Accurate Measurements of Atmospheric Water Vapor Using Lateral Scanning Vibrational Raman Scattering Lidar with Continuous-Wave Laser
Objective More than 50％of atmospheric water vapor exists mainly in the lower atmosphere within 2 km.Vibrational Raman scattering lidar is an important remote sensing tool for atmospheric water vapor measurement.However,the traditional vibrational Raman scattering lidar mainly adopts a coaxial and non-coaxial parallel transceiver system structure,and the system detection blind zone and transition zone limit their effectiveness in ground atmospheric water vapor detection.We propose a novel detection technique of lateral vibrational Raman scattering lidars based on the structure of a bistatic system,where the lateral vibrational Raman scattering signals of N2 and H2O at different heights are detected by the elevation angle scanning of the lateral receiver system.Finally,it realizes fine detection of near-surface atmospheric water vapor without a blind zone from the ground to the height of interest.Methods We study the lateral vibrational Raman scattering lidar technique in the application of accurate measurements of atmospheric water vapor from the ground to the height of interest.First,a novel lateral scanning vibrational Raman scattering lidar technique is proposed and designed.Two telescopes combined with specified narrow-band interference filters are utilized to detect the lateral scattering signals of the vibrational Raman scattering spectra of N2 and H2O respectively.Then,the inversion algorithm of atmospheric water vapor using the lateral vibrational Raman scattering lidar is established.Vibrational Raman scattering spectra of N2 and H2O have large wavelength differences,which lead to large differences between atmospheric transmissivity of the slant path in these two detection channels,and the aerosol extinction coefficients inverted by Raman method are adopted to correct atmospheric transmissivity of the slant path and improve the detection accuracy of the atmospheric water vapor mixing ratio.Finally,the construction of the experimental system is completed,and the preliminary experiments are conducted via the lateral scanning vibrational Raman scattering lidar.Two different rotation schemes including the continuous equidistant resolution and segmented equidistant resolution are employed during the experimental observations.Results and Discussions The detection principle of the lateral vibrational Raman scattering lidar is innovatively proposed.It breaks through the traditional backward vibrational Raman scattering lidar by a monostatic transceiver system structure,which produces the blind zone and transition zone without effective detection of near-surface atmospheric water vapor.Meanwhile,this technology can utilize a continuous-wave laser featuring light weight,portability,mobility,and low cost(Fig.1).Data correction of atmospheric water vapor is realized by analyzing the atmospheric molecular scattering phase function and the difference in slant path atmospheric transmissivity caused by the wavelength difference between the vibrational Raman scattering spectra of N2 and H2O.The aerosol extinction coefficient obtained from the inversion of the lateral N2 vibrational Raman scattering signal is employed for real-time correction of the slant path atmospheric transmissivity,which improves the accuracy of atmospheric water vapor mixing ratio detection(Figs.2-4).Preliminary experimental observational studies of a lateral scanning pure rotational Raman scattering lidar are performed by two different rotation schemes including the continuous equidistant resolution and segmented equidistant resolution,which are employed during the experimental observations.The experimental results show that both rotation schemes can realize atmospheric water vapor detection from the ground to the height of interest.In particular,the segmented equidistant resolution scheme can realize more fine detection of atmospheric water vapor distribution in the ground zone(Figs.5-8).Conclusions We focus on the detection demand for atmospheric water vapor from the ground to the height of interest using the lidar technique.Based on the theoretical basis of vibrational Raman scattering,the innovative technology of lateral scanning Raman scattering lidar for detecting atmospheric water vapor at the ground surface is proposed.This technology combines the elevation angle scanning function of the lateral receiver system to achieve non-blind scanning detection of water vapor in the lower atmosphere.Due to large differences between the wavelengths of the vibrational Raman scattering spectra of N2 and H2O,the aerosol extinction coefficients obtained by inverting the lateral N2 vibrational Raman scattering signals are adopted to make real-time corrections to the slant path atmospheric transmissivity,which improve the accuracy of atmospheric water vapor mixing ratio.If a high-power pulsed laser is applied,it can be simultaneously observed with a backward vibrational Raman scattering lidar to construct a joint detection system to realize the measurement of atmospheric water vapor from the ground to the height of interest.The experimental results show that the lateral vibrational Raman scattering lidar can detect atmospheric water vapor mixing ratios up to 1400 m with a horizontal distance of 60 m between the laser transmitter system and the lateral telescope receiver system.Additionally,the segmented equidistant resolution scheme has variable resolutions at different heights to show more details of water vapor distribution in the ground zone.
lidarnear-surface atmospheric water vaporlateral vibrational Raman scatteringdetection with non-blind zoneelevation angle scanning
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