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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
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
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.
Abstract
Using an airborne lidar, we have measured atmospheric aerosol backscatter coefficients (differential backscatter cross section per unit volume) for 10.6 μm wavelength laser radiation as a function of height to 5200 m for a number of meteorological conditions over the United States high plains. Airborne in situ samplers measured the particle size distribution at the same time and altitude as the lidar measured backscatter. One backscatter coefficient profile at 10.6 μm was compared with a 0.694 μm lidar backscatter profile as well as with the particle size distribution profile. The average infrared backscatter coefficient ranged from ∼8 × 10−9 m−1 sr−1 at the surface to 1 × 10−10 sr−1 at 5200 m altitude.
Abstract
Using an airborne lidar, we have measured atmospheric aerosol backscatter coefficients (differential backscatter cross section per unit volume) for 10.6 μm wavelength laser radiation as a function of height to 5200 m for a number of meteorological conditions over the United States high plains. Airborne in situ samplers measured the particle size distribution at the same time and altitude as the lidar measured backscatter. One backscatter coefficient profile at 10.6 μm was compared with a 0.694 μm lidar backscatter profile as well as with the particle size distribution profile. The average infrared backscatter coefficient ranged from ∼8 × 10−9 m−1 sr−1 at the surface to 1 × 10−10 sr−1 at 5200 m altitude.
Abstract
Measurement of air motion relative to an aircraft by a conically scanned optical Doppler technique has advantages over measurements with conventional gust probes for many applications. Advantages of the laser air motion sensing technique described here include calibration based on physical constants rather than experiment for an accurate measurement of mean wind, freedom from flow distortion effects on turbulence measurements, all-weather performance, reduction in error from mechanical vibrations and ability to measure vertical wind shear. An experiment comparing a single-component laser velocimeter and a differential pressure gust probe shows that the optical approach measures the turbulence spectrum accurately at frequencies up to 10 Hz and that the signal-to-noise ratio is not a limiting factor. In addition, we have observed the effect of spectral skewing caused by airflow distortion in cloud.
Abstract
Measurement of air motion relative to an aircraft by a conically scanned optical Doppler technique has advantages over measurements with conventional gust probes for many applications. Advantages of the laser air motion sensing technique described here include calibration based on physical constants rather than experiment for an accurate measurement of mean wind, freedom from flow distortion effects on turbulence measurements, all-weather performance, reduction in error from mechanical vibrations and ability to measure vertical wind shear. An experiment comparing a single-component laser velocimeter and a differential pressure gust probe shows that the optical approach measures the turbulence spectrum accurately at frequencies up to 10 Hz and that the signal-to-noise ratio is not a limiting factor. In addition, we have observed the effect of spectral skewing caused by airflow distortion in cloud.
