Lidar-based Oblique Range Visibility Inversion Method And Experimental Research

Atmospheric visibility is an important parameter of atmospheric physics and a significant factor of atmospheric pollution,and it also reflects the stability of the atmosphere.Especially,the slant visibility plays an important role in air transportation,and the accurate measurement of slant visibility is of great significance to the safe take-off and landing of aircrafts.At present,the emerged lidar slant visibility meters are mainly based on the traditional Koschmieder’s law to estimate the averaged slant,ignoring the influence of the atmospheric scattered radiation.Taking the advantages of lidar detection,and based on the basic measurement principle of slant visibility,research on a new inversion method and experimental analysis are carried out for the measurement of the slant visibility by lidar in this paper.The specific work is as follows:Firstly,based on the definition of brightness contrast between the object and background,the measurement principles of horizontal visibility and slant visibility are introduced.In view of the shortcomings of current inversion method for lidar slant visibility,a new inversion method considering atmospheric scattered radiance correction is proposed for slant range visibility,and the corresponding design idea and realization process are described in detail.Lidar detection is adopted to realize the detection and inversion of atmospheric aerosol optical parameters,microphysical parameters and scattering characteristic parameters,and the combination of lidar and SBDART model is proposed to obtain the calculation of the actual solar direct radiation and atmospheric scattered radiance,and the next is to get the contrast distributions of the contrast between the object and background by the correction of atmospheric scattered radiance,and thus the slant visibility can be realized.Further,theoretical simulation and analysis of the effect of atmospheric scattered radiance on slant visibility is carried out,and SBDART model is used to simulate the brightness distribution of atmospheric scattered radiance under different aerosol models and weather conditions.The results of slant visibility are discussed from the apparent brightness contrast curves of the object and background.The results show that,different aerosol models with different scattered characteristics determine different distribution trends of atmospheric scattered radiance,and the scattering characteristics of cloud aerosol particles also have a great influence on scattered radiance under cloudy weather conditions.Difference in apparent brightness contrast between the object and background at different observation directions leads to different slant visibility distance as well.The simulation results indicate that atmospheric scattered radiance has an important influence on the daytime slant visibility measurements,which can not be ignored.In addition,aiming at the inherent contrast between the object and background,the influence of the reflectivity of the object and background on the slant visibility is explored,and a reflectivity measurement system composed of a spectrophotometer and a standard reflector is constructed here.The reflectivity of runway,grassland,sand,soil and other underlying surfaces are measured,and their reflectivity and reflectivity spectral characteristic curves are obtained as well.The preliminary analysis of the measurement results and errors provides data support for the subsequent slant visibility inversion.Finally,lidar detection experiment are carried out and result analysis for slant visibility are made.By using a dual-wavelength Raman-Mie scattering lidar system in lidar remote sensing research center of Xi’an University of technology,lidar vertical detection and slant scanning detection are carried out under different weather conditions.The actual atmospheric aerosol parameters are obtained by inversion methods,and the brightness distributions of direct radiation and scattered radiance are calculated by SBDART model.The contrast curve of the apparent brightness is obtained with the playground runway and grassland as the object and background,and thus the slant visibility distances are realized.By comparisons,the slant visibility distances are 6.90 km,15.48 km and 8.74 km for all-sky cloudy,clear and haze weather conditions,under the contrast threshold of 0.05,in the observation direction of zenith angle 600 and azimuth angle 270°.When the observation zenith angle is in the range of 50°-70°,the slope visibility distance gradually increases from 5.60 km to 10.20 km under all-sky cloudy,and the slope visibility distance gradually decreases from 16.65 km to 13.50 km under clear conditions.The slant visibility distance dropped from 10.34 km to 5.68 km under haze weather conditions.The experimental results show that different slant range visibility measurement results are obtained in different observation directions under different weather conditions.