The use of NWP model surface temperatures in cloud detection from satellite
An increasing number of satellite cloud detection methods include the use of Numerical Weather Prediction model (NWP) surface temperatures as a threshold for the thermal infrared cloud detection test. The NWP model surface temperature and the satellite apparent brightness temperature are assumed to correlate well for clear sky. Investigations over Europe in April 1997 indicate that the correlation over land is highly dependent on location and time of day. In this paper, it is shown that this variation of the correlation has a distinct impact on the quality of the infrared cloud detection test. As a result, cloud cover fractions which are retrieved using this thermal infrared test are biased by this effect. This can have serious impact on the quality of cloud climatologies, especially with respect to the diurnal variation of cloud cover fraction. A new method is introduced to equalize the quality of the infrared cloud detection test throughout the day. Threshold values are allowed to be smaller than commonly used and therefore the quality of the infrared test is improved. The method may be applied in both climate research and near real-time processing.
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