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J. C. Kaimal, J. C. Wyngaard, D. A. Haugen, O. R. Coté, Y. Izumi, S. J. Caughey, and C. J. Readings

Abstract

Results from a boundary layer experiment conducted over a flat site in northwestern Minnesota are discussed. Wind and temperature fluctuations near the ground were measured with AFCRL's fast-response instrumentation on a 32 m tower. Measurements between 32 m and the inversion base zi were made with MRU probes attached at five different heights to the tethering cable of a 1300 m2 kite balloon. The daytime convective boundary layer appears to be well-mixed with evidence of significant heat and momentum entrainment through the capping inversion.

The spectra of velocity components are generalized within the framework of mixed-layer similarity. The characteristic wavelength for w increases linearly with height up to z = 0.l zi following free convection prediction, but approaches a limiting value of 1.5 zi, in the upper half of the boundary layer. The characteristic wavelengths for u and v are maintained at approximately 1.5 zi down to heights very close to the ground. This limiting wavelength corresponds to the length scale of large convective elements which extend to the top of the boundary layer.

The behavior of the temperature specra above 0.l zi cannot be generalized in the same manner. Below that height the θ spectra follow behavior observed in the surface layer; z = 0.1 zi is also the upper limit for the free convection predictions of the w and θ variances.

The high-order moments and the structure parameters reveal the strong influence of entrainment at heights above 0.5 zi.

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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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S. F. Zhang, J. C. Wyngaard, J. A. Businger, and S. P. Oncley

Abstract

A new sonic anemometer, called the U.W. sonic anemometer, has been designed to minimize the flow distortion due to the transducer wakes. We present a general analytical model for calculating the effect of these transducer wakes on measured velocity spectra, and show that the effects in the U.W. sonic anemometer are indeed less than in conventional arrays. We suggest a method of correcting for the errors caused by the transducer wakes.

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J. A. Businger, J. C. Wyngaard, Y. Izumi, and E. F. Bradley

Abstract

Wind and temperature profiles for a wide range of stability conditions have been analyzed in the context of Monin-Obukhov similarity theory. Direct measurements of heat and momentum fluxes enabled determination of the Obukhov length L, a key independent variable in the steady-state, horizontally homogeneous, atmospheric surface layer. The free constants in several interpolation formulas can be adjusted to give excellent fits to the wind and temperature gradient data. The behavior of the gradients under neutral conditions is unusual, however, and indicates that von Kármán's constant is ∼0.35, rather than 0.40 as usually assumed, and that the ratio of eddy diffusivities for heat and momentum at neutrality is ∼1.35, compared to the often-suggested value of 1.0. The gradient Richardson number, computed from the profiles, and the Obukhov stability parameter z/L, computed from the measured fluxes, are found to be related approximately linearly under unstable conditions. For stable conditions the Richard on number approaches a limit of ∼0.21 as stability increases. A comparison between profile-derived and measured fluxes shows good agreement over the entire stability range of the observations.

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M. Piper, J. C. Wyngaard, W. H. Snyder, and R. E. Lawson Jr.

Abstract

Large-eddy simulation (LES) results indicate that turbulent scalar diffusion in the convective atmospheric boundary layer (CBL) has interesting properties. A scalar introduced into the bottom of the CBL with no flux through the top (bottom-up diffusion) has a radically different eddy diffusivity profile than a scalar introduced at the CBL top with zero flux through the surface (top-down diffusion).

To test these LES results, a set of experiments was designed and carried out in a replica of the original Deardorff-Willis laboratory convection tank. The authors measured mean gradients and inferred flux profiles for two scalars, temperature, and dye concentration. Using these data, vertical profiles of the top-down, bottom-up gradient functions and eddy diffusivities are calculated. Experimental results for these profiles are in good agreement with the LES predictions.

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J. C. Wyngaard, W. T. Pennell, D. H. Lenschow, and M. A. LeMone

Abstract

The behavior of the temperature-humidity covariance (θq) budget in the convectively driven boundary layer is determined through analysis of data from AMTEX and (to a lesser extent) Kansas and Minnesota. In the near-neutral surface layer a balance is found between production and molecular destruction; in the mixed layer, transport is also important. We extend the Corrsin theory for inertial subrange scalar spectral behavior to the temperature-humidity cospectrum, and thus relate the molecular destruction rate of θq to its inertial range level. Destruction rates inferred from AMTEX cospectra agree with those found from the imbalance of production and transport terms. The budgets within the surface layer and the mixed layer are parameterized separately with appropriate scales.

