利用地基红外高光谱发射率数据进行云参数反演(2): 云滴有效半径和云水路径反演

云滴有效半径和云水路径等微物理参数是了解云的形成过程、辐射效应以及云、气溶胶和降水相互作用等问题的重要数据。利用地基红外高光谱辐射数据开展了云微物理参数反演方法研究。针对光谱数据的特点,进行了基于云层发射率光谱和辐射光谱的敏感性分析,在此基础上建立了云微物理参数与云发射率光谱差值和斜率等特征参数有关的查找表关系。具体特征参数包括：热红外波段862.1和934.9 cm-1的云层发射率之差、中红外波段1 900.1和2 170.1 cm-1的云层发射率之差、热红外波段900～1 000 cm-1区间的发射率光谱斜率和辐射值光谱斜率、1 100～1 200 cm-1区间的发射率光谱斜率和辐射值光谱斜率等。研究了臭氧波段云层透过率的计算方法及对查找关系的约束性,选择了1 050～1 060 cm-1区间的云层透过率平均值作为约束特征参数。实现了基于逐步搜索法的多重查找反演云滴有效半径和光学厚度,并可通过经验关系计算云水路径。研究表明,该算法得到的水云的云滴有效半径与ARM计划中的MICROBASE产品基本相当,冰云的云滴有效半径相对偏小,两者的云水路径反演结果差异较大。该反演算法较适合于光学厚度小于6的薄云。     

星地光通信的天气可用性与地面站选址研究

利用经验公式计算了星地光链路在雾、雨、雪、云天气中的衰减,仿真结果表明:波长越长,雾对激光的衰减越小;雪对激光的衰减量高于雨对激光衰减量1～2个数量级;当光学厚度达到4.7时,云层对激光的衰减已达到20 dB以上。综合考虑我国的地理分布和气候特点以及云层覆盖情况,建议我国星地光通信地面站选址可以考虑西北地区的塔里木盆地-内蒙古西部戈壁沙漠-黄土高原西北部一带。     

利用FTIR和MODIS数据估算塔克拉玛干沙漠宽波段地表比辐射率

分析并提供了一个利用MODIS窄波段数据,估算地表宽波段(8～14 μm)比辐射率的最优估算方程,并根据该方程获得了塔克拉玛干沙漠地区地表比辐射率特征分布情况。首先,沿塔克拉玛干沙漠的两条南北穿越公路,使用傅里叶变换热红外光谱仪(FTIR),选取20个观测点,获取实测的地表宽波段比辐射率。其次,利用MODIS温度产品MOD11A1和MOD11C1热红外区域第29,31和32波段和MOD09A1近红外区域第7波段数据,建立待定系数的地表宽波段比辐射率多元线性回归估算方程。通过FTIR的观测值和MODIS数据确定该估算方程的系数,并进行误差分析。研究发现,使用FTIR观测值,由MODIS第29,31和32波段数据的线性回归方程,可以产生高精度的地表宽波段比辐射率。加入MODIS第7波段后,新的线性回归估算方程的精度更高,均方根误差RMSE为0.004 5,平均偏差Bias为0.000 1。与文献中的其他六种估算方程横向对比,RMSE和Bias分别比其他六种估算方程低1和2个数量级。最后,利用该估算方程获得了研究区的地表比辐射率分布图,结果显示,沙漠中心区域的值为0.880～0.910,平均值为0.906;有稀疏植被区域的值为0.910～0.940;靠近沙漠边缘的绿洲的值为0.950～0.980。     

Optical properties of mixed phase boundary layer clouds observed from a tethered balloon platform in the Arctic

