Comments on "Accuracy of Raman lidar water vapor calibration and its applicability to long-term measurements"

In a recent publication, Leblanc and McDermid [Appl. Opt., 47, 5592 (2008)]APOPAI0003-693510.1364/AO.47.005592 proposed a hybrid calibration technique for Raman water vapor lidar involving a tungsten lamp and radiosondes. Measurements made with the lidar telescope viewing the calibration lamp were used to stabilize the lidar calibration determined by comparison with radiosonde. The technique provided a significantly more stable calibration constant than radiosondes used alone. The technique involves the use of a calibration lamp in a fixed position in front of the lidar receiver aperture. We examine this configuration and find that such a configuration likely does not properly sample the full lidar system optical efficiency. While the technique is a useful addition to the use of radiosondes alone for lidar calibration, it is important to understand the scenarios under which it will not provide an accurate quantification of system optical efficiency changes. We offer examples of these scenarios. Scanning of the full telescope aperture with the calibration lamp can circumvent most of these limitations. Based on the work done to date, it seems likely that the use of multiple calibration lamps in different fixed positions in front of the telescope may provide sufficient redundancy for long-term calibration needs. Further full-aperture scanning experiments, performed over an extended period of time, are needed to determine a "best practice" for the use of multiple calibration lamps in the hybrid technique.     

Study of the statistics of water vapor mixing ratio determined from Raman lidar measurements

The statistical properties of atmospheric water vapor mixing ratio (WVMR) determined as the ratio of Raman lidar signals backscattered from water vapor and nitrogen molecules are studied. It is shown that WVMR estimates can be biased by a small percentage at low signal photon-counting rates due to fluctuations in the nitrogen signal in the denominator of the ratio, the magnitude of the bias being linked to the signal-to-noise ratio of the nitrogen signal. This is particularly important when unbiased estimates are required as in the case of climate studies and global positioning system (GPS) signal calibration. Different bias corrections and a modified ratio formulation are proposed in order to correct or eliminate this bias. The method is successfully applied in processing signals obtained with an experimental Raman lidar system devoted to calibrate GPS signals for slant path delays. It is shown to reduce biases into negligible values in both WVMR and wet path delay estimates in the range interval of 0-7 km.     

Lamp mapping technique for independent determination of the water vapor mixing ratio calibration factor for a Raman lidar system

We have investigated a technique that allows for the independent determination of the water vapor mixing ratio calibration factor for a Raman lidar system. This technique utilizes a procedure whereby a light source of known spectral characteristics is scanned across the aperture of the lidar system's telescope and the overall optical efficiency of the system is determined. Direct analysis of the temperature-dependent differential scattering cross sections for vibration and vibration-rotation transitions (convolved with narrowband filters) along with the measured efficiency of the system, leads to a theoretical determination of the water vapor mixing ratio calibration factor. A calibration factor was also obtained experimentally from lidar measurements and radiosonde data. A comparison of the theoretical and experimentally determined values agrees within 5%. We report on the sensitivity of the water vapor mixing ratio calibration factor to uncertainties in parameters that characterize the narrowband transmission filters, the temperature-dependent differential scattering cross section, and the variability of the system efficiency ratios as the lamp is scanned across the aperture of the telescope used in the Howard University Raman Lidar system.     

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.     

Airborne differential absorption lidar system for measurements of atmospheric water vapor and aerosols

An airborne differential absorption lidar (DIAL) system has been developed at the NASA Langley Research Center for remote measurements of atmospheric water vapor (H(2)O) and aerosols. A solid-state alexandrite laser with a 1-pm linewidth and > 99.85% spectral purity was used as the on-line transmitter. Solid-state avalanche photodiode detector technology has replaced photomultiplier tubes in the receiver system, providing an average increase by a factor of 1.5-2.5 in the signal-to-noise ratio of the H(2)O measurement. By incorporating advanced diagnostic and data-acquisition instrumentation into other subsystems, we achieved additional improvements in system operational reliability and measurement accuracy. Laboratory spectroscopic measurements of H(2)O absorption-line parameters were perfo med to reduce the uncertainties in our knowledge of the absorption cross sections. Line-center H(2)O absorption cross sections were determined, with errors of 3-6%, for more than 120 lines in the 720-nm region. Flight tests of the system were conducted during 1989-1991 on the NASA Wallops Flight Facility Electra aircraft, and extensive intercomparison measurements were performed with dew-point hygrometers and H(2)O radiosondes. The H(2)O distributions measured with the DIAL system differed by ≤ 10% from the profiles determined with the in situ probes in a variety of atmospheric conditions.     