Both temperature and humidity fluctuations contribute to the small-scale refractive index variations which affect acoustic and electromagnetic wave propagation in the atmosphere. Our results indicate that their joint contribution CTq to the refractive index structure parameter is directly related to the molecular destruction rate of θq. The results provide a basis for understanding and predicting the behavior of CTq in the convective boundary layer.

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Nelson L. Seaman, Brian J. Gaudet, David R. Stauffer, Larry Mahrt, Scott J. Richardson, Jeffrey R. Zielonka, and John C. Wyngaard

Abstract

Numerical weather prediction models often perform poorly for weakly forced, highly variable winds in nocturnal stable boundary layers (SBLs). When used as input to air-quality and dispersion models, these wind errors can lead to large errors in subsequent plume forecasts. Finer grid resolution and improved model numerics and physics can help reduce these errors. The Advanced Research Weather Research and Forecasting model (ARW-WRF) has higher-order numerics that may improve predictions of finescale winds (scales <~20 km) in nocturnal SBLs. However, better understanding of the physics controlling SBL flow is needed to take optimal advantage of advanced modeling capabilities.

To facilitate ARW-WRF evaluations, a small network of instrumented towers was deployed in the ridge-and-valley topography of central Pennsylvania (PA). Time series of local observations and model forecasts on 1.333- and 0.444-km grids were filtered to isolate deterministic lower-frequency wind components. The time-filtered SBL winds have substantially reduced root-mean-square errors and biases, compared to raw data. Subkilometer horizontal and very fine vertical resolutions are found to be important for reducing model speed and direction errors. Nonturbulent fluctuations in unfiltered, very finescale winds, parts of which may be resolvable by ARW-WRF, are shown to generate horizontal meandering in stable weakly forced conditions. These submesoscale motions include gravity waves, primarily horizontal 2D motions, and other complex signatures. Vertical structure and low-level biases of SBL variables are shown to be sensitive to parameter settings defining minimum “background” mixing in very stable conditions in two representative turbulence schemes.

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Brian D. Pollard, Samir Khanna, Stephen J. Frasier, John C. Wyngaard, Dennis W. Thomson, and Robert E. McIntosh

Abstract

The local structure and evolution of the convective boundary layer (CBL) are studied through measurements obtained with a volume-imaging radar, the turbulent eddy profiler (TEP). TEP has the unique ability to image the temporal and spatial evolution of both the velocity field and the local refractive index structure-function parameter, 2n. Volumetric images consisting of several thousand pixels are typically formed in as little as 1 s. Spatial resolutions are approximately 30 m by 30 m by 30 m.

CBL data obtained during an August 1996 deployment at Rocks Springs, Pennsylvania, are presented. Measurements of the vertical 2n profile are shown, exhibiting the well-known bright band near the capping inversion at z i, as well as intermittent plumes of high 2n. Horizontal profiles show coherent 100-m-scale 2n and vertical velocity (w) structures that correspond to converging horizontal velocity vectors. To quantify the scales of structures, the vertical and streamwise horizontal correlation distances are calculated within the TEP field of view.

To study the statistics and scales of larger structures, effective volumes larger than the TEP field of view are constructed through Taylor’s hypothesis. Statistics of 2n and w time series are compared to an appropriately scaled large eddy simulation (LES). While w time series comparisons agree very well, the LES 2n predictions agree only with some of the measured data. Finally, the scales of 2n structures in the TEP time series measurements are calculated and compared to the scales in the LES spatial domain. Good agreement is found only near the capping inversion layer, the area of largest structures. This study highlights the unique capabilities of the TEP instrument, and shows what are believed to be the first statistical comparisons of measured 2n data with LES derived results.

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Leonard J. Peltier, Sue Ellen Haupt, John C. Wyngaard, David R. Stauffer, Aijun Deng, Jared A. Lee, Kerrie J. Long, and Andrew J. Annunzio

Abstract

A parameterization of numerical weather prediction uncertainty is presented for use by atmospheric transport and dispersion models. The theoretical development applies Taylor dispersion concepts to diagnose dispersion metrics from numerical wind field ensembles, where the ensemble variability approximates the wind field uncertainty. This analysis identifies persistent wind direction differences in the wind field ensemble as a leading source of enhanced “virtual” dispersion, and thus enhanced uncertainty for the ensemble-mean contaminant plume. This dispersion is characterized by the Lagrangian integral time scale for the grid-resolved, large-scale, “outer” flow that is imposed through the initial and boundary conditions and by the ensemble deviation-velocity variance. Excellent agreement is demonstrated between an explicit ensemble-mean contaminant plume generated from a Gaussian plume model applied to the individual wind field ensemble members and the modeled ensemble-mean plume formed from the one Gaussian plume simulation enhanced with the new ensemble dispersion metrics.

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