Boundary layer scattering measurements with a charge-coupled device camera lidar

A CCD-based bistatic lidar (CLidar) system has been developed and constructed to measure scattering in the atmospheric boundary layer. The system uses a CCD camera, wide-angle optics, and a laser. Imaging a vertical laser beam from the side allows high-altitude resolution in the boundary layer all the way to the ground. The dynamic range needed for the molecular signal is several orders of magnitude in the standard monostatic method, but only approximately 1 order of magnitude with the CLidar method. Other advantages of the Clidar method include low cost and simplicity. Observations at Mauna Loa Observatory, Hawaii, show excellent agreement with the modeled molecular-scattering signal. The scattering depends on angle (altitude) and the polarization plane of the laser.     

Ultraviolet high-spectral-resolution Doppler lidar for measuring wind field and aerosol optical properties

An ultraviolet incoherent Doppler lidar that incorporates the high-spectral-resolution (HSR) technique has been developed for measuring the wind field and aerosol optical properties in the troposphere. An injection seeded and tripled Nd:YAG laser at an ultraviolet wavelength of 355 nm was used in the lidar system. The HRS technique can resolve the aerosol Mie backscatter and the molecular Rayleigh backscatter to derive the signal components. By detecting the Mie backscatter, a great increase in the Doppler filter sensitivity was realized compared to the conventional incoherent Doppler lidars that detected the Rayleigh backscatter. The wind velocity distribution in a two-dimensional cross section was measured. By using the HSR technique, multifunction and absolute value measurements were realized for aerosol extinction, and volume backscatter coefficients; the laser beam transmittance, the lidar ratio, and the backscatter ratio are derived from these measurements.     

High-pulse-repetition-freqyency lidar system using a single telescope for transmission and reception

The design, construction, and operation of a stratospheric Rayleigh lidar system is outlined. The lidar system was designed to operate as a Doppler lidar; however, for the first stage of the project it was set up to operate in a manner similar to a more conventional stratospheric Rayleigh lidar. This system includes a number of unique design features, including a high-pulse-repetition-frequency laser and the use of a single 1-m-diameter telescope for transmission of the laser pulse and reception of the backscattered light. An associated high-speed rotating shutter system switches the optical system from the transmission to the reception mode. The system was operated at Adelaide, Australia (35° S, 138° E). Scattering ratio and temperature profiles are calculated for data collected during the period from 10 March 1992 to 11 May 1993. The scattering ratio profiles clearly show the reduction in the scattering from the stratospheric aerosol layer. This is due to the removal of the aerosol injected by the eruption of Mt. Pinatubo. The measured relative density profiles show very good agreement with the Cospar international reference atmosphere model densities, as do the temperature profiles calculated from these.     

Down looking lidar inversion constrained by ocean reflection and forward scatter of laser light

The laser aureole at 1.06 microm resulting from the redirection of light by sea surface reflection and forward scatter through maritime boundary layer aerosols to a sensor high above the ocean surface is modeled for profiles with typical North Atlantic aerosol size distributions. The magnitude of this laser aureole is highly correlated with the optical depth for these profiles. This optical depth, estimated from the laser aureole, is used to adjust the power of the extinction-backscatter relationship in a Bernoulli-Riccati lidar inversion. Using a lognormal marine aerosol model, 150 profiles of aerosol size distributions are selected by their probability of occurrence in the North Atlantic boundary layer. For these profiles, the lidar inversion using the estimated optical depth predicted the surface extinction 5 times better than the lidar inversions using a climatological backscatter-extinction relationship.     

Backscattering measurements of atmospheric aerosols at CO2 laser wavelengths: implications of aerosol spectral structure on differential-absorption lidar retrievals of molecular species

The volume backscattering coefficients of atmospheric aerosol were measured with a tunable CO2 lidar system at various wavelengths in Utah (a desert environment) along a horizontal path a few meters above the ground. In deducing the aerosol backscattering, a deconvolution (to remove the smearing effect of the long CO2 lidar pulse and the lidar limited bandwidth) and a constrained-slope method were employed. The spectral shape beta(lambda) was similar for all the 13 measurements during a 3-day period. A mean aerosol backscattering-wavelength dependence beta(lambda) was computed from the measurements and used to estimate the error Delta(CL) (concentration-path-length product) in differential-absorption lidar measurements for various gases caused by the systematic aerosol differential backscattering and the error that is due to fluctuations in the aerosol backscattering. The water-vapor concentration-path-length product CL and the average concentration C = /L for a path length L computed from the range-resolved lidar measurements is consistently in good agreement with the water-vapor concentration measured by a meteorological station. However, I was unable to deduce, reliably, the range-resolved water-vapor concentration C(r), which is the derivative of the range-dependent product CL, because of the effect of residual noise caused mainly by errors in the deconvolved lidar measurements.     

