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Atmospheric Measurement Techniques An interactive open-access journal of the European Geosciences Union
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Volume 9, issue 3 | Copyright
Atmos. Meas. Tech., 9, 909-928, 2016
© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 07 Mar 2016

Research article | 07 Mar 2016

Synergy of stereo cloud top height and ORAC optimal estimation cloud retrieval: evaluation and application to AATSR

Daniel Fisher1,a, Caroline A. Poulsen2, Gareth E. Thomas2, and Jan-Peter Muller1 Daniel Fisher et al.
  • 1Imaging Group, Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Dorking, Surrey, RH5 6NT, England, UK
  • 2Rutherford Appleton Laboratory, Didcot, Oxfordshire, UK
  • anow at: Earth and Environmental Dynamics Research Group, Department of Geography, King's College London, The Strand, London, WC2R 2LS, UK

Abstract. In this paper we evaluate the impact on the cloud parameter retrievals of the ORAC (Optimal Retrieval of Aerosol and Cloud) algorithm following the inclusion of stereo-derived cloud top heights as a priori information. This is performed in a mathematically rigorous way using the ORAC optimal estimation retrieval framework, which includes the facility to use such independent a priori information. Key to the use of a priori information is a characterisation of their associated uncertainty.

This paper demonstrates the improvements that are possible using this approach and also considers their impact on the microphysical cloud parameters retrieved. The Along-Track Scanning Radiometer (AATSR) instrument has two views and three thermal channels, so it is well placed to demonstrate the synergy of the two techniques. The stereo retrieval is able to improve the accuracy of the retrieved cloud top height when compared to collocated Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO), particularly in the presence of boundary layer inversions and high clouds. The impact of the stereo a priori information on the microphysical cloud properties of cloud optical thickness (COT) and effective radius (RE) was evaluated and generally found to be very small for single-layer clouds conditions over open water (mean RE differences of 2.2 (±5.9) microns and mean COD differences of 0.5 (±1.8) for single-layer ice clouds over open water at elevations of above 9km, which are most strongly affected by the inclusion of the a priori).

Publications Copernicus
Short summary
Observational data sets of cloud characteristics are of key importance in climate science for reducing uncertainty when predicting the future state of the Earth's climate. Here we present a composite method of observing cloud from the Advanced Along Track Scanning Radiometer, employing both radiometric and geometric approaches. Using this method we find improved accuracy for resolving the cloud top height for very-low and high clouds accompanied by small changes in the microphysical parameters.
Observational data sets of cloud characteristics are of key importance in climate science for...