Narrowband-to-broadband albedo conversion for glacier ice and snow: equations based on modeling and ranges of validity of the equations

In this paper, we propose equations for narrowband-to-broadband (NTB) albedo conversion for glacier ice and snow for four types of satellite sensors: thematic mapper (TM), advanced very high resolution radiometer (AVHRR), moderate resolution imaging spectroradiometer (MODIS), and multi-angle imaging spectroradiometer (MISR). We do this on the basis of spectral albedos and incident spectral irradiances generated with radiative-transfer models of the (sub-)surface (a two-stream model) and the atmosphere, respectively. First, we establish equations for reference values of atmospheric components and the surface elevation. These equations describe measurements with root-mean-square differences of ∼0.016. We then show that the “reference equations” also perform well when total ozone and aerosol optical depth are changed with respect to the reference. The negative effect of humidity and elevation variations on the performance of the equations can be eliminated by adding a correction term. We argue that narrowband albedos are much less sensitive to variations in the incident spectral irradiance than broadband albedos. Hence, our conclusions about the effects of variations in atmospheric composition and elevation are also valid for equations for NTB conversion proposed in other papers.     

MODIS卫星数据地表反照率反演的简化模式

以内蒙西部地区的MODIS遥感图像数据和地表野外同步观测的光谱数据为例,在野外数据量较少且有定标数据的条件下反演地表反照率。使用6S大气1辐射传输模型进行大气校正,并通过MODTRAN4.0模型获取各波段地表入射光通量和窄波段的地表反照率;在窄波段反照率与宽波段反照率之间存在线性关系的前提下,以各波段的入射光通量占总入射通量的比例作为反演参数,实现窄波段到宽波段的反演。反演结果证明此方法简便可行。     

Anisotropic Reflection by Melting Glacier Ice: Measurements and Parametrizations in Landsat TM Bands 2 and 4

This article deals with the anisotropic reflection of radiation by melting glacier ice. Ground-based measurements of the directional distribution of the reflected radiation over the hemisphere (so-called BRDFs=bidirectional reflectance distribution functions) were made on the Morteratschgletscher (Switzerland) in Landsat TM bands 2 (520–600 nm) and 4 (760–900 nm). These BRDFs cover a wide range of solar zenith angles (26–75°) and surface characteristics (quantified by a variation in the spectrally integrated albedo between 0.14 and 0.50). All BRDFs exhibit a similar pattern with a minimum in the nadir direction and a maximum in the forward limb, but the amount of anisotropy increases with increasing wavelength, with increasing solar zenith angle and with decreasing albedo. The data were used to derive parametrizations (one for each TM band) which relate the bidirectional reflectance (the reflectance in a specific direction) to the albedo for a given solar-view geometry. Specific parametrizations (one for each TM band) for “near-nadir reflection” are also presented. All these parametrizations can be used to convert satellite-derived bidirectional reflectances into surface albedos and thus to correct for anisotropic reflectance. The residual uncertainty in the albedo due to inaccuracy of the correction is estimated to be 0.02 in both TM bands.     

Validation of MISR land surface broadband albedo

Land surface broadband albedo is a critical variable for many scientific applications. Due to the scarcity of spectral albedo measurements of the Earth's surface environments, it is useful to construct broadband albedo from spectral albedo data obtained by multi‐angle satellite observations. The Multi‐angle Imaging SpectroRadiometer (MISR) onboard NASA's Earth Observing System (EOS) Terra satellite provides land surface albedo products from multi‐angular observations; however, the products have not been comprehensively validated. We convert MISR spectral albedos to total shortwave albedos and validate them using ground measurements at different validation sites. For most surface types, a published narrowband to broadband conversion formula was used, but a new conversion formula for snow and ice covered sites is developed in this study where the spectral range of the instrument is different. Several comparisons are made: (1) between MISR directional‐hemispherical reflectance (DHR) or albedo and MODIS (Moderate Resolution Imaging Spectroradiometer) DHR; and (2) between MISR spectral DHR and bi‐hemispherical reflectance (BHR). The results show that: (1) both the value and the temporal trends of the MISR shortwave albedo and the ground measured shortwave albedo are in good agreement, with the exception of the snow and ice sites; (2) the MISR DHR conforms well to MODIS DHR; and (3) the values of MISR DHR and BHR are nearly identical.     

