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- Author or Editor: R. M. Hardesty x
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Abstract
We used a continuous-wave (CW) Doppler lidar to measure wind velocity turbulence from a moving frame of reference. By directing the lidar beam to trace the perimeters of vertical-plane disks about horizontal axes parallel to the mean wind direction, we observed turbulence properties similar to those encountered by the tips of revolving turbine blades. As in other measurements made with in situ sensors, turbulence spectra observed from the moving reference points showed a decrease in energy, relative to fixed point observations, at frequencies just below the rotation frequency of the lidar beam, and an increase in energy within discrete spectral bands at higher frequencies. Comparisons with a simple model showed reasonable agreement, although measurement conditions did not correspond to the assumptions of the model. On the basis of the results of this experiment, we conclude that Doppler lidar, with appropriate signal processing, is quite applicable for measurement of turbulence encountered by spinning wind turbines.
Abstract
We used a continuous-wave (CW) Doppler lidar to measure wind velocity turbulence from a moving frame of reference. By directing the lidar beam to trace the perimeters of vertical-plane disks about horizontal axes parallel to the mean wind direction, we observed turbulence properties similar to those encountered by the tips of revolving turbine blades. As in other measurements made with in situ sensors, turbulence spectra observed from the moving reference points showed a decrease in energy, relative to fixed point observations, at frequencies just below the rotation frequency of the lidar beam, and an increase in energy within discrete spectral bands at higher frequencies. Comparisons with a simple model showed reasonable agreement, although measurement conditions did not correspond to the assumptions of the model. On the basis of the results of this experiment, we conclude that Doppler lidar, with appropriate signal processing, is quite applicable for measurement of turbulence encountered by spinning wind turbines.
Abstract
This paper explores the possibilities of using K a -band Doppler radar, microwave radiometer, and lidar as a means of retrieving cloud condensation nucleus (CCN) properties in the stratocumulus-capped marine boundary layer. The retrieval is based on the intimate relationship between the cloud drop number concentration, the vertical air motion at cloud base, and the CCN activation spectrum parameters. The CCN properties that are sought are the C and k parameters in the N = CS k relationship, although activation spectra based on the lognormal distribution of particles is also straightforward. Cloud droplet concentration at cloud base is retrieved from a Doppler cloud radar combined with a microwave radiometer following a previously published technique. Cloud base is determined from a lidar or ceilometer. Vertical velocity just above cloud base is determined from the vertically pointing Doppler cloud radar. By combining the simultaneous retrievals of drop number and vertical velocity, and assuming theoretical relationships between these parameters and the subcloud aerosol parameters, the C parameter can be derived, under the assumption of a fixed k. If a calibrated backscatter lidar measurement is available, retrieval of both C and k parameters is possible. The retrieval is demonstrated for a dataset acquired during the Atlantic Stratocumulus Transition Experiment using a least squares minimization technique. Sensitivity to assumptions used in the retrieval is investigated. It is suggested that this technique may afford the acquisition of long-term datasets for climate monitoring purposes. Further investigation with focused experiments designed to address the issue more rigorously is required.
Abstract
This paper explores the possibilities of using K a -band Doppler radar, microwave radiometer, and lidar as a means of retrieving cloud condensation nucleus (CCN) properties in the stratocumulus-capped marine boundary layer. The retrieval is based on the intimate relationship between the cloud drop number concentration, the vertical air motion at cloud base, and the CCN activation spectrum parameters. The CCN properties that are sought are the C and k parameters in the N = CS k relationship, although activation spectra based on the lognormal distribution of particles is also straightforward. Cloud droplet concentration at cloud base is retrieved from a Doppler cloud radar combined with a microwave radiometer following a previously published technique. Cloud base is determined from a lidar or ceilometer. Vertical velocity just above cloud base is determined from the vertically pointing Doppler cloud radar. By combining the simultaneous retrievals of drop number and vertical velocity, and assuming theoretical relationships between these parameters and the subcloud aerosol parameters, the C parameter can be derived, under the assumption of a fixed k. If a calibrated backscatter lidar measurement is available, retrieval of both C and k parameters is possible. The retrieval is demonstrated for a dataset acquired during the Atlantic Stratocumulus Transition Experiment using a least squares minimization technique. Sensitivity to assumptions used in the retrieval is investigated. It is suggested that this technique may afford the acquisition of long-term datasets for climate monitoring purposes. Further investigation with focused experiments designed to address the issue more rigorously is required.
