An Evaluation of Satellite Estimates of Solar Surface Irradiance Using Ground Observations in San Antonio, Texas, USA
Estimates of solar irradiance at the earth's surface from satellite observations are useful for planning both the deployment of distributed photovoltaic systems and their integration into electricity grids. In order to use surface solar irradiance from satellites for these purposes, validation of its accuracy against ground observations is needed. In this study, satellite estimates of surface solar irradiance from Geostationary Operational Environmental Satellite (GOES) are compared with ground
... observations at two sites, namely the main campus of the University of Texas at San Antonio (UTSA) and the Alamo Solar Farm of San Antonio (ASF). The comparisons are done mostly on an hourly timescale, under different cloud conditions classified by cloud types and cloud layers, and at different solar zenith angle intervals. It is found that satellite estimates and ground observations of surface solar irradiance are significantly correlated (p < 0.05) under all sky conditions (r: 0.80 and 0.87 on an hourly timescale and 0.94 and 0.91 on a daily timescale, respectively for the UTSA and ASF sites); on the hourly timescale, the correlations are 0.77 and 0.86 under clear-sky conditions, and 0.74 and 0.84 under cloudy conditions, respectively for the UTSA and ASF sites, and mostly >0.60 under different cloud types and layers for both sites. The correlations under cloudy-sky conditions are mostly stronger than those under clear-sky conditions at different solar zenith angles. The correlation coefficients are mostly the smallest with solar zenith angle in the range of 75-90 • under all sky, clear-sky and cloudy-sky conditions. At the ASF site, the overall bias of GOES surface solar irradiance is small (+1.77 Wm −2 ) under all sky while relatively larger under clear-sky (−22.29 Wm −2 ) and cloudy-sky (+40.31 Wm −2 ) conditions. The overall good agreement of the satellite estimates with the ground observations underscores the usefulness of the GOES surface solar irradiance estimates for solar energy studies in the San Antonio area. satellite estimates of solar irradiance offer obvious advantages over in situ measurements. Nevertheless, the quality of satellite estimates still need to be ground-truthed with direct in situ measurements. Since January 1996, the National Oceanic and Atmospheric Administration (NOAA)/National Environmental Satellite, Data, and Information Service (NESDIS) has retrieved parameters for evaluating the surface shortwave radiation balance from the Geostationary Operational Environmental Satellite system (GOES) in real time [10, 11] . These satellite observations have allowed the implementation of radiative transfer models for solar radiation. Validation of the GOES-derived satellite surface solar irradiance (G s ), estimated from these types of models, with ground measured global horizontal irradiance (G g ) has been conducted by many previous studies        . For example, Pinker et al.  compared G s , retrieved from the GOES Surface and Insolation Products (GSIP), and G g from about 50 stations and found the correlation coefficients between them on different timescales were all greater than 0.90, with a bias and RMSE (root mean squared error) of −36-5 Wm −2 and 87-104 Wm −2 on an hourly timescale, −11-8 Wm −2 and 22-29 Wm −2 on a daily timescale, and −11-8 Wm −2 and 14-19 Wm −2 on a monthly timescale, respectively. Otkin et al.  found that the correlation coefficient for the seasonal comparison of G s retrieved from a physical model and G g on different timescales at 11 sites was also high (0.95), with a bias of 8 Wm −2 (GOES_E) and −9 Wm −2 (GOES_W) and RMSE of 63 Wm −2 on an hourly timescale, and the bias of 4 Wm −2 (GOES_E) and −17 Wm −2 (GOES_W) and RMSE of 16 Wm −2 on the daily timescale. All the above-mentioned studies have showed that there is overall good agreements between G s and G g under all sky conditions, with RMSE and bias mostly in the ranges of 63-104 Wm −2 and −36-5 Wm −2 on an hourly timescale and 15-49 Wm −2 and −11-8 Wm −2 on a daily timescale, respectively. None of the cited studies, however, has examined the cloud effect on the surface solar irradiance mapping from GOES satellites. The only study that considered the cloud effect was the one by Habte et al.  that compared G s and G g under clear-sky and cloudy-sky conditions on an hourly timescale and found (1) better correlation under clear-sky conditions than that under cloudy-sky conditions and (2) that G s is biased higher (overestimation) under clear-sky conditions than under cloudy-sky conditions, especially at low solar zenith angles. Although many factors (such as aerosol, water vapor and ozone) could attenuate the solar radiation reaching the Earth's surface, clouds are considered as the primary effect    and their attenuation is much larger than that of the atmosphere. Therefore, the objective of this work is to explore the correlation between G s and G g on hourly (and daily) timescales, under different cloud conditions in terms of cloud types and cloud layers, and at different solar zenith angle intervals. The ultimate goal is to determine if real time satellite estimates are reliable and acceptable for future solar forecasting and mapping.