An algorithm for the retrieval of 30-m snow-free albedo from Landsat surface reflectance and MODIS BRDF

We present a new methodology to generate 30-m resolution land surface albedo using Landsat surface reflectance and anisotropy information from concurrent MODIS 500-m observations. Albedo information at fine spatial resolution is particularly useful for quantifying climate impacts associated with land use change and ecosystem disturbance. The derived white-sky and black-sky spectral albedos may be used to estimate actual spectral albedos by taking into account the proportion of direct and diffuse solar radiation arriving at the ground. A further spectral-to-broadband conversion based on extensive radiative transfer simulations is applied to produce the broadband albedos at visible, near infrared, and shortwave regimes. The accuracy of this approach has been evaluated using 270 Landsat scenes covering six field stations supported by the SURFace RADiation Budget Network (SURFRAD) and Atmospheric Radiation Measurement Southern Great Plains (ARM/SGP) network. Comparison with field measurements shows that Landsat 30-m snow-free shortwave albedos from all seasons generally achieve an absolute accuracy of ±0.02-0.05 for these validation sites during available clear days in 2003-2005, with a root mean square error less than 0.03 and a bias less than 0.02. This level of accuracy has been regarded as sufficient for driving global and regional climate models. The Landsat-based retrievals have also been compared to the operational 16-day MODIS albedo produced every 8-days from MODIS on Terra and Aqua (MCD43A). The Landsat albedo provides more detailed landscape texture, and achieves better agreement (correlation and dynamic range) with in-situ data at the validation stations, particularly when the stations include a heterogeneous mix of surface covers.     

Narrowband to broadband conversions of land surface albedo: II. Validation

In the first paper of this series, we developed narrowband to broadband albedo conversion formulae for a series of sensors. These formulae were determined based on extensive radiative transfer simulations under different surface and atmospheric conditions. However, it is important to validate the simulation results using independent measurement data. In this paper, the validation results for three broadband albedos (total-shortwave, -visible and -near-IR albedos) using ground measurement of several cover types on five different days at Beltsville, MD are presented. Results show that the conversion formulae in the previous paper are very accurate and the average residual standard errors of the resulting broadband albedos for most sensors are around 0.02, which meets the required accuracy for land surface modeling.     

Effect of surface properties on the narrow to broadband spectral relationship in clear sky satellite observations

Several computational experiments have been conducted to estimate the difference between clear sky spectral narrowband (0.5–0.7 μm) and broadband (0.3–2.5 μm) planetary albedo for three cases of wavelength-independent surface albedo and four cases of surface wavelength-dependent (snow, dry sand, meadow, water) albedo. The spectral interval of (0.5–0.7 μm) was selected to approximate the bulk of the VISSR visible channel on the COES satellites and Channel 1 of the AVHRR on the NOAA operational satellites. Different atmospheric conditions and solar zenith angles have been simulated. It was demonstrated that the relationship between the spectral narrowband and broadband planetary albedo depends primarily on the assumptions made about the magnitude and wavelength dependence of the surface albedo and less on the atmospheric conditions. Future attempts to parametrize the conversion from narrowband to broadband spectral observations should account for the surface type.     

基于TM影像天山北坡地表反照率反演方法的研究

地表反照率是气候模型和地表能量平衡方程中的重要参数。基于6S模型估算地表反照率,忽略了地形起伏的影响,不同波段组合的地表反照率也有待进一步研究。本文选取干旱区典型流域——天山北坡三工河流域为研究区域,以TM影像为数据源逐步进行地形校正、大气校正等,从而提取窄波段地表反照率。在此基础上,根据亮度、绿度、湿度3个特征变量的物理意义,以各波段能量权重为转换参数对窄波段地表反照率进行组合,实现研究区宽波段地表反照率的反演,得出基于不同波段的物理意义的地表反照率。     

Parametrized BRDF for atmospheric and topographic correction and albedo estimation in Jiangxi rugged terrain,China

A method is presented for bi‐directional reflectance distribution function (BRDF) parametrization for topographic correction and surface reflectance estimation from Landsat Thematic Mapper (TM) over rugged terrain. Following this reflectance, albedo is calculated accurately. BRDF is parametrized using a land‐cover map and Landsat TM to build a BRDF factor to remove the variation of relative solar incident angle and relative sensor viewing angle per pixel. Based on the BRDF factor and radiative transfer model, solar direct radiance correction, sky diffuse radiance and adjacent terrain reflected radiance correction were introduced into the atmospheric‐topographic correction method. Solar direct radiance, sky diffuse radiance and adjacent terrain reflected radiance, as well as atmospheric transmittance and path radiance, are analysed in detail and calculated per pixel using a look‐up table (LUT) with a digital elevation model (DEM). The method is applied to Landsat TM imagery that covers a rugged area in Jiangxi province, China. Results show that atmospheric and topographic correction based on BRDF gives better surface reflectance compared with sole atmospheric correction and two other useful atmospheric‐topographic correction methods. Finally, surface albedo is calculated based on this topography‐corrected reflectance and shows a reasonable accuracy in albedo estimation.     