A tethered balloon system was used to collect data on radiometric and cloud microphysical properties for mixed phase boundary layer clouds, consisting of ice crystals and liquid water droplets during a May–June 2008 experimental campaign in Ny-Ålesund, Norway, located high in the Arctic at 78.9°N, 11.9°E. The balloon instrumentation was controlled and powered from the ground making it possible to fly for long durations and to profile clouds vertically in a systematic manner. We use a radiative transfer model to analyze the radiometric measurements and estimate the optical properties of mixed-phase clouds. The results demonstrate the ability of instruments deployed on a tethered balloon to provide information about optical properties of mixed-phase clouds in the Arctic. Our radiative transfer simulations show that cloud layering has little impact on the total downward irradiance measured at the ground as long as the total optical depth remains unchanged. In contrast, the mean intensity measured by an instrument deployed on a balloon depends on the vertical cloud structure and is thus sensitive to the altitude of the balloon. We use the total downward irradiance measured by a ground-based radiometer to estimate the total optical depth and the mean intensity measured at the balloon to estimate the vertical structure of the cloud optical depth.     

AEDAL在青岛、渤海及黄海探测的定标反演

在不进行大气参数假设和加载其他辅助设备的情况下,通过对AEDAL机载大气环境探测激光雷达探测数据的分析,证明了用Slope法对空基激光雷达进行定标反演的可行性。AEDAL的定标反演结果,生动地展现了青岛地区及渤海、黄海海域气溶胶垂直分布、边界层结构及边界层光学厚度的重要特征。首先,冷锋过境,边界层高度明显下降的同时,气溶胶出现多层分布的现象,海区气溶胶垂直分布较陆地有明显的沉降现象。其次,青岛地区整个边界层光学厚度表现为城区最大、丘陵地区最小、渤海海区介于两者之间的分布特点,海区边界层光学厚度变化相对平缓,迎风坡附近的边界层光学厚度较周围有所增大。青岛至渤海这一区域,晴朗天气下边界层光学厚度的大小介于0.15～0.35之间,雾天则在0.3～0.45之间,而黄海海域则可达到0.55左右。这说明,青岛地区边界层气溶胶主要来自于城市和海洋,在冬季季风对气溶胶分布起作用的同时,山脉及地面风场对边界层气溶胶的输送有重要影响。     

Elastic backscattering lidar system for atmospheric measurements in Antarctica

An elastic backscattering lidar and initial results are described. The lidar system has been designed to operate in Antarctica for the Italian National Programme for Antarctic Research. The first field trials were held during the period from 31 December 1987 to 10 February 1988. Data were collected for both tropospheric and stratospheric atmospheric parameters. Tropospheric measurements included the cloud base, optical depth, extinction and backscattering coefficients for clouds, whereas aerosols and the molecular atmosphere were monitored in the stratosphere. The lidar system was also tested for the determination of temperature and density in ranges up to 40 km. Preliminary results of the lidar measurements carried out at Terra Nova Bay are presented. The lidar system is composed of a Nd–YAG laser, a Newtonian telescope and two receiving channels with computer-controlled data acquisition and display equipment. The system is operated in a mobile container specifically designed for the Antarctic environment.     

New dust aerosol identification method for spaceborne lidar measurements

Classification is a critical step in the backscatter lidar data processing to accurately retrieve extinction and backscatter profiles of atmospheric aerosols and clouds. Different schemes, such as the probability distribution functions (PDFs) method, have been used in the cloud and aerosol classification. In this paper, we attempt to use the support vector machine (SVM) to discriminate aerosols from clouds, with a focus on dust aerosol classification in China. To demonstrate the feasibility of the SVM classifier, we chose dust storms that occurred in the Gobi and Taklimakan deserts and observed by the CALIPSO lidar in spring time 2007. The results show that the SVM can correctly identify the dust storms.     

On the feasibility of optical fibre sensors for strain monitoring in thermoplastic composites under fatigue loading conditions

This study investigates the possibility of using optical fibres with Bragg gratings for measurements in thermoplastic composites under fatigue loading conditions. Two setups are considered: (i) the fibre is embedded in the composite and (ii) the grating is bonded externally. Detailed information is given on the principle of optical fibre measurements and the data acquisition for both setups. To verify the strain derived from the optical fibre, the strain is compared with extensometer measurements. A special design of the blades of the extensometer is presented, since the standard blades suffer from a loss of grip on the surface of the specimen. The material used for this study was a carbon fibre-reinforced polyphenylene sulphide.It can be concluded for both setups that the optical fibre survives over half a million loading cycles, without de-bonding of the fibre. The advantage of the external fibre over the embedded one is that it can be mounted after manufacturing of the plate, but it has a higher risk of being damaged during working conditions of the component.     

