APPLICATION OF INFRARED INTERFEROMETER SPECTROMETER CLEAR-SKY SPECTRAL RADIANCE TO INVESTIGATIONS OF CLIMATE VARIABILITY

An investigation of clear sky spectral radiances and their applicabili ty as a diagnostic for climate variability is described. Global observ ations of outgoing longwave radiation during 1970 from the infrared in terferometer spectrometer (IRIS) aboard Nimbus 4 cover nearly a full a nnual cycle, and the spectral content of the data provides unique info rmation for examining regional and seasonal variations of spectral rad iance. The IRIS radiances have been validated against a line-by-line r adiative transfer model and are found to compare favorably with calcul ated radiances. A brightness temperature threshold technique is used t o separate clear and cloudy spectra, and seasonal means of clear sky s pectra are analyzed for tropical and northern midlatitude ocean areas. Brightness temperature standard deviations are also examined spectral ly. Values of 1-2 K in the tropics and 2-3 K at midlatitudes in the 80 0-1200 cm(-1) window region are consistent with observed ocean tempera ture variability. Spectral features in the standard deviations reflect the variation of surface temperature, tropospheric temperature, and w ater vapor in the tropics and the increased variability of ozone durin g winter in the northern hemisphere. Spectral differences between seas onal and annual mean outgoing radiance are associated with variations in atmospheric parameters, and a linear retrieval algorithm is used to quantify these changes in the tropics. Retrieved seasonal sea surface temperature (SST) differences are typically within 0.7 K of the obser ved SST differences. Interseasonal water column variations during this annual cycle are 5-10% over the tropical Pacific and Atlantic and 10- 20% over the equatorial Indian Ocean. In general, the retrieved season al changes in temperature and water vapor parameters are consistent wi th the known conditions of the 1970 tropical ocean and atmosphere. The se variations are detectable as signatures in outgoing spectral radian ces and provide significant information relevant to climate change.