Ground-truth aerosol lidar observations: can the Klett solutions obtained from ground and space be equal for the same aerosol case?

Upcoming multiyear satellite lidar aerosol observations need strong support by a worldwide ground-truth lidar network. In this context the question arises as to whether the ground stations can deliver the same results as obtained from space when the Klett formalism is applied to elastic backscatter lidar data for the same aerosol case. This question is investigated based on simulations of observed cases of simple and complex aerosol layering. The results show that the differences between spaceborne and ground-based observations can be as large as 20% for the backscatter and extinction coefficients and the optimum estimates of the column lidar ratios. In cases with complex aerosol layering, the application of the two-layer approach can lead to similar results (space, ground) and accurate products provided that horizontally homogeneous aerosol conditions are given.     

Nighttime measurements of atmospheric optical thickness by star photometry with a digital camera

Nighttime stellar photometric measurements have been carried out with a commercial digital single-lens reflex camera to determine the atmospheric optical thickness on large fields of view (FOV). Specific procedures of image analysis allow to extract an equivalent irradiance for a number of stars and for the sky light background; thus, a measure of the optical thickness in each star direction can be retrieved. A larger FOV is obtained by stitching several photographs shot in quick sequence on adjacent regions of the sky: such measurements provide almost instantaneous maps of optical thickness and skylight background that indicate the degree of homogeneity of the aerosol load. Additional information provided by the combined use of the camera and a lidar is presented. The zenithal optical thickness is used with values of the aerosol backscatter provided by a lidar system to obtain the aerosol extinction-to-backscatter ratio.     

Intensity-modulated, stepped frequency cw lidar for distributed aerosol and hard target measurements

A compact frequency-modulated, continuous wave (FM-cw) lidar system for measurement of distributed aerosol plumes and hard targets is presented. The system is based on intensity modulation of a laser diode and quadrature detection of the return signals. The advantages of using laser diode amplitude modulation and quadrature detection is a large reduction in the hardware required for processing and storing return signals as well as the availability of off-the-shelf integrated electronic components from the wireless and telecommunication communities. Equations to invert the quadrature signal components and determine spatial distributions of multiple targets are derived. Spatial scattering intensities are used to extract aerosol backscatter coefficients, which can then be directly compared to microphysics aerosol models for environmental measurements. Finally, results from laboratory measurements with a monostatic FM-cw lidar system with both hard targets and aerosols are discussed.     

Standoff determination of the particle size and concentration of small optical depth clouds based on double-scattering measurements: validation with calibrated target plates and limitations for daytime and nighttime measurements

Diffractive target plates are used to emulate aerosols of known size and concentration. These target plates are used to validate and determine the sensitivity of a multiple-field-of-view lidar signal inversion technique based on double-scattering measurement to retrieve the particle size and the concentration of small optical depth clouds. We estimate that nighttime and daytime quantification (size and concentration) is possible for optical depths as low as 0.005 and 0.016, respectively. The recovery technique limiting factors are the shot noise, the laser features, the optical lens quality, the background illumination level, the background aerosol fluctuations, and the noise introduced by the lidar detector, a gated intensified camera (camera G-ICCD).     

Aerosol and cloud backscatter at 1.06, 1.54, and 0.53 mum by airborne hard-target-calibrated Nd:YAG /methane Raman lidar

A lidar instrument was developed to make simultaneous measurements at three distinct wavelengths in the visible and near infrared at 0.532, 1.064, and 1.54 mum with high cross-sectional calibration accuracy. Aerosol and cloud backscatter cross sections were acquired during November and December 1989 and May and June 1990 by the NASA DC-8 aircraft as part of the Global Backscatter Experiment. The instrument, methodology, and measurement results are described. A Nd:YAG laser produced 1.064- and 0.532-mum energy. The 1.54-mum transmitted pulse was generated by Raman-shifted downconversion of the 1.064-mum pulse through a Raman cell pressured with methane gas. The lidar could be pointed in the nadir or zenith direction from the aircraft. A hard-target-based calibration procedure was used to obtain the ratio of the system calibration between the three wavelengths, and the absolute calibration was referenced to the 0.532-mum lidar molecular backscatter cross section for the clearest scattering regions. From the relative wavelength calibration, the aerosol backscatter cross sections at the longer wavelengths are resolved for values as small as 1% of the molecular cross section. Backscatter measurement accuracies are better than 10(-9) (m sr)(-1) at 1.064 and 1.54 mum. Results from the Pacific Ocean region of the multiwavelength backscatter dependence are presented. Results show extensive structure and variation for the aerosol cross sections. The range of observed aerosol cross section is over 4 orders of magnitude, from less than 10(-9) (m sr)(-1) to greater than 10(-5) (m sr)(-1).     