Implementation and validation of a Raman lidar measurement of middle and upper tropospheric water vapor

Implementation of a Raman lidar measurement of middle and upper tropospheric water vapor is described for a system that uses a 532-nm exciting wavelength, fiber-optic signal transfer, and Q-branch selection. Particular attention is given to the minimizatoin of systematic biases introduced by fluorescent reemission of energy associated with elastic backscatter returns. We compare lidar profiles with collocated radiosonde measurements by using the Vaisala H-Humicap capacitive captor. The variations in the water-vapor concentrations on vertical scales of the order of 1 km in the upper troposphere observed by the two instruments present significant differences. Independent characterization of random and systematic lidar measurement errors and radiosonde sensor response characteristics lead to the conclusion that these differences are due to radiosonde sensor response. These intercomparisons indicate that the lidar measurement can provide important information on water-vapor distributions in the radiatively important 8-11-km region.     

Narrow band fiber-optic phase-shifted Fabry-Perot Bragg grating filters for atmospheric water vapor lidar measurements

A unique ultranarrowband fiber-optic phase-shifted Fabry-Perot Bragg grating filter for atmospheric water vapor lidar measurements was designed, fabricated, and successfully tested. Customized optical fiber Bragg gratings were fabricated so that two transmission filter peaks occurred: one (89% transmission, 8 pm FWHM) near the 946-nm water vapor absorption line and the other peak (80% transmission, 4 pm FWHM) at a region of no absorption. Both transmission peaks were within a 2.66-nm stop band. Demonstration of tension tuning to the 946.0003-nm water vapor line was achieved, and the performance characterization of custom-made optical fiber Bragg grating filters are presented. These measurements are successfully compared to theoretical calculations using a piecewise-matrix form of the coupled-mode equations.     

Atmospheric water vapor differential absorption measurements on vertical paths with a CO2 lidar

Ground based vertical path differential absorption measurements were obtained up to a height of 1.5 km with a CO2 lidar transmitting alternatively on the R(20) (10.247-microm) and R(18) (10.260-microm) lines during daylight in conditions of both strong and weak temperature inversions. The differential absorption between these lines for typical middle latitude lower atmosphere water vapor concentrations appears to be well suited to this type of measurement as the power loss on the more absorbed backscattered line [R(20)] is not too great as to unduly restrict the operating range, while the power differential is still sufficiently large to be readily measureable. In one set of measurements a strong temperature inversion at a height of 1 km resulted in a rapid vertical lapse in aerosol concentration with a consequent loss of SNR on the returns and severe distortion to the differential absorption profiles at this level. Water vapor profiles were derived from all measurements except in the region of the strong temperature inversion where the atmospheric backscattering cross section decayed rapidly. Reasonable results were obtained through the weak inversion region. The measurement capability of the lidar was found to be restricted by the length of the laser pulse tail and an inadequate signal-to-noise performance in regions of strong temperature inversions due to the associated decreases in aerosol concentration.     

Preliminary measurements with an automated compact differential absorption lidar for the profiling of water vapor

The design and preliminary tests of an automated differential absorption lidar (DIAL) that profiles water vapor in the lower troposphere are presented. The instrument, named CODI (for compact DIAL), has been developed to be eye safe, low cost, weatherproof, and portable. The lidar design and its unattended operation are described. Nighttime intercomparisons with in situ sensors and a radiosonde are shown. Desired improvements to the lidar, including a more powerful laser, are also discussed.     

