Techniques for the retrieval of aerosol properties over land andocean using multiangle imaging

Aerosols are believed to play a direct role in the radiation budget of Earth, but their net radiative effect is not well established, particularly on regional scales. Whether aerosols heat or cool a given location depends on their composition and column amount and on the surface albedo, information that is not routinely available, especially over land. Obtaining global information on aerosol and surface radiative characteristics, over both ocean and land, is a task of the Multi-angle Imaging SpectroRadiometer (MISR), an instrument to be launched in 1998 on the Earth Observing System EOS-AM1 platform. Three algorithms are described that will be implemented to retrieve aerosol properties globally using MISR data. Because of the large volume of data to be processed on a daily basis, these algorithms rely on lookup tables of atmospheric radiative parameters and predetermined aerosol mixture models to expedite the radiative transfer (RT) calculations. Over oceans, the “dark water” algorithm is used, taking full advantage of the nature of the MISR data. Over land, a choice of algorithms is made, depending on the surface types within a scene-dark water bodies, heavily vegetated areas, or high-contrast terrain. The retrieval algorithms are tested on simulated MISR data, computed using realistic aerosol and surface reflectance models. The results indicate that aerosol optical depth can be retrieved with an accuracy of 0.05 or 10%, whichever is greater, and some information can be obtained about the aerosol chemical and physical properties     

Regional aerosol retrieval results from MISR

Examples of aerosol retrieval results, derived from the Multi-angle Imaging SpectroRadiometer (MISR) on the Earth Observation Science (EOS) Terra platform, are shown and the performance of the retrieval algorithms are discussed, following the first 18 months of operational data processing. A number of algorithm modifications were implemented, based on an analysis of aerosol retrieval results during this period, and these changes are described. Two cloud-screening algorithms, the angle-to-angle smoothness and angle-to-angle correlation tests, which were used in the preprocessing phase of the analyses are also described. The aerosol retrieval examples cover a wide variety of conditions, both over land and water. Particular aerosol types include dust clouds, forest fire and volcanic plumes, and localized dense haze. Finally, some ideas are discussed for additional improvement of the MISR aerosol data product, based on the experience gained in analyzing multiangle data and the associated geophysical products.     

Improving MODIS surface BRDF/Albedo retrieval with MISR multiangle observations

We explore a synergistic approach to use the complementary angular samplings from the Multi-angle Imaging SpectroRadiometer (MISR) and Moderate Resolution Imaging Spectroradiometer (MODIS) to improve MODIS surface bidirectional reflectance distribution function (BRDF) and albedo retrieval. Preliminary case studies show that MODIS and MISR surface bidirectional reflectance factors (BRFs) are generally comparable in the green, red, and near infrared. An information index is introduced to characterize the information content of directional samplings, and it is found that MISR angular observations can bring additional information to the MODIS retrieval, especially when the MISR observations are close to the principal plane. We use the BRDF parameters derived from the MISR surface BRFs as a priori information and derive a posteriori estimates of surface BRDF parameters with the MODIS observations. Results show that adding MISR angular samplings can reduce the relative BRF prediction error by up to 10% in the red and green, compared to the retrievals from MODIS-only observations which are close to the cross-principal plane.     

Solar zenith angle effects on forest canopy hemisphericalreflectances calculated with a geometric-optical bidirectionalreflectance model

The bidirectional reflectance distribution function (BRDF) provided by the Li-Strahler geometric-optical forest canopy model has been integrated to provide spectral instantaneous hemispherical reflectances of sparsely vegetated surfaces. Further integration over the Sun's zenith angles can yield daily or longer interval hemispherical reflectances as well. A variety of simulated canopies were modeled with varying solar angles. In all cases, as the geometric-optical model introduced increased shadowing of the surface with increased solar zenith angle, the direct-beam hemispherical surface reflectance gradually decreased. The hemispherical reflectance values are direct beam calculations and do not directly include canopy multiple scattering and leaf specularity or consider the impact of diffuse irradiance. These limitations are acceptable for sparse canopies, in which 3D shadowing effects are large. However, radiative transfer calculations have shown that these phenomena must be incorporated before truly realistic modeling of hemispherical surface reflectances can be achieved for dense canopies     

New methods to infer snow albedo from the MISR instrument with applications to the Greenland ice sheet

Snow-covered surfaces have a very high surface albedo, thereby allowing little energy to be absorbed by the snowpack. As the snowpack ages and/or begins to melt, the snow albedo decreases and more solar energy is absorbed by the snowpack. Therefore, accurate estimation of snow albedo is essential for monitoring the state of the cryosphere. This paper examines the retrieval of snow albedo using data from the Multi-angle Imaging SpectroRadiometer (MISR) instrument over the Greenland ice sheet. Two different methods are developed and examined to derive the snow albedo: one based on the spectral information from MISR and one utilizing the angular information from the MISR instrument. The latter method is based on a statistical relationship between in situ albedo measurements and the MISR red channel reflectance at all MISR viewing angles and is found to give good agreement with the ground-based measurements. Good agreement is also found using the spectral information, although the method is more sensitive to instrument calibration, snow bidirectional reflectance distribution function models, and narrowband-to-broadband relationships. In general, using either method retrieves snow surface albedo values that are within about 6% of that measured at the stations in Greenland.     