Ultraviolet Rayleigh-Mie lidar for daytime-temperature profiling of the troposphere

A UV Rayleigh-Mie scattering lidar has been developed for daytime measurement of temperature and aerosol optical properties in the troposphere. The transmitter is a narrowband, injection-seeded, pulsed, third-harmonic Nd:YAG laser at an eye-safe wavelength of 355 nm. Two Fabry-Perot etalons (FPEs) with a dual-pass optical layout filter the molecular Rayleigh scattering components spectrally for retrieval of the temperature and provide a high rejection rate for aerosol Mie scattering in excess of 43 dB. The Mie signal is filtered with a third FPE filter for direct profiling of aerosol optical properties. The Mie scattering component in the Rayleigh signals, which will have influence on temperature measurements, is corrected by using a measure of aerosol scattering because of the relative insufficiency of Mie rejection of Rayleigh filters in the presence of dense aerosols or clouds, and the Mie rejection capability of system is thus improved. A narrowband interference filter is incorporated with the FPEs to block solar radiation. Also, the small field of view (0.1 mrad) of the receiver and the UV wavelength used enhance the ability of the lidar to suppress the solar background signal in daytime measurement. The system is relatively compact, with a power-aperture product of 0.18 W m(-2), and has a high sensitivity to temperature change (0.62%/K). Lidar measurements taken under different weather conditions (winter and summer) are demonstrated. Good agreement between the lidar and the radiosonde measurements was obtained in terms of lapse rates and inversions. Statistical temperature errors of less than 1 K up to a height of 2 km are obtainable, with an averaging time of approximately 12 min for daytime measurements.     

Two-channel direct-detection Doppler lidar employing a charge-coupled device as a detector

A direct-detection Doppler lidar system is demonstrated that uses a CCD as a detector for the first time to our knowledge. The ability to use this linear device with the circular output from a Fabry-Perot etalon comes from use of a circle-to-line converter [Appl. Opt. 29, 1482 (1990)]. In addition to the gains in quantum efficiency obtained through use of this detector, the lidar system described in this paper also has the capability to measure winds from aerosol and molecular backscatter simultaneously in two separate channels by directing the light reflected from one channel into the other. Early measurements with this system are presented; it is shown that, although accurate aerosol wind measurements are easily obtained, molecular measurements require a carefully calibrated inverse model and special hardware to derive accurate wind measurements with this channel.     

High-spectral-resolution lidar with iodine-vapor filters: measurement of atmospheric-state and aerosol profiles

A high-spectral-resolution lidar can measure vertical profiles of atmospheric temperature, pressure, the aerosol backscatter ratio, and the aerosol extinction coefficient simultaneously. We describe a system with these characteristics. The transmitter is a narrow-band (FWHM of the order of 74 MHz), injection-seeded, pulsed, double YAG laser at 532 nm. Iodine-vapor filters in the detection system spectrally separate the molecular and aerosol scattering and greatly reduce the latter (-41 dB). Operating at a selected frequency to take advantage of two neighboring lines in vapor filters, one can obtain a sensitivity of the measured signal-to-air temperature ratio equal to 0.42%/K. Using a relatively modest size transmitter and receiver system (laser power times telescope aperture equals 0.19 Wm(2)), our measured temperature profiles (0.5-15 km) over 11 nights are in agreement with balloon soundings to within 2.0 K over an altitude range of 2-5 km. There is good agreement in the lapse rates, tropopause altitudes, and inversions. In principle, to invert the signal requires a known density at one altitude, but in practice it is convenient to also use a known temperature at that altitude. This is a scalable system for high spatial resolution of vertical temperature profiles in the troposphere and lower stratosphere, even in the presence of aerosols.     

Description and evaluation of a tropospheric ozone lidar implemented on an existing lidar in the southern subtropics

Rayleigh-Mie lidar measurements of stratospheric temperature and aerosol profiles have been carried out at Reunion Island (southern tropics) since 1993. Since June 1998, an operational extension of the system is permitting additional measurements of tropospheric ozone to be made by differential absorption lidar. The emission wavelengths (289 and 316 nm) are obtained by stimulated Raman shifting of the fourth harmonic of a Nd:YAG laser in a high-pressure deuterium cell. A mosaic of four parabolic mirrors collects the backscattered signal, and the transmission is processed by the multiple fiber collector method. The altitude range of ozone profiles obtained with this system is 3?17 km. Technical details of this lidar system working in the southern tropics, comparisons of ozone lidar profiles with radiosondes, and scientific perspectives are presented. The significant lack of tropospheric ozone measurements in the tropical and equatorial regions, the particular scientific interest in these regions, and the altitude range of the ozone measurements to 16?17 km make this lidar supplement useful and its adaptation technically conceivable at many Rayleigh-Mie lidar stations.     

