Narrowband Albedos Research Articles

Retrieval of lake water surface albedo from Sentinel-2 remote sensing imagery

Of the many factors that affect heat flux, water surface albedo can affect the heat flux and energy balance at the water–atmosphere interface. As a result, lake water surface albedo can act on key climate-change factors through its impact on lake–atmosphere heat exchange. Most of the available global water cycle models, however, still use value specific water surface albedos and often produce estimation with relative errors of 5–15 % due to time and location differences. The lake albedo inversion model based on Sentinel-2 MSI sensor was obtained in this research. From the perspective of energy transmission, this research selects an inversion method of converting narrowband albedo to broadband albedo, and using Sentinel-2A spatial resolution image for the study area for 10 m lakes albedo product. Based on the measured albedo data, using the determination coefficient (R2), mean relative error(MRE) and root mean square error (RMSE) for validation, the result of the inversion results show that the R2 is 0.62, MRE is 0.25, RMSE is 0.039. The values of lake albedo in the study area ranged from 0 to 0.45, with an average value of 0.14. The high value of lake albedo appears in the south of the Songnen Plain lake area, with a size between 0.28 and 0.45, and the low-value gathering area appears in the Qiqihar Lake Group. The central lake area of Jilin also has a low value, with a size between 0 and 0.10. From the west to the middle of Jilin Province, the albedo value tends to decrease gradually, and some areas have higher values. In a typical lake, the average albedo is arranged as follows: Yangsha lake > Shitoukoumen reservoir > Chagan lake > Moon lake > Erlong lake > Xinlicheng reservoir.

Non‐Gaussian Distributions of TOA SW Flux as Observed by MISR and CERES

AbstractThe Top of Atmosphere (TOA) shortwave (SW) flux, converted from Terra Multi‐angle Imaging SpectroRadiometer (MISR) narrow band albedos, is compared with that measured from Clouds and the Earth's Radiant Energy System (CERES). We describe the probability density function (PDF) of the monthly TOA SW flux and how the statistical third moment, skewness, can impact the quantification of the flux. The PDF of the SW flux is not normally distributed but positively skewed. In both sets of observations, the near‐global (80 S–80 N) median value of the SW flux is ∼3 W/m2 less than the mean value, due to the positive skewness of the distribution. The near‐global mean TOA SW flux converted from MISR is about 7 W/m2 (∼7%) less than CERES measured flux during the last two decades. Surprisingly, a hemispheric asymmetry exists with TOA SW observations from the Terra platform. SH reflects 3.92 and 1.15 W/m2 more mean SW flux than NH, from MISR and CERES Single Scanner Footprint products, respectively. We can infer that the offsetting by morning clouds in the SH is greater than the effect of hemispheric imbalance of SW flux caused by different land masses in two hemispheres. While the characteristics of the two SW fluxes broadly agree with each other, differences in the regional PDF from two different SW fluxes are substantial over high cloud regions and high altitude regions. Our analysis shows that some parts of the different skewness from the two measurements may be attributed to the different calibration of the radiance anisotropy over high cloud scenes.

Estimating Visible Band Albedo from Aerial Orthophotographs in Urban Areas

Albedo is an important parameter in many environmental and renewable energy models. Satellite sensors can be used to derive broadband or narrowband albedos. However, the spatial resolution of such data can be insufficient in urban areas with complex morphology and land cover diversity. In this study, we propose the use of widely available aerial orthophotographs to derive visible band albedo in urban surfaces that can be effectively used in high-resolution applications. The solution is based on the estimation of the reflected irradiance captured by an RGB sensor and approximated by the brightness component in the hue-saturation-brightness (HSB) color model and incident solar irradiance modelled by the r.sun module in GRASS GIS. The visible band albedo values are calibrated by published reference values for selected land cover classes or, alternatively, by a spectroradiometer. The method is applied to the central part of Košice and compared to visible band albedo derived from the Landsat 8 OLI and Sentinel 2A sensors and previously published typical albedo values for various land cover classes, resulting in reasonable agreement. The proposed methodology is implemented using standard GIS tools that are easily applicable to any high-resolution urban data.

