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J. C. Doran and T. W. Horst

Abstract

Measurements of simple nocturnal slope winds were taken on Rattlesnake Mountain, a nearly ideal two-dimensional ridge. Tower and tethered balloon instrumentation allowed the determination of the wind and temperature characteristics of the katabatic layer as well as the ambient conditions. Two cases were chosen for study; these were marked by well-defined surface-based temperature inversions and a low-level maximum in the downslope wind component. The downslope development of the slope flow could be determined from the tower measurements, and showed a progressive strengthening of the katabatic layer. Hydraulic models developed by Manins and Sawford (1979a) and Briggs (1981) gave useful estimates of drainage layer depths, but were not otherwise applicable. A simple numerical model that relates the eddy diffusivity to the local turbulent kinetic energy was found to give good agreement with the observed wind and temperature profiles of the slope flows.

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T. W. Horst and J. C. Doran

Abstract

Measurements of the turbulence structure of nocturnal slope flow are used to test the hypothesis that slope flow turbulence in the region above the low-level wind maximum is decoupled from the surface and has a local structure similar to that found by Nieuwstadt for stably stratified flow over flat terrain. A turbulence covariance model is used to determine local scaling relations for slope flow. These relations predict that slope-flow turbulence variables, scaled with the local values of the momentum and heat fluxes, are nearly identical to those for stably stratified flow over flat terrain, the principal exception being the normalized eddy diffusivities.

Comparison of observations with local scaling predictions above the slope flow wind maximum indicates that local scaling is a promising approach for the description of slope flow turbulence. In particular, the slope flow turbulent kinetic energy (TKE) is found to be proportional to the local stress, with the constant of proportionality in close agreement with that observed by Nieuwstadt over flat terrain. This result, along with an empirical evaluation of the TKE budget, supports the hypothesis that the slope-flow TKE budget is principally a local balance between shear production and viscous dissipation, and supports the assumption of local scaling that turbulence flux divergence can be neglected in the turbulence second-moment equations.

The observations also support the prediction of local scaling that the normalized eddy diffusivities above the slope-flow wind maximum can be considerably larger than for stably stratified flow over flat terrain. The increased eddy diffusivities are caused by an additional buoyancy term in the TKE budget that depends on the slope-parallel heat flux; the flux Richardson number above the wind maximum can be considerably smaller than that for identical wind and temperature gradients over flat terrain, implying increased TKE production and increased turbulent transport. The gradient Richardson number, however, does not account for this additional term in the TKE budget and does not correctly indicate the stability of the flow.

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J. C. Doran and T. W. Horst

Abstract

Spectra of the time series of wind speeds and temperatures measured in the outflow area of a large drainage region are presented. Peaks are found in these spectra at frequencies corresponding to periods of oscillation of ∼1.5 h. A simple model is given, which accounts qualitatively for some of the observed features.

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T. W. Horst and J. C. Weil

Abstract

Recent model estimates of the flux footprint are used to examine the fetch requirements for accurate micro-meteorological measurement of surface fluxes of passive, conservative scalars within the surface flux layer. The required fetch is quantified by specifying an acceptable ratio of the measured flux to the local surface flux. When normalized by the measurement height zm, the fetch is found to be a strong function of atmospheric stability as quantified by zm/L, where L is the Obukhov length, and a weaker function of the normalized measurement height zm/zo, where zo is the roughness length. Stable conditions are found to require a much greater fetch than do unstable conditions, and the fetch required for even moderately stable conditions is for many situations considerably greater than 100 times the measurement height.

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T. W. Horst and J. C. Weil

Abstract

Correction to Volume 11, Issue 4, pages 1018-1025.

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J. C. Doran, T. W. Horst, and P. W. Nickola

Abstract

The behavior of the quantity S = σ y/x σ θ for surface releases of tracers is examined. Variations in this quantity for different field programs are shown to be attributable largely to variations in sampling time t and averaging time t; larger values of S are found to be associated with longer τ and t. Other previously disregarded quantities, viz., mean wind speed and atmospheric stability, are shown to have potentially important effects. Two spectral models are presented which reproduce many of the observed features.

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L. Mahrt, Jielun Sun, S. P. Oncley, and T. W. Horst

Abstract

Drainage of cold air down a small valley and associated near-surface wind maxima are examined from 20 stations with sonic anemometers at 1 m and from a 20-m tower that includes six sonic anemometers in the lowest 5 m, deployed in the Shallow Cold Pool Experiment (SCP). The small valley is about 270 m wide and 12 m deep with a downvalley slope of 2%–3%. The momentum budget indicates that the flow is driven by the buoyancy deficit of the flow and opposed primarily by the stress divergence while the remaining terms are estimated to be at least an order of magnitude smaller. This analysis also reveals major difficulties in quantifying such a budget due to uncertainties in the measurements, sensitivity to choice of averaging time, and sensitivity to measurement heights.

