely on the basis of trajectories having different updraft velocities; (iii) diameter dispersions are not found to be equal to a third of number dispersions, nor did they relate simply to the dispersion in the cloud-base updraft velocity.

Problems with coupling one- and two-dimensional models to detailed representations of the droplet spectrum are discussed. In the case of the former, the lack of an explicit representation of turbulent eddies requires that the coupling between the microphysics and the dynamics be parameterized. In the case of the latter, boundary layer eddies are represented, thus allowing for a more reasonable coupling between turbulence and microphysics. However, the resolved eddies have a different structure than their three-dimensional counterparts, one consequence of which is that timescales of in-cloud circulations are found to be shorter and have less variability.

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