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

Abstract. Much of the present uncertainty in effective radiative forcing due to aerosol–cloud interactions (ERFaci) arises from aerosol–cloud adjustments. Nevertheless, estimating instantaneous radiative forcing due to aerosol–cloud interactions (IRFaci) remains important because it provides observation-based benchmarks for model evaluation. Such estimates are particularly scarce for Amazonian warm clouds, particularly from approaches combining surface-based remote sensing with in situ aircraft observations. Here, we estimate IRFaci for low-level warm clouds over Amazon using the GoAmazon2014/5 datasets. Cloud microphysical properties were constrained with ground-based remote sensing and in situ measurements, used to configure cloud representations and coupled to libRadtran simulations of daily top-of-atmosphere upward irradiance. To reduce uncertainty in baseline atmosphere, two clean reference states are defined, including one designed to represent the seasonal variability of natural background aerosol conditions. Campaign-mean IRFaci values were -11.8 W m⁻² (interquartile range: -23.0 to -2.4 W m⁻²) and -1.3 W m⁻² (-5.8 to 0.3 W m⁻²) for the two reference-state definitions. The first estimate matches the maximum literature IRFaci per AOD unit in Amazon; the second aligns with IPCC’s global -0.7 ± 0.5 W m⁻². Sensitivity tests showed a strong dependence of IRFaci on aerosol load under clean conditions, decreasing with higher loads. Although it does not quantify aerosol–cloud adjustments or ERFaci, this research provides an observationally constrained estimate of IRFaci in the Amazon, serving as a benchmark for future Amazon-focused studies of ERFaci.