A versatile method for computing optimized snow albedo from spectrally fixed radiative variables: VALHALLA v1.0

Abstract. In climate models, the snow albedo scheme generally calculates only a narrowband or broadband albedo, which leads to significant uncertainties. Here, we present the Versatile ALbedo calculation metHod based on spectrALLy fixed radiative vAriables (VALHALLA version 1.0) to optimize spectral snow albedo calculation. For this optimization, the energy absorbed by the snowpack is calculated by the spectral albedo model Two-streAm Radiative TransfEr in Snow (TARTES) and the spectral irradiance model Santa Barbara DISORT Atmospheric Radiative Transfer (SBDART). This calculation takes into account the spectral characteristics of the incident radiation and the optical properties of the snow based on an analytical approximation of the radiative transfer of snow. For this method, 30 wavelengths, called tie points (TPs), and 16 reference irradiance profiles are calculated to incorporate the absorbed energy and the reference irradiance. The absorbed energy is then interpolated for each wavelength between two TPs with adequate kernel functions derived from radiative transfer theory for snow and the atmosphere. We show that the accuracy of the absorbed energy calculation primarily depends on the adaptation of the irradiance of the reference profile to that of the simulation (absolute difference <1 W m−2 for broadband absorbed energy and absolute difference <0.005 for broadband albedo). In addition to the performance in terms of accuracy and calculation time, the method is adaptable to any atmospheric input (broadband, narrowband) and is easily adaptable for integration into a radiative scheme of a global or regional climate model.

Evaluation of a new snow albedo scheme for the Greenland ice sheet in the Regional Atmospheric Climate Model (RACMO2)

Abstract. Snow and ice albedo schemes in present-day climate models often lack a sophisticated radiation penetration scheme and do not explicitly include spectral albedo variations. In this study, we evaluate a new snow albedo scheme in the Regional Atmospheric Climate Model (RACMO2) for the Greenland ice sheet, version 2.3p3, that includes these processes. The new albedo scheme uses the Two-streAm Radiative TransfEr in Snow (TARTES) model and the Spectral-to-NarrOWBand ALbedo (SNOWBAL) module, version 1.2. Additionally, the bare-ice albedo parameterization has been updated. The snow and ice broadband and narrowband albedo output of the updated version of RACMO2 is evaluated using the Programme for Monitoring of the Greenland Ice Sheet (PROMICE) and Kangerlussuaq transect (K-transect) in situ data and Moderate Resolution Imaging Spectroradiometer (MODIS) remote-sensing observations. Generally, the modeled narrowband and broadband albedo is in very good agreement with satellite observations, leading to a negligible domain-averaged broadband albedo bias for the interior. Some discrepancies are, however, observed close to the ice margin. Compared to the previous model version, RACMO2.3p2, the broadband albedo is considerably higher in the bare-ice zone during the ablation season, as atmospheric conditions now alter the bare-ice broadband albedo. For most other regions, however, the updated broadband albedo is lower due to spectral effects, radiation penetration or enhanced snow metamorphism.

A module to convert spectral to narrowband snow albedo for use in climate models: SNOWBAL v1.2

Abstract. Snow albedo schemes in regional climate models often lack a sophisticated radiation penetration scheme and generally compute only a broadband albedo. Here, we present the Spectral-to-NarrOWBand ALbedo module (SNOWBAL, version 1.2) to couple effectively a spectral albedo model with a narrowband radiation scheme. Specifically, the Two-streAm Radiative TransfEr in Snow model (TARTES) is coupled with the European Centre for Medium-Range Weather Forecasts (ECMWF) Integrated Forecast System (IFS), cycle 33R1, atmospheric radiation scheme based on the Rapid Radiation Transfer Model, which is embedded in the Regional Atmospheric Climate Model version 2.3p2 (RACMO2). This coupling allows to explicitly account for the effect of clouds, water vapor, snow impurities and snow metamorphism on albedo. Firstly, we present a narrowband albedo method to project the spectral albedos of TARTES onto the 14 spectral bands of the IFS shortwave radiation scheme using a representative wavelength (RW) for each band. Using TARTES and spectral downwelling surface irradiance derived with the DIScrete Ordinate Radiative Transfer atmospheric model, we show that RWs primarily depend on the solar zenith angle (SZA), cloud content and water vapor. Secondly, we compare the TARTES narrowband albedo, using offline RACMO2 results for south Greenland, with the broadband albedo parameterizations of Gardner and Sharp (2010), currently implemented in RACMO2, and the multi-layered parameterization of Kuipers Munneke et al. (2011, PKM). The actual absence of radiation penetration in RACMO2 leads on average to a higher albedo compared with TARTES narrowband albedo. Furthermore, large differences between the TARTES narrowband albedo and PKM and RACMO2 are observed for high SZA and clear-sky conditions, and after melt events when the snowpack is very inhomogeneous. This highlights the importance of accounting for spectral albedo and radiation penetration to simulate the energy budget of the Greenland ice sheet.

