参数化的多次散射雷达方程和它的反演理论 II:反演理论

本文根据参数化的多次散射雷达方程,分析了激光多次散射对激光回波信号和一次散射雷达方程反演结果的效应,提出了迭代前向积分法和迭代后向积分法以求解参数化的多次散射雷达方程,并根据多次散射信号对云和气溶胶的前向散射相函数的高度敏感性提出了一个从双接收视场的多次散射激光回波信号,同时确定云和气溶胶消光系数分布和前向散射相函数的方法.此外,本文还根据数值试验分析了这些反演方法在地基激光雷达和空间站激光雷达两种情形下的适用性.     

参数化的多次散射激光雷达方程和它的反演理论 Ⅰ:方程

本文提出了一个参数化的多次散射激光雷达方程,该方程基于本文引出的四个因子,即几何消光因子、消光分布因子、前向散射因子和后向散射因子.前两个因子表征多次散射成分对大气消光系数分布、激光的发散角和接收视场角以及接收孔径的依赖关系,后两个因子表征多次散射成分对散射相函数的依赖关系.这个参数化多次散射雷达方程在241个数值试验中得以检验,这些试验包含很宽的大气条件和雷达几何参数,包括14个大气散射相函数,均匀和不均匀的大气消光系数分布,0.5至1之间变化的一次散射反照率,地基和空间站激光雷达两种情形.数值试验表明,在小于4的光学厚度内参数化的解和Monte-Carlo解之间的标准偏差小于27％,而本模式的计算时间比Monte-Carlo方法偏小4个数量级左右.本模式不仅适合于研究多次散射对激光回波信号和激光大气遥感的效应,而且对于考虑多次散射的激光大气探测而言,是一个合适的应用模式.     

参数化的多次散射激光雷达方程和它的反演理论 I:方程

本文提出了一个参数化的多次散射激光雷达方程,该方程基于本文引出的四个因子,即几何消光因子、消光分布因子、前向散射因子和后向散射因子.前两个因子表征多次散射成分对大气消光系数分布、激光的发散角和接收视场角以及接收孔径的依赖关系,后两个因子表征多次散射成分对散射相函数的依赖关系.这个参数化多次散射雷达方程在241个数值试验中得以检验,这些试验包含很宽的大气条件和雷达几何参数,包括14个大气散射相函数,均匀和不均匀的大气消光系数分布,0.5至1之间变化的一次散射反照率,地基和空间站激光雷达两种情形.数值试验表明     

齐次积分方程数值求解法及在反演气溶胶谱分布上的应用

本文论述了一个数值求解齐次的第一类Fredholm积分方程的方法,并以消光系数和体积散射系数综合反演气溶胶谱分布为例,说明采用比值通道以反演气溶胶谱分布有利于消除多次散射等系统误差.本文还证明了在Junge谱分布下,不同波长的体积消光系数、散射相函数等Mie散射光学参数之比与折射率无关的特性,指出了采用比值通道能够有效地消除折射率的不确定性对于消光——小角散射法反演气溶胶谱分布的影响.     

从反射太阳光谱反演大气气溶胶体积谱分布的理论研究

由大气顶射出太阳辐射的形式解出发,结合球形粒子的米散射理论,由反射太阳光谱推导获得了反演整层大气气溶胶粒子体积谱分布的权重函数。在权重函数特征分析的基础上,根据一个简单的线性反演算法,利用大气辐射传输计算模拟的大气顶反射太阳光谱,反演获得了不同气溶胶浓度和复折射率条件下的气溶胶粒子体积谱。结果表明,初步建立了一套自洽的大气气溶胶体积谱分布的反演方法。通过米散射计算获得的反演参数考虑了波长对粒子半径的权重特征,使气溶胶体积谱的反演具有了明确的物理依据;同目前常用的查算表方法相比,简化了反演方案中参数化的复杂程度。     

大气激光雷达气溶胶光学参数质量可信度评估算法

气溶胶激光雷达由于系统设计、观测场地气候条件、反演过程中参数选取等因素不同,每次测量反演得到的气溶胶光学参数质量不一致,可信度难以评估,进而极大地影响了其在气象、环保等领域业务化应用。为了解决该难题,本文分析了影响激光雷达气溶胶光学参数质量的因素,开发了一套气溶胶光学参数质量可信度评估算法。采用科学、合理的评分和权重分配方案对激光雷达每次测量反演得到的气溶胶光学参数质量可信度进行评估。本文主要讨论对偏振、米散射激光雷达气溶胶光学参数的质量评估,以中国气象局气象探测中心的波长为532 nm的偏振、米散射激光雷达为例,对所提出的质量可信度评估算法进行验证和分析。结果表明:该算法能够有效地区分出不同激光雷达在不同观测场景下每次测量反演得到的气溶胶光学参数质量可信度,具备为气溶胶激光雷达业务化应用提供可靠性评估的能力。     

