Abstract Surface downwelling and upwelling radiative fluxes are important inputs into hydrologic models that evaluate water budgets, and into land surface data assimilation schemes which are driven with radiative fluxes. For large-scale needs, only remote sensing methods can provide such information. The accuracy of the derived fluxes depends on the inference schemes and on the quality of auxiliary input parameters. At present, information on surface short-wave radiative fluxes over the United States is produced in real time by the National Oceanic and Atmospheric Administration (NOAA)/National Environmental Satellite Data and Information Service (NESDIS) at 0.5 degree resolution, at hourly time intervals, using independently derived auxiliary inputs. Information on aerosol properties and their temporal variability is not available, and at best, is only estimated. During 1997 information on aerosol optical properties was collected at the USDA-Agricultural Research Service Walnut Gulch Experimental Watershed, Arizona, in preparation for future validation efforts in support of new satellite observations (e. g., ADEOS-II). This data set was used to test the sensitivity of a radiation inference scheme to aerosols, in particular, on the determination of clear sky fluxes and the surface albedo. Data from the Arizona Meteorological Network (AZMET) have been utilized to evaluate the satellite estimates for 1997. It was found that the current satellite estimates are within 70 Wm -2 of the ground observations on an hourly time scale and within 24 Wm -2 on a daily time scale. In the latter case this is less than 10% of the mean. Use of actual observations of aerosols, as compared to climatological values, reduces the bias substantially, while less significant changes in the rms were found. Keywords: radiative fluxes, aerosols, remote sensing 2 1. Background Site and history The Semi-Arid Land-Surface-Atmosphere (SALSA) Program seeks to evaluate the consequences of natural and human-induced changes in semi-arid environments (this issue; also see the SALSA home-page at http://www.tucson.ars.ag.gov/salsa/salsahome.html). Information on remote sensing activities in this region is presented in Surface downwelling and upwelling radiative fluxes play an important role as inputs into hydrologic models aimed at evaluating water budgets. Therefore, it is important to determine how well such fluxes can be derived from satellite observations. First estimates of short-wave (SW) surface radiative fluxes (global irradiance) by satellite methods for this region were attempted during the Monsoon '90 experiment Linkage to related activities The Upper San Pedro Basin has been established as the North American semi-arid site for assessing the impacts of climatic variation and for calibrating and validating algorithms and process-based models to be implemented with NASA EOS observations. For example, the basin was selected by the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) instrument team as a semi-arid validation site as well as a NASA Global Land Cover Test Site. It was also selected as the primary focus site for the EOS interdisciplinary science hydrology team at the University of Arizona and Centre d'Etudes Spatiales de la Biosphere (CESBIO), France. Two Sonora research groups have been involved in research in the USPB: Instituto del Medio Ambiente y Desarrollo Sustentable del Estado de Sonora (IMADES), and Instituto Tecnológico de Sonora (ITSON). Numerous proposals have been funded for research as well as for remotely sensed data acquisition in the basin, which range from ERS-2, SPOT4, ADEOS-II, and for a mesoscale meteorological modeling initiative. Modeling activity focuses on exploring the potential of coupling parameters derived by methods of remote sensing to mesoscale atmospheric models, to aid in diagnosing the spatial distribution of surface fluxes over the entire San Pedro Basin at a 4 x 4 kilometer grid spacing (Toth, 1997). 4 Radiative fluxes used in this study Remotely sensed data The estimated surface SW radiative fluxes (global irradiance) used in this study are produced by NOAA/NESDIS, using the University of Maryland methodology The full archive maintained at the University of Maryland contains additional parameters, that as yet, are not distributed. Specifically, the following types of information are archived: • satellite based information, used to drive the model; • auxiliary data used to drive the model; • Eta model output products relevant for hydrologic modeling; and, • independently derived satellite products. It is planned to expand the number of parameters currently distributed via the World Wide Web to include,e. g., cloud amounts). Validation data Radiative fluxes A comprehensive evaluation of the model was done using data for the entire year of 1996 from several sources (Pinker et al., 2000). Data available from the Surface Radiation Monitoring Network (SURFRAD) (Hicks et al., 1996), the Illinois State Water Survey (Hollinger et al., 1994) and the Arizona Meteorological Network (AZMET) (Brown, 1989) were used. In this study, only AZMET stations, as illustrated in Aerosol observations at Walnut Gulch In December of 1996 a CIMEL sunphotometer was installed at the USDA-ARS Walnut Gulch Experimental Watershed to provide information on aerosol optical depths and other aerosol optical parameters Aerosol sensitivity experiments Issues Satellite inference schemes that use physical models require information on radiances as observed by the satellite sensor in relevant spectral channels, as well as information on the state of the atmosphere and the surface. Such information has been available for some of the needed parameters from numerical weather prediction models (e. g., on water vapor) or from independently derived satellite quantities (e. g., ozone). Typically