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D. A. Oliver
,
W. S. Lewellen
, and
G. G. Williamson

Abstract

A Turbulent-radiative description of the low-level atmosphere incorporating stratus cloud and fog is developed based on the methodology of second-order closure. Existence criteria for surface layer fog are developed on the basis of the turbulent description. Illustrative application is made to the formation and structure of several fog and cloud episodes including advective-radiative fog, subsidence-capped stratus over the ocean, and surface fog resulting from the nocturnal lowering of stratus.

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R. I. Sykes
,
S. F. Parker
,
D. S. Henn
, and
W. S. Lewellen

Abstract

Detailed statistics of the fluctuating concentration field produced by large-eddy simulations (LES) of the chemically reactive mixing of two species in a convectively driven mixed layer are presented. The effect of the turbulent mixing on the effective reaction rate between the species is analysed. The segregation between the species is shown to be significant for fast reactions, and therefore correct model predictions of the evolution of the species concentration requires an estimate of the segregation coefficient. Some simple modeling concepts for one-point second-order turbulence closure schemes are examined and compared with the LES results. The results are a promising indication that second-order closure schemes can be extended to provide a practical calculation of the turbulent mixing effects on fast chemical reactions.

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R. I. Sykes
,
S. F. Parker
,
D. S. Henn
, and
W. S. Lewellen

Abstract

A long-range transport model based on turbulence closure concepts is described. The model extends the description of planetary boundary layer turbulent diffusion to the larger scales and uses statistical wind information to predict contaminant dispersion. The model also contains a prediction of the statistical fluctuations in the tracer concentration resulting from the unresolved velocity fluctuations. The dispersion calculation is made by means of a Lagrangian puff representation, allowing the use of time-dependent three-dimensional flow fields. Predictions of the ANATEX (Across North America Tracer Experiment) releases are compared with observations. Both 24-h average surface and short-term aircraft sampler concentrations are calculated using the high-resolution wind fields from the NMC Nested Grid Model. The statistical prediction is also tested using long-term average wind data.

Statistical uncertainty in the predictions, due to the unresolved wind fluctuations, is found to be small for the 24-h average surface concentrations obtained with the high-resolution winds but is very significant for the short-term aircraft sampler concentrations. A clipped normal probability distribution provides a reasonably good description of the overall cumulative distribution of the aircraft sampler concentrations. A reasonably good description of the 24-h surface concentrations is also obtained using only the long-term average wind statistics and a lognormal probability distribution for the concentration values.

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P. Bechtold
,
S. K. Krueger
,
W. S. Lewellen
,
E. van Meijgaard
,
C.-H. Moeng
,
D. A. Randall
,
A. van Ulden
, and
S. Wang

Several one-dimensional (ID) cloud/turbulence ensemble modeling results of an idealized nighttime marine stratocumulus case are compared to large eddy simulation (LES). This type of model intercomparison was one of the objects of the first Global Energy and Water Cycle Experiment Cloud System Study boundary layer modeling workshop held at the National Center for Atmospheric Research on 16–18 August 1994.

Presented are results obtained with different 1D models, ranging from bulk models (including only one or two vertical layers) to various types (first order to third order) of multilayer turbulence closure models. The ID results fall within the scatter of the LES results. It is shown that ID models can reasonably represent the main features (cloud water content, cloud fraction, and some turbulence statistics) of a well-mixed stratocumulus-topped boundary layer.

Also addressed is the question of what model complexity is necessary and can be afforded for a reasonable representation of stratocumulus clouds in mesoscale or global-scale operational models. Bulk models seem to be more appropriate for climate studies, whereas a multilayer turbulence scheme is best suited in mesoscale models having at least 100- to 200-m vertical resolution inside the boundary layer.

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C.-H. Moeng
,
W. R. Cotton
,
C. Bretherton
,
A. Chlond
,
M. Khairoutdinov
,
S. Krueger
,
W. S. Lewellen
,
M. K. MacVean
,
J. R. M. Pasquier
,
H. A. Rand
,
A. P. Siebesma
,
B. Stevens
, and
R. I. Sykes

This paper reports an intercomparison study of a stratocumulus-topped planetary boundary layer (PBL) generated from ten 3D large eddy simulation (LES) codes and four 2D cloud-resolving models (CRMs). These models vary in the numerics, the parameterizations of the subgrid-scale (SGS) turbulence and condensation processes, and the calculation of longwave radiative cooling. Cloud-top radiative cooling is often the major source of buoyant production of turbulent kinetic energy in the stratocumulus-topped PBL. An idealized nocturnal stratocumulus case was selected for this study. It featured a statistically horizontally homogeneous and nearly solid cloud deck with no drizzle, no solar radiation, little wind shear, and little surface heating.

Results of the two-hour simulations showed that the overall cloud structure, including cloud-top height, cloud fraction, and the vertical distributions of many turbulence statistics, compared well among all LESs despite the code variations. However, the entrainment rate was found to differ significantly among the simulations. Among the model uncertainties due to numerics, SGS turbulence, SGS condensation, and radiation, none could be identified to explain such differences. Therefore, a follow-up study will focus on simulating the entrainment process. The liquid water mixing ratio profiles also varied significantly among the simulations; these profiles are sensitive to the algorithm used for computing the saturation mixing ratio.

Despite the obvious differences in eddy structure in two- and three-dimensional simulations, the cloud structure predicted by the 2D CRMs was similar to that obtained by the 3D LESs, even though the momentum fluxes, the vertical and horizontal velocity variances, and the turbulence kinetic energy profiles predicted by the 2D CRMs all differ significantly from those of the LESs.

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