MISR: A multiangle imaging spectroradiometer for geophysical andclimatological research from Eos

The scientific objectives, instrument concept, and data plan for the multiangle imaging spectroradiometer (MISR), an experiment proposed for the Eos (Earth Observing System) mission, are described. MISR is a pushbroom imaging system designed to obtain continuous imagery of the sunlit Earth at four different view angles (25.8°, 45.6°, 60.0°, and 72.5° relative to the vertical at the Earth's surface), in both the forward and aftward directions relative to nadir, using eight separate cameras. Observations will be acquired in four spectral bands, centered at 440, 550, 670, and 860 nm. Data analysis algorithms will be applied to MISR imagery to retrieve the optical, geometric, and radiative properties of complex, three-dimensional scenes, such as aerosol-laden atmospheres above a heterogeneously reflecting surface, nonstratified cloud systems, and vegetation canopies. The MISR investigation will address a number of scientific questions concerning the climatic and ecological consequences of many natural and anthropogenic processes, and will furnish the aerosol information necessary     

Algorithm for global leaf area index retrieval using satellite imagery

Leaf area index (LAI) is one of the most important Earth surface parameters in modeling ecosystems and their interaction with climate. Based on a geometrical optical model (Four-Scale) and LAI algorithms previously derived for Canada-wide applications, this paper presents a new algorithm for the global retrieval of LAI where the bidirectional reflectance distribution function (BRDF) is considered explicitly in the algorithm and hence removing the need of doing BRDF corrections and normalizations to the input images. The core problem of integrating BRDF into the LAI algorithm is that nonlinear BRDF kernels that are used to relate spectral reflectances to LAI are also LAI dependent, and no analytical solution is found to derive directly LAI from reflectance data. This problem is solved through developing a simple iteration procedure. The relationships between LAI and reflectances of various spectral bands (red, near infrared, and shortwave infrared) are simulated with Four-Scale with a multiple scattering scheme. Based on the model simulations, the key coefficients in the BRDF kernels are fitted with Chebyshev polynomials of the second kind. Spectral indices - the simple ratio and the reduced simple ratio - are used to effectively combine the spectral bands for LAI retrieval. Example regional and global LAI maps are produced. Accuracy assessment on a Canada-wide LAI map is made in comparison with a previously validated 1998 LAI map and ground measurements made in seven Landsat scenes.     

Validation and refinement of hyperspectral/multispectral atmospheric compensation using shadowband radiometers

First-principles atmospheric compensation of Earth-viewing spectral imagery requires atmospheric property information derived from the image itself or measured independently. A field experiment was conducted in May, 2003 at Davis, CA to investigate the consistency of atmospheric properties and surface reflectances derived from simultaneous ground-, aircraft- and satellite-based spectral measurements. The experiment involved the simultaneous collection of HyMap hyperspectral and Landsat-7 multispectral imagery, in situ reflectance spectra of calibration surfaces, and sun and sky radiances from ultraviolet and visible multifilter rotating shadowband radiometers (MFRSRs). The data were analyzed using several different radiation transport and atmospheric compensation algorithms. Reasonable self-consistency was found between aerosol property retrievals from the radiometers and from dark pixels of the imagery, and, when using the most accurate algorithm, there was excellent agreement between the retrieved surface spectra and the ground truth measurements.     

基于DPC的中国部分区域陆地气溶胶光学厚度反演

大气气溶胶是气候变化与遥感定量化等的重要影响因素,研究气溶胶光学特性具有重要意义.基于高分五号卫星搭载的大气气溶胶多角度偏振探测仪(Directional polarimetric camera,DPC)过境中国区域的数据,开展了气溶胶光学厚度的反演研究.基于矢量辐射传输模型6SV计算构建气溶胶光学特性查找表,采用Ro...     