EXAMINATION OF THE TERRAIN EFFECT FOR TERRESTRIAL ALBEDO ESTIMATION VIA BRDF MODEL PARAMETERS

Abstract. In this paper, we examine the effect of terrain on terrestrial albedo estimation. Terrestrial albedo is one of the most important parameters for understanding the global heat balance. The existing approach for estimating terrestrial albedo involves the estimation of model parameters of the bidirectional reflectance distribution function (BRDF) based on measurements obtained at different geometries. Then, narrowband albedos are estimated from the BRDF model parameters and the broadband albedo is finally estimated by narrowband-tobroadband conversion. Previous studies have not considered the terrain effect for generating the terrestrial albedo. Experiments using in situ measurements showed that the BRDF model, which transforms the geocoordinate of the reflectance of the shadowed terrain, generates the best accuracy. The improvement in the accuracy by the terrain effect correction is limited, and therefore further investigations using more in situ and simulated data are necessary for operational products.

Evaluation of satellite remote sensing albedo retrievals over the ablation area of the southwestern Greenland ice sheet

MODerate resolution Imaging Spectroradiometer (MODIS) albedo products have been validated over spatially uniform, snow-covered areas of the Greenland ice sheet (GrIS) using the so-called single ‘point-to-pixel’ method. This study expands on this methodology by applying a ‘multiple-point-to-pixel’ method and examination of spatial autocorrelation (here using semivariogram analysis) by using in situ observations, high-resolution WorldView-2 (WV-2) surface reflectances, and MODIS Collection V006 daily blue-sky albedo over a spatially heterogeneous surfaces in the lower ablation zone in southwest Greenland. Our results using 232 ground-based samples within two MODIS pixels, one being more spatial heterogeneous than the other, show little difference in accuracy among narrow and broad band albedos (except for Band 2). Within the more homogenous pixel area, in situ and MODIS albedos were very close (error varied from −4% to +7%) and within the range of ASD standard errors. The semivariogram analysis revealed that the minimum observational footprint needed for a spatially representative sample is 30m. In contrast, over the more spatially heterogeneous surface pixel, a minimum footprint size was not quantifiable due to spatial autocorrelation, and far exceeds the effective resolution of the MODIS retrievals. Over the high spatial heterogeneity surface pixel, MODIS is lower than ground measurements by 4–7%, partly due to a known in situ undersampling of darker surfaces that often are impassable by foot (e.g., meltwater features and shadowing effects over crevasses). Despite the sampling issue, our analysis errors are very close to the stated general accuracy of the MODIS product of 5%. Thus, our study suggests that the MODIS albedo product performs well in a very heterogeneous, low-albedo, area of the ice sheet ablation zone.Furthermore, we demonstrate that single ‘point-to-pixel’ methods alone are insufficient in characterizing and validating the variation of surface albedo displayed in the lower ablation area. This is true because the distribution of in situ data deviations from MODIS albedo show a substantial range, with the average values for the 10th and 90th percentiles being −0.30 and 0.43 across all bands. Thus, if only single point is taken for ground validation, and is randomly selected from either distribution tails, the error would appear to be considerable. Given the need for multiple in-situ points, concurrent albedo measurements derived from existing AWSs, (low-flying vehicles (airborne or unmanned) and high-resolution imagery (WV-2)) are needed to resolve high sub-pixel variability in the ablation zone, and thus, further improve our characterization of Greenland's surface albedo.

