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Martin J. Otte and John C. Wyngaard

Abstract

The structure of the interfacial layer capping the atmospheric boundary layer is not well understood. The dominant influence on turbulence within the interfacial layer is the stable stratification induced by the capping inversion. A series of 26 high-resolution large eddy simulation runs ranging from neutral, inversion-capped to free-convection cases are used to study interfacial layer turbulence. The interfacial layer is found to be similar in many aspects to a classic stable boundary layer. For example, the shapes of interfacial layer spectra and cospectra, including the locations of the spectral peaks, agree with previous observations from nocturnal PBLs. The eddy diffusivities, variances, structure-function parameters, and dissipation rates within the interfacial layer, suitably nondimensionalized using local scaling, also agree with observations from nocturnal PBLs. These results may lead to improved models of the interfacial layer and entrainment, and may also have implications for remote sensing of the interfacial layer.

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Martin J. Otte and John C. Wyngaard

Abstract

Mixed-layer models are computationally efficient, but they do not realistically represent the structure of the boundary layer under many conditions. Many of the deficiencies of the mixed-layer model can be attributed to the assumed flat profiles. A new method is proposed that, by relaxing the assumption of well-mixed profiles, makes possible an integral PBL parameterization that is computationally efficient, yet accurately describes the mean structure of the boundary layer. The vertical structure of the mean variables in the PBL is represented by a truncated series of Legendre polynomials. The first Legendre mode, the layer average, is identically a mixed-layer model. Additional modes add structure to the vertical profiles and represent corrections to the mixed-layer model. Only a few modes an necessary to produce vertical profiles comparable to the predictions of high-resolution models. Results of the model are shown for a variety of PBL stability regimes.

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J. C. Wyngaard and M. A. LeMone

Abstract

The refractive index structure parameter CN 2 has contributions from the temperature and humidity structure parameters Cr 2 and CQ 2 and from the joint structure parameter CTQ. We briefly review the behavior of these structure parameters in the surface layer. We show that the surface-layer similarity expressions for Cr 2, CTQ and CQ 2 yield, in the unstable limit, mixed-layer scaling laws which are in good agreement with data at small z/zi, where zi is the mixed-layer depth. However, we show that entrainment effects cause large departures from these laws in mid and upper regions of the convective boundary layer.

Using Deardorff's idealization of the structure of the interfacial region at the top of a convective boundary layer, we use a “mean-field closure” approach to develop scaling expressions for the structure parameters generated by the entrainment process there. The available data on CT 2, CTQ and CQ 2 near the convective boundary-layer top, from both steady and evolving cases, are shown to be consistent with these new scaling expressions.

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R. A. Brost and J. C. Wyngaard

Abstract

No abstract available.

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R. A. Brost and J. C. Wyngaard

Abstract

A second-order turbulence model is used to study the stable boundary layer (SBL). Over a horizontal surface, a constant surface cooling rate drives the SBL to a steady state within a few hours. Parameterizations are developed for eddy diffusivities, the kinetic energy dissipation rate and the geostrophic drag law in this idealized case. Over a sloped surface, a constant cooling rate produces a quasi-steady-state SBL in which some flow properties continue to vary but h(|f|/u * L)½ becomes constant; however, this constant is a function of the wind direction relative to the slope and the baroclinity, as measured by the cooling rate times the slope. Calculated eddy diffusivity profiles in the baroclinic (sloping terrain) case compare well with recent data from Antarctica. If a surface energy budget is used rather than a constant cooling rate, the SBL does not reach a steady state even over a horizontal surface; the nondimensional height slowly decays. We conclude that equilibrium models of the SBL are likely to be much less applicable to the real world than are their counterparts for the convective boundary layer.

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J. C. Wyngaard and O. R. Coté

Abstract

Measurements of the shear production, buoyant production, turbulent transport (flux divergence) and dissipation terms in the budget of turbulent kinetic energy, and production and turbulent transport terms in the temperature variance budget are presented. Direct observations of the surface stress and heat flux over a horizontally uniform site enable presentation of the data in terms of surface layer similarity theory.

The dissipation term, obtained from differentiated hot-wire anemometer signals, agrees with estimates made from the inertial subrange levels of longitudinal velocity spectra with a value of 0.5 for the spectral constant. Under stable conditions dissipation essentially balances shear production, while turbulent transport and buoyant production are of secondary importance. Under unstable conditions, dissipation slightly exceeds the total production, and energy is also lost at a substantial rate due to upward export by the turbulence.

The large imbalance among the measured terms in the energy budget under unstable conditions is discussed. The cause of the imbalance cannot at this point be determined with certainty, but an interesting possibility is that pressure transport is significant under very unstable conditions.

The production rate of temperature variance exceeds its rate of vertical transport by an order of magnitude. Estimates of the universal temperature spectral constant were made with the assumption that temperature variance dissipation and production rates are equal; the average value, 0.8, falls within the range reported by other workers.