Aerosol retrieval over land from AVHRR data-application foratmospheric correction

Correction of Advanced Very High Resolution Radiometer (AVHRR) imagery for the aerosol effect requires retrieval of the aerosol loading from the images. Two retrieval algorithms that were previously developed for Landsat are modified for the AVHRR. The methods determine the aerosol optical thickness over land surfaces from AVHRR band one data independently of ancillary information. The first method retrieves aerosols based on the atmospheric effect on the path radiance. This method requires the surface reflectance to be 0.02±0.01, which is found over forests in the red channel. Two techniques are used to screen an AVHRR scene for pixels that have this low reflectance. The qualifying requirements for these techniques are discussed, and the method is demonstrated to retrieve aerosol optical thicknesses to ~±0.1. The second method uses the change in contrast for several scenes to determine the change in the optical thickness between the scenes. A reference scene allows absolute determination. The method has an rms error of ~0.1     

基于短波红外波段的Ⅱ类水体MODIS影像大气校正算法

提出了一种基于短波红外(SWIR,short wave infrared)波段离水反射率为0的Ⅱ类水体大气校正算法.采用MODIS的1.240μm和1.640μm两个SWIR波段的反射率计算出了可见光及近红外(NIR, near infrared)波段气溶胶散射反射率,进而反演得到了这些波段的离水反射率.应用该算法对中国东部近海及湖泊的Ⅱ类水体进行了大气校正,并与实测数据和常用的大气校正算法进行了比较分析,结果表明该算法能够有效地去除大气的影响.     

基于航空偏振数据的地表BPDF模型优化研究

针对航空偏振气溶胶反演涉及的地表双向偏振辐射分布函数（BPDF）特性问题,分析了地表双向偏振反射分布物理机理,结合经典地表BPDF模型参数特点,提出了基于阴影方程和归一化植被指数的优化地表BPDF模型。利用航空辐射偏振遥感数据,验证了新模型的精度,相对于实测结果,平均相对偏差小于9.9%。与三种经典模型模拟结果相比,新模型值随不同观测角度变化时,其一致性很好。根据新模型精度,还模拟了地表偏振反射率偏差与气溶胶光学厚度反演误差的关系,发现地表偏振反射率偏差为4×10-4时,带来反演气溶胶光学厚度误差小于0.03,满足气溶胶偏振反演应用要求。     

Mapping surface mineralogy and scattering behavior usingbackscattered reflectance from a hyperspectral midinfrared airborne CO2 laser system (MIRACO2LAS)

Airborne, high-spectral resolution, thermal-infrared (TIR) MIRACO ₂LAS reflectance data were evaluated for mapping surface mineralogy and scattering behavior for a variety of semi-arid, geological test sites in Australia. MIRACO₂LAS is a rapidly tuned, airborne CO₂ laser system that measures backscattered (bidirectional) reflectance at 100 wavelengths between 9.1 and 11.2 μm for 2-m footprints in line profile mode. An operational methodology is described that permits reduction of the raw airborne signal-to-ground reflectance. This ground reflectance has two major properties, namely, wavelength-dependent mineralogical variations and reflection albedo variations related to surface roughness. Comparisons between the airborne MIRACO₂LAS spectra and laboratory directional hemispherical reflectance (DHR) spectra show the same spectral shapes, though differences in average reflectance (albedo) occur for some types of rocks. The minerals identified using MIRACO₂LAS include silicates (for example, quartz, microcline, plagiodase, almandine, spessartine, talc, tremolite, and kaolinite) and carbonates (dolomite and magnesite) as well as vegetation (dry and green). Many of the diagnostic spectral features that allow identification of these materials are narrow (<0.2 μm), making them difficult to detect with broadband TIR systems, like the airborne TIMS and satelliteborne ASTER. Based on an empirical relationship between the minimum and maximum reflectance established using laboratory DHR spectra, a method is proposed that allows the use of MIRACO₂ LAS data to identify surfaces that are characterized by Lambertian or specular scattering. The MIRACO₂LAS results show that Lambertian-type scatterers include soils and many types of isotropic rocks     

Topographic effects on bidirectional and hemispherical reflectancescalculated with a geometric-optical canopy model

The effects of topography on both the bidirectional reflectance distribution function (BRDF) and the hemispherical reflectance (surface albedo) of a forested scene are investigated with the Li-Strahler geometric-optical model. The Li-Strahler geometric-optical model treats a vegetation canopy as an assemblage of partially illuminated tree crowns of spheroidal shape, and through geometric optics and Boolean set theory, models the proportion of sunlit or shadowed canopy and background as functions of view angle, illumination angle, and crown geometry. The model has been modified to accommodate a sloping surface in its computation of bidirectional and hemispherical reflectance. When the BRDF of a flat vegetated surface is compared to the BRDF of a sloping surface that is similarly vegetated, the interaction of the illumination angle and the slope distort the shape of the BRDF. A hemispherical integration of this distorted BRDF provides an albedo for the sloping surface     