A New Method for the Estimation of Broadband Apparent Albedo Using Hyperspectral Airborne Hemispherical Directional Reflectance Factor Values

The broadband albedo values retrieved from satellite sensors are usually compared directly to ground measurements. Some authors have noted the necessity of high spatial resolution albedo estimates to fill the gap between ground measurements and satellite retrievals. In this respect, hyperspectral airborne data with high spatial resolution is a powerful tool. Here, a new operational method for the calculation of airborne broadband apparent albedo over the spectral range of 350–2500 nm is presented. This new method uses the Hemispherical Directional Reflectance Factor (HDRF) as a proxy for the narrowband albedo, assuming a Lambertian approximation. The broadband apparent albedo obtained is compared to that estimated using theapparent albedo equation devised for the Moderate Resolution Imaging Spectroradiometer (MODIS). Airborne data were collected using the Airborne Hyperspectral Scanner (AHS). Field data were acquired at three sites: a camelina field, a green grass field, and a vineyard. The HDRF can be used to approximate the narrowband albedo for all View Zenith Angle (VZA) values for flights parallel to the solar principal plane (SPP); for flights orthogonal to the SPP, discrepancies are observed when the VZA approaches −45°. Root Mean Square Error (RMSE) values in the range 0.009–0.018 were obtained using the new method, improving upon previous results over the same area (RMSEs of 0.01–0.03). The relative error in the albedo estimation using the new method is 12% for −36.2° < VZA < 40.8° in the case of flights parallel to the SPP and less than 15% for −13° < VZA < 45° and 45% for VZA = −45° for flights orthogonal to the SPP. The good performance of the new method lies in the use of the at-surface solar irradiance and the proposed integration method.

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.

An improved derivation of the top‐of‐atmosphere albedo from POLDER/ADEOS‐2: 2. Broadband albedo

The narrowband albedos derived from Polarization and Directionality of Earth Reflectances (POLDER) measurements have been described in a companion paper (Buriez et al., 2005). Here, they are used to estimate the broadband shortwave albedo. Except for the gaseous absorption, the albedos at 443 and 670 nm are considered as representative of the UV‐visible range, and the albedo at 865 nm is considered as representative of the near infrared. The gaseous absorption is estimated from TOMS data and from the POLDER 910‐ to 865‐nm reflectance ratio. In a previous approach, the respective weights of the three narrowband albedos were based on radiative transfer simulations. Now, we take advantage of spatiotemporal coincidences between the second Advanced Earth Observing Satellite (ADEOS‐2) and Terra to adjust these weights from the comparison between POLDER narrowband and CERES broadband reflectances. With no adjustment, the POLDER albedos are underestimated by 2% (in relative value) whereas the associated reflectances are underestimated by 4%. With an adjustment to the CERES reflectances, the POLDER shortwave albedos are overestimated by 2%. With or without adjustment the RMS difference between the POLDER and the CERES 1°‐regional instantaneous shortwave albedos is 6%; this is quite satisfactory since it is comparable to the CERES SW albedo consistency between nadir and oblique viewing zenith angles. When considering only homogeneous areas, the agreement between POLDER and CERES estimates was rather less good in the previous approach; it is now more than twice better. The difference between POLDER and CERES shortwave albedo estimates appears, for a large part, due to our plane‐parallel assumption that is crucial for the heterogeneous liquid water clouds. It results in a bias in monthly mean shortwave flux around 2 W m−2 that is found to present only small variations in latitude.

Correcting first‐order errors in snow water equivalent estimates using a multifrequency, multiscale radiometric data assimilation scheme

A season‐long, multiscale, multifrequency radiometric data assimilation experiment is performed to test the feasibility of snow water equivalent (SWE) estimation. Synthetic passive microwave (PM) observations at Advanced Microwave Scanning Radiometer‐Earth Observing System frequencies and 25 km resolution and synthetic near infrared (NIR) narrowband albedo observations corresponding to Moderate Resolution Imaging Spectroradiometer band 5 (1230–1250 μm) and 1 km resolution are assimilated into a land surface model snow scheme using the ensemble Kalman filter. First‐order sources of model uncertainty, including error in precipitation quantity, grain size evolution, precipitation spatial distribution, and vegetation leaf area index are modeled. SWE remote sensing retrieval schemes would be of limited value for these scenarios where snow depth ranged between 1.0 and 2.0 m, grain size varied in space, significant vegetation was present, and the snowpack sometimes contained liquid water. Nevertheless, the true basinwide SWE is recovered, in general, within a root‐mean‐square error (RMSE) of approximately 2 cm. Synergy is observed between the PM and NIR measurements because of the complementary nature of the multiscale, multifrequency measurements. Results from the assimilation are compared to those from a pure modeling approach and from a remote sensing retrieval approach. The effects of model uncertainty, measurement error, and ensemble size are investigated.

