Satellite Retrieval of Downwelling Shortwave Surface Flux and Diffuse Fraction under All Sky Conditions in the Framework of the LSA SAF Program (Part 2: Evaluation)

Abstract

High frequency knowledge of the spatio-temporal distribution of the downwelling surface shortwave flux (DSSF) and its diffuse fraction (fd) at the surface is nowadays essential for understanding climate processes at the surface–atmosphere interface, plant photosynthesis and carbon cycle, and for the solar energy sector. The European Organization for the Exploitation of Meteorological Satellites (EUMETSAT) Satellite Application Facility for Land Surface Analysis operationally delivers estimation of the MDSSFTD (MSG Downwelling Surface Short-wave radiation Fluxes—Total and Diffuse fraction) product with an operational status since the year 2019. The method for retrieval was presented in a companion paper. Part 2 now focuses on the evaluation of the MDSSFTD algorithm and presents a comparison of the corresponding outputs, i.e., total DSSF and diffuse fraction (fd) components, against in situ measurements acquired at four Baseline Surface Radiation Network (BSRN) stations over a seven-month period. The validation is performed on an instantaneous basis. We show that the satellite estimates of DSSF and fd meet the target requirements defined by the user community for all-sky (clear and cloudy) conditions. For DSSF, the requirements are 20 Wm−2 for DSSF < 200 Wm−2, and 10% for DSSF ≥ 200 Wm−2. The mean bias error (MBE) and relative mean bias error (rMBE) compared to the ground measurements are 3.618 Wm−2 and 0.252%, respectively. For fd, the requirements are 0.1 for fd < 0.5, and 20% for fd ≥ 0.5. The MBE and rMBE compared to the ground measurements are −0.044% and −17.699%, respectively. The study also provides a separate analysis of the product performances for clear sky and cloudy sky conditions. The importance of representing the cloud–aerosol radiative coupling in the MDSSFTD method is discussed. Finally, it is concluded that the quality of the aerosol optical depth (AOD) forecasts currently available is accurate enough to obtain reliable diffuse solar flux estimates. This quality of AOD forecasts was still a limitation a few years ago.

Satellite Retrieval of Downwelling Shortwave Surface Flux and Diffuse Fraction under All Sky Conditions in the Framework of the LSA SAF Program (Part 1: Methodology)

“…The validation of the method presented in this article is reported in the companion paper [73]. The evaluation was conducted by comparing the instantaneously retrieved total DSSF and diffuse fraction (fd) against ground measurements and other satellite-based products.…”

“…Consequently, this is an interesting case that illustrates the major importance of considering the attenuation of the radiation due to aerosols not only in clearsky conditions. A more detailed comparison between the operational MDSSF product and the upcoming MDSSFTD will be given in the companion article [73].…”

Improved estimation of global solar radiation over rugged terrains by the disaggregation of Satellite Applications Facility on Land Surface Analysis data (LSA SAF)

“…Finally, in the near future, ERAD will be able to use, in place of the current DSSF LSA‐201 product, the new DSSF LSA‐207 product, which became operational from April 3, 2020. The latter product, available every 15 min, shows some important improvements particularly concerning the definition of the aerosol scheme used in a cloudless atmosphere (Ceamanos et al ., 2014a; 2014b; Carrer et al ., 2019a; 2019b). Moreover, this new product directly provides the diffuse component of solar radiation derived from a standard decomposition model (Reindl et al ., 1990).…”

Satellite Retrieval of Downwelling Shortwave Surface Flux and Diffuse Fraction under All Sky Conditions in the Framework of the LSA SAF Program (Part 1: Methodology)

“…The LSA SAF dissemination methods are described in reference [49]. The validation of the method presented in this article is reported in the companion paper [73]. The evaluation was conducted by comparing the instantaneously retrieved total DSSF and diffuse fraction (fd) against ground measurements and other satellite-based products.…”

“…Consequently, this is an interesting case that illustrates the major importance of considering the attenuation of the radiation due to aerosols not only in clear-sky conditions. A more detailed comparison between the operational MDSSF product and the upcoming MDSSFTD will be given in the companion article [73]. this region and for this period of the year (see equivalent AOD in Figure 7d), resulting from biomass burning (see Figure 5).…”