The impact of instrument field of view on measurements of cloudy-sky spectral radiances from space: application to IRIS and IMG

Spatially resolved images from the MODerate Resolution Imaging Spectrometer (MODIS) instrument are used to investigate the impact of a change in spatial field of view, from that typical of the Nimbus 4 Infrared Interferometer Spectrometer (IRIS) to that of the Interferometric Monitor for Greenhouse Gases (IMG), upon the spectral outgoing longwave radiation (OLR). Considering all-sky conditions it is found that for a typical tropical scene, approximately 150 paired measurements are required to obtain agreement to within ±2 K in the average brightness temperature ( T B), in the most transparent window channels. At mid-latitudes, the reduced scene variability means that fewer observations are required to meet the same criterion. For clear- and cloudy-sky separation a simple threshold technique based on the window T B and underlying sea-surface temperature tends to result in a systematic underestimate of the average cloudy T B by the larger field of view. A better estimate can be obtained by applying a double threshold to discriminate against the most mixed scenes.