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Coupling Diffuse Sky Radiation and Surface Albedo Ϩ BERNARD PINTY,* ALESSIO LATTANZIO, JOHN V

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Coupling Diffuse Sky Radiation and Surface Albedo Ϩ BERNARD PINTY,* ALESSIO LATTANZIO, JOHN V 2580 JOURNAL OF THE ATMOSPHERIC SCIENCES VOLUME 62 Coupling Diffuse Sky Radiation and Surface Albedo ϩ BERNARD PINTY,* ALESSIO LATTANZIO, JOHN V. MARTONCHIK,# MICHEL M. VERSTRAETE,* NADINE GOBRON,* MALCOLM TABERNER,* JEAN-LUC WIDLOWSKI,* ROBERT E. DICKINSON,@ AND YVES GOVAERTS& *Global Vegetation Monitoring Unit, IES, EC Joint Research Centre, Ispra, Italy ϩMakalumedia gmbh, Darmstadt, Germany #Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California @School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, Georgia &EUMETSAT, Am Kavalleriesand, Darmstadt, Germany (Manuscript received 29 July 2004, in final form 3 November 2004) ABSTRACT New satellite instruments have been delivering a wealth of information regarding land surface albedo. This basic quantity describes what fraction of solar radiation is reflected from the earth’s surface. However, its concept and measurements have some ambiguity resulting from its dependence on the incidence angles of both the direct and diffuse solar radiation. At any time of day, a surface receives direct radiation in the direction of the sun, and diffuse radiation from the various other directions in which it may have been scattered by air molecules, aerosols, and cloud droplets. This contribution proposes a complete description of the distribution of incident radiation with angles, and the implications in terms of surface albedo are given in a mathematical form, which is suitable for climate models that require evaluating surface albedo many times. The different definitions of observed albedos are explained in terms of the coupling between surface and atmospheric scattering properties. The analytical development in this paper relates the various quan- tities that are retrieved from orbiting platforms to what is needed by an atmospheric model. It provides a physically simple and practical approach to evaluation of land surface albedo values at any condition of sun illumination irrespective of the current range of surface anisotropic conditions and atmospheric aerosol load. The numerical differences between the various definitions of albedo for a set of typical atmospheric and surface scattering conditions are illustrated through numerical computation. 1. Introduction angular distribution of the field of downwelling inten- sities originally available at the top of the atmosphere. Albedo at some level z of a geophysical system is This situation occurs when estimating the albedo of a defined as the ratio between the upward flux density or land surface intercepting solar radiation at level z0 after irradiance exiting that particular level and the down- it has traveled through the entire column of an absorb- z ward flux density impinging on that same level . This ing and scattering atmosphere: the surface albedo, so quantity is needed for many purposes including an as- defined as a radiant flux ratio, is not an intrinsic prop- sessment of the partitioning between the radiant fluxes erty of the surface but rather determined by both the absorbed by the atmosphere and the surface. It can be surface and the overlying atmospheric layers. estimated at any wavelength and over any spectrally A proper estimation of land surface albedo requires, integrated region of the spectrum. Its dependency on therefore, consideration of the absorbing and scattering the vertical coordinate z complicates such estimates properties of the atmosphere that are controlling the when the medium located above that level z is able to field of downwelling intensities at level z . This modi- interact and thus to modify both the intensity and the 0 fication of the radiation depends on minor constituents (i.e., water vapor, carbon dioxide, ozone and other pol- lutants, clouds, and aerosols), whose concentrations Corresponding author address: B. Pinty, Global Vegetation Monitoring Unit, IES, EC Joint Research Centre, TP 440, via E. may be highly variable in space and time, or with poorly Fermi, I-21020 Ispra (VA), Italy. constrained optical properties (e.g., gaseous and par- E-mail: [email protected] ticulate emissions from industrial, agricultural and © 2005 American Meteorological Society Unauthenticated | Downloaded 10/07/21 11:02 AM UTC JAS3479 JULY 2005 P I N T Y E T A L . 