NASA multipurpose airborne DIAL system and measurements of ozone and aerosol profiles

An airborne differential absorption lidar (DIAL) system has been developed for the remote measurement of gas and aerosol profiles in the troposphere and lower stratosphere. The multipurpose DIAL system can operate from 280 to 1064 nm for measurements of ozone, sulfur dioxide, nitrogen dioxide, water vapor, temperature,pressure, and aerosol backscattering. The laser transmitter consists of two narrow linewidth Nd: YAG pumped dye lasers with automatic wavelength control. The DIAL wavelengths are transmitted with a 00-,usec temporal separation to reduce receiver system complexity. A coaxial receiver system is used to collect and optically separate the DIAL and aerosol lidar returns. Photomultiplier tubes detect the backscattered laser returns after optical filtering, and the analog signals from three tubes are digitized and stored on high-speed magnetic tape. Real-time gas concentration profiles or aerosol backscatter distributions are calculated and displayed for experiment control. Operational parameters for the airborne DIAL system are presented for measurements of ozone, water vapor, and aerosols in the 290-, 720-, and 600-nm wavelength regions, respectively. The first ozone profile measurements from an aircraft using the DIAL technique are discussed in this paper. Comparisons between DIAL and in situ ozone measurements show agreement to within +/-5 ppbv in the lower troposphere. Lidar aerosol data obtained simultaneously with DIAL ozone measurements are presented for a flight over Virginia and the Chesapeake Bay. DIAL system performance for profiling ozone in a tropopause folding experiment is evaluated, and the applications of the DIAL system to regional and global-scale tropospheric investigations are discussed.     

Airborne lidar imaging of salmon

Lidar images of adult salmon are presented. The lidar system is built around a pulsed green laser and a gated intensified CCD camera. The camera gating is timed to collect light scattered from the turbid water below the fish to produce shadows in the images. Image processing increases the estimated contrast-to-noise ratio from 3.4 in the original image to 16.4 by means of a matched filter.     

Relativistic effects in earth-orbiting Doppler lidar return signals

Frequency shifts of side-ranging lidar signals are calculated to high order in the small quantities (v/c), where v is the velocity of a spacecraft carrying a lidar laser or of an aerosol particle that scatters the radiation back into a detector (c is the speed of light). Frequency shift measurements determine horizontal components of ground velocity of the scattering particle, but measured fractional frequency shifts are large because of the large velocities of the spacecraft and of the rotating earth. Subtractions of large terms cause a loss of significant digits and magnify the effect of relativistic corrections in determination of wind velocity. Spacecraft acceleration is also considered. Calculations are performed in an earth-centered inertial frame, and appropriate transformations are applied giving the velocities of scatterers relative to the ground.     

Two-wavelength lidar inversion algorithm for a two-component atmosphere

A method for the boundary-value determination of aerosol extinction profiles from backscatter lidar measurements is presented. Artificially generated lidar signals from two-component inhomogeneous model atmospheres are inverted with the information from two wavelengths (532 and 1064 nm) simultaneously. The solution for the vertical aerosol extinction profile is formulated with Klett's far-end solution. The boundary value is expressed in terms of aerosol transmission along the lidar line according to Fernald's solution of the lidar equation. The aerosol transmission is determined iteratively with a transcendental equation on the assumption that a linear relationship exists between the extinction coefficients at both wavelengths. Inversion calculations are applied to model atmospheres with range-dependent lidar ratios representing the growth of aerosol particles caused by increasing relative humidity in the planetary boundary layer. For the inversion constant lidar ratios are assumed that vary between 40 and 70 sr. The numerical procedure turns out to be stable enough to provide meaningful results even in cases of misestimated lidar ratios. The application of the method is of less use for misestimated background radiation and low aerosol concentrations.     

Development of a high spectral resolution lidar based on confocal Fabry-Perot spectral filters

The high spectral resolution lidar (HSRL) instrument described in this paper utilizes the fundamental and second-harmonic output from an injection seeded Nd:YAG laser as the laser transmitter. The light scattered in the atmosphere is collected using a commercial Schmidt-Cassegrain telescope with the optical receiver train first splitting the fundamental and second-harmonic return signal with the fundament light monitored using an avalanche photodiode. The second-harmonic return signal is mode matched into a tunable confocal Fabry-Perot (CFP) interferometer with a free spectral range of 7.5?GHz and a finesse of 50.7 (312) at 532?nm (1064?nm) placed in the optical receiver for spectrally filtering the molecular and aerosol return signals. The light transmitted through the CFP is used to monitor the aerosol return signal while the light reflected from the CFP is used to monitor the molecular return signal. Data collected with the HSRL are presented and inversion results are compared to a co-located solar radiometer, demonstrating the successful operation of the instrument. The CFP-based filtering technique successfully employed by this HSRL instrument is easily portable to other arbitrary wavelengths, thus allowing for the future development of multiwavelength HSRL instruments.