Wind maxima occur as low as 0.5 m in the downvalley drainage flow—the lowest observational level. The downvalley cold air drainage and wind maxima are frequently disrupted by transient modes that sometimes lead to significant vertical mixing. On average, the downvalley drainage of cold air occurs with particularly weak turbulence with stronger turbulence above the drainage flow. The momentum flux profile responds to the shear reversal at the wind maximum on a vertical scale of 1 m or less, suggesting the important role of finescale turbulent diffusion.

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S. P. Burns, J. Sun, A. C. Delany, S. R. Semmer, S. P. Oncley, and T. W. Horst

Abstract

Techniques for improving the relative accuracy of longwave radiation measurements by a set of pyrgeometers [the Eppley Laboratory Precision Infrared Radiometer (Model PIR)] are presented using 10 PIRs from the 1999 Cooperative Atmosphere–Surface Exchange Study (CASES-99). The least squares–based optimization technique uses a field intercomparison (i.e., a time period during which all the PIRs were upward looking and set up side by side) to determine a set of optimization coefficients for each PIR. For the 10 CASES-99 PIRs, the optimization technique improved the standard deviation of the difference of downwelling irradiance between the PIRs from ±0.75 to ±0.4 W m−2 (for nighttime data). In addition to presenting the optimization method, various PIR data quality checks are outlined and applied to the PIR data. Based on these quality checks, the measured case and dome temperatures of the CASES-99 PIRs were all reasonable. Using the 10 CASES-99 PIRs, simple estimates of the average nighttime net radiative flux divergence within the layer between 2 and 48 m were determined and resulted in cooling rates over a range from 0 to −1.3°C h−1, depending on the assumptions made for the upwelling irradiance at 2 m. The effect of the coefficient optimization on the calculated net radiative flux divergence is explored.

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J. A. Businger, W. F. Dabberdt, A. C. Delany, T. W. Horst, C. L. Martin, S. P. Oncley, and S. R. Semmer

The Atmosphere-Surface Turbulent Exchange Research (ASTER) facility developed at the National Center for Atmospheric Research (NCAR) will support observational research on the structure of the atmospheric surface layer. ASTER will provide state-of-the-art measurements of surface fluxes of momentum, sensible heat, and water vapor, and support measurements of surface fluxes of trace chemical species. The facility will be available to the scientific community in the spring of 1990. The motivation for the development of ASTER and the elements that constitute this new national facility are briefly discussed.

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T. W. Horst, J. Kleissl, D. H. Lenschow, C. Meneveau, C.-H. Moeng, M. B. Parlange, P. P. Sullivan, and J. C. Weil

Abstract

The Horizontal Array Turbulence Study (HATS) field program utilized horizontal, crosswind arrays of sonic anemometers to calculate estimates of spatially filtered and subfilter-scale (SFS) turbulence, corresponding to its partitioning in large-eddy simulations (LESs) of atmospheric flows. Measurements were made over a wide range of atmospheric stability and for zf nominally equal to 0.25, 0.5, 1.0, and 2.0, where z is height and Δf is the width of the spatial filter. This paper examines the viability of the crosswind array technique by analyzing uncertainties in the filtered turbulence fields. Aliasing in the crosswind direction, caused by the discrete spacing of the sonic anemometers, is found to be minimal except for the spatially filtered vertical velocity and for SFS second moments. In those cases, aliasing errors become significant when the sonic spacing is greater than the wavelength at the peak in the crosswind spectrum of vertical velocity. Aliasing errors are estimated to be of a similar magnitude for the crosswind gradients of filtered variables. Surrogate streamwise filtering is performed by assuming Taylor's hypothesis and using the mean wind speed U to interpret sonic time series as spatial data. The actual turbulence advection velocity U c is estimated from the cross correlation between data from HATS sonics separated in the streamwise direction. These estimates suggest that, for near-neutral stratification, the ratio U c/U depends on the turbulence variable and is typically between 1.0 and 1.2. Analysis of LES turbulence fields for a neutrally stratified boundary layer finds that the correlation between the true spatially filtered SFS stress component τ 13 and the same variable obtained with surrogate streamwise filtering exceeds 0.98 for zf > 0.25. Within the limits noted, it is concluded that the horizontal array technique is sufficient for the estimation of resolved and SFS turbulence variables.

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