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Cloud Climatology from ATSR

Caroline Poulsen(1), Richard Siddans(1), Andy Sayer(1), Gareth Thomas(2), R G Grainger(2), Chris Arnold(2) and Phillip Watts(3)

(1) Rutherford appleton laboratory, Chilton, OX110QZ, United Kingdom
(2) University of Oxford, Oxford, OX13PU, United Kingdom
(3) EUMETSAT, Darmstadt, Darmstadt, Germany


Clouds have long been recognised as one of the key moderators of the earths atmosphere, low clouds such as Stratus effectively reflect incoming solar radiation, an overall cooling effect while high clouds such as thin Cirrus can transmit the solar radiation but trap the outgoing thermal radiation, an overall warming effect. These responses and the complex response of clouds to climate change in the presence of aerosol (man-made or natural) is not well understood.

Scientists at Oxford and Rutherford Appleton Laboratory have developed a cloud retrieval algorithm. The ORAC (Oxford, RAL Aerosol and Cloud) algorithm is an application of Optimal Estimation (OE) : all available measurements and all available a priori information are combined using fast radiative transfer and known error characteristics to find the most probable values of all cloud parameters simultaneously. The cloud parameters retrieved are cloud fraction, optical depth, cloud top height/pressure and effective radius. The radiative transfer includes cloud, atmosphere and surface effects.

The algorithm has been applied to measurements from the A/ATSR instruments. The data has been processed with this algorithm 1995 to the present providing a long timeseries of cloud properties from a well calibrated instrument. The results from this data set will be presented and compared with similar measurements from AVHRR and MODIS as well as validation results.

Applications of the data and Future algorithm developments will also be discussed.


Workshop presentation