Application of infrared interferometer spectrometer clear sky spectral radiance to investigations of climate variability

Abstract

An investigation of clear sky spectral radiances and their applicability as a diagnostic for climate variability is described. Global observations of outgoing longwave radiation during 1970 from the infrared interferometer spectrometer (IRIS) aboard Nimbus 4 cover nearly a full annual cycle, and the spectral content of the data provides unique information for examining regional and seasonal variations of spectral radiance. The IRIS radiances have been validated against a line‐by‐line radiative transfer model and are found to compare favorably with calculated radiances. A brightness temperature threshold technique is used to separate clear and cloudy spectra, and seasonal means of clear sky spectra are analyzed for tropical and northern midlatitude ocean areas. Brightness temperature standard deviations are also examined spectrally. Values of 1–2 K in the tropics and 2–3 K at midlatitudes in the 800–1200 cm−1 window region are consistent with observed ocean temperature variability. Spectral features in the standard deviations reflect the variation of surface temperature, tropospheric temperature, and water vapor in the tropics and the increased variability of ozone during winter in the northern hemisphere. Spectral differences between seasonal 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 observed 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 seasonal changes in temperature and water vapor parameters are consistent with the known conditions of the 1970 tropical ocean and atmosphere. These variations are detectable as signatures in outgoing spectral radiances and provide significant information relevant to climate change.