Infrared spectral radiance measurements in the tropical Pacific atmosphere

Downwelling thermal infrared emission from the tropical atmosphere is affected strongly by the typically large amounts of water vapor. In two experiments within the last 2 years we have used a Fourier transform spectroradiometer to measure tropical atmospheric emission, concentrating on the “window” region between about 800 and 1200 cm. Shortly after the first of these experiments, substantial differences between measured and calculated radiances led to the development of a new water vapor continuum model. This model subsequently has been incorporated into several widely distributed radiative transfer codes (LBLRTM, MODTRAN, FASCODE). Measurements from the second tropical experiment, which occurred during March and April 1996, validate this new continuum model. This is an important comparison because the new measurements were taken with an improved instrument under better defined clear-sky conditions than the original tropical data on which the continuum correction was based. Model residuals are of the order of the uncertainty in measurements, especially of the atmospheric water vapor and temperature profiles: