Sensitivity of depolarized lidar signals to cloud and aerosol particle properties

Measurements from depolarized lidars provide a promising method to retrieve both cloud and aerosol properties and a versatile complement to passive satellite-based sensors. For lidar observations of clouds and aerosols, multiple scattering plays an important role in the scattering process. Monte Carlo simulations are carried out to investigate the sensitivity of lidar backscattering depolarization to cloud and aerosol properties. Lidar parameters are chosen to be similar to those of the upcoming space-based CALIPSO lidar. Cases are considered that consist of a single cloud or aerosol layer, as well as a case in which cirrus clouds overlay different types of aerosols. It is demonstrated that besides thermodynamic cloud phase, the depolarized lidar signal may provide additional information on ice or aerosol particle shapes. However, our results show little sensitivity to ice or aerosol particle sizes. Additionally, for the case of multiple but overlapping layers involving both clouds and aerosols, the depolarized lidar contains information that can help identify the particle properties of each layer.     

Identification of cloud phase from PICASSO-CENA lidar depolarization: a multiple scattering sensitivity study

A fast Monte Carlo simulation scheme is developed to assess the impact of multiple scattering on space-based lidar backscattering depolarization measurements. The specific application of our methodology is to determine cloud thermodynamic phase from satellite-based lidar depolarization measurements. Model results indicate that multiple scattering significantly depolarizes backscatter return from water clouds. Multiple scattering depolarization is less significant for non-spherical particles. There are sharp contrasts in the depolarization profile between a layer of spherical particles and a layer of non-spherical particles. Although it is not as obvious as ground-based lidar observations, it is likely that we can identify cloud phase not only for a uniform cloud layer, but also for overlapping cloud layers where one layer contains ice and the other water droplets.     

Signal Penetration into Thick Clouds Studied by Multi-Layer Data Observed with a Micro-Pulse Lidar

Lidar return signals are studied for a micro-pulse lidar under sky conditions with multi-layer clouds. From theoretical considerations on the lidar signal-to-noise ratio, it is estimated that the maximum cloud optical thickness detectable is about 3.7. This result reasonably agrees with the actual multi-layer cloud data obtained from observations in Sukhothai, Thailand. Deviations from theoretical prediction, however, are found for a geometrically thin but dense cloud, and for a moderately concentrated but geometrically thick cloud. The effect of multiple scattering is also discussed.     

Simple relation between lidar multiple scattering and depolarization for water clouds

An empirical relationship is derived between the multiple-scattering fraction and the linear depolarization ratio by using Monte Carlo simulations of water clouds measured by backscatter lidar. This relationship is shown to hold for clouds having a wide range of extinction coefficients, mean droplet sizes, and droplet size distribution widths. The relationship is also shown to persist for various instrument fields of view and for measurements made within broken cloud fields. The results obtained from the Monte Carlo simulations are verified by using multiple-field-of-view lidar measurements. For space-based lidars equipped to measure linear depolarization ratios, this new relationship can be used to accurately assess signal perturbations due to multiple scattering within nonprecipitating water clouds.     

Dependence of the lidar signal depolarization on the receiver’s field of view in the sounding of fog and clouds

We report an experimental study of the lidar signal depolarization as a function of the relative contribution of the multiple scattering in case of optically dense objects in the atmospheric planetary boundary layer. Results of the observation of fog and stratus clouds are presented, as well as those obtained by sounding of stratocumulus clouds during a snowfall. The lidar data point to a rise of the depolarization coefficient as the influence of the multiple scattering increases in consequence of both viewing angle enlargement and penetration into the object sounded. The variations of the depolarization coefficient are studied as a function of the field of view. In the case of fog, this dependence is approximated by a three-parameter exponential law; it is found that the depolarization increases steeply when the viewing angle is increased from 9 mrad to 12.5 mrad. The relationships between the approximation parameters and the microphysical characteristics of the scattering medium are considered. The experimentally determined size of the area where multiple scattering occurs is in good agreement with that calculated according to the diffusion model. The results obtained on the multiple scattering effect on the depolarization can also be employed in determining the extinction coefficient profiles in optically dense objects, as well as in evaluating the characteristic size of the scattering particles. Received: 6 September 1999 / Revised version: 7 February 2000 / Published online: 6 September 2000     

Polarization of Lidar Returns from Water Clouds: Calculations and Laboratory Scaled Measurements

The main features of a Monte Carlo code developed to deal with polarization of lidar returns from water clouds in multiple scattering regime are described. The paper presents an example of results pertaining to a stratified cloud structure, and a series of comparisons of calculation results with measurements carried out in the laboratory. For double scattering a comparison of Monte Carlo results with those of an analytical formula is also given.Presented at the 7th International Workshop on Multiple Scattering Lidar/Light Experiments (MUSCLE7), July 21–23 1994, Chiba, Japan.     

Two-Color Dual-Polarization Pulsed Bistatic Lidar for Measuring Water Cloud Droplet Size

The concept of a pulsed bistatic lidar for measuring water cloud particle size is presented. The method uses a two-color laser and a receiver with a polarization analyzer located at a suitable scattering angle. The dependence of Mie scattering on scattering angle, wavelength, and polarization is used to derive water cloud droplet size. The measurement was simulated for the C₁ and C₂ clouds, and the technique for determining mode radius was studied. The result shows the lidar system with a two-wavelength laser (1064 nm and 532 nm) and a dual-polarization receiver fixed at a scattering angle of around 178 deg can be used to measure a cloud particle size (mode radius) of 4 to 12 μm. Evaluation of the effect of multiple scattering showed that the method can be applied not only for the measurement at the cloud base but also in the cloud where multiple scattering is not negligible.     

Spaceborne Laser Rangefinder “LORA” Used as a Cloud Lidar

First, in this report an overview is given on the use of the Russian laser altimeter in space “LORA” as a lidar for cloud measurements. Secondly, the influence of multiple scattering is verified by analysis of the backscatter signals from clouds having the excellent range resolution of a rangefinder.Presented at the 7th International Workshop on Multiple Scattering Lidar/Light Experiments (MUSCLE7), July 21–23 1994, Chiba, Japan.     

Measurement of water cloud particle size with a dual-polarization pulsed bistatic lidar

A new bistatic lidar was developed for measuring water cloud particle size at the base of lower clouds. The lidar uses a pulsed Nd:YAG laser at 532 nm and a receiver having a polarization analyzer located at a suitable scattering angle. Cloud particle size (mode radius of the assumed size distribution) was derived from the ratio of the polarization components of the scattered light based on the single scattering Mie theory. The experiment was performed on board the research vessel Mirai in the northwestern Pacific. Particle size at the bottom of maritime cumulus and stratus was measured, and the difference between the internal structures of cumulus and stratus was observed. The effect of multiple scattering was studied by changing the observing scattering angle. The effect was not significant when the penetration depth was less than 50 m.     

An algorithm to determine cirrus properties from analysis of multiple-scattering influence on lidar signals

A Monte Carlo-based evaluation of the multiple-scattering influence on ground-based Raman lidar measurements is presented. The lidar returns from cirrus clouds are analyzed in order to evaluate vertical profiles of the extinction and backscattering coefficients. Results show that for the typical cirrus cloud, the presence of the multiple scattering can lead to an underestimation of the extinction coefficient by as large as 200% whereas the backscattering coefficient is almost unaffected for the Raman lidar technique. An algorithm to select one or a set of phase functions which fit to the lidar data is also presented. It is an iterative procedure based on Monte Carlo scattering simulation. By comparison of the experimental value of the lidar ratio, corrected for the multiple scattering influence, and the phase function used in the Monte Carlo simulation, one can determine a suitable phase function. The validity and sensitivity of the algorithm have been demonstrated by applying it to simulated cases. The application to some real cases indicates that our procedure allows for the establishing of a practical scattering model for the cirrus clouds.     

Lidar model with parameterized multiple scattering for retrieving cloud optical properties

Cirrus clouds play an important role in the climate through their optical and microphysical properties. The problem with measuring optical properties of these clouds can be partially addressed by using lidar systems. This paper presents a new model for describing the multiple scattering contribution to the backscatter signal measured by the lidar system. The new lidar equation introduced this way, expresses the backscatter signal in terms of a polynomial function of the cloud scattering coefficient. Cloud optical properties such as the extinction coefficient and lidar ratio can be deduced from the new proposed lidar equation. Moreover, some cloud microphysical properties can also be inferred from these optical properties. The method is applied to lidar data collected by the micropulsed lidar operating at Nauru under the auspices of the US Department of Energy ARM program.     

高斯光束在双层云中传输的蒙特卡罗模拟

基于辐射传输理论, 利用蒙特卡罗方法模拟了无限窄(冲击函数)准直光束入射到典型水云以及冰水双层云时的后向散射特性, 进而将得到的冲击响应与高斯光束卷积, 得到高斯光束在云层中传输的多次散射特性. 文中给出了两种波束入射时水云以及冰水双层云的反射函数随径向r和天顶角α的变化关系, 并给出了光强在云层内部的二维分布图. 计算结果表明, 高斯光束入射时, 云层反射函数的特点与无限窄准直光束入射时有较大区别. 因此在利用激光雷达进行云层探测时需要考虑激光的散斑, 文中的方法可以为此提供理论依据.     

