Preliminary exploration of atmospheric water vapor, liquid water and ice water by ultraviolet Raman lidar

Water is the only atmospheric component with three phases. In this work, ultraviolet Raman lidar is developed for synchronous measurements of water vapor, liquid water and ice water in Xi'an (34.233°N, 108.911°E), China. Different interference filters are designed to construct individual water Raman channels, and the corresponding central wavelength and bandwidth are determined by 399.0 nm (3.1 nm), 403.0 nm (5.0 nm) and 407.6 nm (0.6 nm) in ice water, liquid water and water vapor Raman channels, respectively. The mutual interference effect originating from the overlapping characteristics of water Raman spectra is further analyzed, and an accurate retrieval method based on linear simultaneous equations and mutual interference degrees is proposed for synchronous three-phase water mixing ratio profiles. Preliminary measurements are carried out in the Centre for lidar remote sensing research of Xi'an University of Technology, and representative measurement examples are obtained and validated for the performance of the Raman lidar system. Synchronous mixing ratio profiles in water vapor, liquid water and ice water are retrieved, and the corresponding extinction coefficient and relative humidity profiles are also combined to reveal the variation characteristics in three-phase waters. The possible aerosol fluorescence are analyzed as well, and it is inferred that the aerosol fluorescence might affect (possibly overestimate) the derived mixing ratio values of the liquid water and ice water. The effective detection can reach up to a height of 5 km under cloudy weather, and synchronized growth in water vapor and liquid water content is obtained in cloud layers. Continuous observations are also made under hazy weather conditions, and the temporal and spatial evolution trends of three-phase waters in clouds are successfully realized. Preliminary exploration and results validate the feasibility of ultraviolet Raman lidar for synchronous measurements of atmospheric water vapor, liquid water and ice water.

Measuring cloud droplet effective radius and liquid water content using changes in degree of linear polarization along cloud depth

Two important parameters of liquid clouds are the cloud effective size (CES) and liquid water content (LWC). To measure these parameters, we have used two multiple scattering depolarization effects: (1) the slope of the degree of linear polarization (SLDLP) at the cloud base, and (2) the saturated degree of linear polarization (SADLP) at infinite altitude. We used Monte Carlo simulation to validate this method, with the assumption that the water cloud droplet size follows a Gamma distribution. From our calculation, we find that although the SADLP varies with both extinction coefficient (or LWC) and the CES, the SLDLP varies only with the extinction coefficient. After extracting the extinction coefficient using the SLDLP, we can easily obtain the CES using the SADLP. As a result, we found that the CES and the LWC can be extracted from the experimental parameters of SLDLP and SADLP, which can be easily measured using a single wavelength depolarization LIDAR.

Spectrally resolved Raman lidar measurements of gaseous and liquid water in the atmosphere

A spectrally resolved Raman lidar based on a tripled Nd:YAG laser is built for measuring gaseous and liquid water in the atmosphere. A double-grating polychromator with a reciprocal linear dispersion of ~0.237 nm mm(-1) is designed to achieve the wavelength separation and the suppression of elastic backscatter. A 32-channel linear-array photomultiplier tube is employed to sample atmospheric Raman water spectrum between 401.65 and 408.99 nm. The lidar-observed Raman water spectrum in the very clear atmosphere is nearly invariable in shape. It is dominated by water vapor, and can serve as background reference for Raman lidar identification of the phase state of atmospheric water under various weather conditions. The lidar has measured also the Raman water spectrum of an aerosol/liquid water layer. The spectrum showed a moderate increase of the signal on both sides of the Q-branch of water vapor. Noting that under clear weather conditions the Raman water spectrum intensity stays at a very low level in the 401.6-404.7 nm range, the Raman water signal in this portion can be used to estimate the liquid water content in the layer.

Raman and fluorescent scattering matrix of spherical microparticles

In this paper, we have investigated the main properties of the Raman and fluorescent matrix of scattering by microspheres using the matrix scattering formalism. The coherent and incoherent inelastic scattering of incident light by a microsphere is described by the Stokes parameters. We demonstrate the main symmetry properties of the coherent and incoherent Raman and fluorescent scattering matrices. Numerical results are presented to illustrate the Raman scattering efficiency, cross-phase coefficient, and some other parameters of scattering by microspheres.