Retrieval of tropospheric NO2 columns from satellite measurements in presence of cirrus: A theoretical sensitivity study using SCIATRAN and prospect application for the A-Train

A theoretical sensitivity study of the influence of cirrus cloud properties on tropospheric NO₂ columns retrieved from the spaceborne Ozone Monitoring Instrument (OMI) measurements is performed. It is conducted within the framework of the synergetic use of A-Train sensors to derive more representative trace gas products. We aim to study the potential effects of cirrus clouds on tropospheric NO₂ retrievals using a retrieval algorithm that, unlike the OMI Standard and DOMINO algorithms, does not correct for the effects of clouds. The sensitivity study is based on the radiative transfer code SCIATRAN that performs both simulations of top of atmosphere (TOA) reflectances as measured by an OMI-like band and tropospheric NO₂ column retrievals based on the differential optical absorption spectroscopy (DOAS) method. The results of the sensitivity study show that if a correction for cirrus clouds is not included in our simple retrieval that does not account for clouds in the first place, the tropospheric column can be underestimated by 55%. This underestimation depends strongly on cirrus parameters as, in order of importance, cloud fraction, cloud optical depth, asymmetry factor of cirrus cloud phase function and cloud top height. The perspective of the synergy between OMI and cloud information obtained from cloud-derived products of the A-Train is evaluated in two parts by applying a simple cloud correction scheme based on the independent pixel approximation (IPA). Firstly, we evaluated the tropospheric NO₂ column retrievals error caused by uncertainties in cirrus cloud properties. Secondly we studied the influence of subpixel cloud optical depth variability on NO₂ retrievals. From our simulations, it is demonstrated that the error will be reduced significantly if the cloud fraction is lower or equal to 0.5. In this case, the cloud fraction and the cloud optical depth must be known within accuracy less than 0.05% and 50%, respectively. The cloud top height and the asymmetry factor must be known within uncertainty of at least 1 km and less than 0.05, respectively. The latter result shows that the uncertainty of the asymmetry factor is a major source of error in the cloud correction for tropospheric NO₂ retrieval in the presence of cirrus.     

Microwave and Millimeter Wave Characteristics and Attenuation of Clouds over some Malaysian Equatorial Stations

Based on radar range height indicator (RHI) measurements, cloud characteristics in relation to radiowave propagation over three locations in different geographical region in western Malaysia have been presented. It is seen that low cloud occurrence over these locations are quite significant. Cloud attenuation and noise temperature can result in serious degradation of telecommunication link performances. This paper presents cloud coverage in different months, 0°C isotherm height and cloud attenuation results at 12 GHz, 20 GHz, 36 GHz, 50 GHz, 70 GHz and 100 GHz over measurement site. The low level cloud over the measurement sites has been found to occur for many days and nights and particularly in the months of April to May and October to December. Such results are useful for satellite communication and remote sensing application in Malaysia.     

On the retrieval of ice cloud particle shapes from POLDER measurements

Shapes of ice crystals can significantly affect the radiative transfer in ice clouds. The angular distribution of the polarized reflectance over ice clouds strongly depends on ice crystal shapes. Although the angular-distribution features of the total or polarized reflectance over ice clouds imply a possibility of retrieving ice cloud particle shapes by use of remote sensing data, the accuracy of the retrieval must be evaluated. In this study, a technique that applies single ice crystal habit and multidirectional polarized radiance to retrieve ice cloud particle shapes is assessed. Our sensitivity studies show that the retrieved particle shapes from this algorithm can be considered good approximations to those in actual clouds in calculation of the phase matrix elements. However, this algorithm can only work well under the following conditions: (1) the retrievable must be overcast and thick ice cloud pixels, (2) the particles in the cloud must be randomly oriented, (3) the particle shapes and size distributions used in the lookup tables must be representative, and (4) the multi-angle polarized measurements must be accurate and sufficient to identify ice cloud pixels of randomly oriented particles. In practice, these conditions will exclude most of the measured cloud pixels. Additionally, because the polarized measurements are only sensitive to the upper cloud part not deeper than an optical thickness of 4, the retrieved particle shapes with the polarized radiance may only approximate those in the upper parts of the clouds. In other words, for thicker clouds with vertical inhomogeneity in particle shapes, these retrieved particle shapes cannot represent those of whole clouds. More robust algorithm is needed in accurate retrieval of ice cloud particle shapes.     

