Abstract
Aerosol absorption constitutes a significant component of the total radiative effect of aerosols, and hence its representation in general circulation models is crucial to radiative forcing estimates. We use here multiple observations to evaluate the performance of CAM5.3-Oslo with respect to its aerosol representation. CAM5.3-Oslo is the atmospheric component of the earth system model NorESM1.2 and shows on average an underestimation of aerosol absorption in the focus region over East and South Asia and a strong aerosol absorption overestimation in desert and arid regions compared to observations and other AeroCom phase III models. We explore the reasons of the model spread and find that it is related to the column burden and residence time of absorbing aerosols, in particular black carbon and dust. We conduct further sensitivity simulations with CAM5.3-Oslo to identify processes which are most important for modelled aerosol absorption. The sensitivity experiments target aerosol optical properties, and contrast their impact with effects from changes in emissions and deposition processes, and the driving meteorology. An improved agreement with observations was found with the use of a refined emission data set, transient emissions and assimilation of meteorological observations. Changes in optical properties of absorbing aerosols can also reduce the under- and overestimation of aerosol absorption in the model. However, changes in aerosol absorption strength between the sensitivity experiments are small compared to the inter-model spread among the AeroCom phase III models.
Acknowledgements
Model simulations, data post-processing and data analysis of model output were performed on resources provided by the Swedish National Infrastructure for Computing (SNIC) at the National Supercomputer Centre at Linköping University (NSC). The authors acknowledge furthermore the open database AERONET (https://aeronet.gsfc.nasa.gov/) for providing remote sensing measurements of aerosol physical properties. We are also grateful for the openly available data sets from the satellite based MODIS instrument as well as the MERRA-2 model output. We would further like to acknowledge the NASA Goddard Space Flight Center, the Met Office Hadley Centre, the Norwegian Institute of Meteorology and the University of Oslo for providing model output of the AeroCom phase III control experiment. We would like to thank Inger Helene H. Karset from the University of Oslo for providing ERA-Interim data used for the nudging of CAM5.3-Oslo and Dirk Olivié from the Norwegian Meteorological Institute for providing the CMIP6 emission data set. Further, we would like to thank Annica Ekman and Dirk Olivié for valuable input during the writing process. We would like to acknowledge the Swedish Research Council (Vetenskapsrådet), DNR 2018-04274.
Disclosure statement
The authors declare that they have no conflict of interest.
Data availability statement
Data produced with model simulations using the model CAM5.3-Oslo is available from the corresponding author upon request. Observational data from AERONET and MODIS as well as the MERRA-2 reanalysis is openly available. AeroCom model output is available via the AeroCom archive.