反演,地表反照率,比辐射率

In retrieving the surface albedo from satellite data (e.g. Greuell & Oerlemans, 2003; Key, Wang, Stroeve, & Fowler, 2001; Knap, Brock, Oerlemans, & Willis, 1999; Stroeve, Nolin, & Steffen, 1997), one of the processing steps is narrowband-to-broadband conversion (NTB conversion). This step is necessary because satellite sensors used for the retrieval of the albedo carry out their measure-
* Corresponding author. E-mail address: w.greuell@phys.uu.nl (W. Greuell).
0034-4257/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.rse.2003.10.010
Since both narrowband and broadband albedos depend on the spectral albedo (i.e. variation of the surface reflectivity with wavelength) and the incident spectral irradiance (i.e. spectral variation of the incoming solar radiative flux with wavelength), equations for NTB conversion do as well. The spectral albedo is determined by the optical properties of the surface and of the sub-surface material, and by the solar zenith angle (Wiscombe & Warren, 1980). The incident spectral irradiance is the result of the optical properties of the atmosphere and, owing to multiple reflections between the surface and the atmosphere, the spectral albedo. Since the optical depth of the atmosphere is a function of surface elevation and solar zenith angle, these variables also affect the incident spectral irradiance. Of course, it is desirable that equa-
ments in spectral bands with widths ranging typically between 20 and 300 nm (called ‘‘narrowbands’’), whereas the albedo refers to reflection of solar radiation integrated over all wavelengths (almost equal to the ‘‘broadband albedo’’, a term in used remote sensing, where the integration is confined to the wavelength interval covered by ordinary pyranometers).
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W. Greuell, J. Oerlemans / Remote Sensing of Environment 89 (2004) 95–105
tions for NTB conversion are valid for as wide as possible ranges of (sub-)surface and atmospheric conditions, elevations and solar zenith angles. Recently, Greuell, Reijmer, and Oerlemans (2002) proposed equations for NTB conversion for snow and ice, valid for thematic mapper (TM) and advanced very high resolution radiometer (AVHRR) albedos. Their TM equation is a fit to ground-based and helicopter measurements acquired in Greenland, Iceland, the Alps, and Antarctica. These measurements represent a wide variety of snow and ice surfaces and the variability in atmospheric conditions, surface elevations, and solar zenith angles during the measurements was considerable. Therefore, the TM equation of Greuell et al. has a large range of applicability. On the other hand, their AVHRR equation was based only on the data from helicopter experiments performed in Greenland and Iceland. Both experiments lasted a few hours, during which the observed surfaces were all melting. Moreover, the atmospheric conditions were stable and similar for the two experiments and the ranges in elevation (100 – 1600 m a.s.l.) and solar zenith angle (41 – 53j) limited. The question arises whether the AVHRR equations are valid under conditions that are outside the limited ranges encountered during these experiments.
Keywords: Surface albedo; Satellite data; Retrieval method; Narrowband-to-broadband conversion; Snow; Ice
1. Introduction
The surface albedo is the fraction of the incident solar radiative flux reflected by the surface. For snow and ice surfaces, variations in the surface albedo (shortly: albedo) can have an enormous influence on the amount of melt because the albedo regulates absorption of solar radiation, which usually is the dominant term in the surface energy balance, especially during the melt season (e.g. Ambach, 1979; Greuell & Smeets, 2001; Hock & Holmgren, 1996; Van den Broeke, 1996).
Remote Sensing of Environment 89 (200rowband-to-broadband albedo conversion for glacier ice and snow: equations based on modeling and ranges of validity of the equations
Abstract
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 f 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. D 2003 Elsevier B.V. All rights reserved.