Abstract
Solar surface radiation data of high quality is essential for the appropriate monitoring and analysis of the Earth's radiation budget and the climate system. Further, they are crucial for the efficient planning and operation of solar energy systems. However, well maintained surface measurements are rare in many regions of the world and over the oceans. There, satellite derived information is the exclusive observational source. This emphasizes the important role of satellite based surface radiation data. Within this scope, the new satellite based CM-SAF SARAH (Solar surfAce RAdiation Heliosat) data record is discussed as well as the retrieval method used. The SARAH data are retrieved with the sophisticated SPECMAGIC method, which is based on radiative transfer modeling. The resulting climate data of solar surface irradiance, direct irradiance (horizontal and direct normal) and clear sky irradiance are covering 3 decades. The SARAH data set is validated with surface measurements of the Baseline Surface Radiation Network (BSRN) and of the Global Energy and Balance Archive (GEBA). Comparison with BSRN data is performed in order to estimate the accuracy and precision of the monthly and daily means of solar surface irradiance. The SARAH solar surface irradiance shows a bias of 1.3 \(W/m^2\) and a mean absolute bias (MAB) of 5.5 \(W/m^2\) for monthly means. For direct irradiance the bias and MAB is 1 \(W/m^2\) and 8.2 \(W/m^2\) respectively. Thus, the uncertainty of the SARAH data is in the range of the uncertainty of ground based measurements. In order to evaluate the uncertainty of SARAH based trend analysis the time series of SARAH monthly means are compared to GEBA. It has been found that SARAH enables the analysis of trends with an uncertainty of 1 \(W/m^2/dec\); a remarkable good result for a satellite based climate data record. SARAH has been also compared to its legacy version, the satellite based CM-SAF MVIRI climate data record. Overall, SARAH shows a significant higher accuracy and homogeneity than its legacy version. With its high accuracy and temporal and spatial resolution SARAH is well suited for regional climate monitoring and analysis as well as for solar energy applications.
Highlights
The surface solar irradiance (I) is defined as the incoming solar radiation at the surface in the0.2–4.0 μm wavelength region
The comparison results in a mean bias, mean absolute difference, anomaly correlation, standard deviation and fraction of months above a given limit for each individual station and for all stations together
surface incoming solar radiation (SIS) and surface direct radiation (SID) data are averaged by application of an equation by Diekmann [73], which is preferable because of the large dependency on the solar zenith angle of SIS and SID, which makes the arithmetic averaging very sensitive to data gaps
Summary
The surface solar irradiance (I) is defined as the incoming solar radiation at the surface in the. One of the first climate data records of solar surface radiation (SIS) has been provided by the International Satellite Cloud Climatology project (ISCCP FD, [9] and citations within) and by the Global Energy and Water Cycle Experiment ([GEWEX SRB, [10]) These data sets lack in homogeneity [11,12] and are only of limited value for the analysis of climate trends. Validation of SARAH is done against ground based measurements and against its well established legacy version, the CM-SAF MVIRI data set [25,26] This data set is already used by several hundred users in different fields, covering climate applications and solar energy (e.g., [27,28,29,30]) and is an appropriate benchmark for SARAH.
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