An Evaluation of Urban Land Use Regulations from View of Urban Surface Albedo

abstractUrban surface albedo, which is one of the important factors of the urban heat island phenomenon, was used to evaluate urban land use regulations. Broadband albedo at the surface level of Sakai City, Japan, was calculated, and effects of land use regulations and actual land use were estimated using satellite remote sensing data, a land use regulation map and detailed digital information expressing actual land use. Land use regulations define land use, maximum building coverage ratios and maximum floor area ratios, which relate the heights of buildings to the areas of their sites. These factors have effects upon broadband albedo and each narrowband albedo corresponding to wavelengths observed by Landsat ETM+. Albedo values differed according to land use regulation zones. The combination of land use regulation and actual land use is also meaningful in grasping urban surface albedo. But this relationship may provide only slight insight into the urban heat island phenomenon.

An improved derivation of the top‐of‐atmosphere albedo from POLDER/ADEOS‐2: Narrowband albedos

The Polarization and Directionality of Earth Reflectances (POLDER) instrument was routinely functioning aboard the second Advanced Earth Observing Satellite (ADEOS‐2) from April to October 2003. A series of algorithms dedicated to Earth radiation budget, water vapor, and clouds are applied to the POLDER data. This paper presents the derivation scheme of the narrowband albedos at the top of the atmosphere from POLDER measurements at 443, 670, and 865 nm. These narrowband albedos are used to estimate the broadband shortwave albedo at the top of atmosphere in a companion paper. Here we focus on the 670 nm (over land)/865 nm (over ocean) albedo. Although our derivation is based on the plane‐parallel hypothesis, and thanks to the multidirectional capability of POLDER, results appear reliable for both cloudy and clear‐sky scenes. A quality index is introduced from the comparison between the angular variability of the retrieved “directional” albedo values and that of the measured reflectances. This quality index is “good” in 80% of the cases. Moreover, the retrieved albedo values are found to be statistically little dependent on the viewing direction.

A Simplified Surface Albedo Inverse Model with MODIS Data

Land surface albedo, which is a fundamental component needed for determining the radiation balance of the Earth-atmosphere system, is the surface hemispherical reflectivity integrated over the solar spectrum. Briefly said, it is the ratio of the total upwelling irradiance to the total downward irradiance. Given the limit of experiment field and instrument equipment, it is very difficult to obtain a great deal of surface measured data. Thereof, in our research, from the point of view that surface albedos are the ratio of flux, surface albedos can be reversed with less calibrated field synchronous observed data. \;During the conversion process of surface albdeos, the first is to obtain surface reflectance from remotely sensed data, which can be completed by running 6S atmospheric radiative transfer model; the second is to reverse surface narrowband albedos, which is fulfilled by MODTRAN model. The outputs of MODTRAN simulations are upwelling and downwelling irradiance. The sensor spectral response functions are integrated with these outputs to generate narrowband albedos; the third is to calculate surface broadband albedos, which is focused on determining the converting coefficients from narrowbands to broadbands under the the linear relationship between narrowband albedos and broadband albedos. The spectral distribution of solar irradiance at the surface is set as the weighting function for comverting the narrowband albedos to broadband albedos in the research. The conversion of broadband albedos is fulfilled finally. The simplified surface broadband albedo reverse model is tested for the west of Inner Mongolia using MODIS image data and synchronous observed surface spectral data based on the above conversion algorithm. The results show that the method is convenient and feasible under a specific surface/atmospheric condition.

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.