Validation of the Raman lidar algorithm for quantifying aerosol extinction

To calculate aerosol extinction from Raman lidar data, it is necessary to evaluate the derivative of a molecular Raman signal with respect to range. The typical approach taken in the lidar community is to make an a priori assumption about the functional behavior of the data to calculate the derivative. It has previously been shown that the use of the chi-squared technique to determine the most likely functional behavior of the data prior to actually calculating the derivative eliminates the need for making a priori assumptions. Here that technique is validated through numerical simulation and by application to a significant body of Raman lidar measurements. In general, we show that the chi-squared approach for evaluating extinction yields lower extinction uncertainty than traditional techniques. We also use the technique to study the feasibility of developing a general characterization of the extinction uncertainty that could permit the uncertainty in Raman lidar aerosol extinction measurements to be estimated accurately without the need of the chi-squared technique.

Active Raman sounding of the earth's water vapor field

The typically weak cross-sections characteristic of Raman processes has historically limited their use in atmospheric remote sensing to nighttime application. However, with advances in instrumentation and techniques, it is now possible to apply Raman lidar to the monitoring of atmospheric water vapor, aerosols and clouds throughout the diurnal cycle. Upper tropospheric and lower stratospheric measurements of water vapor using Raman lidar are also possible but are limited to nighttime and require long integration times. However, boundary layer studies of water vapor variability can now be performed with high temporal and spatial resolution. This paper will review the current state-of-the-art of Raman lidar for high-resolution measurements of the atmospheric water vapor, aerosol and cloud fields. In particular, we describe the use of Raman lidar for mapping the vertical distribution and variability of atmospheric water vapor, aerosols and clouds throughout the evolution of dynamic meteorological events. The ability of Raman lidar to detect and characterize water in the region of the tropopause and the importance of high-altitude water vapor for climate-related studies and meteorological satellite performance are discussed.

Radar Refractivity Retrieval: Validation and Application to Short-Term Forecasting

This study will validate the S-band dual-polarization Doppler radar (S-Pol) radar refractivity retrieval using measurements from the International H2O Project conducted in the southern Great Plains in May–June 2002. The range of refractivity measurements during this project extended out to 40–60 km from the radar. Comparisons between the radar refractivity field and fixed and mobile mesonet refractivity values within the S-Pol refractivity domain show a strong correlation. Comparisons between the radar refractivity field and low-flying aircraft also show high correlations. Thus, the radar refractivity retrieval provides a good representation of low-level atmospheric refractivity. Numerous instruments that profile the temperature and moisture are also compared with the refractivity field. Radiosonde measurements, Atmospheric Emitted Radiance Interferometers, and a vertical-pointing Raman lidar show good agreement, especially at low levels. Under most daytime summertime conditions, radar refractivity measurements are representative of an ∼250-m-deep layer. Analyses are also performed on the utility of refractivity for short-term forecasting applications. It is found that the refractivity field may detect low-level boundaries prior to the more traditional radar reflectivity and Doppler velocity fields showing their existence. Data from two days on which convection initiated within S-Pol refractivity range suggest that the refractivity field may exhibit some potential utility in forecasting convection initiation. This study suggests that unprecedented advances in mapping near-surface water vapor and subsequent improvements in predicting convective storms could result from implementing the radar refractivity retrieval on the national network of operational radars.

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.

Inversion of multiwavelength Raman lidar data for retrieval of bimodal aerosol size distribution

