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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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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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S. P. Oncley, K. Schwenz, S. P. Burns, J. Sun, and R. K. Monson

Abstract

A system to make atmospheric measurements from a moving trolley suspended by a stretched cable has been developed. At present, these measurements consist of wind velocity, temperature, humidity, and carbon dioxide concentration, though other sensors may be added. The track consists of cable segments attached to turns mounted on standard triangular towers. Using this approach, the path can be a closed (three dimensional) polygon of arbitrary length. This tool allows continuous, high spatial and temporal resolution sampling in environments, such as within forest canopies, not possible with other platforms. This system was used at the Niwot Ridge AmeriFlux site to obtain insight into the spatial and temporal structure of CO2, wind, and humidity fields in a natural forest ecosystem. Specifically, cool, moist, and CO2-rich air was observed to move in thin blobs downslope along the local water drainage through the subcanopy space at night.

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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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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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Steven P. Oncley, Carl A. Friehe, John C. Larue, Joost A. Businger, Eric C. Itsweire, and Sam S. Chang

Abstract

An atmospheric surface-layer experiment over a nearly uniform plowed field was performed to determine the constants in the flux-profile similarity formulas, particularly the von Kármán constant. New instruments were constructed to minimize flow distortion effects on the turbulence measurements and to provide high-resolution gradient measurements. In addition, a hot-wire anemometer directly measured the turbulent kinetic energy dissipation rate.

An average value of the von Kármán constant of 0.365 ± 0.015 was obtained from 91 runs (31 h) in near-neutral stability conditions. However, four near-neutral runs when snow covered the ground gave an average value of 0.42. This result suggests that the von Kármán constant depends on the roughness Reynolds number, which may resolve some of the differences in previous determinations over different surfaces. The one-dimensional Kolmogorov inertial subrange constant was found to have a value of 0.54 ± 0.03, slightly larger than previous results.

The flux-profile relations for momentum and temperature variance were evaluated, and humidity variance data behaved similarly to temperature. Dissipation of turbulent kinetic energy was found to be less than production under near-neutral conditions, which suggests that turbulent or pressure transport may be significant.

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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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Jielun Sun, Steven P. Oncley, Sean P. Burns, Britton B. Stephens, Donald H. Lenschow, Teresa Campos, Russell K. Monson, David S. Schimel, William J. Sacks, Stephan F. J. De Wekker, Chun-Ta Lai, Brian Lamb, Dennis Ojima, Patrick Z. Ellsworth, Leonel S. L. Sternberg, Sharon Zhong, Craig Clements, David J. P. Moore, Dean E. Anderson, Andrew S. Watt, Jia Hu, Mark Tschudi, Steven Aulenbach, Eugene Allwine, and Teresa Coons

A significant fraction of Earth consists of mountainous terrain. However, the question of how to monitor the surface–atmosphere carbon exchange over complex terrain has not been fully explored. This article reports on studies by a team of investigators from U.S. universities and research institutes who carried out a multiscale and multidisciplinary field and modeling investigation of the CO2 exchange between ecosystems and the atmosphere and of CO2 transport over complex mountainous terrain in the Rocky Mountain region of Colorado. The goals of the field campaign, which included ground and airborne in situ and remote-sensing measurements, were to characterize unique features of the local CO2 exchange and to find effective methods to measure regional ecosystem–atmosphere CO2 exchange over complex terrain. The modeling effort included atmospheric and ecological numerical modeling and data assimilation to investigate regional CO2 transport and biological processes involved in ecosystem–atmosphere carbon exchange. In this report, we document our approaches, demonstrate some preliminary results, and discuss principal patterns and conclusions concerning ecosystem–atmosphere carbon exchange over complex terrain and its relation to past studies that have considered these processes over much simpler terrain.

