Satellite retrievals of the ice crystal number concentration
Résumé
Recent assessments of the climate research community globally converge towards the conclusion that a better understanding of aerosol-cloud interactions is needed to further constrain current radiative forcing estimates. As a consequence, significant efforts have been made during the last decade to answer this question with regard to liquid clouds, but interactions between aerosols and ice clouds still remain poorly understood despite their high importance for long-wave forcing estimates. One important reason is the current lack of adequate satellite observations to perform such studies. Indeed, despite that the number of cloud particles is often viewed as one of the most important parameters to quantify aerosol-cloud interactions, there exists to date no space-borne product of the ice crystal number concentration (ICNC).
In this study, we present a novel product of ICNC retrievals from satellite observations, based on the liDAR-raDAR (DARDAR) operational algorithm. As a first step towards the validation of this product, we show comparisons to in-situ measurements from recent airborne campaigns. Good agreements are found in the overall variability of ICNC, despite a possible overestimation of its absolute value in our product at warm temperatures (greater than -40°C). Further analyses show that this overestimation can be explained by the misrepresentation of the small mode (ice crystals smaller then 100 microns) in current particle size distribution (PSD) parameterizations, which therefore need to be further improved. Finally, a climatology obtained from several years of ICNC retrievals is presented and analyzed.