Satellite-based estimate of the climate forcing due to aerosol - ice cloud interactions

Substantial efforts have been led over the last decades to improve our understanding of the interactions between clouds and anthropogenic aerosols (aci). The effective radiative forcing associated with these interactions (ERFaci), which combines the radiative forcing (i.e. Twomey effect) and cloud adjustments, still constitutes a large part of our current uncertainties on climate predictions.

Important progress has been made in the assessment of ERFaci for liquid clouds, partly due to advances in the joint use of satellite and modelling data to tackle this problem. More particularly, the retrieval of the droplet number concentration from satellite remote sensing - a property closely related to droplet nucleation processes - has been extremely helpful to better quantify ERFaci. However, similar estimations for ice clouds have for long suffered from a lack of observational constraint on the ice crystal number concentration (N_i), a challenging task due to the high complexity of the physical processes associated with the nucleation and growth of ice crystals. However, a novel long-term global dataset of N_i from active satellite measurements has recently (DARDAR-Nice) opened the door to new observation-based estimates of RFaci for ice clouds.

This study investigates aerosol - ice clouds interactions using N_i profiles from the DARDAR-Nice product together with collocated aerosol information from the Copernicus Atmospheric Monitoring Service (CAMS) reanalyses. A multitude of cloud regimes, subdivided into seasonal and regional bins, are considered in order to disentangle meteorological effects from the aci signature. First results of joint-histograms between N_i and the aerosol mass show an overall positive sensitivity of N_i to the aerosols load. This response is particularly strong towards to cloud-top and flattens towards cloud-base, consistently with expectations for ice nucleation processes. In terms of adjustments, the relation between IWP and Ni is also investigated. Preliminary results suggest a slightly negative global ERFaci for ice clouds, with important regional variations, but a precise quantifications of these effects will require further statistics.