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- Author or Editor: C. A. Friehe x
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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.
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.
Abstract
We use a Taylor series expansion technique to calculate the effects of mean-flow distortion on turbulence measurements ahead of an axisymmetric body. The approach is valid when the integral scale of the turbulence is large compared to the maximum body diameter, which is usually the case for energy-containing turbulence statistics measured ahead of aircraft and the towed bodies used in oceanography. Using a 5:1 ellipsoid as a representative body, we show the nature of the attenuation and crosstalk error terms which the flow distortion induces in the measured turbulence components. The contours of the resulting errors in velocity covariances measured ahead of the ellipsoid and a paraboloid of revolution reveal that, in general, the errors in turbulent Roynolds stress can be the most severe. For sufficiently small distortion effects, which typically means more than one diameter away from the nose, the distortion matrix can easily be inverted analytically, giving explicit expressions for the true covariances in terms of the measured ones. We assess the effects of averaging measurements taken in different directions along the same path, as is sometimes done with aircraft data, and of nonzero angle of attack.
Abstract
We use a Taylor series expansion technique to calculate the effects of mean-flow distortion on turbulence measurements ahead of an axisymmetric body. The approach is valid when the integral scale of the turbulence is large compared to the maximum body diameter, which is usually the case for energy-containing turbulence statistics measured ahead of aircraft and the towed bodies used in oceanography. Using a 5:1 ellipsoid as a representative body, we show the nature of the attenuation and crosstalk error terms which the flow distortion induces in the measured turbulence components. The contours of the resulting errors in velocity covariances measured ahead of the ellipsoid and a paraboloid of revolution reveal that, in general, the errors in turbulent Roynolds stress can be the most severe. For sufficiently small distortion effects, which typically means more than one diameter away from the nose, the distortion matrix can easily be inverted analytically, giving explicit expressions for the true covariances in terms of the measured ones. We assess the effects of averaging measurements taken in different directions along the same path, as is sometimes done with aircraft data, and of nonzero angle of attack.
Abstract
An improved calibration technique for an airborne Lyman-alpha hygrometer is presented. Like previous methods, it relies upon simultaneous measurement of absolute humidity determined from a slower response hygrometer. We show that a substantial improvement in the Lyman-alpha calibration is obtained by accounting for the time lag of the slower instrument.
To show our technique we use data from Lyman-alpha and thermoelectric devices on the NCAR Electra during an investigation of the nearly neutral boundary layer over the Arabian Sea as part of the WMO/ICSU Summer Monsoon Experiment. We also show that for near-neutral conditions the eddy-correlation water vapor flux can be adequately estimated using the fast response vertical velocity data from a gust probe and slower response data from the thermoelectric device, which has been properly advanced to account for the time lag.
Abstract
An improved calibration technique for an airborne Lyman-alpha hygrometer is presented. Like previous methods, it relies upon simultaneous measurement of absolute humidity determined from a slower response hygrometer. We show that a substantial improvement in the Lyman-alpha calibration is obtained by accounting for the time lag of the slower instrument.
To show our technique we use data from Lyman-alpha and thermoelectric devices on the NCAR Electra during an investigation of the nearly neutral boundary layer over the Arabian Sea as part of the WMO/ICSU Summer Monsoon Experiment. We also show that for near-neutral conditions the eddy-correlation water vapor flux can be adequately estimated using the fast response vertical velocity data from a gust probe and slower response data from the thermoelectric device, which has been properly advanced to account for the time lag.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