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J. C. Kaimal, J. C. Wyngaard, and D. A. Haugen

Abstract

The paper describes the general characteristics of a non-orthogonal sonic anemometer array. The effects of line-averaging and spatial separation (between mid-points of the horizontal paths) are analyzed and spatial transfer functions are derived for power spectra of the longitudinal, lateral and vertical velocity components. While line-averaging always causes spectral attenuation at wavenumbers larger than 1/l (where l is the sonic path length), spatial separation produces cross-contamination between the horizontal velocity spectra at wavenumbers exceeding 1/d (where d is the separation distance). For an array with a 120° angle between the horizontal sonic paths the net effect of this cross-contamination is to overestimate the longitudinal velocity spectrum and underestimate the lateral velocity spectrum. The separation distance which yields maximum flatness in the transfer function for the longitudinal component is found to be 0.6 l.

Also discussed are the effects of aliasing and long-term trends on the shape of the computed spectrum and spectral correction for the spatial transfer function in the context of these effects.

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J. R. Garratt, J. C. Wyngaard, and R. J. Francey

Abstract

A simple, but realistic, three-layer model of the unstable atmospheric boundary layer (ABL) comprising surface layer, deep mixed layer and transition layer, within which geostrophic adjustment takes place, is utilized. Relations for the mixed-layer wind components and velocity defects are derived, with the latter depending upon entrainment, baroclinity, advection and local acceleration.

These predicted quantities are compared with observations from three field experiments in the unstable ABL, including conditions of moderate baroclinity, strong horizontal advection and rapid growth of the mixed layer. The observations cover a range in scale-height ratio fh/u *, of approximately 0.025–0.5, and in normalized height h/z 0 of 103 to 107.

Within the limits of experimental errors in wind speed measurements the observed and predicted wind components are in good agreement, implying an internal equilibrium with the turbulent field and no significant influence from, what are observed to be, strong entrainment and advective effects. They are described simply by mixed-layer “drag” laws also appropriate to a steady, horizontally homogeneous, barotropic ABL.

Overall, the observed and predicted velocity defects are in reasonable agreement, and show significant influences of entrainment and advection which far exceed the baroclinity effect. The results demonstrate the applicability of the three-layer model to real atmospheric situations, and imply that the main response of the ABL flow to entrainment and advection is a rotation of the average mixed-layer flow toward the externally imposed pressure-gradient field. Examples are given of the dependence of cross-isobar flow angle upon an entrainment parameter and the scale-height ratio.

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L. Kristensen, J. Mann, S. P. Oncley, and J. C. Wyngaard

Abstract

To improve the quality of scalar-flux measurements, the two-point covariance between the vertical velocity and a scalar s̃, separated in space both horizontally and vertically, is studied. The measurements of such two-point covariances between vertical velocity and temperature with horizontal and vertical separations show good agreement with a symmetric turbulence model when the displacement is horizontal. However, a similar model does not work for vertical displacements because up–down asymmetry exists; that is, there is a lack of reflection symmetry of the covariance function. The second-order equation for conservation of two-point covariance of and reveals the reason for this up–down asymmetry and determines its character. On the basis of our measurements, the “loss of flux” for a given lateral displacement decreases with increasing height of the sensors. For example, at a height of z = 10 m with a sensor displacement of D = 0.2 m, less than 1% of the flux is lost, whereas at z = 1 m the same instrument configuration gives rise to a loss of 13%. Also, when the displacement is vertical, the “flux loss” decreases with height if the displacement is kept constant, but in this case the asymmetry causes the loss to be much smaller if the scalar sensor is positioned below the anemometer: if the mean height is 1 m and the displacement 0.2 m, the loss is 18% with the scalar sensor over the anemometer and only 2% if the instrument positions are interchanged. The authors conclude that when measuring close to the ground, the separation should be vertical with the scalar sensor below the anemometer. In this way a symmetric (omnidirectional) configuration with a minimum of flux loss is obtained.

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Shari J. Kimmel, John C. Wyngaard, and Martin J. Otte

Abstract

Turbulent fluctuations of a conservative scalar in the atmospheric boundary layer (ABL) can be generated by a scalar flux at the surface, a scalar flux of entrainment at the ABL top, and the “chewing up” of scalar variations on the mesoscale. The first two have been previously studied, while the third is examined in this paper through large-eddy simulation (LES). The LES results show that the scalar fluctuations due to the breakdown of mesoscale variations in advected conservative scalar fields, which the authors call the “log-chipper” component of scalar fluctuations, are uniformly distributed through the depth of the convective ABL, unlike the top–down and bottom–up components.

A similarity function, similar to those for the top–down and bottom–up scalars, is derived for the log-chipper scalar variance in the convective ABL and used to compare the relative importance of these three processes for generating scalar fluctuations. Representative mesoscale gradients for water vapor mixing ratio and potential temperature are computed from airplane measurements over both land and water. In situations where the entrainment and surface fluxes are sufficiently small, or the ABL depth, turbulence intensity, or the mesoscale scalar gradient is sufficiently large, the variance of the log-chipper scalar fluctuations in mid-ABL can be of the order of the variance of top–down and bottom–up scalars.

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