Abstract
Three cases of colliding outflow boundaries are examined using data collected from the NOAA Doppler lidar and a meteorological tower during the summer of 1986 near Boulder, Colorado. The data are unique because the lidar and the 300 m tower were colocated, providing measurements of both kinematic and thermodynamic properties. Lidar data reveal small-scale vortex roll instabilities within the leading edge of the outflow. Observations of the post-collision interactions showed that the warmer of the two outflows was deflected upward by the colder outflow to heights of 2 km. In all cases, this forced mechanical lifting was sufficient to produce convection. A simple model of two colliding density currents also suggests that deeper outflows are more efficient in initiating convection.
Abstract
Three cases of colliding outflow boundaries are examined using data collected from the NOAA Doppler lidar and a meteorological tower during the summer of 1986 near Boulder, Colorado. The data are unique because the lidar and the 300 m tower were colocated, providing measurements of both kinematic and thermodynamic properties. Lidar data reveal small-scale vortex roll instabilities within the leading edge of the outflow. Observations of the post-collision interactions showed that the warmer of the two outflows was deflected upward by the colder outflow to heights of 2 km. In all cases, this forced mechanical lifting was sufficient to produce convection. A simple model of two colliding density currents also suggests that deeper outflows are more efficient in initiating convection.
The Optical Autocovariance Wind Lidar. Part II: Green OAWL (GrOAWL) Airborne Performance and ValidationAbstract
A two-look airborne Doppler wind lidar operating at the 532-nm laser wavelength, the Green Optical Autocovariance Wind Lidar (GrOAWL), was built and flown aboard the NASA WB-57 research aircraft. Flight campaign goals were to validate the instrument wind measurements and to demonstrate the two-look measurement concept proposed for spaceborne mission concepts such as the Atmospheric Transport, Hurricanes, and Extratropical Numerical Weather Prediction with the Optical Autocovariance Wind Lidar (ATHENA-OAWL) mission. The GrOAWL-measured winds were compared with collocated dropsonde measurements. Line-of-sight velocity (LOSV) measurements for the individual GrOAWL looks showed excellent agreement with dropsondes (R 2 > 0.9). The LOSV biases were very small and not statistically different from 0 m s−1 at the 95% confidence interval (−0.07 ± 0.07 m s−1 and 0.01 ± 0.07 m s−1 for look 1 and look 2, respectively). The wind speed and direction profiles retrieved by combining the two GrOAWL looks were also in very good agreement (R 2 > 0.85). An instrument performance model indicated the instrument wind measurement precision was likely lowered (uncertainty was increased) by a factor of ~3.3 during the flights relative to predicted “as built” instrument performance. The reduced performance was not observed during ground-based atmospheric testing and thus has been attributed to impacts of the harsh operating conditions of the WB-57 aircraft (high vibration, thermal gradients, and high humidity). The exercise of scaling the GrOAWL instrument performance and grid scale to space showed space-based OAWL wind measurements would yield products with precision at least as good as the GrOAWL instrument.
Abstract
A two-look airborne Doppler wind lidar operating at the 532-nm laser wavelength, the Green Optical Autocovariance Wind Lidar (GrOAWL), was built and flown aboard the NASA WB-57 research aircraft. Flight campaign goals were to validate the instrument wind measurements and to demonstrate the two-look measurement concept proposed for spaceborne mission concepts such as the Atmospheric Transport, Hurricanes, and Extratropical Numerical Weather Prediction with the Optical Autocovariance Wind Lidar (ATHENA-OAWL) mission. The GrOAWL-measured winds were compared with collocated dropsonde measurements. Line-of-sight velocity (LOSV) measurements for the individual GrOAWL looks showed excellent agreement with dropsondes (R 2 > 0.9). The LOSV biases were very small and not statistically different from 0 m s−1 at the 95% confidence interval (−0.07 ± 0.07 m s−1 and 0.01 ± 0.07 m s−1 for look 1 and look 2, respectively). The wind speed and direction profiles retrieved by combining the two GrOAWL looks were also in very good agreement (R 2 > 0.85). An instrument performance model indicated the instrument wind measurement precision was likely lowered (uncertainty was increased) by a factor of ~3.3 during the flights relative to predicted “as built” instrument performance. The reduced performance was not observed during ground-based atmospheric testing and thus has been attributed to impacts of the harsh operating conditions of the WB-57 aircraft (high vibration, thermal gradients, and high humidity). The exercise of scaling the GrOAWL instrument performance and grid scale to space showed space-based OAWL wind measurements would yield products with precision at least as good as the GrOAWL instrument.