Raman lidar observations of cloud liquid water

We report the design and the performances of a Raman lidar for long-term monitoring of tropospheric aerosol backscattering and extinction coefficients, water vapor mixing ratio, and cloud liquid water. We focus on the system's capabilities of detecting Raman backscattering from cloud liquid water. After describing the system components, along with the current limitations and options for improvement, we report examples of observations in the case of low-level cumulus clouds. The measurements of the cloud liquid water content, as well as the estimations of the cloud droplet effective radii and number densities, obtained by combining the extinction coefficient and cloud water content within the clouds, are critically discussed.     

Methodology for the independent calibration of Raman backscatter water-vapor lidar systems

We present a method for the independent calibration of Raman backscatter water-vapor lidar systems. Particular attention is given to the resolution of instrumental changes in the short and the long terms. The method reposes on the decomposition of the instrument function, which allows the lidar calibration coefficient to be re-expressed as the product of two terms, one describing the instrumental transmission and detection efficiency and the other describing the wavelength-dependent convolution of the Raman backscatter cross sections with the instrument function. The origins of changes in instrument response necessitate the experimental determination of the system detection efficiency. Two external light sources for calibration are assessed: zenith observation of diffuse sunlight and a xenon arc lamp. The results favor use of the diffuse-sunlight measurement but highlight the need for simultaneous sunphotometer measurements to constrain modeled aerosol optical properties. Quantum mechanical models of the Raman cross sections are described, and errors in determining the cross sections and their convolution with the instrument function are discussed in detail. The calibration coefficients deduced by using the independent method are compared with coefficients deduced from Vaisala H-Humicap radiosonde measurements. These results agree to within current calibration errors (15%, unconstrained aerosol parameters), and a change in calibration coefficient following instrument modification is reproduced satisfactorily. Results from modeling and intercomparison studies are extended to estimate the calibration accuracy and the precision of the diffuse-sunlight method with constrained modeled aerosol parameters. Changes in the calibration coefficient in the short and the long terms should be resolved to 4(6)% and 6(9)%, respectively, which is comparable or better than the precision of existing dependent methods of calibration. The reduction of the absolute calibration error remains an outstanding issue for all calibration methods.     

Temperature measurements made with a combined Rayleigh -Mie and Raman lidar

The NASA Goddard Space Flight Center stratospheric ozone lidar system has the capability of collecting both Rayleigh -Mie and Raman backscatter data simultaneously at a number of wavelengths. Here we report on an improved method by which temperature can be derived from a combination of the Rayleigh -Mie return at 351-nm lidar channels and the Raman nitrogen return at 382-nm lidar channels. We also examine some common techniques by which temperatures are retrieved from lidar data. Finally, we show results obtained in 1995 during two Network for the Detection of Stratospheric Change intercomparison campaigns at Lauder, New Zealand and Mauna Loa, Hawaii.     

Upper tropospheric temperature measurements with the use of a Raman lidar

Upper tropospheric temperature profiles were measured with the NASA Goddard Space Flight Center scanning Raman lidar five months after the eruption of Mt. Pinatubo. To derive temperatures in regions of high aerosol content, the aerosol transmission is calculated for the Raman N(2) return signals under cloud-free conditions. The lidar-derived aerosol backscattering ratio and an estimate of the aerosol extinction-to-backscatter ratio were used to compute the aerosol transmission. With a model reference temperature at 25 km, temperature profiles with a root-mean-square difference between the lidar and radiosonde temperatures of <2 K were obtained over an altitude range of 5-10 km for a 10-min integrated measurement with 300-m resolution.     

探测低空大气温度分布的转动拉曼激光雷达

提出了一种新的探测对流层低层大气温度的转动拉曼激光雷达方法,通过测量N2和O2的后向散射的纯转动拉曼谱的强度,计算它们的比值来确定大气温度的垂直分布,并对其性能进行了数值模拟。转动拉曼激光雷达的光源是一个调Q的Nd:YAG激光器,经扩束器后输出能量200mJ;采用双光栅单色仪提取所需要的氮气和氧气的转动拉曼谱;接收机采用光电倍增管和双通道光子计数器,量子效率是10%(48000个脉冲累加)。夜晚它对近地面10.2km高度内的探测信噪比在10:1以上,白天它对近地面3.6km高度内的探测信噪比在10:1以上,计算的温度与模拟用的温度真值阔线相差约0.3K。     

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.