A new calculation of the radiative cooling by carbon-dioxide in the middle atmosphere and the lower thermosphere.

Abstract

A description of an improved non-LTE model together with earlier two-level and multi-level models is presented. Effects of collisional and radiative energy transfer paths among CO\sb2 vibrational levels on source functions are discussed. We take into account the 10 $\mu$m bands, i.e. (00\sp01) - (02\sp00/10\sp00) bands, which were previously neglected. A line-by-line calculation of transmission functions with Voigt line shape is tested and employed for evaluation of the mean specific intensity and flux divergence for a vibrational-rotational band. The cooling rate from the atmospheric carbon dioxide is evaluated for altitudes ranging from 40 km to 140 km. Above 105 km, departures of source functions for the 15 $\mu$m fundamental and hot bands from LTE are so significant that non-LTE effects on band strength and transmission function cannot be neglected. The non-LTE effects increase the cooling rate for the 15 $\mu$m fundamental band, but decrease the cooling rates for the hot bands. The mixing ratio of CO\sb2 above 100 km during the day is smaller than at night, due to photodissociation of CO\sb2. This reduces the cooling rate, in particular, for the fundamental band. Contributions to the total cooling rate from the hot bands for the 40 to 75 km range are comparable to the contribution from the fundamental band. Above 110 km, the hot and 4.3 $\mu$m bands make considerable contributions to the cooling, but cooling from the 10 $\mu$m band is negligible. However, the 10 $\mu$m band contribution would be significant if the non-LTE effects on the band strength and transmission function were neglected. Cooling rates of minor isotopic species are small, but are not negligible. We obtain a maximum total cooling rate of 7.4 K day\sp{-1} at the stratopause for the U.S. Standard Atmosphere (1976) and a minimum cooling of 1.7 K day\sp{-1} at about 70 km, in good agreement with previous results. Above about 105 km, however, our total cooling rate is, in general, larger than those of earlier workers. During the night, a second maximum at about 135 km is as large as 29.8 K day\sp{-1}. For day-time, this second maximum appears at about 125 km with the smaller cooling rate of 17.5 K day\sp{-1}. Solar radiation corresponding to the 4.3 $\mu$m band gives rise to heating in the stratosphere and mesosphere, and enhances the vibrational temperature of the (00\sp01) state particularly at the mesopause, where the vibrational temperature is about 105 K higher than the kinetic temperature. The N\sb2(v = 1) and CO\sb2(00\sp01) states are in LTE up to about 100 km, and above this altitude the vibrational temperature for N\sb2(v = 1) tends to follow the kinetic temperature. These vibrational temperatures also agree well with earlier results.Ph.D.Nuclear physics and radiationPhysics, Atmospheric SciencePure SciencesUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/128329/2/8920574.pd