An improved cloud detection scheme for Nimrod
Watkin Sarah
A new cloud detection scheme has been developed for Nimrod which uses infrared and visible image data from Meteosat. The visible-based component has been operational since 1996 and remains unchanged, whilst the infrared-based component has been significantly upgraded. The new infrared-based scheme uses RTTOV, a radiative transfer model, to simulate the clear sky brightness temperature for each grid point using Mesoscale Model forecast data as inputs. These simulated brightness temperatures minus an offset, which is a function of surface type, are compared with the Meteosat infrared brightness temperature to determine whether a pixel is mainly clear or mainly cloudy. This scheme replaces the old infrared-based scheme that used the Mesoscale Model surface temperature minus an offset (ignoring effects of the gaseous atmosphere) as the threshold for each pixel. During the daytime, estimates of cloud cover from the infrared and visible-based components are combined to produce a cloud cover analysis. However, at night only the infrared-based scheme is used to generate a cloud cover analysis. Validation ...
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