双基地激光测风雷达回波信号仿真研究

本文针对传统的体散射模型并未考虑大气不均匀性对信号传输的影响等问题,通过引入垂直非均匀的大气参数改进了模型,并利用其建立了体目标的双基地激光测风雷达方程,仿真了侧向散射回波信号,并与单基地雷达进行了对比分析。研究表明:水平方向上,双基地激光测风雷达的回波信号分布特征与单基地雷达差异较大,其回波信号等值线在近地面为卵形线,随着探测高度的增加,回波信号等值线逐渐变为以主、被动雷达为焦点的椭圆形,并最终趋近于圆形;垂直方向上,双基地激光测风雷达的回波信号随高度衰减剧烈,近地面的回波能量约为10^-10 J,4 km高度的回波能量约为10^-15J,在中低层大气(0~10 km),回波信号中气溶胶散射占比大,在高层大气(10 km以上),分子散射占比大。     

利用毫米波测云雷达反演层状云中过冷水

毫米波测云雷达已成为研究云内微物理参数的有效工具,利用其从混合相云中识别出过冷水,对人工影响天气及预防飞机积冰具有重要意义,对我国毫米波雷达的数据处理也具有借鉴作用。本文利用英国的35 GHz、94 GHz测云雷达,结合激光雷达和探空资料,采用阈值法,反演分析了层状云中的过冷水。结果表明:(1)毫米波雷达联合激光雷达可以识别层状云中的过冷水,其结果与微波辐射计测量的液态水路径或毫米波雷达的双峰谱相符合;(2)利用多普勒速度的双峰谱可以反演混合相云中的过冷水含量、冰晶含水量。混合相云的雷达反射率因子主要取决于冰晶,根据雷达反射率因子反演会低估云内液态水含量;(3)本次层状云降水的亮带以上含有较多过冷水,此处35 GHz的雷达回波强度随冰晶的增大而减弱,且冰晶的含水量主导了总液态水含量。     

第六届国际激光雷达会议

第六届国际激光雷达会议于1974年9月8日至6日在日本仙台举行。会议涉及的问题有能见度和云底高度的测量;激光雷达光谱学应用拉曼旋转后向散射谱测量气温;用激光观测大气消光作用;气溶胶对光的散射的物理特征;从云、霾反射回来的激光的退极性观测;行星边界层和大气污染问题,用激光雷达监测大气污染,研究大气污染的扩散,监测环境污染;平流层气溶胶的测量;多次散射问题。     

基于激光雷达资料的小波变换法反演边界层高度的方法

利用小波变换法反演边界层高度时,不同小波母函数的选取可能得到不同的边界层高度。因此,对构造的白天及夜间激光雷达后向散射信号理想廓线进行Haar小波协方差变换,并对后向散射信号梯度廓线进行Morlet与Mexican Hat小波变换反演边界层高度。结果表明,宜采用Haar函数与Mexican Hat函数作为小波母函数,其中Haar函数准确性优于Mexican Hat函数,而Mexican Hat函数更易稳定。同时为了进一步检验3种小波变换法的反演结果对小波振幅的敏感性,通过改变小波母函数的小波振幅,发现无论是理想廓线还是叠加扰动的廓线,较大的小波振幅易得到比较稳定准确的白天边界层高度与夜间混合层高度。     

Constraint Inversion Algorithm of lidar equation for deriving aerosol optical property

A key question of the backward integration algorithm to lidar equation is how to determine the far-end boundary value. This paper develops a Constraint Inversion Algorithm (CIA) for deriving the value and then the aerosol extinction profile from lidar signals, which uses the ground-level horizontal lidar signals as the constraint information. The smaller the wavelength is, the more sensitive to the variation of aerosol ex-tinction to backscatter ratio solved by CIA. According to the property an algorithm is further proposed to simultaneously retrieve the aerosol extinction profile, the size distribution and the imaginary part of its re-flective index from the multi-wavelength lidar observations. CIA is tested in the inversion simulations with satisfactory result.     

拉曼雷达重叠函数的模拟与校准

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。     

Sensitivity of lidar equation solution to boundary values and determination of the values

An analytical dependence of the optical depth solution to lidar equation on boundary values was con-firmed. According to the dependence this paper analyzed the sensitivity of lidar equation solutions obtained by forward and backward integration algorithms to the boundary values and quantitatively expounded an error limit to the boundary values under a given inversion accuracy. Furthermore, this paper presented a method for determination of the far-end boundary value in the case of inhomogeneous atmosphere, improving the accuracy of lidar equation solution.     

