Raman lidar observations of cloud liquid water

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.

Concept for maritime near-surface surveillance using water Raman scattering

We discuss a maritime surveillance and detection concept based on Raman scattering of water molecules. Using a range-gated scanning lidar that detects Raman scattered photons from water, the absence or change of signal indicates the presence of a non-water object. With sufficient spatial resolution, a two-dimensional outline of the object can be generated by the scanning lidar. Because Raman scattering is an inelastic process with a relatively large wavelength shift for water, this concept avoids the often problematic elastic scattering for objects at or very close to the water surface or from the bottom surface for shallow waters. The maximum detection depth for this concept is limited by the attenuation of the excitation and return Raman light in water. If excitation in the UV is used, fluorescence can be used for discrimination between organic and non-organic objects. In this paper, we present a lidar model for this concept and discuss results of proof-of-concept measurements. Using published cross section values, the model and measurements are in reasonable agreement and show that a sufficient number of Raman photons can be generated for modest lidar parameters to make this concept useful for near-surface detection.

Multipass optical system for a Raman gas spectrometer

In the present work, a multipass optical system intended for increasing the sensitivity of a Raman gas spectrometer based on the 90° geometry of scattered light collection is described. The system is characterized by an adjustment stability and an increased number of laser beams that pass through a small scattering volume, thus allowing the intensities of Raman signals from components of the gas medium in this volume to be increased. It is demonstrated that the application of this multipass optical system allows the sensitivity of the Raman gas spectrometer to be increased practically by 20 times (to several ppm for the 30-s registration time).

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.

Suggestion for qualitative lidar identification of different types of aerosol using the two-wavelength rotational Raman and elastic lidar

Aerosols are important parameters in the meteorological and environmental fields, and remote aerosol identification is extremely desirable. We have proposed and designed a two-wavelength (355 and 532 nm) rotational Raman and elastic lidar that can measure the wavelength dependence of the aerosol backscattering coefficient without any assumptions about the Angström coefficient or the overlapping function from low (100 m) to high (10 km) altitude, depending on the weather conditions. We have measured the differences in the backscattering ratios (BRs) among a cloud, aerosol in the boundary layer, and Asian dust. The ratio of the aerosol backscattering coefficients between two wavelengths is a fingerprint of an aerosol, which is similar to the Angström coefficient. The BR value for a typical aerosol ranged from 0.56 to 0.4 in the boundary layer and from 0.5 to 0.1 for Asian dust. The BR value of water droplet was not unique but was spread over a wide range because of its size distributions.