Correcting atmospheric masking to retrieve the spectral albedo of land surfaces from satellite measurements

A radiative transfer approach to the problem of atmospheric correction of satellite images in the solar spectral range is presented which includes all multiple scattering processes without any approximation. The numerical solution is accepted as satisfying, if the numerical accuracy is better than I per cent. This means that the accuracy of the atmospheric correction depends almost exclusively on the quality of the auxiliary data on the atmospheric state and the surface reflection indicatrix. Byextensivemodel calculations these parameter driven error bounds have been quantified. Thus the calculation results in a corrected albedo image with specified error bounds. This seems to be the first algorithm available for atmospheric correction of real imagery data which relies on a numerical exact treatment of multiple scattering. The program EXACT (EXact Atmospheric Correction Technique) has so far been used with Landsat Thematic Mapper (TM), NOAA AVHRR (National Oceanic and Atmospheric Administration Advanced Very High Resolution Radiometer) and also with airborne Daedalus ATM images. The algorithm has been validated by comparison of satellite data to ground measurements and between different sensors. Errors of the derived albedos were found to remain below 0·01 for visible and near-infrared sensor channels of this set of radiometers.     

Glacier changes in Glacier Bay,Alaska, during 2000–2012

This article presents measurements of glacier surface areas, mean snow line altitude (MSLA) values, mean snow accumulation area ratio (MAAR) values, and elevation changes in the Glacier Bay, Alaska, using Landsat Thematic Mapper (TM)/Landsat Enhanced Thematic Mapper Plus (ETM+) images and digital elevation models (DEMs) from Shuttle Radar Topography Mission (SRTM) and interferometric synthetic aperture radar (IFSAR) data during 2000–2012. Glacier area estimation results showed that Desolation glacier and Fairweather glacier have lost 2.6% and 2.2% of the glacier area, respectively. Only minor surface area changes were seen in Cascade glacier, Crillon glacier, and Lituya glacier during the study period. The results of MSLA and MAAR showed that the MSLA of Fairweather glacier, Lituya glacier, and Desolation glacier increased by about 120–289 m and the MAAR of Fairweather glacier, Lituya glacier, and Desolation glacier decreased by about 3–6%. In contrast, MSLA and MAAR of Crillon glacier decreased by about 70 m and increased by about 1%, respectively. Glacier elevation change results showed that 7.7 m, 4.6 m and 1.5 m of mean thinning change were observed, respectively, on Fairweather glacier, Lituya glacier, and Desolation glacier. However, 7 m and 0.65 m of mean thickening were, respectively, experienced on Cascade glacier and Crillon glacier in the same period. Results from the study indicated that glacier retreat (Fairweather glacier, Lituya glacier, and Desolation glacier) affected by higher temperatures probably dominates with over-increased precipitation. However, increasing debris cover on the glacier surface can also modify the glacier dynamic, resulting in a different response to global warming.     

Snow surface albedo estimation using terrestrial photography

A flexible and inexpensive remote sensing tool for albedo estimation using conventional terrestrial photography and its validation on an Alpine glacier are described. The proposed technique involves georeferencing oblique photographs to a digital elevation model (DEM), defining a mapping function between the information contained on a given pixel of the image and the corresponding cell of the DEM. This is attained by performing a perspective projection of the DEM after a viewing transformation into the camera coordinate system. Once the image is georeferenced, the reflectance values recorded by the film or digital camera are corrected for topographic and atmospheric influences and for the effect of the photographic process (lens-film-developing-scanning). Atmospheric transmittance is evaluated using the MODTRAN radiative transfer model. Diffuse and direct irradiation are estimated using a parametric solar irradiation model. The solar-ground geometry, anisotropy of reflected radiation, the effect of surrounding topography and the portion of visible sky are evaluated using terrain algorithms applied to the DEM. The response of the camera-film-scanner system is evaluated using an empirical approach. The result is a geographically correct map of normalized reflectance values. By comparing these to a surface of known albedo, the spatial distribution of albedos is calculated. Comparisons to in situ measurements on the Mer de Glace glacier, French Alps, show good agreement. Sources of error are identified and ways of improvement addressed. The georeferencing algorithm, implemented into the Interactive Data Language (IDL) is available from the author and at the user contributed IDL library at www.rsinc.com.     

