Didier Bruneau

Airborne lidar LEANDRE II for water-vapor profiling in the troposphere. I. System description

The airborne differential absorption lidar LEANDRE II, developed for profiling tropospheric water-vapor mixing ratios, is described. The emitter is a flash-lamp-pumped alexandrite laser, which operates in a double-pulse, dual-wavelength mode in the 727-736 nm spectral domain. Two 50-mJ successive on-line and off-line pulses with an output linewidth of 2.4 x 10(-2) cm(-1) and a spectral purity larger than 99.99% are emitted at a 50-mus time interval. The spectral positioning is controlled in real time by a wavemeter with an absolute accuracy of 5 x 10(-3) cm(-1). The receiver is a 30-cm aperture telescope with a 3.5-mrad field of view and a 1-nm filter bandwidth. These instrument characteristics are defined for measuring the water-vapor mixing ratio with an accuracy better than 0.5 g kg(-1) in the first 5 km of the atmosphere with a range resolution of 300 m, integration on 100 shots, and an instrumental systematic error of less than 2%. The sensitivity study and first results are presented in part II [Appl. Opt. 40, 3462-3475 (2001)].

Airborne lidar measurements of aerosol spatial distribution and optical properties over the Atlantic Ocean during a European pollution outbreak of ACE-2

Airborne lidar measurements of the aerosol spatial distribution and optical properties associated with an European pollution outbreak which occured during the Second Aerosol Characterization Experiment (ACE-2) are presented. Size distribution spectra measured over the ocean near Sagres (Portugal), on-board the Research Vessel Vodyanitsky and on-board the Avion de Recherche Atmosphérique et Télédétection (ARAT) have been used to parameterize the aerosol vertical distribution. This parameterization, which is essential to the analysis of airborne lidar measurements, has been validated via closure experiments on extinction coefficient profiles and aerosol optical depth (AOD). During the studied event, AOD7’s retrieved from lidar measurements at 0.73 μm range between 0.055 and 0.10. The parameterized aerosol vertical distribution has been used to shift AOD retrievals from 0.73 to 0.55 μm to enable comparison with other remote sensing instruments. At the latter wavelength, AOD’s retrieved from lidar measurements range between 0.08 and 0.14. An agreement better than 20% is obtained between AOD’s derived from lidar and sunphotometer measurements made at the same time and place over the ocean near the coast. However, large differences are observed with the AOD estimated from Meteosat imagery in the same area. These differences are thought to be caused by large uncertainties associated with the Meteosat sensitivity for small AOD’s or by the presence of thin scattered clouds. Lidar-derived particulate extinction profiles and scattering coefficient profiles measured by a nephelometer mounted on the ARAT, in a different part of the plume, were found in good agreement, which could be an indication that absorption by pollution aerosols is small and/or that soot is present in small amounts in the European pollution plume. Lidar measurements have also been used to differentiate the contribution of different aerosol layers to the total AOD. It is shown that the AOD in the marine atmospheric boundary layer (MABL) can contribute as much as 70% of the total AOD in some regions. At 0.73 μm, the AOD in the continental plume was observed to diminish with the distance to the coastline from 0.04 to 0.03.

Vertical 2-μm Heterodyne Differential Absorption Lidar Measurements of Mean CO2 Mixing Ratio in the Troposphere

Abstract Vertical mean CO2 mixing ratio measurements are reported in the atmospheric boundary layer (ABL) and in the lower free troposphere (FT), using a 2-μm heterodyne differential absorption lidar (HDIAL). The mean CO2 mixing ratio in the ABL is determined using 1) aerosol backscatter signal and a mean derivative of the increasing optical depth as a function of altitude and 2) optical depth measurements from cloud target returns. For a 1-km vertical long path in the ABL, 2% measurement precision with a time resolution of 30 min is demonstrated for the retrieved mean CO2 absorption. Spectroscopic calculations are reported in details using new spectroscopic data in the 2-μm domain and the outputs of the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5). Then, using both aerosols in the ABL and midaltitude dense clouds in the free troposphere, preliminary HDIAL measurements of mean CO2 mixing ratio in the free troposphere are also presented. The ...

Mach–Zehnder interferometer as a spectral analyzer for molecular Doppler wind lidar

The theoretical performance of a Mach-Zehnder interferometer used as a spectral analyzer for wind-speed measurement by direct-detection Doppler lidar is presented. The interferometer is optimized for measurement of wind velocity from the signal backscattered by the molecules. Two arrangements are proposed, involving two detection channels (DMZ) or four detection channels (QMZ). Using the assumption of a pure molecular signal with a Gaussian spectral profile, we derive an analytic expression for the standard deviation of the measurement error for each arrangement. They are then compared with the ideal spectral analyzer (ISA) and with the double-edge Fabry-Perot (DFP) in the case of a shot-noise-limited signal. The DMZ measurement error is shown to be only 1.65 times that of the ISA and is 1.4 times lower than that given by the DFP. The QMZ arrangement provides a measurement that is insensitive to the aerosol scattering contribution but gives a measurement error that is 1.4 times higher than that of the DMZ.

Double-pulse dual-wavelength alexandrite laser for atmospheric water vapor measurement

We describe a new alexandrite laser source arrangement designed to measure atmospheric water vapor using the differential absorption lidar technique. This laser is capable of emitting two pulses at two appropriately selected wavelengths within a single flash lamp discharge. A narrow spectral linewidth of Deltalambda < 1 pm is obtained for each pulse by intracavity filtering with a birefringent filter and two Fabry-Perot interferometers. Wavelength commutation between the two pulses is performed by electro-optically tuning the birefringent filter. The temporal separation between the two pulses can be chosen between 50 and 70 micros and each pulse duration is <250-ns (full width at half-maximum). Typical output energies of 50 mJ/pulse at each wavelength are obtained with this laser system at a 10-Hz repetition rate for a 1.3-kW input electrical power.

