Monitoring O3 with solar-blind Raman lidars

The benefits of retrieving ozone concentration profiles by a use of a single Raman signal rather than the Raman differential absorption lidar (DIAL) technique are investigated by numerical simulations applied either to KrF- (248 nm) or to quadrupled Nd:YAG- (266 nm) based Raman lidars, which are used for both daytime and nighttime monitoring of the tropospheric water-vapor mixing ratio. It is demonstrated that ozone concentration profiles of adequate accuracy and spatial and temporal resolution can be retrieved under low aerosol loading by a single Raman lidar because of the large value of the ozone absorption cross section both at 248 nm and at 266 nm. Then experimental measurements of Raman signals provided by the KrF-based lidar operating at the University of Lecce (40 degrees 20'N, 18 degrees 6'E) are used to retrieve ozone concentration profiles by use of the Raman DIAL technique and the nitrogen Raman signal.     

Ground-based differential absorption lidar for water-vapor profiling: assessment of accuracy, resolution, and meteorological applications

The accuracy and the resolution of water-vapor measurements by use of the ground-based differential absorption lidar (DIAL) system of the Max-Planck-Institute (MPI) are determined. A theoretical analysis, intercomparisons with radiosondes, and measurements in high-altitude clouds allow the conclusion that, with the MPI DIAL system, water-vapor measurements with a systematic error of <5% in the whole troposphere can be performed. Special emphasis is laid on the outstanding daytime and nighttime performance of the DIAL system in the lower troposphere. With a time resolution of 1 min the statistical error varies between 0.05 g/m(3) in the near range using 75 m and-depending on the meteorological conditions-approximately 0.25 g/m(3) at 2 km using 150-m vertical resolution. When the eddy correlation method is applied, this accuracy and resolution are sufficient to determine water-vapor flux profiles in the convective boundary layer with a statistical error of <10% in each data point to approximately 1700 m. The results have contributed to the fact that the DIAL method has finally won recognition as an excellent tool for tropospheric research, in particular for boundary layer research and as a calibration standard for radiosondes and satellites.     

Ozone and water-vapor measurements by Raman lidar in the planetary boundary layer: error sources and field measurements

A new lidar instrument has been developed to measure tropospheric ozone and water vapor at low altitude. The lidar uses Raman scattering of an UV beam from atmospheric nitrogen, oxygen, and water vapor to retrieve ozone and water-vapor vertical profiles. By numerical simulation we investigate the sensitivity of the method to both atmospheric and device perturbations. The aerosol optical effect in the planetary boundary layer, ozone interference in water-vapor retrieval, statistical error, optical cross talk between Raman-shifted channels, and optical cross talk between an elastically backscattered signal in Raman-shifted signals and an afterpulse effect are studied in detail. In support of the main conclusions of this model study, time series of ozone and water vapor obtained at the Swiss Federal Institute of Technology in Lausanne and during a field campaign in Crete are presented. They are compared with point monitor and balloon sounding measurements for daytime and nighttime conditions.     

Narrow-band, narrow-field-of-view Raman lidar with combined day and night capability for tropospheric water-vapor profile measurements

We describe a high-performance Raman lidar system with combined day and night capability for tropospheric water-vapor profile measurements. The system incorporates high-performance UV interference filters and a narrow-band, dual-field-of-view receiver for rejection of background sunlight. Daytime performance has been demonstrated up to 5 km with 150-m vertical and 5-min temporal averaging. The nighttime performance is significantly better with measurements routinely extending from 10 to 12 km with 75-m range resolution and a 5-min temporal average. We describe design issues for daytime operation and a novel daytime calibration technique.     

Spaceborne profiling of atmospheric temperature and particle extinction with pure rotational Raman lidar and of relative humidity in combination with differential absorption lidar: performance simulations

The performance of a spaceborne temperature lidar based on the pure rotational Raman (RR) technique in the UV has been simulated. Results show that such a system deployed onboard a low-Earth-orbit satellite would provide global-scale clear-sky temperature measurements in the troposphere and lower stratosphere with precisions that satisfy World Meteorological Organization (WMO) threshold observational requirements for numerical weather prediction and climate research applications. Furthermore, nighttime temperature measurements would still be within the WMO threshold observational requirements in the presence of several cloud structures. The performance of aerosol extinction measurements from space, which can be carried out simultaneously with temperature measurements by RR lidar, is also assessed. Furthermore, we discuss simulations of relative humidity measurements from space obtained from RR temperature measurements and water-vapor data measured with the differential absorption lidar (DIAL) technique.     