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.     

基于后向轨迹追踪模式分析SACOL气溶胶 来源及其光学特性

利用2007年11月至2010年10月NCEP/NCAR再分析资料和HYSPLIT模式模拟了后向气流轨迹, 分析了兰州大学半干旱气候环境观测站(SACOL)所在地区的大气输送特征. 采用聚类分析方法将1096条轨迹分类, 得到15簇反映主要特征的典型气流, 研究了不同气流特征、出现频数及季节分布特点, 与主要天气形势和天气系统的关系. 统计主要气流路径所占比例, 其中源于四川和重庆的气流占总路径的16%, 为影响SACOL的主要路径, 其次为源于局地和陕西的气流, 路径最少的是东欧和孟加拉国. 结合同期MFRSR地基辐射计反演的870nm气溶胶光学厚度和波长指数, 分析了不同气团源控制下的气溶胶光学特性和粒子性质特征. 揭示了最大平均气溶胶光学厚度达0.29± 0.12(平均值± 标准差), 气团源于塔克拉玛干沙漠; 最小平均光学厚度的气团源于孟加拉国, 为0.14± 0.02. 根据研究站点周围环境条件和气团源区地理条件以及移动轨迹, 分析其可能影响, 并定量给出不同源区气团对SACOL三年平均气溶胶光学厚度的贡献率. 结果表明, 源于局地和区域性气团贡献平均光学厚度的41.1%, 沙尘区贡献28.4%, 中亚为17.9%, 远距离东欧和中东为12.6%.     

The use of low density high accuracy (LDHA) data for correction of high density low accuracy (HDLA) point cloud

Coordinate measuring techniques rely on computer processing of coordinate values of points gathered from physical surfaces using contact or non-contact methods. Contact measurements are characterized by low density and high accuracy. On the other hand optical methods gather high density data of the whole object in a short time but with accuracy at least one order of magnitude lower than for contact measurements. Thus the drawback of contact methods is low density of data, while for non-contact methods it is low accuracy. In this paper a method for fusion of data from two measurements of fundamentally different nature: high density low accuracy (HDLA) and low density high accuracy (LDHA) is presented to overcome the limitations of both measuring methods. In the proposed method the concept of virtual markers is used to find a representation of pairs of corresponding characteristic points in both sets of data. In each pair the coordinates of the point from contact measurements is treated as a reference for the corresponding point from non-contact measurement. Transformation enabling displacement of characteristic points from optical measurement to their match from contact measurements is determined and applied to the whole point cloud. The efficiency of the proposed algorithm was evaluated by comparison with data from a coordinate measuring machine (CMM). Three surfaces were used for this evaluation: plane, turbine blade and engine cover. For the planar surface the achieved improvement was of around 200 µm. Similar results were obtained for the turbine blade but for the engine cover the improvement was smaller. For both freeform surfaces the improvement was higher for raw data than for data after creation of mesh of triangles.     

Methods for estimating the similarity parameter of clouds from internal measurements of the scattered radiation field

The similarity parameter is a combination of the single scattering albedo and the scattering asymmetry factor and can be used in conjunction with particle-size information to estimate the fractional absorption of a cloud layer. Methods are presented for estimating the parameter without estimating the optical thickness of the cloud or the ground albedo, but radiance measurements must be made at two elevations deep within the cloud layer. If the radiance data is for a sufficient number of directions, then one method can be used to give the parameter without using any correlation of the parameter with properties of the cloud. With the appropriate correlations, the parameter can be obtained using either the mean radiance and net flux or the radiance in only the zenith and nadir directions. The methods are based on an asymptotic transport theory and are shown to be a special case of more general transport results.     