Simultaneous measurements of particle backscattering and extinction coefficients and wind velocity by lidar with a Mach–Zehnder interferometer: principle of operation and performance assessment

The development of remote-sensing instruments that can be used to monitor several parameters at the same time is important for the study of complex processes such as those that control climate and environment. In this paper the performance of a new concept of lidar receiver that allows for the direct measurement of aerosol and cloud optical properties simultaneously with wind velocity is investigated. This receiver uses a Mach-Zehnder interferometer. Two different configurations, either with four photometric output channels or with fringe imaging on a multichannel detector, are studied. Analytical expressions of the statistical errors are given under the assumption of Gaussian signal spectra. It is shown that similar accuracies can be achieved for both configurations. Performance modeling of the retrieval of semitransparent cloud optical scattering properties and wind velocity was done at different operation wavelengths for a Nd:YAG laser source. Results for such a lidar system onboard an aircraft flying at an altitude of 12 km show that for semitransparent clouds the best results were obtained at 355 nm, with relative standard deviations of 0.5% and 5% for the backscatter and extinction coefficients, respectively, together with a velocity accuracy of 0.2 ms(-1). The accuracy of optical properties retrieved for boundary layer aerosols are comparable, whereas the velocity accuracy is decreased to 1 ms(-1). Finally, an extrapolation to a large 355-nm spaceborne lidar shows accuracies in the range from 2.5% to 5% for the backscatter coefficient and from 10% to 15% for the extinction coefficient together with a vertical wind speed accuracy of better than 0.5 ms(-1) for semitransparent clouds and boundary layer, with a vertical resolution of 500 m and a 100 shot averaging.

Airborne lidar LEANDRE II for water-vapor profiling in the troposphere. II. First results

The airborne lidar LEANDRE II, described in part I [Appl. Opt. 40, 3450-3461 (2001)], has been flown on the French Atmospheric Research Aircraft to perform lower-troposphere (0-3.5-km) measurements of the water-vapor mixing ratio. We present and discuss the method used for retrieval of the water-vapor mixing ratio and analyze systematic and random measurement errors in relation to instrument design and performance. The results of a series of test flights are presented. With a 0.8-km horizontal resolution and a 300-m vertical resolution, the standard deviation of the measurement error ranges from approximately 0.05 g kg(-1) at 3.5 km to 0.3-0.4 g kg(-1) near the ground, in agreement with the predicted random error. Comparisons with dew-point hygrometer measurements show a vertically averaged difference of ?0.15 g kg(-1), approximately equal to the observed water-vapor variability.

Laser diode absorption spectroscopy for accurate CO 2 line parameters at 2 μm: consequences for space-based DIAL measurements and potential biases

Space-based active sensing of CO(2) concentration is a very promising technique for the derivation of CO(2) surface fluxes. There is a need for accurate spectroscopic parameters to enable accurate space-based measurements to address global climatic issues. New spectroscopic measurements using laser diode absorption spectroscopy are presented for the preselected R30 CO(2) absorption line ((20(0)1)(III)<--(000) band) and four others. The line strength, air-broadening halfwidth, and its temperature dependence have been investigated. The results exhibit significant improvement for the R30 CO(2) absorption line: 0.4% on the line strength, 0.15% on the air-broadening coefficient, and 0.45% on its temperature dependence. Analysis of potential biases of space-based DIAL CO(2) mixing ratio measurements associated to spectroscopic parameter uncertainties are presented.

Injection-seeded pulsed alexandrite laser for differential absorption lidar application

We describe a Q-switched alexandrite laser injection seeded with a cw single-mode titanium-sapphire laser. The reported experimental results show that this system meets the frequency stabilization required for differential absorption lidar measurement of humidity, pressure, and temperature. The emission of the cw titanium-sapphire master oscillator is locked to an atmospheric absorption line by means of a servoloop with derivative spectroscopy. The spectral position is stabilized within ±3.5 × 10(-4) cm(-1) (10 MHz) of the peak of the line over 1 hr. The alexandrite laser emits pulses of 30 mJ in 500 ns, with a spectral linewidth of ≈ 3.3 × 10(-3) cm(-1) (100 MHz). The position of the centroid of the emitted spectrum has a standard deviation of 6 × 10(-4) cm(-1) (18 MHz) and is held within ±1.3 × 10(-3) cm(-1) (40 MHz) of the peak of the absorption line over 1 h.

Fringe-imaging Mach-Zehnder interferometer as a spectral analyzer for molecular Doppler wind lidar

The theoretical performance of a Mach-Zehnder interferometer used as a spectral analyzer for wind-speed measurement by direct-detection Doppler lidar is presented. The interferometer is optimized for the measurement of wind velocity from the signal that is backscattered by the molecules. In the proposed fringe-imaging Mach-Zehnder (FIMZ) interferometer, a pattern of equally spaced linear fringes is formed and detected by two conventional detector linear arrays. Assuming a pure molecular signal with Gaussian spectral profile, an analytic expression for the standard deviation of the measurement error is obtained and compared with the Cramer-Rao lower bound given by an ideal spectral analyzer (ISA) in the case of shot-noise-limited signal. The FIMZ measurement error is shown to be 2.3 times that of the ISA and is comparable with the error given by previously developed multichannel spectral analyzers that are based on Fabry-Perot interferometers that, in contrast, have the disadvantages of producing unequally spaced circular fringes and requiring dedicated detectors. The optimal path difference for a FIMZ operating at 355 nm is approximately 3 cm. The interferometer is shown to match important lidar beam étendues without significant performance reduction.