Multisensor Data Product Fusion for Aerosol Research

Combining data sets from multiple satellite sensors is a powerful method for studying Earth-atmosphere problems. By fusing data, we can utilize the strengths of the individual sensors that may not be otherwise possible. In this paper, we provide the framework for combining level 2 data products, using data from three sensors aboard the National Aeronautics and Space Administration (NASA)'s Terra satellite. These data include top-of-the-atmosphere (TOA) radiative energy fluxes obtained from the Clouds and the Earth's Radiant Energy System (CERES), aerosol optical thickness from the multispectral Moderate Resolution Imaging Spectroradiometer (MODIS), and aerosol properties from the Multi-angle Imaging SpectroRadiometer (MISR). The CERES Single Scanner Footprint (SSF) contains the pixel level CERES TOA fluxes and the level 2 MODIS aerosol data. We specifically focus upon fusing the CERES SSF with the MISR aerosol products. Although this project was undertaken specifically to address aerosol research, the methods employed for fusing data products can be used for other problems requiring synergistic data sets. We present selected case studies over different aerosol regimes and indicate that multisensor information provides value-added information for aerosol research that is not available from a single sensor.     

Sun and view angle corrections on reflectances derived fromNOAA/AVHRR data

A new method is proposed for the reduction of the noise-like fluctuations associated with variations of Sun-target-sensor geometry in multitemporal AVHRR data in the visible and near-infrared bands. Its principle is to adjust, over a monthly period, a three-parameter model of surface bidirectional reflectance on a time series of cloud-free atmospherically corrected AVHRR data. One parameter of the model represents the surface reflectance corrected for angular effects. Time profiles of corrected reflectances are obtained by making the monthly period slide over the annual vegetation cycle. This procedure is applied to an annual cycle of AVHRR data on seven test sites in France representative of bare soils, agricultural crops, and forested thematic areas. The method is evaluated according to two criteria, which are the ability of the bidirectional reflectance model to reduce the amount of noise-like temporal fluctuations of AVHRR data, and the stability of the retrieved parameters when random noise is artificially added to the original AVHRR data set. The perturbations induced by the coupling between diffuse sky radiance effects and the non-Lambertian behavior of ground reflectance are also discussed. The method is proved to give satisfactory results, and can potentially be used to compare satellite-derived reflectances obtained not only at different times, but also at different places and with different sensors     

基于MODIS数据反演高反射率地区气溶胶光学厚度的方法研究

针对高反射率地区地表反射率难以确定的难点,假设在不同年间同一天相同地物的两个不同波段的比值近似相等的前提下,借助几何光学模型理论,应用MODIS历史产品和数据,即MODIS地表反射率产品(MOD09)、历史MODIS图像第七波段的地表反射率和待反演地区的MODIS第七波段的地表反射率数据计算出待反演地区的MODIS其他波段的地表反射率,然后结合利用6S辐射传输模式计算建立的查找表,可实现待反演区域的气溶胶光学厚度的反演.以北京市区为例,应用该方法实现了北京市区的气溶胶光学厚度反演,并把反演结果与AERONET观测站的结果进行了对比,对比发现该反演方法对亮区域有效,其误差在30％以内.     

利用MODIS近红外陆地通道反演浑浊水体上空气溶胶光学厚度

目前MODIS海洋气溶胶反演算法能够很好地给出远海气溶胶性质,但近海结果并不理想。这是因为近海浑浊水体对0.55 µm、0.646 µm波段不能视为暗目标,对于0.86 µm波段也并不是总能视为暗目标。本研究采用MODIS近红外陆地通道对中国东南近海浑浊水体上空的气溶胶进行了反演研究,结果与AEROET符合得较好,这种算法可以很容易的与现行算法相结合,从而能够获得更多宝贵的气溶胶数据。     

基于反射率统计模型填充MODIS AOD中国陆地缺值区域

针对MODIS气溶胶产品受算法局限性、厚云层造成缺值问题,提出基于反射率统计模型的普通克里金-自然邻近插值方法,利用6S辐射传输模型模拟MODIS蓝光波段表观反射率随地表反射率、气溶胶光学厚度(aerosol optical depth, AOD)的响应变化,建立反射率与AOD的统计模型。采用2017年11月的中国区域气溶胶产品MOD04_L2、反射率数据做实验,并利用时空匹配的AERONET地基站数据做交叉验证,结果表明：利用该方法填充MOD04_L2中国缺值区域精度较好,60%以上的插补结果处于期望误差界限内,不仅能有效解决低反射率地区AOD有效值缺失问题,而且不受气溶胶类型假设不当影响。