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  • Author or Editor: M. Khairoutdinov x
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Y. L. Kogan, M. P. Khairoutdinov, D. K. Lilly, Z. N. Kogan, and Qingfu Liu

Abstract

A new large eddy simulation (LES) stratocumulus cloud model with an explicit formulation of micro-physical processes has been developed, and the results from three large eddy simulations are presented to illustrate the effects of the stratocumulus-topped boundary layer (STBL) dynamics on cloud microphysical parameters. The simulations represent cases of a well-mixed and a radiatively driven STBL. Two of the simulations differ only in the ambient aerosol concentration and show its effect on cloud microphysics. The third simulation is based on the data obtained by Nicholls, and the simulation results from this case are contrasted with his measurements. Cloud-layer dynamical parameters and cloud droplet spectra are in reasonably good agreement with observations.

As demonstrated by the results of three large eddy simulations presented in the paper, the cloud microphysical parameters are significantly affected by cloud dynamics. This is evidenced by the sensitivity of the cloud drop spectra itself, as well as by that of the integral parameters of the spectra, such as mean radii and droplet concentration. Experiments presented here also show that cloud microstructure is significantly asymmetric between updrafts and downdrafts. Mixing with dry air from the inversion may significantly enhance evaporation and result in cloud-free zones within the cloud. As a result of mixing, the cloud layer is very inhomogeneous, especially near its top and bottom.

The authors analyze in detail the fine structure of the supersaturation field and suggest an explanation for the formation of the model-predicted supersaturation peak near the cloud top. The LES results suggest that super-saturation may have a sharp increase in near-saturated parcels that undergo forced vertical displacement at the cloud-layer top. The main forcing mechanism that may supply the additional energy for the forced convection in the case presented is from propagating gravity waves. Although radiative cooling may also result in increased convective activity at cloud top, the sensitivity tests presented here suggest that, at least in these simulations, this effect is not dominant.

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C.-H. Moeng, P. P. Sullivan, M. F. Khairoutdinov, and D. A. Randall

Abstract

A large-domain large-eddy simulation of a tropical deep convection system is used as a benchmark to derive and test a mixed subgrid-scale (SGS) scheme for scalar and momentum fluxes in cloud-resolving models (CRMs). The benchmark simulation resolves a broad range of scales ranging from mesoscale organizations, through gravity waves and individual clouds, down to energy-containing turbulent eddies. A spectral analysis shows that the vertical-velocity kinetic energy peaks at scales from hundreds of meters in the lower cloud layer to several kilometers higher up; these scales are typical grid sizes of today’s CRMs. The analysis also shows that a significant portion of the scalar and momentum fluxes in the benchmark simulation are carried by motions smaller than several kilometers (i.e., smaller than a typical grid resolution of CRMs). The broad range of scales of the benchmark simulation is split into two components: filter scale (mimicking CRM resolvable scale) and subfilter scale (mimicking CRM SGS), using filter widths characteristic of a typical CRM grid spacing. The local relationship of the subfilter-scale fluxes to the filter-scale variables is examined. This leads to a mixed SGS scheme to represent the SGS fluxes of scalars and momentum in CRMs. A priori tests show that the mixed SGS scheme yields spatial distributions of subfilter-scale fluxes that correlate much better with those retrieved from the benchmark when compared with an eddy viscosity/diffusivity scheme that is commonly used in today’s CRMs.

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