Infrared Radiative Cooling in the Middle Atmosphere (30–110 km)

Abstract

The infrared contributions to the heat budget by the 15 µ CO2, 9.6 µ O2, and 80 µ H2O bands are evaluated for the upper stratosphere, mesosphere and lower thermosphere as a function of latitude for both summer and winter. Flux divergences are numerically evaluated for a quasi-random band model with the appropriate line-broadening mechanism. A general discussion of the source function applicable to a multi-vibrational level molecule is given, and this formulation is applied to the 15 µ band of carbon dioxide. The flux divergence of infrared radiation acts to cool the atmosphere in the 30–110 km height region except in the vicinity of the mesopause. Here there is a small, but nevertheless significant heating which increases in value toward the summer pole (∼4K day−1). Centers of cooling appear near the stratopause for low latitudes (∼10K day−1) and in the lower thermosphere over the winter pole. Thermospheric values may vary by a factor of 4 because of uncertainties in the collisional lifetime of the 15 µ transition; the rates of temperature change in this region have been parametrized in terms of the collisional and the radiative rates. Ozone makes a significant contribution to the cooling in the vicinity of the stratopause (∼3K day−1). The water vapor contribution is approximately 1K day−1for a mixing ratio of 10−6 gm gm−1. Our calculations indicate that both these gases, when compared with carbon dioxide, give a negligible contribution to the flux divergence in the upper mesosphere.