Do 1D and 3D radiative transfer estimates of aerosol direct radiative effects differ? A sensitivity study using realistic cloudy EarthCARE scenes

Aerosols and clouds play key roles in climate through their direct radiative effects (DREs) by modulating the Earth-atmosphere radiative energy budget. Satellite-based retrievals of DREs are essential for quantifying Earth’s radiative energy budget. They are, however, subject to much uncertainty due to difficulties in characterizing the spatiotemporal variability of aerosols and clouds and their optical properties. In addition, DRE quantification relies predominantly on one-dimensional (1D) radiative transfer (RT) simulations. According to Cole et al. (2023), differences between 1D and three-dimensional (3D) RT calculations of upwelling shortwave fluxes at 20 km altitude are expected to exceed EarthCARE’s scientific goal (differences between predicted and “observed” fluxes of less than ±10 W m⁻²) in at least 50% of cases. While several studies have quantified differences in cloud DREs between 1D and 3D RT simulations, to our knowledge no such studies exist for aerosol DREs.

The EarthCARE (EC) mission aims to improve our understanding of how aerosols and clouds modify radiative fluxes by providing collocated observations of aerosols, clouds, precipitation, and radiation, enabling a three-dimensional representation of the atmosphere. In this study, we use these novel datasets to quantify differences in aerosol shortwave DREs between 1D and 3D RT simulations under clear- and cloudy-sky conditions. Aerosol DREs are calculated using1D and 3D RT solvers from the libRadtran package (Mayer & Kylling, 2005; Emde et al., 2016; Mayer 2009) for selected scenes from pre-operational EC test frames. The scenes are chosen to represent a range of aerosol types and cloud conditions. The sensitivity of the 3D effect (defined as the difference between 3D and 1D calculations of the DRE) is investigated as a function of aerosol optical depth and solar zenith angle. To assess the influence of the relative vertical positioning of aerosols and clouds on 3D effects, artificial aerosol layers placed above and below cloud layers are examined.  Overall, our analysis provides insights into how more realistic 3D representations of atmospheric constituents can improve understanding of the role of aerosols in modifying Earth’s radiative energy fluxes.  

 

Acknowledgements:

This research was financially supported by the CERTAINTY (Cloud aERosol inTeractions & their impActs IN The earth sYstem ) project funded from Horizon Europe programme under Grant Agreement No 101137680, the project RACE-ECV, (SBFI-633.4-2021-2024/PMOD - EarthCARE 202/2) supported by SBFI (State Secretariat of Research and Innovation Switzerland),  and the Obs3RvE (Optimising 3D RT EarthCARE product using geostationary observations and AI) project, funded from the European Space Agency under Contract No. 4000147848/25/I/AG. We would like also to acknowledge the COST Action HARMONIA, CA21119.

 

References:

Cole, J. N. S. et al, (2023) Broadband radiative quantities for the EarthCARE mission: the ACM-COM and ACM-RT products, Atmos. Meas. Tech., 16, 4271–4288.

Emde, C. et al, (2016) The libRadtran software package for radiative transfer calculations (version 2.0.1), Geoscientific Model Development, 9(5), 1647–1672.

Mayer, B., A. Kylling, (2005) Technical note: The libRadtran software package for radiative transfer calculations - description and examples of use. Atmos. Chem. Phys., 5(7), 1855–1877.

Mayer, B. (2009) Radiative transfer in the cloudy atmosphere, in: EPJ Web of Conferences, 75–99.