Abstract
During January and February 1987, the NOAA/WPL pulsed Doppler lidar was deployed in the foothills west of Boulder, Colorado, to study orographically induced flows over the Continental Divide. On 29 January 1987, the lidar, with its unique spatial and temporal data-gathering capabilities, documented a downslope windstorm affecting the Boulder area and the rest of the Front Range. The lidar recorded in detail 1) a low-level leeside wind maximum, 2) propagating wind gusts exhibiting two distinct periodicities, and 3) the eastern edge of a mountain wave feature where a jumplike flow reversal occurred. Such structures have not previously been observed with comparable detail by conventional in situ and remote sensing instruments. The observed phenomena were similar to results obtained from mountain-wave numerical models. The most notable of the structural similarities was between the observed and modeled wind gusts.
Abstract
During January and February 1987, the NOAA/WPL pulsed Doppler lidar was deployed in the foothills west of Boulder, Colorado, to study orographically induced flows over the Continental Divide. On 29 January 1987, the lidar, with its unique spatial and temporal data-gathering capabilities, documented a downslope windstorm affecting the Boulder area and the rest of the Front Range. The lidar recorded in detail 1) a low-level leeside wind maximum, 2) propagating wind gusts exhibiting two distinct periodicities, and 3) the eastern edge of a mountain wave feature where a jumplike flow reversal occurred. Such structures have not previously been observed with comparable detail by conventional in situ and remote sensing instruments. The observed phenomena were similar to results obtained from mountain-wave numerical models. The most notable of the structural similarities was between the observed and modeled wind gusts.
Wind shear has been recognized as a major aviation hazard in the airport environment. A dual, acoustic Doppler–microwave Doppler radar system has been installed at Dulles International Airport near Washington, D.C., to measure the vertical profile of wind from the surface to 510 m in 30 m height increments. The acoustic system gathers data under clear-air conditions, and the microwave radar takes over automatically when precipitation is present. System performance is being assessed by comparing its output with National Weather Service radiosondes and with balloon-borne anemometers and by intercomparing the acoustic- and microwave-measured winds under light precipitation conditions. The dual-sensor system has been operating for several months, registering the passage of fronts, some with potentially hazardous wind shears.
Wind shear has been recognized as a major aviation hazard in the airport environment. A dual, acoustic Doppler–microwave Doppler radar system has been installed at Dulles International Airport near Washington, D.C., to measure the vertical profile of wind from the surface to 510 m in 30 m height increments. The acoustic system gathers data under clear-air conditions, and the microwave radar takes over automatically when precipitation is present. System performance is being assessed by comparing its output with National Weather Service radiosondes and with balloon-borne anemometers and by intercomparing the acoustic- and microwave-measured winds under light precipitation conditions. The dual-sensor system has been operating for several months, registering the passage of fronts, some with potentially hazardous wind shears.
Abstract
Accurate measurement of wind speed profiles aloft in the marine boundary layer is a difficult challenge. The development of offshore wind energy requires accurate information on wind speeds above the surface at least at the levels occupied by turbine blades. Few measured data are available at these heights, and the temporal and spatial behavior of near-surface winds is often unrepresentative of that at the required heights. As a consequence, numerical model data, another potential source of information, are essentially unverified at these levels of the atmosphere. In this paper, a motion-compensated, high-resolution Doppler lidar–based wind measurement system that is capable of providing needed information on offshore winds at several heights is described. The system has been evaluated and verified in several ways. A sampling of data from the 2004 New England Air Quality Study shows the kind of analyses and information available. Examples include time–height cross sections, time series, profiles, and distributions of quantities such as winds and shear. These analyses show that there is strong spatial and temporal variability associated with the wind field in the marine boundary layer. Winds near the coast show diurnal variations, and frequent occurrences of low-level jets are evident, especially during nocturnal periods. Persistent patterns of spatial variability in the flow field that are due to coastal irregularities should be of particular concern for wind-energy planning, because they affect the representativeness of fixed-location measurements and imply that some areas would be favored for wind-energy production whereas others would not.
Abstract
Accurate measurement of wind speed profiles aloft in the marine boundary layer is a difficult challenge. The development of offshore wind energy requires accurate information on wind speeds above the surface at least at the levels occupied by turbine blades. Few measured data are available at these heights, and the temporal and spatial behavior of near-surface winds is often unrepresentative of that at the required heights. As a consequence, numerical model data, another potential source of information, are essentially unverified at these levels of the atmosphere. In this paper, a motion-compensated, high-resolution Doppler lidar–based wind measurement system that is capable of providing needed information on offshore winds at several heights is described. The system has been evaluated and verified in several ways. A sampling of data from the 2004 New England Air Quality Study shows the kind of analyses and information available. Examples include time–height cross sections, time series, profiles, and distributions of quantities such as winds and shear. These analyses show that there is strong spatial and temporal variability associated with the wind field in the marine boundary layer. Winds near the coast show diurnal variations, and frequent occurrences of low-level jets are evident, especially during nocturnal periods. Persistent patterns of spatial variability in the flow field that are due to coastal irregularities should be of particular concern for wind-energy planning, because they affect the representativeness of fixed-location measurements and imply that some areas would be favored for wind-energy production whereas others would not.
Addressing the need for high-quality wind information aloft in the layer occupied by turbine rotors (~30–150 m above ground level) is one of many significant challenges facing the wind energy industry. Without wind measurements at heights within the rotor sweep of the turbines, characteristics of the flow in this layer are unknown for wind energy and modeling purposes. Since flow in this layer is often decoupled from the surface, near-surface measurements are prone to errant extrapolation to these heights, and the behavior of the near-surface winds may not reflect that of the upper-level flow.
Addressing the need for high-quality wind information aloft in the layer occupied by turbine rotors (~30–150 m above ground level) is one of many significant challenges facing the wind energy industry. Without wind measurements at heights within the rotor sweep of the turbines, characteristics of the flow in this layer are unknown for wind energy and modeling purposes. Since flow in this layer is often decoupled from the surface, near-surface measurements are prone to errant extrapolation to these heights, and the behavior of the near-surface winds may not reflect that of the upper-level flow.
Abstract
During spring and early summer, a surface confluence zone, often referred to as the dryline, forms in the midwestern United States between continental and maritime air masses. The dewpoint temperature across the dryline can vary in excess of 18°C in a distance of less than 10 km. The movement of the dryline varies diurnally with boundary layer growth over sloping terrain leading to an eastward apparent propagation of the dryline during the day and a westward advection or retrogression during the evening. In this study, we examine the finescale structure of a retrogressing, dryline using data taken by a Doppler lidar, a dual-channel radiometer, and serial rawinsonde ascents. While many previous studies were unable to accurately measure the vertical motions in the vicinity of the dryline, our lidar measurements suggest that the convergence at the dryline is intense with maximum vertical motions of ∼5 m s−1. The winds obtained from the Doppler lidar Measurements were combined with the equations of motion to derive perturbation fields of pressure and virtual potential temperature θ v . Our observations indicate that the circulations associated with this retrogressing dryline were dominated by hot, dry air riding over a westward moving denser, moist flow in a manner similar to a density current. Gravity waves were observed above the dryline interface. Previous observational and numerical studies have shown that differential heating across the dryline may sometimes enhance regional pressure gradients and thus impact dryline movement. We propose that this regional gradient in surface heating in the presence of a confluent flow results in observed intense wind shifts and large horizontal gradients in θ v across the dryline. The local gradient in θ v influences the movement and flow characteristics of the dryline interface. This study is one of the most complete and novel uses of Doppler lidar to date.