Constraint inversion Algorithm of Lidar Equationfor Deriving Aerosol Optical Property

1.IntroductionLidarisincreasinglyusedinmeasuringtheopticalpropertiesofaerosolsandcloudssincethe1960's.Ithasbeenshownthatthedeterminationoftheextinctioncoefficientasafunctionofheightisfeasible(Klett,1980;Fernald,1984;LuDarenetal.,1977;Spinhirneetal.,...     

A New Method for Aerosol Retrieval Based on Lidar Observations in Beijing

Lidar has been used extensively in the area of atmospheric aerosol measurement. Two unknowns at the reference altitude, the lidar ratio and the backscatter coefficient, need to be resolved from the lidar equation. In the actual application, these two values are difficult to obtain, particularly the backscatter coefficient. To better characterize the optical properties of aerosols, optical thickness, and attenuated backscatter obtained by other instruments are usually used as the input for joint inversion. However, this method is limited by location and time. In this study, the authors propose a new method for aerosol retrieval by using Mie scatter- ing lidar data to solve this problem. The authors take the horizontal aerosol extinction coefficient as the con- straint to begin the iteration until a self-consistent aerosol vertical profile was obtained. By comparing their results with Aerosol Robotic Network （AERONET） data, the authours determine that the aerosol extinction coefficient obtained by combining horizontal and vertical lidar observations is more pre- cise than that obtained by using the traditional Fernald method. This new method has been adopted for re- trieving the extinction coefficient of aerosols during the observation days.     

Integration method and ratio method for retrieving extinction coefficient from lidar signals

New solution techniques to improve the accuracy of quantitatively determining the atmospheric extinction coefficient and the backscattering-to-extinction ratio from lidar signals are developed. The integration method is proposed to analytically retrieve the extinction coefficient at ground level, which has the advantage of eliminating the effect of backscattering fluctuations on the inversion results.The ratio method, on the other hand, deals with the inversion of the vertical distribution of the extinction coefficient. The main idea of this method is to begin with a calculation of the transmittance by eliminating the backscattering through ratioing lidar signals at two elevations, and subsequently derive the extinction coefficient from the transmittance, thus avert from ambiguous results caused by inappropriate assumptions on the backscattering-to-extinction ratio. Observational investigations have demonstrated that the integration method is superior to the slope method in terms of accuracy and stability, and the ratio method is reasonable and feasible as well.     

Characteristics of cirrus clouds and its radiative properties based on lidar observation over Chung-Li, Taiwan

In this paper, characterization of cirrus clouds are made by using data from ground based polarization lidar and radiosonde measurements over Chung-Li (24.58°N, 121.10°E), Taiwan for a period of 1999–2006. During this period, the occurrence of cirrus clouds is about 37% of the total measurement nights over the lidar site. Analysis of the measurement gives the statistical characteristics about the macrophysical properties such as occurrence height, ambient temperature, and its geometrical thickness while the microphysical properties are interpreted in terms of extinction coefficient, optical depth, effective lidar ratio and depolarization ratio. The effective lidar ratio has been retrieved by using the simulation technique of backscattered lidar signals. The effect of multiple scattering has been taken into the account by a model calculation. Summer (Jun–Aug) shows the maximum appearances of cirrus due to its formation mechanism. It is shown that tropopause cirrus clouds may occur with a probability of about 24%. These clouds are usually optically thin and having laminar in structure with some cases resembling the characteristics similar to that of polar stratospheric clouds (PSCs). The radiative properties of the cirrus clouds are also discussed in detail by the empirical equations with results show a positive feedback on any climate change.     

Lidar Methods for Observing Mineral Dust

Lidar methods for observing mineral dust aerosols are reviewed.These methods include Mie scattering lidars,polarization lidars,Raman scattering lidars,high-spectral-resolution lidars,and fluorescence lidars.Some of the lidar systems developed by the authors and the results of the observations and applications are introduced.The largest advantage of the lidar methods is that they can observe vertical distribution of aerosols continuously with high temporal and spatial resolutions.Networks of ground-based lidars provide useful data for understanding the distribution and movement of mineral dust and other aerosols.The lidar network data are actually used for validation and assimilation of dust transport models,which can evaluate emission,transport,and deposition of mineral dust.The lidar methods are also useful for measuring the optical characteristics of aerosols that are essential to assess the radiative effects of aerosols.Evolution of the lidar data analysis methods for aerosol characterization is also reviewed.Observations from space and ground-based networks are two important approaches with the lidar methods in the studies of the effects of mineral dust and other aerosols on climate and the environment.Directions of the researches with lidar methods in the near future are discussed.