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L. J. Peltier and J. C. Wyngaard

Abstract

The conventional theory of the scattering of transmitted acoustic, microwave, and optical radiation by turbulence uses the refractive index structure-function parameter C N2. The authors calculate the vertical profiles of the structure function parameters on which C N2 depends, those for temperature and humility, C T2 and C Q2, and their joint structure-function parameter, C TQ, from large-eddy simulation (LES) data for convective boundary layers. The results agree well with experimental measurements.

Modern views of wave propagation through turbulence with substantial intermittency, such as that found in the high Reynolds number flows in geophysics, suggest that the structure–function parameter be interpreted as a local flow variable rather than the traditional ensemble average. Through the refined Kolmogorov–Obukhov similarity hypotheses, a set of local structure–function parameters is defined that depends on locally averaged values of the molecular destruction rates of velocity and scalar variances. Through analysis of the locally averaged variance budgets, the coupling between the resolvable-scale fields in LFS and these local destruction rates are outlined, with the focus on scalars. Using data from direct numerical simulation, we test two models of the locally averaged destruction rate of scalar variance. Each emulates its approximately lognormal statistics and can be used with LES codes, enabling predictions of local structure–function parameter fields.

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J. C. Wyngaard, L. J. Peltier, and S. Khanna

Abstract

The surface fluxes in the fine-mesh numerical codes used in small-scale meteorology are typically diagnosed from resolvable-scale variables through surface-exchange coefficients. This is appropriate if the aspect ratio (length/height) of the grid volume adjacent to the surface is very large, as in mesoscale models. The aspect ratio can approach unity in large-eddy simulation (LES) codes for the planetary boundary layer, however. In that limit the surface-exchange coefficients are random variables, and it is shown through analysis of surface-layer measurements and LES results that their fluctuation levels can be large.

As an alternative to surface-exchange coefficients, the authors derive conservation equations for the surface scalar and momentum fluxes in LES. Scaling relations for resolvable-scale variables in the surface layer are developed and used to simplify these equations. It is shown that, as the grid aspect ratio decreases toward unity, local time change, horizontal advection, production due to horizontal velocity convergence, and random noise terms cause the local surface-exchange coefficients to fluctuate. A simple closure of the equations is adopted, which has little effect on surface-layer structure calculated through LES with a Smagorinsky-based subgrid-scale (SGS) model. Through analysis of very high-resolution LES fields, the authors find the SGS model to be a poor representation of surface-layer physics and conclude that the surface-flux conservation equations need to be coupled with a greatly improved SGS model in the surface layer.

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L. J. Peltier, J. C. Wyngaard, S. Khanna, and J. O. Brasseur

Abstract

A simple approach to modeling spectra in unstable atmospheric surface layers is presented. The authors use a single form for the two-dimensional spectrum of horizontal velocity, vertical velocity, and a scalar in the horizontal plane; it has two free constants, a length scale, and an intensity scale. Continuity is used to relate the vertical and horizontal velocity spectra. The two free constants are determined by matching the variance and the inertial-subrange spectral level with observations. The scales are chosen so that the spectra follow law of the wall and mixed-layer scaling in the neutral and free-convection limits, respectively. The authors model the stability dependence of the spectra by combining these two limiting forms. The one-dimensional spectra, obtained by integration over one wavenumber component, and their variances agree well with observations. Near the surface the vertical velocity variance follows Monin-Obukhov (M–O) similarity and shows a realistic local free-convection asymptote; at greater heights it shows departures from M–O similarity that also agree well with observations. Finally, the two-dimensional spectra are used to calculate the valances of the resolvable and subgrid-scale components of large eddy simulations and their dependence on grid mesh size, distance from the surface, boundary layer depth, and stability.

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R. J. Stouffer, J. Yin, J. M. Gregory, K. W. Dixon, M. J. Spelman, W. Hurlin, A. J. Weaver, M. Eby, G. M. Flato, H. Hasumi, A. Hu, J. H. Jungclaus, I. V. Kamenkovich, A. Levermann, M. Montoya, S. Murakami, S. Nawrath, A. Oka, W. R. Peltier, D. Y. Robitaille, A. Sokolov, G. Vettoretti, and S. L. Weber

Abstract

The Atlantic thermohaline circulation (THC) is an important part of the earth's climate system. Previous research has shown large uncertainties in simulating future changes in this critical system. The simulated THC response to idealized freshwater perturbations and the associated climate changes have been intercompared as an activity of World Climate Research Program (WCRP) Coupled Model Intercomparison Project/Paleo-Modeling Intercomparison Project (CMIP/PMIP) committees. This intercomparison among models ranging from the earth system models of intermediate complexity (EMICs) to the fully coupled atmosphere–ocean general circulation models (AOGCMs) seeks to document and improve understanding of the causes of the wide variations in the modeled THC response. The robustness of particular simulation features has been evaluated across the model results. In response to 0.1-Sv (1 Sv ≡ 106 m3 s−1) freshwater input in the northern North Atlantic, the multimodel ensemble mean THC weakens by 30% after 100 yr. All models simulate some weakening of the THC, but no model simulates a complete shutdown of the THC. The multimodel ensemble indicates that the surface air temperature could present a complex anomaly pattern with cooling south of Greenland and warming over the Barents and Nordic Seas. The Atlantic ITCZ tends to shift southward. In response to 1.0-Sv freshwater input, the THC switches off rapidly in all model simulations. A large cooling occurs over the North Atlantic. The annual mean Atlantic ITCZ moves into the Southern Hemisphere. Models disagree in terms of the reversibility of the THC after its shutdown. In general, the EMICs and AOGCMs obtain similar THC responses and climate changes with more pronounced and sharper patterns in the AOGCMs.

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