Effective lidar ratios of dense dust layers over North Africa derived from the CALIOP measurements

Lidar ratio (i.e., extinction-to-backscatter ratio) is a key parameter required for retrieving extinction profiles and optical depths from elastic backscatter lidar measurements, and the quality of any extinction retrieval depends critically on the accuracy of the assumed or measured lidar ratio. In this study, we analyze the first two and a half years of the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) data acquired during nighttime. Distributions of the effective lidar ratio (ELR), which is the product of the lidar ratio and an instrument-dependent multiple scattering factor, are derived for opaque dust layers observed by CALIOP over the North Africa. The median and mean ELR values are, respectively, 36.4 and 38.5 sr at 532 nm and 47.7 and 50.3 sr at 1064 nm. For these opaque dust layers, the derived ELR decreases as the volume depolarization ratio (VDR) increases, reflecting the impact of multiple scattering within the dense layers. The particulate depolarization ratio is typically ∼0.3 at 532 nm for African dust observed by CALIOP. This ratio can increase to ∼0.4 in the presence of significant multiple scattering. Correspondingly, the calculated ELR will decrease to ∼20 sr at 532 nm and to ∼30 sr at 1064 nm. The median and mean effective lidar ratio values approach, respectively, to 38 and 40 sr at 532 nm and 52 and 55 sr at 1064 nm for smaller VDR values measured in less dense layers where the multiple scattering is relatively insignificant. These values are very close to those derived in previous case studies for moderately dense dust. Case studies are also performed to examine the impacts of multiple scattering. The results obtained are generally consistent with Monte-Carlo simulations.     

Mie理论递推公式计算散射相位函数

在激光雷达探测中,关于多次散射雷达回波的研究,散射相位函数是个非常重要的物理量。本文利用Mie理论的递推公式,对单一粒径介质的散射相位函数进行了计算,计算结果与散射理论中前、后向散射峰值大小随粒子半径的增大而增大相一致。同时,对非单一粒径介质的散射相位函数进行了计算,可用于大气、雾和云等气溶胶多次散射的研究。     

A method for estimating optical properties of dusty cloud

Based on the scattering properties of nonspherical dust aerosol, a new method is developed for retrieving dust aerosol optical depths of dusty clouds. The dusty clouds are defined as the hybrid system of dust plume and cloud. The new method is based on transmittance measurements from surface-based instruments multi-filter rotating shadowband radiometer （MFRSR） and cloud parameters from lidar measurements. It uses the difference of absorption between dust aerosols and water droplets for distinguishing and estimating the optical properties of dusts and clouds, respectively. This new retrieval method is not sensitive to the retrieval error of cloud properties and the maximum absolute deviations of dust aerosol and total optical depths for thin dusty cloud retrieval algorithm are only 0.056 and 0.1, respectively, for given possible uncertainties. The retrieval error for thick dusty cloud mainly depends on lidar-based total dusty cloud properties.     

激光雷达回波信号的蒙特卡罗模拟研究:I.均匀薄雾的多重散射强度分布(英文)

本文简要报道了激光雷达回波信号及多重散射强度分布的蒙特卡罗模拟计算结果。激光雷达系统采用共轴设计 ,计算中采用具有一定厚度、均匀分布的球形薄雾粒子来模拟大气环境 ,研究了粒子尺寸对回波信号的影响 ,获得了不同粒子半径、不同接受视场角条件下多重散射信号强度分布的经验公式。以上两套经验公式具有良好的校正系数 ,可以直接用于激光雷达系统设计中多重散射效应的研究 ,节省大量的模拟计算时间 ,提高设计效率     

Retrieval of water cloud characteristic from active sensor data using the analytical solution of radiative transfer equation

An analytical forward model and numerical algorithm for retrieving the parameters of water cloud of earth atmosphere from optical measurements carried out by satellite-based lidars is presented. The forward model, based on the analytical solution of the radiative transfer equation, is used to fit the temporal profile of the laser light pulses backscattered from the cloud layers. The cloud parameters extracted from the analysis at each position on earth include the transport mean free path, the average radius of water drops, the density of drops, the scattering length, the scattering cross section, the anisotropy factor, and the altitude of top level of major clouds. Also estimated is the possible thickness of cloud layers. The efficacy of the approach is demonstrated by generating parameters of water cloud using the data collected by NASA's cloud-aerosol lidar and infrared pathfinder satellite observations (CALIPSO) satellite when it passed through North America on August 7, 2007.     

Feasibility of a Lidar Utilizing the Glory for Measuring Particle Size of Water Clouds

A new lidar method for measuring water cloud particle size is proposed, and the feasibility of the measurement is discussed. The method utilizes the phenomenon known as the glory which is observed in open air. The proposed lidar consists of a multicolor laser transmitter and two receiver systems looking at the scattering from the target cloud with different scattering angles. Results of the theoretical study show that a system with five laser wavelengths (355, 532, 750, 1064 and 1500 nm) and two receivers located at scattering angles of 180 and 177.5–179 deg is useful for measuring particle size (mode radius of the size distribution) in a range of 4 to 12μm.     

激光雷达回波信号的蒙特卡罗模拟研究:Ⅰ.均匀薄雾的多重散射强度分布

本文简要报道了激光雷达回波信号及多重散射强度分布的蒙特卡罗模拟计算结果。激光雷达系统采用共轴设计,计算中采用具有一定厚度、均匀分布的球形薄雾粒子来模拟大气环境,研究了粒子尺寸对回波信号的影响,获得了不同粒子半径、不同接受视场角条件下多重散射信号强度分布的经验公式。以上两套经验公式具有良好的校正系数,可以直接用于激光雷达系统设计中多重散射效应的研究,节省大量的模拟计算时间,提高设计效率。