ESA Cloud Climate Change Initiative: Generation of an Optimal Estimation, Multi-sensor Cloud Property Data Set.
Hollmann, Rainer1; Stengel, Martin2; Poulsen, Caroline3; Lindstrot, Rasmus4; Stapelberg, Stefan2

The Clouds' project, embedded in ESA's Climate Change Initiative (CCI) program, main objective is to provide long-term coherent cloud property data sets exploiting the synergic capabilities of different European and non-European Earth observation satellite missions. To reach this aim, an open community cloud property retrieval system based on an optimal estimation (OE) approach has been developed. It yields cloud top properties as well as cloud optical thickness and cloud droplet effective radius and further derived parameters like cloud liquid and ice water path. The OE approach, unlike traditional methods, ensures a simultaneous, radiatively consistent and coherent estimation of those parameters together with associated uncertainties of the retrieved parameters.
In the current first project phase the system is applied to process a global, inter-calibrated MODIS, AATSR and AVHRR heritage radiance dataset, spanning 2007-2009. The resulting monthly averages in a global 0.5 degree grid and daily composites at higher resolution are characterized by high temporal and spatial sampling and are expected to lead to a global, multi-sensor cloud climatology of high accuracy, stability and homogeneity compared to single sensor equivalents.
They are being compared to other satellite based climatologies, climate models, ground based and reanalysis data. In the second project phase (beyond 2014), the complete available data of the above sensors will be processed leading to a climatology comprising several decades.
The presentation will give an introduction and overview to the project of the retrieval system as well as present the project's first results and their evaluation against the reference datasets.