A Multispectral Cloud Type Identification Method Developed for Tropical Ocean Areas with Nimbus-3 MRIR Measurements

Abstract

Four registered channels of the Nimbus-3 Medium Resolution Infrared Radiometer (MRIR) have been employed in developing a technique for the identification of unique individual cloud types and cloud type combinations over tropical oceans. With slight modifications the technique is applicable over ocean areas in middle and high latitudes. The four channels are spectrally located at 0.2–4.0 μm, 6.5–7.0 μm,10–11μm and 20–23 μm. A cloud type decision matrix was established, based on radiative transfer theory, an average Barbados temperature and moisture sounding for the summer of 1969, and assumed maximum cloud top levels for low and middle clouds. Cloud type maps were generated in which a unique cloud type or combination was identified when the measurement for each channel was within specified equivalent blackbody temperature or reflectance limits. For verification, 20 case were selected during the BOMEX period (summer, 1969) when high resolution cloud photography was available for comparison with the cloud type identification maps within 2 h of the satellite observations. The comparisons were good for all 20 cases. The effects on discriminating capability, coverage, and accuracy were evaluted when some of the channels were deleted from the method. Elimination of the 20–23 μm channel resulted in no degradation. Decision matrices were also tested for the combination of the 6.5–7.0 μm and 10–11 μm channels, and for the 6.5–7.0 μm channel alone. The 6.5–7.0 μm channel essentially discriminated between cirrus and clear conditions in the upper troposphere.