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- Author or Editor: R. L. Schwiesow x
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Abstract
Lidar measurements of wind, temperature and water vapor, using a variety of techniques that rely on the detection and analysis of laser light backscattered from the atmosphere, allow data to be obtained that are similar to those hypothetically available from a meteorologically instrumented lower extending to 1 km altitude (or more). This paper reviews these various recent accomplishments in lidar instrumentation without attempting historical completeness. Based on criteria of 1) altitude resolution to 50 m, 2) tower-like measurement geometry, 3) hardware commonality between techniques and 4) daytime as well as nighttime operation, the intercomparison results in recommended techniques to be combined for a compact, mobile lidar “tower.” For horizontal wind, recommendations include pulsed time-of-flight lidar, for vertical wind, pulsed direct Doppler lidar at visible or shorter wavelengths; for temperature, Cabannes-scattering linewidth or rotational Raman band shape; and for water vapor, vibrational Raman scattering. Although further development of some of these techniques is needed to achieve the desired range and resolution, results in the literature support the conclusion that a lidar tower is a feasible concept for meteorological measurements under conditions allowing direct optical propagation.
Abstract
Lidar measurements of wind, temperature and water vapor, using a variety of techniques that rely on the detection and analysis of laser light backscattered from the atmosphere, allow data to be obtained that are similar to those hypothetically available from a meteorologically instrumented lower extending to 1 km altitude (or more). This paper reviews these various recent accomplishments in lidar instrumentation without attempting historical completeness. Based on criteria of 1) altitude resolution to 50 m, 2) tower-like measurement geometry, 3) hardware commonality between techniques and 4) daytime as well as nighttime operation, the intercomparison results in recommended techniques to be combined for a compact, mobile lidar “tower.” For horizontal wind, recommendations include pulsed time-of-flight lidar, for vertical wind, pulsed direct Doppler lidar at visible or shorter wavelengths; for temperature, Cabannes-scattering linewidth or rotational Raman band shape; and for water vapor, vibrational Raman scattering. Although further development of some of these techniques is needed to achieve the desired range and resolution, results in the literature support the conclusion that a lidar tower is a feasible concept for meteorological measurements under conditions allowing direct optical propagation.
Abstract
We have measured the spatial variation of a single horizontal component of the velocity in a number of waterspouts using an airborne infrared Doppler lidar. In 21 data sets, maximum velocities range from 4.2 to 33.6 m s−1 and visible funnel diameters from 6.6 to 90 m. Data were taken at altitudes between 675 m, near cloud base, and 95 m above the surface. The sequences show time development of the velocity as a function of radius at a fixed altitude and the velocity structure at different altitudes and sequential times with a horizontal resolution of ∼0.75 m between data points. The variation in velocity structure between waterspouts is large, with some showing marked azimuthal asymmetry and mixing with the ambient flow, and others showing multiple concentric vortex shells.
Abstract
We have measured the spatial variation of a single horizontal component of the velocity in a number of waterspouts using an airborne infrared Doppler lidar. In 21 data sets, maximum velocities range from 4.2 to 33.6 m s−1 and visible funnel diameters from 6.6 to 90 m. Data were taken at altitudes between 675 m, near cloud base, and 95 m above the surface. The sequences show time development of the velocity as a function of radius at a fixed altitude and the velocity structure at different altitudes and sequential times with a horizontal resolution of ∼0.75 m between data points. The variation in velocity structure between waterspouts is large, with some showing marked azimuthal asymmetry and mixing with the ambient flow, and others showing multiple concentric vortex shells.
Abstract
Misalignment of pyranometers used for airborne measurements can lead to serious errors in the determination of downwelling radiation flux. The magnitude of these errors depends strongly on the elevation angle of the sun. This note presents an iterative numerical procedure for determining the angles of misalignment of upward-facing pyranometers. Deviations in pitch and roll of the instrument with respect to the aircraft's inertial navigation system (INS) must be added to the pitch and roll angles measured by the INS before the radiometric data are corrected for the attitude of the aircraft. For successful determination of the two angles of deviation, a calibration flight must be performed in which the aircraft flies in at least three directions at the same altitude under clear skies and above any haze.
Abstract
Misalignment of pyranometers used for airborne measurements can lead to serious errors in the determination of downwelling radiation flux. The magnitude of these errors depends strongly on the elevation angle of the sun. This note presents an iterative numerical procedure for determining the angles of misalignment of upward-facing pyranometers. Deviations in pitch and roll of the instrument with respect to the aircraft's inertial navigation system (INS) must be added to the pitch and roll angles measured by the INS before the radiometric data are corrected for the attitude of the aircraft. For successful determination of the two angles of deviation, a calibration flight must be performed in which the aircraft flies in at least three directions at the same altitude under clear skies and above any haze.