Turn-key Raman lidar for profiling atmospheric water vapor, clouds, and aerosols

We describe an operational, self-contained, fully autonomous Raman lidar system that has been developed for unattended, around-the-clock atmospheric profiling of water vapor, aerosols, and clouds. During a 1996 three-week intensive observational period, the system operated during all periods of good weather (339 out of 504 h), including one continuous five-day period. The system is based on a dual-field-of-view design that provides excellent daytime capability without sacrificing nighttime performance. It is fully computer automated and runs unattended following a simple, brief (~5-min) start-up period. We discuss the theory and design of the system and present detailed analyses of the derivation of water-vapor profiles from the lidar measurements.     

Differential absorption lidar system for routine monitoring of tropospheric ozone

A differential absorption lidar system for routine profiling of tropospheric ozone for daytime and nighttime operation is described. The system uses stimulated Raman scattering in hydrogen and deuterium of 266-nm radiation from a quadrupled Nd:YAG laser. Ozone profiles from altitudes of 600 m to approximately 5 km have been obtained with analog detection. Implementing corrections for differential Rayleigh scattering, differential absorption from oxygen, sulphur dioxide, and nitrogen dioxide, and differential aerosol extinction and backscatter can reduce the total system inaccuracy to 5-15% for a clear day and 20-30% for a hazy day, except at the top of the mixed layer. Photon counting must be installed to increase the measurement range from 5 to 15 km. An example of an application of routine measurements of tropospheric ozone profiles is given.     

Compact airborne lidar for tropospheric ozone: description and field measurements

An airborne lidar has been developed for tropospheric ozone monitoring. The transmitter module is based on a solid-state Nd:YAG laser and stimulated Raman scattering in deuterium to generate three wavelengths (266, 289, and 316 nm) that are used for differential ozone measurements. Both analog and photon-counting detection methods are used to produce a measurement range up to 8 km. The system has been flown on the French Fokker 27 aircraft to perform both lower tropospheric (0.5-4-km) and upper tropospheric (4-12-km) measurements, with a 1-min temporal resolution corresponding to a 5-km spatial resolution. The vertical resolution of the ozone profile can vary from 300 to 1000 m to accommodate either a large-altitude range or optimum ozone accuracy. Comparisons with in situ ozone measurements performed by an aircraft UV photometer or ozone sondes and with ozone vertical profiles obtained by a ground-based lidar are presented. The accuracy of the tropospheric ozone measurements is generally better than 10-15%, except when aerosol interferences cannot be corrected. Examples of ozone profiles for different atmospheric conditions demonstrate the utility of the airborne lidar in the study of dynamic or photochemical mesoscale processes that control tropospheric ozone.     

Modeling the performance of direct-detection Doppler lidar systems including cloud and solar background variability

Previous modeling of the performance of spaceborne direct-detection Doppler lidar systems assumed extremely idealized atmospheric models. Here we develop a technique for modeling the performance of these systems in a more realistic atmosphere, based on actual airborne lidar observations. The resulting atmospheric model contains cloud and aerosol variability that is absent in other simulations of spaceborne Doppler lidar instruments. To produce a realistic simulation of daytime performance, we include solar radiance values that are based on actual measurements and are allowed to vary as the viewing scene changes. Simulations are performed for two types of direct-detection Doppler lidar system: the double-edge and the multichannel techniques. Both systems were optimized to measure winds from Rayleigh backscatter at 355 nm. Simulations show that the measurement uncertainty during daytime is degraded by only approximately 10-20% compared with nighttime performance, provided that a proper solar filter is included in the instrument design.     

Future Performance of Ground-Based and Airborne Water-Vapor Differential Absorption Lidar. II. Simulations of the Precision of a Near-Infrared, High-Power System