Abstract
During spring and early summer, a surface confluence zone, often referred to as the dryline, forms in the midwestern United States between continental and maritime air masses. The dewpoint temperature across the dryline can vary in excess of 18°C in a distance of less than 10 km. The movement of the dryline varies diurnally with boundary layer growth over sloping terrain leading to an eastward apparent propagation of the dryline during the day and a westward advection or retrogression during the evening. In this study, we examine the finescale structure of a retrogressing, dryline using data taken by a Doppler lidar, a dual-channel radiometer, and serial rawinsonde ascents. While many previous studies were unable to accurately measure the vertical motions in the vicinity of the dryline, our lidar measurements suggest that the convergence at the dryline is intense with maximum vertical motions of ∼5 m s−1. The winds obtained from the Doppler lidar Measurements were combined with the equations of motion to derive perturbation fields of pressure and virtual potential temperature θ v . Our observations indicate that the circulations associated with this retrogressing dryline were dominated by hot, dry air riding over a westward moving denser, moist flow in a manner similar to a density current. Gravity waves were observed above the dryline interface. Previous observational and numerical studies have shown that differential heating across the dryline may sometimes enhance regional pressure gradients and thus impact dryline movement. We propose that this regional gradient in surface heating in the presence of a confluent flow results in observed intense wind shifts and large horizontal gradients in θ v across the dryline. The local gradient in θ v influences the movement and flow characteristics of the dryline interface. This study is one of the most complete and novel uses of Doppler lidar to date.
Abstract
The NOAA airborne ozone lidar system [Tunable Optical Profiler for Aerosol and Ozone (TOPAZ)] is compared with the fast-response chemiluminescence sensor flown aboard the NOAA WP-3D during the 2006 Texas Air Quality Study (TexAQS). TOPAZ measurements made from the NOAA Twin Otter, flying at an altitude of ~3300 m MSL in the Houston, Texas, area on 31 August, and the Dallas, Texas, area on 13 September, show that the overall uncertainty in the 10-s (~600-m horizontal resolution) TOPAZ profiles is dominated by statistical uncertainties (1σ) of ~8 ppbv (6%–10%) at ranges of ~2300 m from the aircraft (~1000 m MSL), and ~11–27 ppbv (12%–30%) at ranges of ~2800 m (~500 m MSL). These uncertainties are substantially reduced by spatial averaging, and the averages of 11 profiles (of 110 s or 6.6-km horizontal resolution) at ~1000 m MSL are in excellent agreement (±2%) with the in situ measurements at ~500 m MSL. The TOPAZ measurements at lower altitudes on 31 August exhibit a negative bias of up to ~15%, however, when the lidar signals were strongly attenuated by very high ozone levels in the plume from the Houston Ship Channel. This bias appears to result from nonlinear behavior in the TOPAZ signal amplifiers, which is described in the companion paper by Alvarez et al. An empirical correction is presented.
Abstract
The NOAA airborne ozone lidar system [Tunable Optical Profiler for Aerosol and Ozone (TOPAZ)] is compared with the fast-response chemiluminescence sensor flown aboard the NOAA WP-3D during the 2006 Texas Air Quality Study (TexAQS). TOPAZ measurements made from the NOAA Twin Otter, flying at an altitude of ~3300 m MSL in the Houston, Texas, area on 31 August, and the Dallas, Texas, area on 13 September, show that the overall uncertainty in the 10-s (~600-m horizontal resolution) TOPAZ profiles is dominated by statistical uncertainties (1σ) of ~8 ppbv (6%–10%) at ranges of ~2300 m from the aircraft (~1000 m MSL), and ~11–27 ppbv (12%–30%) at ranges of ~2800 m (~500 m MSL). These uncertainties are substantially reduced by spatial averaging, and the averages of 11 profiles (of 110 s or 6.6-km horizontal resolution) at ~1000 m MSL are in excellent agreement (±2%) with the in situ measurements at ~500 m MSL. The TOPAZ measurements at lower altitudes on 31 August exhibit a negative bias of up to ~15%, however, when the lidar signals were strongly attenuated by very high ozone levels in the plume from the Houston Ship Channel. This bias appears to result from nonlinear behavior in the TOPAZ signal amplifiers, which is described in the companion paper by Alvarez et al. An empirical correction is presented.