Abstract
The National Center for Atmospheric Research Airborne Infrared Lidar System is being developed for Doppler wind measurements using heterodyne detection. Its design is based on a pulsed CO2 laser transmitter and a single continuous-wave CO2 laser as local oscillator and injection seed for the pulsed laser. Research during the system development has shown the workability of the two-laser concept, the need for optical isolation between the lasers, and advantages of averaging complex autocorrelation functions rather than velocity estimates. The authors describe the system and show calculated performance for wind measurements.
Abstract
The National Center for Atmospheric Research Airborne Infrared Lidar System is being developed for Doppler wind measurements using heterodyne detection. Its design is based on a pulsed CO2 laser transmitter and a single continuous-wave CO2 laser as local oscillator and injection seed for the pulsed laser. Research during the system development has shown the workability of the two-laser concept, the need for optical isolation between the lasers, and advantages of averaging complex autocorrelation functions rather than velocity estimates. The authors describe the system and show calculated performance for wind measurements.
Abstract
We have demonstrated practical measurement of profiles of horizontal wind magnitude and direction to altitudes of 750 m by making radial velocity measurements with a Doppler lidar using conical scanning. Comparison with surface anemometers and with profiles measured by balloon sondes allows one to evaluate the consistency between lidar measurements and more conventional sensors. Overall we find a correlation coefficient of 0.83 and an rms difference of 1.3 m s−1 for magnitude and a correlation coefficient of 0.91 and an rms difference of 12° for direction when the lidar and sonde profiles are compared. The differences are not a result of lidar errors because comparisons of 20 s averages between the lidar and a sonic anemometer show a correlation coefficient of 0.98, an rms difference of 0.19 m s−1, and a long-term average difference of 0.05 m s−1 for a single component. Profile differences are attributable to horizontal inhomogeneity in the wind field and uncertainty inherent in balloon sondes. Impaired visibility reduces the effective range of the lidar, and the vertical extent of the lidar sample region increases with height.s
Abstract
We have demonstrated practical measurement of profiles of horizontal wind magnitude and direction to altitudes of 750 m by making radial velocity measurements with a Doppler lidar using conical scanning. Comparison with surface anemometers and with profiles measured by balloon sondes allows one to evaluate the consistency between lidar measurements and more conventional sensors. Overall we find a correlation coefficient of 0.83 and an rms difference of 1.3 m s−1 for magnitude and a correlation coefficient of 0.91 and an rms difference of 12° for direction when the lidar and sonde profiles are compared. The differences are not a result of lidar errors because comparisons of 20 s averages between the lidar and a sonic anemometer show a correlation coefficient of 0.98, an rms difference of 0.19 m s−1, and a long-term average difference of 0.05 m s−1 for a single component. Profile differences are attributable to horizontal inhomogeneity in the wind field and uncertainty inherent in balloon sondes. Impaired visibility reduces the effective range of the lidar, and the vertical extent of the lidar sample region increases with height.s
Abstract
Continuous-wave (CW) Doppler lidar measurements of wind magnitude and direction that are based on radial velocity data on only a part of a full azimuth circle compare favorably with measurements based on a full circle. Winds were measured over an altitude range of 750 m. For example, the rms difference between 76 wind data pairs at various altitudes, taken from a full circle and from a ¼-circle sector is 0.43 m s−1 in magnitude (correlation coefficient 0.98) and 4.2° in direction, even when only 12 s of measurement time is used for the ¼-circle sector. Increased integration time leads to an even closer comparison. Useful velocity estimates can be obtained from sector scans as small as ⅙ of a circle when a weighted least-squares fitting program is used to analyze the radial velocity versus azimuth data. Results from a two-point scan technique compare less favorably with the full-scan results than do results from a sector-scan technique. A scan employing a π/2, two-point azimuth difference results in an rms difference of 0.78 m s−1 (correlation coefficient 0.95) for 2 s of measurement time when compared with a full circle scan. We conclude that even if data are available or of interest over only part of an azimuth circle, good wind estimates are still possible.
Abstract
Continuous-wave (CW) Doppler lidar measurements of wind magnitude and direction that are based on radial velocity data on only a part of a full azimuth circle compare favorably with measurements based on a full circle. Winds were measured over an altitude range of 750 m. For example, the rms difference between 76 wind data pairs at various altitudes, taken from a full circle and from a ¼-circle sector is 0.43 m s−1 in magnitude (correlation coefficient 0.98) and 4.2° in direction, even when only 12 s of measurement time is used for the ¼-circle sector. Increased integration time leads to an even closer comparison. Useful velocity estimates can be obtained from sector scans as small as ⅙ of a circle when a weighted least-squares fitting program is used to analyze the radial velocity versus azimuth data. Results from a two-point scan technique compare less favorably with the full-scan results than do results from a sector-scan technique. A scan employing a π/2, two-point azimuth difference results in an rms difference of 0.78 m s−1 (correlation coefficient 0.95) for 2 s of measurement time when compared with a full circle scan. We conclude that even if data are available or of interest over only part of an azimuth circle, good wind estimates are still possible.
