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D. Khelif, S. P. Burns, and C. A. Friehe

Abstract

Improved techniques for measuring horizontal and vertical wind components and state variables on research aircraft are presented. They include a filtering method for correcting ground speed and position Inertial Navigation System data with Global Positioning System data, use of moist-air thermodynamic properties in the true airspeed calculation, postflight calculation of the aircraft vertical velocity, and calibration of airflow attack and sideslip angles from the two air-data systems on each aircraft—a radome gust probe and a pair of fuselage-mounted Rosemount 858Y probes. Winds from the two air-data systems are compared for the National Oceanic and Atmospheric Administration WP-3D aircraft.

As an evaluation of these techniques, data from the two aircraft during side-by-side low-level constant-heading runs are compared for mean and turbulent measurements of wind, ambient temperature, and absolute humidity. Small empirical offsets were determined and applied to the two latter scalars as well as to static and dynamic pressures. Median differences between mean horizontal wind components from nine comparisons were within 0.1 ± 0.4 m s−1. Median differences in latent heat and sensible heat fluxes and momentum flux components were 3.5 ± 15 W m−2, 0 ± 2.5 W m−2, and 0 ± 0.015 Pa, respectively.

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R. A. Antonia, A. J. Chambers, C. A. Friehe, and C. W. Van Atta

Abstract

A review of the evidence for the organized temperature structure observed in both the atmospheric surface layer and the laboratory boundary layer reveals similar features between the two turbulent flows. This similarity suggests that the atmospheric temperature ramp may be interpreted as the signature of an organized large-scale motion rather than a necessary consequence of the presence of buoyant plumes. An experiment was conducted in which the translation velocity Ut of the sharp edge of the temperature ramp is determined from the transit time of the ramp between two thermistors placed at the same height in the marine surface layer but separated in a direction parallel to the wind. Ut was found to be in more nearly constant ratio to the local velocity than to the friction velocity. Velocities determined from the phase angle of the temperature cross spectrum and from the optimum temperature cross correlation obtained from the two thermistors are in reasonable agreement with Ut. Cross correlations of temperature signals from thermistors separated in either vertical or lateral directions are briefly discussed in the context of the spatial geometry of the organized temperature structure.

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C. D. Winant, C. E. Dorman, C. A. Friehe, and R. C. Beardsley

Abstract

During the spring and summer, northerly winds driven by the North Pacific high pressure system are prevalent over the Northern California continental shelf, only interrupted for periods of a few days, when weak or southerly winds occur. In the course of the Coastal Ocean Dynamics Experiment (CODE), fixed station and observations were made to describe the temporal and spatial structure of the lower atmosphere, and their relation to the strong upwelling of coastal waters in a region extending up to 40 km offshore and 100 km along the coast. These observations suggest that atmospheric conditions during the spring and summer usually fall into one of three categories: the surface wind can be everywhere weak (Pattern 1), it can blow at large speeds in a uniform pattern (Pattern 2), or finally the structure of the northerly surface wind can be complex, with large changes in the wind speed and corresponding changes in the surface pressure over short spatial scales (Pattern 3), The latter pattern, which occurs with generally northerly winds, is characterized by a strong low-level inversion and the spatial structure of the surface wind is correlated with the coastal topography. The inversion acts as a material interface, and the marine layer behaves as a supercritical channel flow, when the Froude number is greater than one: oblique expansion waves and hydraulic jumps, associated with changes in the orientation of the coastline, account for the observed spatial structure of the flow. Observations from mid-latitudes on the eastern side of other ocean basins suggest that similar supercritical conditions in the marine layer may prevail there also.

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F. H. Champagne, C. A. Friehe, J. C. LaRue, and J. C. Wynagaard

Abstract

An AFCRL-UCSD joint experiment in Minnesota in 1973 has provided a comparison of direct and indirect measurements of the surface-layer fluxes of momentum, heat and moisture under unstable conditions. The direct momentum and heat flux measurements of the two groups agreed well, and also agreed well with values inferred by the direct dissipation technique. The moisture flux estimates from the inertial-dissipation technique also agreed well with the directly measured values.

Several of the important terms in the budgets of turbulent kinetic energy and turbulent scalar variances were evaluated directly. The imbalance (or pressure transport) term in the energy budget was estimated, and the ratio of the imbalance term to the dissipation term determined from the present experiment agrees well with the Kansas results. The dissipation rate of temperature variance exceeded its production rate, in contrast with the Kansas results, implying an imbalanced temperature variance budget. Several possible contributors to this imbalance are discussed.

The one-dimensional spectra of the temperature and streamwise velocity fluctuations are presented in Kolmogorov normalized form. Spectral moments to fourth order are shown to agree with earlier results. Values of the universal velocity and temperature spectral constants

of α1=0.50±0.02 and β=0.45±0.02 were obtained.