Narrowband-to-broadband albedo conversion for glacier ice and snow based on aircraft and near-surface measurements

This article presents albedo measurements of snow and glacier ice at Vatnajökull (Iceland) and the Kangerlussuaq transect (Greenland). Radiative fluxes were measured in the broadband and in four narrowbands, namely, Thematic Mapper (TM) Bands 2 and 4, and Advanced Very High Resolution Radiometer (AVHRR) Bands 1 and 2. The incoming fluxes were measured near the ground and the outgoing fluxes from a helicopter. Extracts of the data collected over snow, ice, supraglacial moraines, supraglacial lakes, and tundra are discussed. Using the data sets from Iceland and Greenland, and data sets with entirely ground-based albedo measurements from the Morteratschgletscher (Switzerland) and Scharffenbergbotnen (Antarctica), new equations for narrowband-to-broadband (NTB) conversion were developed. They have a residual standard deviation of 0.011 for TM and 0.008 for AVHRR and can be applied without having to classify the surface. The helicopter data are also used to develop criteria for distinguishing different types of surfaces, which are needed for the application of Bidirectional Reflectance Distribution Functions (BRDFs). Snow can be distinguished from ice by defining a threshold for a single narrowband albedo. The ratio of the albedos in TM4 and TM2, and to a lesser extent, the ratio of the albedos in AVHRR2 and AVHRR1, may serve as a proxy for the surface characteristics of glacier ice in terms of concentrations of water, debris, and dust.

Surface albedo measurements over snow and blue ice in thematic mapper bands 2 and 4 in Dronning Maud Land, Antarctica

Ground‐based observations of broadband, narrowband (in Landsat thematic mapper bands 2, 0.52–0.60 μm, and 4, 0.76–0.90 μm), and bidirectional reflectance are used to study the albedo of blue ice and snow. The measured broadband albedos for blue ice and snow are 0.60 and 0.78, respectively. The narrowband albedos in bands 2 and 4 for snow are 0.97 and 0.86, respectively, and for blue ice are 0.87 and 0.51, respectively. The measurements are point values and used to verify existing weighting functions between the broadband and narrowband albedos. Blue ice data do not comply with the existing functions. Furthermore, a new function for blue ice is derived. A simple two‐stream model is used to illustrate the behavior of the snow and ice albedo as a function of reflection and absorption. The bidirectional reflectance measurements are used to derive bidirectional reflectance distribution functions (BRDFs) for snow and blue ice. The BRDF of blue ice exhibits a maximum brightening in the forward limb around a zenith angle close to the solar zenith angle, indicating specular reflectance of blue ice which is not visible for snow and glacier ice surfaces.

Retrieval of Land Surface Albedo from Satellite Observations: A Simulation Study

Land surface albedo is a critical parameter affecting the earth's climate and is required by global and regional climatic modeling and surface energy balance monitoring. Surface albedo retrieved from satellite observations at one atmospheric condition may not be suitable for applicati to other atmospheric conditions. In this paper the authors separate the apparent surface albedo from the inherent surface albedo, which is independent of atmospheric conditions, based on extensive radiative transfer simulations under a variety of atmospheric conditions. The results show that spectral inherent albedos are different from spectral apparent albedos in many cases. Total shortwave apparent albedos under both clear and cloudy conditions are also significantly different from their inherent total shortwave albedos. The conversion coefficients of the surface inherent narrowband albedos derived from the MODIS (Moderate-Resolution Imaging Spectroradiometer) and the MISR (Multiangle Imaging Spectroradiometer) instruments to the surface broadband inherent albedo are reported. A new approach of predicting broadband surface inherent albedos from MODIS or MISR top of atmosphere (TOA) narrowband albedos using a neural network is proposed. The simulations show that surface total shortwave and near-infrared inherent albedos can be predicted accurately from TOA narrowband albedos without atmospheric information, whereas visible inherent albedo cannot.

Narrowband to broadband conversion of Landsat TM glacier albedos

In this paper we present an empirical relationship between the broadband glacier albedo (alpha) and the narrowband glacier albedos in Landsat TM bands 2 and 4 (alpha2 and alpha4, respectively). The relationship was established on the basis of multiple linear regression analysis of 112 ground-based simultaneous measurements of alpha, alpha2 and alpha4 made at 32 sites on the tongue of the Morteratschgletscher, Switzerland. The measurements were carried out over a representative set of glacier surface types ranging from completely debris-covered glacier ice (alpha=0.08)to dry snow (alpha=0.86). The regression model explains more than 99% of the variance of the broadband albedo and the root-mean-square value of the residuals is only 0.009. The relationship enables users of Landsat TM data to make an accurate estimate of the broadband albedo on the basis of narrowband albedos without having to classify the glacier surface.