Abstract
Significant Doppler lidar returns have been observed from snow and rain. This demonstrates the feasibility of measuring velocity and range of hydrometeors with 10.6-μm wavelength CO2 laser lidar.
Abstract
Significant Doppler lidar returns have been observed from snow and rain. This demonstrates the feasibility of measuring velocity and range of hydrometeors with 10.6-μm wavelength CO2 laser lidar.
Abstract
A Doppler lidar measures the line-of-sight velocity of cloud droplets in a waterspout much as a meteorological Doppler radar measures the velocity of larger hydrometeors. We discuss details of the application of an airborne Doppler lidar to waterspout velocity measurements, including intensity weighting and limitations of the technique. One type of result available from the lidar data is the velocity spectrum of the line-of-sight velocity component of scatterers in the flow, integrated along the lidar axis, as a function of distance from the vortex axis. From the velocity spectra, peak winds in the portion of the waterspout marked by cloud droplets, turbulence levels, and interaction with the ambient flow can be inferred. In one example the maximum velocity observed in the visible part of the waterspout is 10 m s−1. This double-walled waterspout showed a two-peaked velocity spectrum, which we interpret as a dynamic difference between the two coaxial components of the vortex.
Abstract
A Doppler lidar measures the line-of-sight velocity of cloud droplets in a waterspout much as a meteorological Doppler radar measures the velocity of larger hydrometeors. We discuss details of the application of an airborne Doppler lidar to waterspout velocity measurements, including intensity weighting and limitations of the technique. One type of result available from the lidar data is the velocity spectrum of the line-of-sight velocity component of scatterers in the flow, integrated along the lidar axis, as a function of distance from the vortex axis. From the velocity spectra, peak winds in the portion of the waterspout marked by cloud droplets, turbulence levels, and interaction with the ambient flow can be inferred. In one example the maximum velocity observed in the visible part of the waterspout is 10 m s−1. This double-walled waterspout showed a two-peaked velocity spectrum, which we interpret as a dynamic difference between the two coaxial components of the vortex.
Abstract
Comparisons between measurements of a wind component by a Doppler lidar and by a conventional anemometer are presented. The two measurement techniques provided thirteen 15 min data sets which agreed within 0.04 m s−1 on the average. The maximum difference was 0.12 m s−1, which constitutes less than 3% discrepancy, referred to the period average. The results conclusively demonstrate the ability of Doppler lidar to measure winds with a high degree of velocity resolution and accuracy.
Abstract
Comparisons between measurements of a wind component by a Doppler lidar and by a conventional anemometer are presented. The two measurement techniques provided thirteen 15 min data sets which agreed within 0.04 m s−1 on the average. The maximum difference was 0.12 m s−1, which constitutes less than 3% discrepancy, referred to the period average. The results conclusively demonstrate the ability of Doppler lidar to measure winds with a high degree of velocity resolution and accuracy.
Abstract
The NCAR Airborne Infrared Lidar System (NAILS) observed the edge of an extended, sloping aerosol layer that intersected a stratocumulus cloud deck over the Pacific Ocean during the First ISCCP (International Satellite Cloud Climatology Project) Regional Experiment, 260 km WNW of San Diego. In situ measurements support the interpretation of the lidar observations as arising from a particle-laden layer with relatively clean air above, below, and to the SW. Intersection of these sloping layers with cloud top leads to substantial horizontal variability of boundary-layer structure in the intersection region. The intersection of the aerosol layer with cloud top also corresponded closely to a quasi-linear trough in the cloud top that showed enhanced brightness and an enhanced number of small particles.
Abstract
The NCAR Airborne Infrared Lidar System (NAILS) observed the edge of an extended, sloping aerosol layer that intersected a stratocumulus cloud deck over the Pacific Ocean during the First ISCCP (International Satellite Cloud Climatology Project) Regional Experiment, 260 km WNW of San Diego. In situ measurements support the interpretation of the lidar observations as arising from a particle-laden layer with relatively clean air above, below, and to the SW. Intersection of these sloping layers with cloud top leads to substantial horizontal variability of boundary-layer structure in the intersection region. The intersection of the aerosol layer with cloud top also corresponded closely to a quasi-linear trough in the cloud top that showed enhanced brightness and an enhanced number of small particles.