Remote-sensing-based study of impact of a rock avalanche on North Terong Glacier in Karakorum Himalaya

ABSTRACT

In this article, we report the results of a study on occurrence of a mass-movement event classified as ‘rock avalanche’ over the North Terong glacier (a tributary of the Siachen Glacier in the Nubra Valley), using multi-date remote-sensing data of Landsat Thematic Mapper (TM), Enhanced Thematic Mapper Plus (ETM+), and Operational Land Imager (OLI) and its effects on glacier-surface velocity. Normalized cross-correlation image matching method for displacement measurements at sub-pixel level using Computer Imaging Analysis Software (CIAS), an add-on module of Environment for Visualizing Images (ENVI) software, was used to map changes in the position of debris-deposits of the rock avalanche and assess variations in glacier-surface velocities. Well-defined debris margins gave positions of debris on different dates. Study indicated that this rock avalanche must have occurred between 21 April 2000 and 8 May 2000 (possibly on 3 or 4 May). It induced a kinematic wave or a ‘mini-surge’ in the glacier with its surface velocity showing almost twofold increase in post-event period.     

High resolution planetary albedos—values and variability

The spectral variability of high-resolution planetary albedo is shown to be a useful tool in surface-type analysis. The possibility of using comparison between planetary albedos in selected wavelength regions for cirrus cloud identification has been investigated. The results of an analysis of digital data streams from the polar orbiter satellite, TIROS-N, in three wavelength regions. Channel 1 (0.55–0.90 μm), Channel 2 (0.725–1.10 μm), and Channel 3 (3.55–3.93 μm) are described. These preliminary findings suggest that a spectral relationship exists for high-resolution planetary albedos in clear sky situations which permits surface type/cover discrimination. However the variability of planetary albedo in regions of cirrus cloud cover appears to be a strong function of the underlying surface or cloud type particularly when the cirrus cloud is tenous and/or it overlies highly reflective surfaces or clouds.     

Mapping leaf area index over a mixed natural forest area in the flooding season using ground-based measurements and Landsat TM imagery

Leaf area index (LAI) is an important structural parameter in terrestrial ecosystem modelling and management. Therefore, it is necessary to conduct an investigation on using moderate-resolution satellite imagery to estimate and map LAI in mixed natural forests in southeastern USA. In this study, along with ground-measured LAI and Landsat TM imagery, the potential of Landsat 5 TM data for estimating LAI in a mixed natural forest ecosystem in southeastern USA was investigated and a modelling method for mapping LAI in a flooding season was developed. To do so, first, 70 ground-based LAI measurements were collected on 8 April 2008 and again on 1 August 2008 and 30 July 2009; TM data were calibrated to ground surface reflectance. Then univariate correlation and multivariate regression analyses were conducted between the LAI measurement and 13 spectral variables, including seven spectral vegetation indices (VIs) and six single TM bands. Finally, April 08 and August 08 LAI maps were made by using TM image data, a multivariate regression model and relationships between April 08 and August 08 LAI measurements. The experimental results indicate that Landsat TM imagery could be used for mapping LAI in a mixed natural forest ecosystem in southeastern USA. Furthermore, TM4 and TM3 single bands (R ² > 0.45) and the soil adjusted vegetation index, transformed soil adjusted vegetation index and non-linear vegetation index (R ² > 0.64) have produced the highest and second highest correlation with ground-measured LAI. A better modelling result (R ²?=?0.78, accuracy?=?73%, root mean square error (RMSE)?=?0.66) of the 10-predictor multiple regression model was obtained for estimating and mapping April 08 LAI from TM data. With a linear model and a power model, August 08 LAI maps were successfully produced from the April 08 LAI map (accuracy?=?79%, RMSE?=?0.57), although only 58–65% of total variance could be accounted for by the linear and non-linear models.     

An Albedo Downscaling Method based on Stratified Linear Regression

In recent years,remote sensing technology has been widely used in the field of surface energy,thermal environment,and climate research in small and medium-sized regions.The demand for high resolution,high precision albedo products has increased.In view of this,a physical-based downscaling method is proposed for efficiently and accurately generating high-resolution albedo results.First,under the Lambert hypothesis,the primary albedo of Landsat 8 could be obtained based on Landsat 8 reflectance at 30m resolution.On the 500 m scale,it is found that after classification,the primary albedo of Landsat 8 has a better correlation with the broad-band albedo of MODIS MCD43A3 product.Therefore,a linear regression function based on surface classification is established to integrate the high-resolution primary albedo of Landsat 8 with high-precision MCD43A3 albedo to obtain downscaled albedo.Compared to MODIS albedo,the downscaled albedo provided more rich details.Verification experience based on SURFRAD observation data shows that Bias of the albedo downscaling is 0.01,and standard deviation is 0.012,which has good adaptability for different surface categories.It shows that the algorithm has great application value for producing high-resolution albedo products.     