Tritium in precipitation of Vostok (Antarctica): conclusions on the tritium latitude effect

During the Antarctic summer of 1985 near the Soviet Antarctic station Vostok, firn samples for tritium measurements were obtained down to a depth of 2.40 m. The results of the tritium measurements are presented and discussed. Based on this and other data, conclusions regarding the tritium latitude effect are derived.     

Optoheterodyne Doppler measurements of the ballistic expansion of the products of the shock wave-induced surface destruction: Experiment and theory

The results of optoheterodyne Doppler measurements of the ballistic expansion of the products of surface destruction under shock-wave loading are presented. The possibility of determining the physical characteristics of a rapidly flying dust cloud, including the microparticle velocities, the microparticle sizes, and the areal density of the dust cloud, is shown. A compact stand for performing experiments on shock-wave loading of metallic samples is described. Shock-wave loading is performed by a 100-µm-thick tantalum flyer plate accelerated to a velocity of 2.8 km/s. As the samples, lead plates having various thicknesses and the same surface roughness are used. At a shock-wave pressure of 31.5 GPa, the destruction products are solid microparticles about 50 µm in size. At a pressure of 42 and 88 GPa, a liquid-drop dust cloud with a particle size of 10–15 µm is formed. To interpret the spectral data on the optoheterodyne Doppler measurements of the expansion of the surface destruction products (spalled fragments, dust microparticles), a transport equation for the function of mutual coherence of a multiply scattered field is used. The Doppler spectra of a backscattered signal are calculated with the model developed for the dust cloud that appears when a shock wave reaches the sample surface at the parameters that are typical of an experimental situation. Qualitative changes are found in the spectra, depending on the optical thickness of the dust cloud. The obtained theoretical results are in agreement with the experimental data.     

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.     

利用太阳光度计CE318反演气溶胶光学厚度改进算法的研究

针对影响太阳分光光度计精度的各种因素误差分析,分别在太阳天顶角,大气光学质量,Rayleigh光学厚度,臭氧总量等方面做了方法上的改进,以提高气溶胶光学厚度的反演精度。利用一般的文献上的处理方法做为比较,先考虑各种影响因素单独影响,再综合考虑各因素的影响结果,提出了太阳分光光度计处理的高精度算法,并应用于太湖等实际测量的反演中。实验处理表明本文改进算法对提高气溶胶光学厚度反演的精度有着良好的效果。将为仪器的使用,数据的处理提供有益的参考。     

霾光谱特性分析与卫星遥感识别算法

近年来,我国频繁发生的灰霾污染事件和常态性的高细颗粒物浓度(PM_2.5),已经引起了全世界范围的广泛关注。卫星遥感能够对大气污染进行快速准确地监测。然而在大气遥感领域具有代表性的中分辨率成像光谱仪(NASA/MODIS),其云监测和暗像元反演算法通常把灰霾当做薄云、雾或地表亮目标处理,无法反演霾天的气溶胶光学厚度(aerosol optical depth, AOD)。笔者研究了云、雾、霾、地表覆盖等不同像元在可见光、近红外以及红外通道的光谱特性。基于MODIS数据,参考相关的云监测和气溶胶反演算法,选取多个对灰霾敏感的光谱通道,计算表观反射率和亮度温度。针对不同波段,分别探讨了霾与薄云、低层云、雾、浓密植被和地表亮目标等像元之间的光谱差异,统计灰霾分布的阈值区间,并设计基于MODIS卫星遥感数据的灰霾识别自动处理流程。通过对2008年华北平原春夏两个重霾事件进行测试,该算法的霾分布监测结果与卫星真彩图具有较好的一致性。基于北京和香河AERONET站点观测的高AOD数据,验证了本算法的霾识别率接近80%,在一定程度能够弥补MODIS标准气溶胶算法用于灰霾天的不足。最后,分析了灰霾识别过程中的主要误差来源,并提出了基于霾纹理特征,以及其他辅助数据支撑的改进方法。