Comment on gi-2022-6

Abstract

MDAL is the operational Meteosat Second Generation (MSG) derived daily surface albedo product that is generated and disseminated in near real time by the EUMETSAT Satellite Application Facility for Land Surface Analysis (LSA-SAF) since 2005. We propose and evaluate an update to the MDAL retrieval algorithm which introduces the accounting for aerosol effects, as well as other scientific developments: pre-processing recalibration of radiances acquired by the SEVIRI instrument aboard MSG and improved coefficients for atmospheric correction as well as for albedo conversion from narrow- to broad-band. We compare the performance of MDAL broad-band albedos pre- and post upgrade with respect to two types of reference data: EPS-Metop based 10-day albedo product ETAL (which was found to be comparable to MODIS-based albedo in terms of accuracy) and albedo derived from in-situ flux measurements acquired by ground stations. For the comparison to ETAL — based on differences over the whole coverage area of SEVIRI — we see a reduction of average white-sky albedo mean bias error (MBE) from -0.02 to negligible levels (< 0.001), and a reduction of average mean absolute error (MAE) from 0.034 to 0.026 (–24 %). Improvements can be seen for black-sky albedo as well, albeit less pronounced (14 % reduction in MAE). Further analysis distinguishing individual seasons, regions, and land covers show that performance changes have spatial and temporal dependence: for white-sky albedo we see improvements over almost all regions and seasons relative to ETAL, except for Eurasia in winter; resolved by land cover we see a similar effect with improvements for all types for all seasons except winter, where some types exhibit slightly worse results (crop-, grass- and shrublands). For black-sky albedo we similarly see improvements for all seasons when averaged over the full dataset, although sub-regions exhibit clear seasonal dependence: performance of the upgraded MDAL version is generally diminished in local winter but better in local summer. The comparison against in-situ observations is less conclusive due to the well known problem of spatial representativeness of near-ground observations with respect to satellite pixel footprint sizes. Considering all evidence presented in this study, the updated algorithm version is considered to be able to deliver a valuable improvement of the operational MDAL product.