Determination of chlorophyll concentration changes in Lake Garda using an image-based radiative transfer code for Landsat TM images

The distribution of phytoplankton chlorophyll concentration in Lake Garda (Italy) was estimated using Landsat Thematic Mapper (TM) data acquired at two different times, February 1992 and March 1993. To investigate the waterleaving radiance adequately, the contribution of the atmospheric path radiance reaching the sensor should be removed. In this work a completely image-based atmospheric correction method was applied by means of an inversion technique based on a simplified radiative transfer code (RTC). A semi-empirical approach of relating atmospherically corrected TM spectral reflectances to in situ measurements through regression analysis was used. Limnological parameters were measured near to the TM images dates; some of the in situ measurements were used to define algorithms relating chlorophyll concentration measurements to water surface reflectance and the others too were used to validate the results of the predictive model. The models developed, which performed better (r² = 0.818) when concentrations were higher than > 3.0 mg m³, were used to map chlorophyll concentration throughout the lake. Spatial distribution maps of chlorophyll concentration and concentration changes were produced with contour intervals of 1 mg m³.     

Inter-calibration of the Moderate-Resolution Imaging Spectroradiometer and the AlongTrack Scanning Radiometer-2

The performance of the on board calibrator for bands 1 ( , 0.66 µm), 2 ( , 0.86 µm) and 4 ( , 0.55 µm) of the Moderate-Resolution Imaging Spectroradiometer (MODIS Terra) has been evaluated by comparison of the top-of-atmosphere (TOA) albedos measured in the three bands over a six-day period with a three-year record of TOA albedos measured in the 0.56, 0.66 and 0.86 µm channels of the Along-Track Scanning Radiometer-2 (ATSR-2). The albedo measurements were made over two radiometrically stable sites located in the Libyan desert (22°0'N, 28°30'E), Sudan, and in the Sonoran desert (32°0'N, 114°6'W), Mexico. MODIS Terra albedos are within - 2.5 per cent of those measured in the corresponding channels of ATSR-2. Analysis of the measurements, and of model-derived albedos in the three channels (bands) of the two instruments, indicates that the MODIS Terra on board calibrator for bands 1, 2 and 4 is functioning as expected, and that either of the two instruments can be used to monitor the in-orbit performance of the other.     

Linear 3-D transformations of Landsat 5 TM satellite images for the enhancement of archaeological signatures during the phenological cycle of crops

A linear 3-D transformation that can be used for the enhancement of crop marks related to buried archaeological features is developed and presented in this article. The methodology is based on three steps: (1) recalculation of the ground narrowband spectroradiometric measurements to the multispectral Landsat 5 Thematic Mapper (TM) sensor, based on the relative response filter of the sensor, (2) application of principal component analysis (PCA) transformation in order to determine the initial axes used for the orthogonal transformation, followed by (3) a 3-D rotation of the PCA axes. The linear coefficients of the transformation were retrieved and adjusted to different phenological stages of the crops. The transformation was successfully evaluated using both in situ measurements and Landsat 5 TM images in two different archaeological case studies. The proposed transformation tends to enhance archaeological signatures better than other established vegetation indices or algorithms, while the methodology can be expanded to any other multispectral satellite images using only the visible and very near-infrared part of the spectrum.     

Obtaining global land-surface broadband emissivity from MODIS collection 5 spectral albedos using a dynamic learning neural network

Surface broadband emissivity (BBE) is a key parameter for estimating surface radiation budget, but it is treated crudely in land-surface models because of a lack of global-scale observational BBE data. In this study, the non-linear relationship between the BBE that is calculated from the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) emissivity product and the seven Moderate Resolution Imaging Spectroradiometer (MODIS) narrowband albedos was established individually for bare soils, transition areas, and vegetated areas using a dynamic learning neural network (DLNN). The trained DLNN was tested using a vast array of independent samples, and the results are robust with a bias and root-mean square error (RMSE) of –1e^?4 and 0.012 for bare soils, 2e^?4 and 0.012 for transition areas, and 7e^?4 and 0.010 for vegetated areas. Two independent field-measured emissivity data sets that were measured over sand dunes were used to validate the DLNN. With respect to the BBE that was calculated from the field-measured emissivities, the bias was 0.019. Ultimately, we introduced the strategy of generating a global land-surface BBE product and presented an example of a global BBE map.