Taking into account Poisson, background, amplifier, and speckle noise, we can simulate the precision of water-vapor measurements by using a 10-W average-power differential absorption lidar (DIAL) system. This system is currently under development at Hohenheim University, Germany, and at the American National Center for Atmospheric Research. For operation in the 940-nm region, a large set of measurement situations is described, including configurations that are considered for the first time to the authors' knowledge. They include ultrahigh-resolution measurements in the surface layer (resolutions, 1.5 m and 0.1 s) and vertically pointing measurements (resolutions, 30 m and 1 s) from the ground to 2 km in the atmospheric boundary layer. Even during daytime, the DIAL system will have a measurement range from the ground to the upper troposphere (300 m, 10 min) that can be extended from a mountain site to the lower stratosphere. From the ground, for the first time of which the authors are aware, three-dimensional fields of water vapor in the boundary layer can be investigated within a range of the order of 15 km and with an averaging time of 10 min. From an aircraft, measurements of the atmospheric boundary layer (60 m, 1 s) can be performed from a height of 4 km to the ground. At higher altitudes, up to 18 km, water-vapor profiles can still be obtained from aircraft height level to the ground. When it is being flown either in the free troposphere or in the stratosphere, the system will measure horizontal water-vapor profiles up to 12 km. We are not aware of another remote-sensing technique that provides, simultaneously, such high resolution and accuracy.     

Combined Raman lidar for the measurement of atmospheric temperature,water vapor,particle extinction coefficient,and particle backscatter coefficient

The lidar of the Radio Science Center for Space and Atmosphere (RASC; Kyoto, Japan) make use of two pure rotational Raman (MR) signals for both the measurement of the atmospheric temperature profile and the derivation of a temperature-independent Raman reference signal. The latter technique is new and leads to significant smaller measurement uncertainties compared with the commonly used vibrational Raman lidar technique. For the measurement of temperature, particle extinction coefficient, particle backscatter coefficient, and humidity simultaneously, only four lidar signal are needed the elastic Cabannes backscatter signal, two RR signals, and the vibrational Raman water vapor signal. The RASC lidar provides RR signals of unprecedented intensity. Although only 25% of the RR signal intensities can be used with the present data-acquisition electronics, the 1-s -statistical uncertainty of nighttime temperature measurements is lower than for previous systems and is < 1K up to 11-km height for, e.g., a resolution of 500 m and 9 min. In addition, RR measurements in daytime also have become feasible.     

Retrieval of atmospheric-temperature and water-vapor profiles by use of combined satellite and ground-based infrared spectral-radiance measurements

A nonlinear sounding retrieval algorithm is used to produce vertical-temperature and water-vapor profiles from coincident observations taken by the airborne High-resolution Interferometer Sounder (HIS) and the ground-based Atmospheric Emitted Radiance Interferometer (AERI) during the SUbsonic Contrails and Clouds Effects Special Study (SUCCESS). Also, clear sky Geostationary Operational Environmental Satellite (GOES) and AERI radiance measurements, achieved on a daily real-time basis at the Department of Energy's Oklahoma CART (Cloud and Radiation Testbed) site, are used to demonstrate the current profiling capability by use of simultaneous geostationary satellite and ground-based remote sensing observations under clear-sky conditions. The discrepancy principle, a method to find the proper smoothing parameters from the minimum value between the normalized spectral residual norm and the a priori upper bound, is used to demonstrate the feasibility and effectiveness of on-line simultaneous tuning of the multiple weighting and smoothing parameters from the combined satellite/airborne and ground-based measurements for the temperature and water-vapor retrieval in this nonlinear-retrieval process. An objective method to determine the degrees of freedom (d.f.) of the observation signal is derived. The d.f. of the radiance signal for the combined GOES and AERI measurements is larger than that for either instrument alone; while the d.f. of the observation signal for the combined GOES and AERI measurements is larger than that for either instrument alone and of the combined GOES and AERI measurements. The use of simultaneous clear-sky AERI and GOES data now provides improved vertical temperature and moisture soundings on an hourly basis for use in the Atmospheric Radiation Measurement program [J. Appl. Meteorol. 37, 875 (1998)].     

Ground-based differential absorption lidar for water-vapor and temperature profiling: methodology

A comprehensive formulation of the differential absorption lidar (DIAL) methodology is presented that explicitly includes details of the spectral distributions of both the transmitted and the backscattered light. The method is important for high-accuracy water-vapor retrievals and in particular for temperature measurements. Probability estimates of the error that is due to Doppler-broadened Rayleigh scattering based on an extended experimental data set are presented, as is an analytical treatment of errors that are due to averaging in the nonlinear retrieval scheme. System performance requirements are derived that show that water-vapor retrievals with an accuracy of better than 5% and temperature retrievals with an accuracy of better than 1 K in the entire troposphere are feasible if the error that results from Rayleigh-Doppler correction can be avoided. A modification of the DIAL technique, high-spectral-resolution DIAL avoids errors that are due to Doppler-broadened Rayleigh backscatter and permits simultaneous water-vapor and wind measurements with the same system.     

