ABSTRACT
A high spectral resolution (HSR) measurement capability in the ultraviolet has been added to the 3-wavelength-2-polarization-backscatter lidar LNG (lidar aerosols nouvelle génération) and tested during several flights. The system includes a Mach-Zehnder interferometer (MZI) as a spectral discriminator and does not require any frequency locking between the emitter and the interferometer. Results obtained during test flights show that the backscatter and extinction coefficients at 355 nm can be measured with a relative precision of 10% for 60 m and 240 m vertical resolution, respectively, in aerosol layers of 10-6 m-1 sr-1 backscatter coefficient with a 30-km horizontal resolution. The same relative precision is obtained in cirrus clouds of a 2×10-5 m-1 sr-1 backscatter coefficient for the same vertical resolution and a horizontal resolution reduced to 5 km. The capacity of the system to perform wind velocity measurements is also demonstrated with precisions in the range of 1 to 2 ms-1. Particle-to-total backscatter ratio and line-of-sight speed measurements have been performed on ground echoes; averaged data show biases less than 1% and 0.15 ms-1, respectively.
ABSTRACT
We demonstrate the first emitter, based on a single optical source device, capable of addressing three species of interest (CO2, CH4, and H2O) for differential absorption Lidar remote sensing of atmospheric greenhouse gases from space in the 2 µm region. It is based on an amplified nested cavity optical parametric oscillator. The single frequency source shows a total conversion efficiency of 37% and covers the 2.05-2.3 µm range.
ABSTRACT
Spaceborne active lidar systems are under development to give new insight into the vertical distribution of clouds and aerosols in the atmosphere and to provide new information on variables required for improvement of forecast models and for understanding the radiative and dynamic processes that are linked to the dynamics of climate change. However, when they are operated from space, lidar systems are limited by atmospheric backscattered signals that have low signal-to-noise ratios (SNRs) on optically thin targets. Therefore specific methods of analysis have to be developed to ensure accurate determination of the geometric and optical properties of scattering layers in the atmosphere. A first approach to retrieving the geometric properties of semitransparent cloud and aerosol layers is presented as a function of false-alarm and no-detection probabilities for a given SNR. Simulations show that the geometric properties of thin cirrus clouds and the altitude of the top of the unstable atmospheric boundary layer can be retrieved with standard deviations smaller than 150 m for a vertical resolution of the lidar system in the 50-100-m range and a SNR of 3. The altitudes of the top of dense clouds are retrieved with a precision in altitude of better than 50 m, as this retrieval corresponds to a higher SNR value. Such methods have an important potential application to future spaceborne lidar missions.
ABSTRACT
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)].
ABSTRACT
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.
ABSTRACT
The Paris area is strongly urbanized and is exposed to atmospheric pollution events. To understand the chemical and physical processes that are taking place in this area it is necessary to describe correctly the atmospheric boundary-layer (ABL) dynamics and the ABL height evolution. During the winter of 1994-1995, within the framework of the Etude de la Couche Limite Atmosphérique en Agglomération Parisienne (ECLAP) experiment, the vertical structure of the ABL over Paris and its immediate suburbs was extensively documented by means of lidar measurements. We present methods suited for precise determination of the ABL structure's temporal evolution in a dynamic environment as complex as the Paris area. The purpose is to identify a method that can be used on a large set of lidar data. We compare commonly used methods that permit ABL height retrievals from backscatter lidar signals under different meteorological conditions. Incorrect tracking of the ABL depth's diurnal cycle caused by limitations in the methods is analyzed. The study uses four days of the ECLAP experiment characterized by different meteorological and synoptic conditions.
ABSTRACT
An optical technique is described that determines the path-averaged value of a refractive-index structure parameter at 10.6 microm by use of a pulsed coherent CO(2) lidar in direct detection and hard-target returns. The lidar measurements are compared with measurements taken by a 0.9-microm scintillometer and temperature probe (with humidity corrections). The experimental results show good agreement for C(n)(2) >or= (-14) m(-2/3). With respect to practical applications the new technique permits C(n)(2) lidar measurements in a neutral meteorological situation to an unstably stratified convective boundary layer over long ranges (1 km or more).
ABSTRACT
A preliminary study of the synergism between active and passive spaceborne remote sensing systems has been conducted on the basis of new prospects for the implementation of lidar systems on space platforms for global scale measurements. Assuming a quasi-simultaneity in the measurements performed with an active backscatter lidar and with operational meteorological packages such as the Television Infrared Operational Satellite (TIROS)-N Operational Vertical Sounder radiometers, it is shown that combining both measurements could lead to an improvement in the accuracy of the retrieved vertical temperature profile in the lower troposphere. We used a modified version of the improved initialization inversion operational algorithm, to process the TIROS-N Operational Vertical Sounder data, taking into account the lidar measurements of cloud heights to define a temperature reference. New perspectives for the coupling of lidar and passive radiometers are discussed.
ABSTRACT
The development of a model for 2-mum laser operation in Tm, Ho:YAG and YLF crystals pumped in the near infrared is reported. This model, based on a simplified spectroscopic scheme, is fitted to a set of characterization experiments by means of three adjustable parameters. Results show that the excited-state populations are predicted with a relative accuracy of approximately 10% for a large range of pump levels. Using this model, we calculate the extractable energy on short-laser-pulse interactions for the two materials under different operation conditions. We study the sensitivity to pump duration and the optimization of dopant concentrations. We investigate the improvement of the extractable-energy efficiency with multiple-pulse operation. For double-pulse operation the improvement is approximately a factor of 1.5 and leads to overall extractable-energy efficiencies of 16% in YAG and 15% in YLF for an absorbed pump energy of 10 J cm(-3).
ABSTRACT
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.
ABSTRACT
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.
ABSTRACT
To study the ozone spatial and temporal evolution in the atmosphere, lidar systems have proved to be adequate but have remained complex. We define in this paper the main characteristics of a UVDIAL system for ground based and airborne ozone measurements in the troposphere and the lower stratosphere both for daytime and nighttime operation. A multiwavelength lidar system using either Rayleigh/Mie signals or the Raman nitrogen signal, is discussed as a way to efficiently correct the ozone measurements from the systematic bias due to aerosol and other interference gases (i.e. SO(2)) in the lower troposphere. Two types of lasers (solid state and excimer) are compared, as both lasers are suitable for long term field operation and airborne use.