the least amount of information in known about aerosols. Therefore, most inference schemes use some type of aerosol climatology. We have followed a two-step approach in the process of inferring surface SW radiative fluxes. Initially, we use an average value of clear sky radiance as derived from about two weeks of clear sky observations. We assume a climatological value of aerosol optical depth 6 Subsequently, we use each clear sky pixel from the beginning of the retrieval time interval (one month segments at a time) and the initially derived surface albedo to subsequently derive an aerosol optical thickness from each clear sky pixel. The corresponding flux at the surface will be selected from a look-up table as the one that is appropriate for all the derived values of input parameters, as well as the inferred aerosol optical depth. This approach was used to produce the surface fluxes presented in Aerosol experiments We have performed an experiment to evaluate the sensitivity of surface SW radiative flux parameters to aerosol information. An off-line version of the GCIP/SRB model was run for the entire year of 1997. All the satellite input parameters, as well as the atmospheric and surface parameters were the same as used by NOAA/NESDIS in the real time runs for 1997 and as archived at the University of Maryland. The only difference was that the climatological aerosol optical depth values used to initialize the retrieval process were replaced by the monthly mean observed values, as presented in Results Surface fluxes In A comparison of hourly mean estimated global irradiance with ground truth as obtained from the control run, is presented in Figures 6 for all sky (a) and clear sky (b) cases independently. In for clear sky). Surface albedo The GCIP/SRB model produces surface downwelling and upwelling SW radiative fluxes (global and reflected radiation), and their ratio is termed "albedo". At instantaneous time scales, the albedo represents the value at the time of the observation. In order to derive a daily value, the downwelling and upwelling fluxes are averaged and their ratio is taken. Since the surface fluxes are computed independently for clear and cloudy pixels, it is possible to produce "clear sky" albedos and all-sky albedos. Preliminary evaluations show that these two values are quite close to each other. In Discussion Radiative fluxes at the earth's surface determine the surface energy budget, and therefore, the rate of evapotranspiration (Dickinson, 1986; Avissar and Verstraete, 1990; Henderson-Sellers, 1993; Sellers et al., 1996; In addition to better aerosol information, there is a need for improved calibration of satellite sensors. It is believed that some degradation of satellite instruments might have occurred. Preliminary estimates of such degradation are as high as 15%. Experiments were performed to evaluate the possible impact of such degradation on the derived surface fluxes Summary Regions classified as semi-arid or arid constitute about one-third of the total global land cover. Often these regions are subjected to soil erosion, wind-storms, and variable aerosol loading. Aerosols are important in altering the radiation that reaches the surface and therefore, they are a source of error in the interpretation of satellite signals. This is particularly true in the visible region of the spectrum. Routine and continuous information on atmospheric aerosol content is lacking. However, such data are becoming available on regional scales under observational initiatives like the Tropospheric Aerosol Radiative Forcing Observational Experiment (TARFOX) (Russel et al., 1999), AERONET (Holben et al., 1998), SKYNET (Takamura, 1996); and will become available on global scale under new satellite observational programs like MODIS (King et al., 1999), ADEOS and ADEOS-II (http://www.eorc.nasda.go.jp/index.html); and under integrating initiatives like the Global Aerosol Climatology Project (GACP) (Curran et al., 1998; Curran, 1999). Many landscapes in the southwest United States and northern Mexico are being altered from activities such as groundwater mining and overgrazing. Lack of information on aerosols can therefore introduce errors in our ability to estimate from space how much the surface has changed. In the framework of the SALSA Program objectives for long-term monitoring of human-induced change on the hydrological and ecological resources of semi-arid regions, we have conducted an experiment to assess the current uncertainties in aerosol optical depths on such parameters as surface short-wave fluxes and surface albedos. This is important because these parameters influence the modeling of hydrological processes that control the exchange of heat, water vapor and CO2. It was found that using observed aerosol climatology improved radiative flux retrieval from satellite observations and subsequent computation of flux estimates. In addition, it was found that using measured values to initialize the aerosol optical depth in the retrieval of surface global irradiance, the surface albedo increased by about 0.02 on the average. Comparison of satellite estimates of radiative flux were made with data from the Arizona Meteorological Network (AZMET) for 1997 to evaluate the procedures described. It was found that the current satellite estimates are within 70 Wm -2 of the ground observations on an hourly time scale and within 24 Wm -2 on a daily time scale. In the latter case this is less than 10% of the mean. 10 Use of actual observations of aerosols, as compared to climatological values, reduces the bias substantially, while less significant changes in the rms were found. In summary, this study demonstrated that on a local scale, characterizations of aerosols, based even on a limited observational periods is preferred to estimates based on large-scale climatologies