2581 transportation activities, biomass burning, and dust). the European Organisation for the Exploitation of Me- Consequently, the field of downward radiation at the teorological Satellites (EUMETSAT) delivers surface bottom of the atmosphere (z0) is spatially heteroge- albedo product corresponding to DHRs for a fixed sun neous, temporally dynamic, and spectrally and angu- zenith angle. larly dependent. The difficulty of accurately estimating These products are, in fact, complementary since the upward flux density at the bottom of the atmo- they do not represent the same physical quantities and sphere (z0) is further compounded by the fact that all the selection of one particular product in this panoply is land surfaces themselves exhibit substantial spectral thus driven by the application at hand. They provide and anisotropic properties: they absorb and scatter so- means to better represent land surface processes in gen- lar radiation differently in different directions, and eral, and land surface albedo in particular, in atmo- they do so differently at different wavelengths. These spheric general circulation models (see, e.g., Zhou et al. complex processes depend not only on the nature and 2003; Knorr et al. 2001; Roesch et al. 2004). The lower optical properties of the objects that constitute the sur- atmospheric boundary conditions are increasingly rec- face, but also on their structure and spatial heteroge- ognized as of importance for weather prediction and neity. The interaction between the downward irradi- even more so for longer climatic time scales. As the ance reaching the bottom of the atmosphere at level z0 duration of the time integration increases (multiple and the surface is thus complicated and its representa- days and beyond), the relative importance of surface tion through a single number, often called albedo, can processes becomes progressively more critical because mask this complexity. of their cumulative effects on the overall energetics of Space-based measurements now document the state the atmosphere (see, e.g., Verstraete 1989). and evolution of the global land surface under cloud- Surface albedo products derived from remote sensing free conditions, as well as characterize properties of the are currently being used to improve the accuracy of the atmosphere. Regional and global surface albedo prod- simulations of global climate models provided they can ucts are thus becoming available that are useful for prescribe or generate physical quantities equivalent to better representing land surface processes, such as in those currently delivered by space agencies. To do so climate models. All such products are based on ratios requires that the models include the coupling between between upward and downward radiant fluxes. They the atmosphere and the surface scattering processes make various assumptions as to how atmospheric scat- needed to represent accurately the ratios of upward to tering processes determine the downward diffuse inten- downward radiant fluxes, that is, the BHRs 1) for any sities. If the atmospheric scattering effects are removed, given solar position, that is, for any grid cell in the the measured radiant fluxes become datasets of direc- model at any time of the day or season, and 2) for any tional hemispherical reflectance (DHR), a quantity that arbitrary state and composition of the overlying atmo- is an intrinsic surface property. If the atmospheric scat- sphere, that is, for any particular irradiance field that tering contributions are included, the resulting datasets may result from the distribution of clouds and aerosols are the bihemispherical reflectances (BHR). These de- computed by the atmospheric model. pend on the ambient atmospheric and sun zenith angle This paper discusses the scientific issues associated conditions (see, e.g., Martonchik et al. 2000). If all the with the coupling between the atmospheric and surface radiation reaching the surface can be assumed to be layers, evaluates the differences between the various isotropic, retrieval algorithms deliver a BHRiso value albedo products, and helps bridge the gap between the corresponding to a flux ratio that is, again, independent remote sensing and the climate simulation communi- of ambient conditions. For instance, the National Aero- ties. Specifically, it investigates the analytical relation- nautics and Space Administration (NASA) Moderate ships that exist between the DHRs, the BHRs, and the Resolution Imaging Spectroradiometer (MODIS) sen- BHRiso and then evaluates the differences between sor on Terra delivers DHRs and BHRiso [these prod- these products based on a series of radiation transfer ucts are called black sky and white sky albedos, respec- simulations. It proposes an analytical, highly accurate tively; Schaaf et al. (2002)], while the retrieval algo- procedure to couple land surfaces with the overlying rithm selected for the processing of data acquired by atmospheric models for a wide range of sun zenith the NASA Multiangle Imaging Spectroradiometer angle values and aerosol load conditions. (MISR) sensor on Terra generates DHRs, as well as BHRs but for the particular sun illumination conditions 2. From downward to upward intensity fields at the time of the satellite measurement
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