We report on the feasibility of deriving microphysical parameters of bimodal particle size distributions from Mie-Raman lidar based on a triple Nd:YAG laser. Such an instrument provides backscatter coefficients at 355, 532, and 1064 nm and extinction coefficients at 355 and 532 nm. The inversion method employed is Tikhonov's inversion with regularization. Special attention has been paid to extend the particle size range for which this inversion scheme works to approximately 10 microm, which makes this algorithm applicable to large particles, e.g., investigations concerning the hygroscopic growth of aerosols. Simulations showed that surface area, volume concentration, and effective radius are derived to an accuracy of approximately 50% for a variety of bimodal particle size distributions. For particle size distributions with an effective radius of < 1 microm the real part of the complex refractive index was retrieved to an accuracy of +/- 0.05, the imaginary part was retrieved to 50% uncertainty. Simulations dealing with a mode-dependent complex refractive index showed that an average complex refractive index is derived that lies between the values for the two individual modes. Thus it becomes possible to investigate external mixtures of particle size distributions, which, for example, might be present along continental rims along which anthropogenic pollution mixes with marine aerosols. Measurement cases obtained from the Institute for Tropospheric Research six-wavelength aerosol lidar observations during the Indian Ocean Experiment were used to test the capabilities of the algorithm for experimental data sets. A benchmark test was attempted for the case representing anthropogenic aerosols between a broken cloud deck. A strong contribution of particle volume in the coarse mode of the particle size distribution was found.

Method for integrating the absorption cross sections of spheres over wavelength or diameter

The absorption cross sections of spherical particles and droplets must be integrated over frequency or droplet size or both for various applications. Morphology-dependent resonances (MDRs) of the spheres can make evaluation of such integrals difficult because the MDRs can contribute significantly to the integrals even when their linewidths are extremely narrow, especially when the absorption is weak. A method of evaluating these integrals by use of Lorentzian approximations near MDRs is described. Calculated frequency-integrated absorption cross sections illustrate how the method obtains accurate cross sections with far fewer integration points than a method that uses equally spaced points. The method reported here suggests a way to integrate over frequency in more-complicated scattering and emission problems and should also be useful for integrating scattering and absorption by other shapes, e.g., spheroids and cylinders, for which the MDR positions and linewidths can be calculated.

Examination of the traditional Raman lidar technique. I. Evaluating the temperature-dependent lidar equations

The essential information required for the analysis of Raman lidar water vapor and aerosol data acquired by use of a single laser wavelength is compiled here and in a companion paper [Appl. Opt. 42, 2593 (2003)]. Various details concerning the evaluation of the lidar equations when Raman scattering is measured are covered. These details include the influence of the temperature dependence of both pure rotational and vibrational-rotational Raman scattering on the lidar profile. The full temperature dependence of the Rayleigh-Mie and Raman lidar equations are evaluated by use of a new form of the lidar equation where all the temperature dependence is carried in a single term. The results indicate that, for the range of temperatures encountered in the troposphere, the magnitude of the temperature-dependent effect can reach 10% or more for narrowband Raman water-vapor measurements. Also, the calculation of atmospheric transmission, including the effects of depolarization, is examined carefully. Various formulations of Rayleigh cross-section determination commonly used in the lidar field are compared and reveal differences of as much as 5% among the formulations. The influence of multiple scattering on the measurement of aerosol extinction with the Raman lidar technique is considered, as are several photon pulse pileup-correction techniques.

Examination of the traditional raman lidar technique. II. Evaluating the ratios for water vapor and aerosols

In a companion paper [Appl. Opt. 42, 2571 (2003)] the temperature dependence of Raman scattering and its influence on the Raman and Rayleigh-Mie lidar equations were examined. New forms of the lidar equation were developed to account for this temperature sensitivity.Here those results are used to derive the temperature-dependent forms of the equations for the water vapor mixing ratio, the aerosol scattering ratio, the aerosol backscatter coefficient, and the extinction-to-backscatter ratio. The error equations are developed, the influence of differential transmission is studied, and several laser sources are considered in the analysis. The results indicate that the temperature functions become significant when narrowband detection is used. Errors of 5% and more can be introduced into the water-vapor mixing ratio calculation at high altitudes, and errors larger than 10% are possible for calculations of aerosol scattering ratio and thus of aerosol backscatter coefficient and of extinction-to-backscatter ratio.

Analytical modeling of Raman lidar return, including multiple scattering

An analytical approach to modeling Raman lidar return with multiple scattering in presented. This approach is based on a small-angle quasi-single-scattering approximation developed earlier for elastic lidar sounding. An approximation of isotropic backscattering for the Raman-scattering case is proposed and tested. The computed results are presented and compared with known data. The approximation was found to be quite simple and provided a high accuracy of Raman lidar return calculations.

