New aerosol and cloud satellite observations from PACE and EarthCARE consistently constrain model uncertainties

NASA and ESA launched the PACE and Earthcare satellites in 2024 to provide unique aerosol and cloud measurements. We use these measurements to constrain model uncertainty on aerosol Effective Radiative Forcing (ERF). Perturbed Parameter Ensembles (PPEs) are extremely powerful tools that offer an effective approach to evaluate and constrain the model uncertainty of aerosol using observations.

We create a PPE for  July 2024-August 2025 based on 250 simulations by the aerosol -climate model ECHAM-HAM and co-locate the 3-hourly output with aerosol and cloud products from PACE and Earthcare. We define regional monthly mean observations for 19 regions of fine- and coarse Aerosol Optical Depth (AOD), Aerosol Index (AI), Single Scattering Albedo (SSA), cloud droplet number concentration (Nd), Cloud Effective Radius (CER), and fraction of extinction below 2km altitude, resulting in almost 1600 observations. An emulator is used to extend the PPE to simulate these observations to 2 million PPE members and constrain the PPE by applying least-squares minimization. resulting in 0.2% of accepted ensemble members.

Both PACE and EarthCARE independently and consistently constrain several model parameters that affect ERFaci and RFari. These observations fundamentally and consistently change where and how ERF uncertainty is controlled and alter the global spatial ERF distribution. The constrained ensemble indicates a stronger negative global aerosol ERF than previous mean estimates, alongside a more positive forcing over Central Africa. The observations suggest a reduction of emission of DMS, Organic, and Black Carbon (anthropogenic and biomass burning), and accumulation mode sea salt.  Also, the absorption capability (imaginary refractive index) of different aerosol species is reduced.  Cloud observations constrain ‘activation’ and ‘vertical velocity’ parameters, resulting in smaller aerosol-Nd susceptibility. However, some parameter uncertainties, such as biomass burning emission particle size, remain mostly unchanged. These results demonstrate that new satellite observations can robustly and consistently constrain aerosol ERF uncertainty, while also identifying key processes where additional or complementary observations are required.