Assessment of present-day estimates of AOD from global reanalyses against different satellite products and multi-model ensembles

Aerosols are known to affect atmospheric processes on a wide range of spatio-temporal scales, from dust storms reducing incoming solar radiation to aerosol-climate feedbacks. Although plenty of studies address aerosol radiative forcing, there are persistent differences in current aerosol estimates from both, observations and models. Global reanalyses are able to provide consistent estimates of aerosol distributions by combining these two data sources. However, continuous assimilation of single satellite products forces the analyses towards the satellites climatology including possible inaccuracies. This study investigates differences between current estimates of aerosol optical depth (AOD) by addressing two questions: (1.) How well do we know the large-scale spatio-temporal pattern of present-day AOD across state-of-the-art data? (2.) How does current global aerosol reanalyses perform in comparison to other model- and observation-based data sets? To answer these questions, AOD from the global CAMS and MERRA-2 reanalyses is compared to 8 satellite products, 1 established climatology and 4 multi-model ensembles. The comprehensive data set used in this study allows to evaluate the performance of individual products concerning different spatial and temporal aspects. The evaluation covers results from 1998 to 2019, including most recently available products like the climate model inter-comparison project CMIP6.

Spatially and temporally averaged AOD from MERRA-2 agrees well with the mean satellite climatology, while the CAMS climatology is higher than most other products. With relative standard deviations of about 11%, temporal variations of CAMS and MERRA-2 agree well with the mean satellite variation. However, averaged AOD from the individual satellites show large differences, ranging from 0.124 for MISR to 0.164 for MODIS. In addition to average differences, spatial patterns vary significantly between the individual data sets. Because the CAMS reanalysis only assimilates AOD from MODIS, it remains close to the MODIS climatology which overestimates AOD in most regions in comparison to other products. This overestimation is considerably increased over eastern China were CAMS simulates regional values of more than 1.2 during summer. By assimilating both, MODIS and MISR data, the MERRA-2 reanalysis is closer to the satellite mean under most conditions. Although annual deviations remain small compared to other models, MERRA-2 tends to underestimate AOD at the equator and overestimates AOD at higher latitudes especially during the winter-season. The spatio-temporal differences between individual aerosol data sets underline the need for further research on both satellite retrievals and model simulations for aerosols. For example, integrating multiple observations in a reanalysis system would allow to compensate for inaccuracies of the individual products. Further developing the multi-scale coupled ICON-ART system at the German Weather Service provides a promising environment to achieve accurate aerosol climatologies on high spatial resolution.