Angle- and size-dependent characteristics of incoherent Raman and fluorescent scattering by microspheres. 2. Numerical simulation

The results of numerical simulation of inelastic scattering by microspheres with the use of a dipole model are presented. The formulas that are derived speed up the computation, thereby permitting larger-sized microspheres to be studied. The angular scattering cross section and depolarization are calculated for a wide range of size parameters as well as for different orientations of incident wave polarization. Calculations performed with small incremental changes in size permit the influence of morphology-dependent resonance (MDR) on the power and angular distribution of scattered radiation to be studied. TM and TE types of MDR produce enhanced scattering of the incident wave with vertical and horizontal polarization; the corresponding shape of the phase function becomes oscillatory. Special attention is paid to the simulation of backward scattering by water droplets, which is important for Raman lidar applications.

Inversion with regularization for the retrieval of tropospheric aerosol parameters from multiwavelength lidar sounding

We present an inversion algorithm for the retrieval of particle size distribution parameters, i.e., mean (effective) radius, number, surface area, and volume concentration, and complex refractive index from multiwavelength lidar data. In contrast to the classical Tikhonov method, which accepts only that solution for which the discrepancy reaches its global minimum, in our algorithm we perform the averaging of solutions in the vicinity of this minimum. This averaging stabilizes the underlying ill-posed inverse problem, particularly with respect to the retrieval of number concentration. Results show that, for typical tropospheric particles and 10% error in the optical data, the mean radius could be retrieved to better than 20% from a lidar on the basis of a Nd:YAG laser, which provides a combination of backscatter coefficients at 355, 532, and 1064 nm and extinction coefficients at 355 and 532 nm. The accuracy is improved if the lidar is also equipped with a hydrogen Raman shifter. In this case two additional backscatter coefficients at 416 and 683 nm are available. The combination of two extinction coefficients and five backscatter coefficients then allows one to retrieve not only averaged aerosol parameters but also the size distribution function. There was acceptable agreement between physical particle properties obtained from the evaluation of multiwavelength lidar data taken during the Lindenberg Aerosol Characterization Experiment in 1998 (LACE 98) and in situ data, which were taken aboard aircraft.

Fluorescence from airborne microparticles: dependence on size, concentration of fluorophores, and illumination intensity

We measured fluorescence from spherical water droplets containing tryptophan and from aggregates of bacterial cells and compared these measurements with calculations of fluorescence of dielectric spheres. The measured dependence of fluorescence on size, from both droplets and dry-particle aggregates of bacteria, is proportional to the absorption cross section calculated for homogeneous spheres containing the appropriate percentage of tryptophan. However, as the tryptophan concentration of the water droplets is increased, the measured fluorescence from droplets increases less than predicted, probably because of concentration quenching. We model the dependence of the fluorescence on input intensity by assuming that the average time between fluorescence emission events is the sum of the fluorescence lifetime and the excitation lifetime (the average time it takes for an illuminated molecule to be excited), which we calculated assuming that the intensity inside the particle is uniform. Even though the intensity inside the particles spatially varies, this assumption of uniform intensity still leads to results consistent with the measured intensity dependence.

Performance modeling of an airborne Raman water-vapor lidar

We have developed a sophisticated Raman lidar numerical model to simulate the performance of two ground-based Raman water-vapor lidar systems. After verifying the model using these ground-based measurements, we then used the model to simulate the water-vapor measurement capability of an airborne Raman lidar under both daytime and nighttime conditions for a wide range of water-vapor conditions. The results indicate that, under many circumstances, the daytime measurements possess comparable quality to an existing airborne differential absorption water-vapor lidar whereas the nighttime measurements have improved spatial and temporal resolution. In addition, an airborne Raman lidar can offer measurements that are difficult or impossible with the differential absorption lidar technique.

Remote detection of Raman scattering by use of a holographic optical element as a dispersive telescope

We describe the retrieval of nighttime lidar profiles by use of a large holographic optical element to simultaneously collect and spectrally disperse Raman-shifted return signals. Results obtained with a 20-Hz, 6-mJ/pulse , frequency-tripled Nd:YAG source demonstrate profiles for atmospheric nitrogen with a range greater than 1 km for a time average of 26 s.