拉曼雷达重叠函数的模拟与校准

Simulation and calibration of Raman lidar overlap function

重叠函数的校准对地基激光雷达低空大气探测的准确性至关重要。目前被广泛用于校准拉曼激光雷达重叠函数的双通道实验标定法,需要满足弹性散射通道与氮气拉曼散射通道的重叠函数近似相等的前置条件。但实际仪器光路往往会偏离理想状态,使得该前置条件无法得到满足,导致校准失败。本文使用光线追踪法模拟重叠函数,计算了各种光路失调情况下弹性散射通道与氮气拉曼散射通道的比值,并引入了一种镜头遮蔽实验来评估双通道实验标定法的前置条件是否得到满足。数值模拟结果表明,当雷达接收面的不同象限被遮蔽时,如果弹性散射通道与氮气通道的信号强度比值基本保持不变,则满足前置条件;如果弹性散射通道与氮气通道的信号强度比值有显著差异,则可判定仪器光学系统失调,需要调整直至满足上述前置条件时才能使用双通道实验标定法标定。利用该方法对南京市气象局安装的拉曼激光雷达进行了光路测评和调整,并与CCD(charge-coupled device)侧向成像激光雷达观测信号做对比,结果显示调整后的定标效果更好。

Atmospheric detection lidar has been widely used in air pollutant monitoring,aerosol detection,cloud parameter retrieval,boundary layer height inversion and other important fields due to its high detection accuracy,fine time resolution and far vertical detection ability.In recent years,many new ground-based lidars have been applied to observation networks in China,providing many continuous vertical-profiling observations.However,the quality of ground-based lidar data is inconsistent,thus the quality control of lidar detection data is important.Specifically,unrealistic near-surface aerosol profiles can be found in some lidar products,indicating that the calibration of overlap function is crucial to improving the data quality of ground lidars.The experimental method proposed by Wandinger and Ansmann(2002),based on observations from two channels,is widely used to determine the overlap function of Raman lidars when external calibration instruments are unavailable.This method assumes that the overlap function in the elastic channel is approximately equal to that in the nitrogen channel.This assumption is valid when the lidar system is well aligned,thus Wandinger and Ansmann(2002)did not create a method to determine it.However,misalignments often occur in practice.After a lidar is initially calibrated after production,slight vibrations in the process of transportation,handling and installation may lead to misalignments,which may result in the precondition for the overlap function calibration method being invalid.It is unclear whether the method proposed by Wandinger and Ansmann(2002)can be used to calibrate the overlap function in reality,thus this paper introduces a telecover test to evaluate the validity of precondition for the calibration method.A ray tracing method is applied to the Raman lidar at the Nanjing Meteorological Bureau,and simulations are conducted to determine the overlap functions of both the elastic and nitrogen channels under various alignment conditions and during different telecover tests.The simulation results reveal that the precondition of the method proposed by Wandinger and Ansmann(2002)is valid when the ratio of elastic signals to nitrogen signals remains consistent across different quadrants of the telescope aperture.However,the precondition can be invalid when the ratio of elastic signals to nitrogen signals differs during different telecover tests.Using this method,the Raman lidar of Nanjing Meteorological Bureau was evaluated and adjusted.After the Raman lidar was initially installed,the results from telecover tests show that the signal strength of elastic scattering channel differs greatly from that of nitrogen channel.A test using the CCD-lidar(charge-coupled device lidar)system was applied to verify the overlap function determined by the dual-channel experimental method,and the results suggest that the error is high due to the invalidity of the precondition.To solve this problem,we adjusted the lidar optical system to improve its collimation.After this,we performed the telecover test again,and the signal strength of elastic scattering channel was basically the same as that of nitrogen channel.Our numerical simulations suggest that the preconditions of Raman radar overlapping function calibration method can be basically satisfied after the adjustment.The CCD-lidar test was performed again,and this time the overlap function determined by the dual-channel experimental calibration method was constant with CCD-lidar.The method proposed in this paper can effectively verify the applicability of the dual-channel experimental calibration method,and has good operability,thus this method could be applied to other Raman lidars in the future.Compared with the CCD-lidar observation calibration method which requires outdoor operations at clear-sky nighttime,the dual-channel experimental calibration method has greater advantages when the preconditions are met.