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R. A. Antonia, A. J. Chambers, S. Rajagopalan, K. R. Sreenivasan, and C. A. Friehe

Abstract

Measurements of turbulent momentum, heat and moisture fluxes have been made in Bass Strait from a stable platform, at a height of approximately 5 m above water. Direct measurements of these fluxes are compared with estimates obtained from spectra of velocity, temperature and humidity fluctuations with the use of the inertial dissipation technique. Directly measured momentum and moisture flux values are in reasonable agreement with inertial dissipation values. The sensible heal flux obtained by the inertial dissipation technique is about twice as large as the directly measured heat flux. The dependence on wind speed of bulk transfer coefficients of momentum, heat and moisture and of variances of velocity and scalar fluctuations is discussed and compared with available data.

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Robert C. Beardsley, Amelito G. Enriquez, Carl A. Friehe, and Carol A. Alessi

Abstract

An intercomparison between low-level aircraft measurements of wind and wind stress and buoy measurements of wind and estimated wind stress was made using data collected over the northern California shelf in the Shelf Mixed Layer Experiment (SMILE). Twenty-five buoy overflights were made with the NCAR King Air at a nominal altitude of 30 m over NOAA Data Buoy Center (NDBC) environmental buoys 46013 and 46014 between 13 February and 17 March 1989; meteorological conditions during this period were varied, with both up- and downcoast winds and variable stability. The buoy winds measured at 10 m were adjusted to the aircraft altitude using flux profile relations, and the surface fluxes and stability were estimated using both the TOGA COARE and bulk parameterizations.

The agreement between the King Air wind speed and direction measurements and the adjusted NDBC buoy wind speed and direction measurements was good. Average differences (aircraft − buoy) and standard deviations were 0.6 ± 0.8 m s−1 for wind speed and 0.0° ± 10.5° for direction (adjusted for buoy offset), independent of parameterization used.

The comparisons of aircraft and buoy wind stress components also showed good agreement, especially at larger values of the wind stress (>0.1 Pa) when the wind stress field appeared to be more spatially organized. For the east component, the average difference and standard deviation were 0.018 ± 0.029 Pa using TOGA COARE and −0.018 ± 0.027 Pa using . For the north component, the average difference and standard deviation are 0.003 ± 0.018 Pa using TOGA COARE and 0.003 ± 0.017 Pa using . These results support the idea that low-flying research aircraft like the King Air can be used to accurately map both the surface wind and the surface wind stress fields during even moderate wind conditions.

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E. N. Brown, M. A. Shapiro, P. J. Kennedy, and C. A. Friehe

Abstract

The aeronautical use of electronic altimeters is to measure the absolute clearance of an aircraft above the earth's surface. In the support of atmospheric research, accurate high-range altimeters, in conjunction with accurate static pressure and navigation data, also can provide a means for measuring the heights of constant-pressure surfaces. From the derivatives of the measurements, surface slopes and dynamical quantities such as the geostrophic wind may be obtained. Although the technique is easiest over oceans or large bodies of water, it can be successfully used over land, if detailed terrain heights are known.

This paper describes the operational and research use of a high-altitude pulse-type radar altimeter system installed on the NCAR Sabreliner for jet stream research. An error analysis for “D-value”, derived from radar altitude and pressure measurements, gave an estimated error of ±6.0 m, which surpasses measurements from conventional balloon soundings or satellite-derived height analyses. For a case study of jet stream dynamics, the above error in D-value corresponded to an error of ±5% in the computed geostrophic wind.

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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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Carl A. Friehe, Robert C. Beardsley, Clinton D. Winant, and Jerome P. Dean

Abstract

Intercomparisons of meteorological data—wind speed and direction, surface temperature and surface pressure—were obtained for NCAR Queen Air overflights of four buoys during the CODE-1 experiment. The overflights were at a nominal altitude of 33 m. Wind and air temperature sensors were at 10 m on two National Data Buoy Office (NDBO) buoys and at 3.5 m on two Woods Hole Oceanographic Institution (WHOI) buoys. The buoy wind speeds were adjusted to the aircraft altitude using diabatic flux-profile relations and bulk aerodynamic formulas to estimate the surface fluxes and stability. For the experimental period (22 April-23 May 1981) and location (northern coast of California), the atmospheric surface layer was generally stable, with the Monin-Obukhov length on average 500 m with large variability.

The results of the intercomparisons of the above variables were in general good. Average differences (aircraft - buoy) and standard deviations were +0.1 m s−1 (±1.8) for wind speed, 3.3 deg (±11.2) for wind direction, +0.02°C (±1.7) for air temperature and +0.8 mb (+1.0) for surface pressure. The aircraft downward-looking infrared radiometer indicated a surface temperature 1°C lower than the buoy hull (NDBO) and 1 m immersion (WHOI) sea temperature sensors.

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