Instantaneous Direct Radiative Effects of Aerosols in Cloud and Clear-sky Scenes from Passive Space-borne Multi-spectral Observations

Extreme wildfire events are likely to increase in number in the near future and observations of regional radiative effects of smoke can be used to challenge and improve climate model simulations, which are currently relying almost exclusively on model-model intercomparisons, while models disagree on the magnitude and sign of the radiative forcing by aerosols.

A methodology is presented to measure the instantaneous aerosol direct radiative effect of wildfire smoke using satellite observations of aerosol optical thickness and a radiative transfer model, and determine the radiative heating and cooling of the smoke in both clear-sky and cloud scenes. Radiative effects of smoke are defined as the radiative effect with and without smoke in the atmosphere, which are necessarily computed using model simulations. Regionally, aerosol-radiation interactions can be an order of magnitude larger than their global mean values, especially during extreme events. Results at the top of the atmosphere and at the surface for a case of extreme wildfires in Chile in 2023 will be shown. The results are compared to retrievals of the aerosol direct radiative effects of smoke above clouds using hyperspectral measurements, to show the ability of direct retrievals of aerosol effects from satellite measurements. These results are important to validate and challenge climate models, and will help the development of radiative effect retrievals from more dedicated missions, like PACE and EarthCARE. The SpexONE instrument of PACE is capable of separating aerosols and clouds using the polarisation of light, while the ATmospheric LIDar (ATLID) on EarthCARE is an active instrument, which provides profiles of aerosol extinction and heating rates. In addition, the upcoming EUMETSAT mission Metop-Second Generation A (Metop-SG A) will carry ahyperspectral spectrometer Sentinel-5 and the Multi-viewing Multi-channel Multi-polarisation Imager (3MI). On this mission, the hyper- and multi-spectral and polarization capabilities are combined and improved with mutli-viewing infromation. The approach presented here will be useful for the quantification of aerosol direct and semi-direct effects from space, which is urgently needed to improve the radiative interaction schemes between aerosols and clouds in climate models. Better observations of both aerosols and cloud properties, and direct observations of aerosol-cloud- radiation interactions will improve our ability to attribute climate change, quantify climate sensitivity, and improve the accuracy of future climate change projections.