Wide-range sounding of free-tropospheric water vapor with a differential-absorption lidar (DIAL) at a high-altitude station

A differential absorption lidar (DIAL) system has been developed for the measurement of water vapor throughout the free troposphere [3 to 12 km above sea level (asl.)] with high vertical resolution varied from 50 m next to the ground to 300 m above an altitude of 10 km. The system was installed at the Schneefernerhaus high-altitude research station (2675 m asl., Zugspitze, Germany). The DIAL system is based on a tunable single-mode laser system with a high pulse energy of currently 250 mJ and a repetition rate of 20 s(-1). For lidar operation with energies typically between 100 mJ and 150 mJ and an integration time of 1000 s (10000 laser shots for both DIAL wavelengths) a vertical range of at least 10 km has been demonstrated even under dry conditions and during daytime, while daytime measurements up to 12 km have been possible under humid conditions. The system was intercompared with radiosondes, which suggests an agreement within 5% in a major part of the operating range. Further improvements are planned in the upper troposphere to approach the accuracy requirements needed in climate research.     

Statistical-uncertainty-based adaptive filtering of lidar signals

An adaptive filter signal processing technique is developed to overcome the problem of Raman lidar water-vapor mixing ratio (the ratio of the water-vapor density to the dry-air density) with a highly variable statistical uncertainty that increases with decreasing photomultiplier-tube signal strength and masks the true desired water-vapor structure. The technique, applied to horizontal scans, assumes only statistical horizontal homogeneity. The result is a variable spatial resolution water-vapor signal with a constant variance out to a range limit set by a specified signal-to-noise ratio. The technique was applied to Raman water-vapor lidar data obtained at a coastal pier site together with in situ instruments located 320 m from the lidar. The micrometeorological humidity data were used to calibrate the ratio of the lidar gains of the H(2)O and the N(2) photomultiplier tubes and set the water-vapor mixing ratio variance for the adaptive filter. For the coastal experiment the effective limit of the lidar range was found to be approximately 200 m for a maximum noise-to-signal variance ratio of 0.1 with the implemented data-reduction procedure. The technique can be adapted to off-horizontal scans with a small reduction in the constraints and is also applicable to other remote-sensing devices that exhibit the same inherent range-dependent signal-to-noise ratio problem.     

Numerical simulation of light backscattering by spheres with off-center inclusion. Application to the lidar case

A Mie backscattering model for spherical particles with off-center inclusion has been developed and tested. The program is capable of dealing with size parameter values up to approximately 1000, thus allowing one to simulate the optical behavior of a large variety of atmospheric aerosols, as well as cloud and precipitation particles. On the basis of this model, we simulated the optical properties of polydisperse composite atmospheric particles as observed by ground-based and airborne lidar systems. We have characterized optical properties in terms of host and inclusion radii, considering water particles with different composition inclusions. The performed modeling provides some insight into the so-called lidar bright- and dark-band phenomenon.     

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.     

Rotational vibrational-rotational Raman differential absorption lidar for atmospheric ozone measurements: methodology and experiment

A single-laser Raman differential absorption lidar (DIAL) for ozone measurements in clouds is proposed. An injection-locked XeCl excimer laser serves as the radiation source. The ozone molecule number density is calculated from the differential absorption of the anti-Stokes rotational Raman return signals from molecular nitrogen and oxygen as the on-resonance wavelength and the vibrational-rotational Raman backscattering from molecular nitrogen or oxygen as the off-resonance wavelength. Model calculations show that the main advantage of the new rotational vibrational-rotational (RVR) Raman DIAL over conventional Raman DIAL is a 70-85% reduction in the wavelength-dependent effects of cloud-particle scattering on the measured ozone concentration; furthermore the complexity of the apparatus is reduced substantially. We describe a RVR Raman DIAL setup that uses a narrow-band interference-filter polychromator as the lidar receiver. Single-laser ozone measurements in the troposphere and lower stratosphere are presented, and it is shown that on further improvement of the receiver performance, ozone measurements in clouds are attainable with the filter-polychromator approach.     

Measurements of high resolution atmospheric water-vapor profiles by use of a solar blind Raman lidar

Presented here are preliminary results of measurements of atmospheric water-vapor profiles which were obtained by use of a solar blind Raman lidar. Interesting new features of the data gathered include high spatial resolution during daylight hours along with associated measurement errors.