Instantaneous radiative forcings due to the first indirect effect of aerosols linked to warm clouds in the Amazon

Instantaneous Radiative Forcings due to Aerosol-Cloud Interactions (IRFaci) occur due to anthropogenic aerosols' first indirect effect on the global radiative balance. These forcings represent a significant source of uncertainty regarding human impact on climate, mainly when warm clouds act as the mediating element [1]. Studies quantifying IRFaci have focused primarily on stratiform clouds over the oceans (e.g. [2]). It is particularly noteworthy the lack of studies about the first indirect effect (i.e., Twomey effect) over the Amazon. This study uses datasets from the GoAmazon2014/5 campaign (collected both in situ and via ground-based remote sensing) to configure warm cloud models that serve as inputs to a radiative transfer code (libRadtran). This allows the calculation of daily values of ascending irradiance at the Top of Atmosphere (TOA) for 2014 and 2015. Given that a detailed evaluation of aerosol conditions in the reference atmosphere can reduce the uncertainties associated with RFaci estimates [3], the IRFaci values were calculated based on two clean atmospheric reference states. The annual distributions of IRFaci derived from these references show interannual variation, with the 2014 forcings being more negative than in 2015. The average IRFaci values (and the average values of the 25th and 75th percentiles in the brackets) for the entire duration of the GoAmazon2014/5 campaign relative to the two reference states were -11.8 [-23.0; -2.4] W/m² and -1.3 [-5.8; 0.3] W/m², respectively. These values align with the maximum IRFaci amounts per aerosol optical depth (AOD) unit documented in the literature [4] for the Amazon region. The value obtained for the second reference state corresponds to the most recent estimate provided by the IPCC, which is -0.7 ± 0.5 W/m² on a global scale. Sensitivity tests of IRFaci revealed a strong dependence on aerosol load for clean background conditions. Further increases in aerosol load reduced the sensitivity. The techniques and results presented here offer a unique approach to calculating indirect radiative forcings related to the Twomey effect of warm clouds over the Amazon, contributing to a better understanding of human impact on the region's climate.

 

[1] Mülmenstädt, J. and Feingold, G.: The Radiative Forcing of Aerosol–Cloud Interactions in Liquid Clouds: Wrestling and Embracing Uncertainty, Curr Clim Change Rep, 4, 23–40, https://doi.org/10.1007/s40641-018-0089-y, 2018. 

[2] Wall, C. J., Storelvmo, T., and Possner, A.: Global observations of aerosol indirect effects from marine liquid clouds, Atmospheric Chemistry and Physics, 23, 13125–13141, https://doi.org/10.5194/acp-23-13125-2023, 2023. 

[3] Gryspeerdt, E., Povey, A. C., Grainger, R. G., Hasekamp, O., Hsu, N. C., Mulcahy, J. P., Sayer, A. M., and Sorooshian, A.: Uncertainty in aerosol–cloud radiative forcing is driven by clean conditions, Atmospheric Chemistry and Physics, 23, 4115–4122, https://doi.org/10.5194/acp-23-4115-2023, 2023. 

[4] Herbert, R. and Stier, P.: Satellite observations of smoke–cloud–radiation interactions over the Amazon rainforest, Atmospheric Chemistry and Physics, 23, 4595–4616, https://doi.org/10.5194/acp-23-4595-2023, 2023.