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- Author or Editor: C. A. Friehe x
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Abstract
The heat transfer characteristics of an aircraft-mounted resistance-wire atmospheric temperature sensor are modeled to determine the time and frequency responses. The sensor element (Rosemount 102E4AL) consists of a 25-μm-diameter platinum wire wound around a cruciform mica support with approximately 143 diameters of wire between contacts with the mica. A longitudinally distributed, radially lumped capacitance model provided for the convective heat transfer to the wire and the transient heat conduction along it. Similarly, the temperature gradient across the thin dimension of the mica support was neglected, and a radially distributed model provided for the convective heat transfer to the mica and the transient conduction within it. The two solutions are coupled by the boundary conditions at the wire-mica contact. The equations were solved to produce the temperature distribution along the wire and in the mica support as a function of the frequency of a free-stream sinusoidal temperature fluctuation. The frequency response transfer function was determined and fit to a two-time-constant transfer function by regression analysis. The two-time-constant model fits the general solution very well. The small (fast response) time constant is essentially determined by the wire itself. The larger (slow response) time constant is due to conduction into and out of the mica supports. The model predicts that the effects of the mica supports are important for frequencies greater than about 0.1 Hz. The responses to five different temperature waveform inputs (sinusoid, step, pulse, ramp, and ramp level) are derived using the two-time-constant model with Laplace transform techniques for both infinite-length wire (no mica support effects) and the finite-length wire of the 102 probe. The actual temperature signals are distorted by the larger time constant of the mica supports, especially for the pulse and ramp inputs that are typical of aircraft measurements of thermals and inversions.
Abstract
The heat transfer characteristics of an aircraft-mounted resistance-wire atmospheric temperature sensor are modeled to determine the time and frequency responses. The sensor element (Rosemount 102E4AL) consists of a 25-μm-diameter platinum wire wound around a cruciform mica support with approximately 143 diameters of wire between contacts with the mica. A longitudinally distributed, radially lumped capacitance model provided for the convective heat transfer to the wire and the transient heat conduction along it. Similarly, the temperature gradient across the thin dimension of the mica support was neglected, and a radially distributed model provided for the convective heat transfer to the mica and the transient conduction within it. The two solutions are coupled by the boundary conditions at the wire-mica contact. The equations were solved to produce the temperature distribution along the wire and in the mica support as a function of the frequency of a free-stream sinusoidal temperature fluctuation. The frequency response transfer function was determined and fit to a two-time-constant transfer function by regression analysis. The two-time-constant model fits the general solution very well. The small (fast response) time constant is essentially determined by the wire itself. The larger (slow response) time constant is due to conduction into and out of the mica supports. The model predicts that the effects of the mica supports are important for frequencies greater than about 0.1 Hz. The responses to five different temperature waveform inputs (sinusoid, step, pulse, ramp, and ramp level) are derived using the two-time-constant model with Laplace transform techniques for both infinite-length wire (no mica support effects) and the finite-length wire of the 102 probe. The actual temperature signals are distorted by the larger time constant of the mica supports, especially for the pulse and ramp inputs that are typical of aircraft measurements of thermals and inversions.
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Abstract
An analytical study was conducted of the thermal frequency response of an atmospheric temperature probe consisting of a thermistor bead with two lead wires soldered to thin support posts. Such probes are used in aircraft temperature sensors and for surface-layer turbulence studies. The results show the effects of the lead wires on the frequency response (amplitude and phase) of the probe for two end conditions of the lead wires: 1) fixed temperature at the mean free-stream value, and 2) adiabatic. For the smallest commercially available thermistor bead of approximately 200-µm diameter and for 20-µm-diameter platinum lead-wire lengths of about 0.8 mm, the conduction to the supports was found to be minimal for both end conditions. It was determined, however, that the lead wires themselves act as heat transfer fins and actually improve the frequency response over that of an ideal isolated bead. Model calculations show that the inclusion of multiple lead wires (four and six) connected mechanically, but not electrically or thermally, to supports would further improve the response. The thermal analysis is also applied to small type-E thermocouple junctions made of 12.5-, 25-, and 50-µm- diameter wires, and the results show that the lead wires also improve the frequency response.
Abstract
An analytical study was conducted of the thermal frequency response of an atmospheric temperature probe consisting of a thermistor bead with two lead wires soldered to thin support posts. Such probes are used in aircraft temperature sensors and for surface-layer turbulence studies. The results show the effects of the lead wires on the frequency response (amplitude and phase) of the probe for two end conditions of the lead wires: 1) fixed temperature at the mean free-stream value, and 2) adiabatic. For the smallest commercially available thermistor bead of approximately 200-µm diameter and for 20-µm-diameter platinum lead-wire lengths of about 0.8 mm, the conduction to the supports was found to be minimal for both end conditions. It was determined, however, that the lead wires themselves act as heat transfer fins and actually improve the frequency response over that of an ideal isolated bead. Model calculations show that the inclusion of multiple lead wires (four and six) connected mechanically, but not electrically or thermally, to supports would further improve the response. The thermal analysis is also applied to small type-E thermocouple junctions made of 12.5-, 25-, and 50-µm- diameter wires, and the results show that the lead wires also improve the frequency response.
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 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.
