Cloud Droplet Number Concentration: Satellite Retrievals Improved by Advanced Atmospheric Modelling

The cloud droplet number concentration (CDNC) is one of the most important microphysical properties of liquid clouds for understanding and quantifying the effective radiative forcing by aerosol-cloud interactions (ERFaci). Indeed, CDNC is linked to the relevant processes of the cloud formation and evolution. CDNC is closely related to the chemical composition of the condensation nucleation nuclei and the cloud droplet size distribution. Nevertheless, this key parameter remains poorly known. CDNC is not yet operationally provided from current standard satellite retrievals. Our approach relies on an innovative determination of CDNC from satellite observations in combination with atmospheric cloud-resolving modelling. We introduce our new, community-based tool: the Satellite Simulator and Sandbox for Cloud Observation and Modelling (S3COM). Briefly, S3COM aims to simulate realistic satellite observations and cloud products from model outputs, to quantify the sensitivity of radiative quantities to cloud parameters, and to assist the development of retrieval algorithms using output fields from high-resolution models. We use realistic cloud situations (stratocumulus, cumulus, marine and continental clouds) obtained from the ICOsahedral Nonhydrostatic Large Eddy Model (ICON-LEM) to simulate top of atmosphere radiances with the Radiative Transfer for TOVS (RTTOV), from visible to infrared, observed by the Moderate Resolution Imaging Spectroradiometer (MODIS). Performance of MODIS-type algorithms coupled with ICON-LEM simulations is described and characterization of error sources is given. Results on CDNC retrievals for warm liquid water stratocumulus clouds are presented and discussed for the study case of the 02 May 2013.