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Margaret A. LeMone, Robert L. Grossman, Richard L. Coulter, Marvin L. Wesley, Gerard E. Klazura, Gregory S. PouIos, William Blumen, Julie K. Lundquist, Richard H. Cuenca, Shaun F. Kelly, Edward A. Brandes, Steven P. Oncley, Robert T. McMillen, and Bruce B. Hicks

This paper describes the development of the Cooperative Atmosphere Surface Exchange Study (CASES), its synergism with the development of the Atmosphere Boundary Layer Experiments (ABLE) and related efforts, CASES field programs, some early results, and future plans and opportunities. CASES is a grassroots multidisciplinary effort to study the interaction of the lower atmosphere with the land surface, the subsurface, and vegetation over timescales ranging from nearly instantaneous to years. CASES scientists developed a consensus that observations should be taken in a watershed between 50 and 100 km across; practical considerations led to an approach combining long-term data collection with episodic intensive field campaigns addressing specific objectives that should always include improvement of the design of the long-term instrumentation. In 1997, long-term measurements were initiated in the Walnut River Watershed east of Wichita, Kansas. Argonne National Laboratory started setting up the ABLE array. The first of the long-term hydrological enhancements was installed starting in May by the Hydrologic Science Team of Oregon State University. CASES-97, the first episodic field effort, was held during April–June to study the role of surface processes in the diurnal variation of the boundary layer, to test radar precipitation algorithms, and to define relevant scaling for precipitation and soil properties. The second episodic experiment, CASES-99, was conducted during October 1999, and focused on the stable boundary layer. Enhancements to both the atmospheric and hydrological arrays continue. The data from and information regarding both the long-term and episodic experiments are available on the World Wide Web. Scientists are invited to use the data and to consider the Walnut River Watershed for future field programs.

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H. J. S. Fernando, J. Mann, J. M. L. M. Palma, J. K. Lundquist, R. J. Barthelmie, M. Belo-Pereira, W. O. J. Brown, F. K. Chow, T. Gerz, C. M. Hocut, P. M. Klein, L. S. Leo, J. C. Matos, S. P. Oncley, S. C. Pryor, L. Bariteau, T. M. Bell, N. Bodini, M. B. Carney, M. S. Courtney, E. D. Creegan, R. Dimitrova, S. Gomes, M. Hagen, J. O. Hyde, S. Kigle, R. Krishnamurthy, J. C. Lopes, L. Mazzaro, J. M. T. Neher, R. Menke, P. Murphy, L. Oswald, S. Otarola-Bustos, A. K. Pattantyus, C. Veiga Rodrigues, A. Schady, N. Sirin, S. Spuler, E. Svensson, J. Tomaszewski, D. D. Turner, L. van Veen, N. Vasiljević, D. Vassallo, S. Voss, N. Wildmann, and Y. Wang

Abstract

A grand challenge from the wind energy industry is to provide reliable forecasts on mountain winds several hours in advance at microscale (∼100 m) resolution. This requires better microscale wind-energy physics included in forecasting tools, for which field observations are imperative. While mesoscale (∼1 km) measurements abound, microscale processes are not monitored in practice nor do plentiful measurements exist at this scale. After a decade of preparation, a group of European and U.S. collaborators conducted a field campaign during 1 May–15 June 2017 in Vale Cobrão in central Portugal to delve into microscale processes in complex terrain. This valley is nestled within a parallel double ridge near the town of Perdigão with dominant wind climatology normal to the ridges, offering a nominally simple yet natural setting for fundamental studies. The dense instrument ensemble deployed covered a ∼4 km × 4 km swath horizontally and ∼10 km vertically, with measurement resolutions of tens of meters and seconds. Meteorological data were collected continuously, capturing multiscale flow interactions from synoptic to microscales, diurnal variability, thermal circulation, turbine wake and acoustics, waves, and turbulence. Particularly noteworthy are the extensiveness of the instrument array, space–time scales covered, use of leading-edge multiple-lidar technology alongside conventional tower and remote sensors, fruitful cross-Atlantic partnership, and adaptive management of the campaign. Preliminary data analysis uncovered interesting new phenomena. All data are being archived for public use.

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