The Radiative-Photochemical Response of the Mesosphere to Fluctuations in Radiation

Abstract

The response of the mesosphere to slow fluctuations in ultraviolet and visible radiation intensity is investigated using the simplified models for photochemistry and radiative transfer developed by Lindzen and Goody (1965). Fluctuations in oxygen's ultraviolet bands, ozone's ultraviolet bands and ozone's visible band are separately considered. It is found that the mesosphere is most sensitive to fluctuations in ozone's ultraviolet bands above 35 km and to fluctuations in oxygen's ultraviolet bands above 30 km. At levels of peak sensitivity, fluctuations of about 12 per cent in either of these bands will give rise to temperature fluctuations of 2 deg K. This appears to rule out minute changes in solar ultraviolet emission as a cause for the ?26-month? oscillation in the equatorial mesosphere. It is also found that the mesosphere is quite sensitive to fluctuations in visible light in the region between 20 and 35 km where fluctuations of 3?6 per cent in visible radiation can give rise to fluctuations of 2 deg K in temperature. On the average, about 26 per cent of the visible radiation in the mesosphere is received via reflection from below. Much of the reflection is from clouds and hence, variation in cloud cover forms an effective way of varying visible light in the mesosphere. In this connection it is found that the winter distribution of cloud cover in the subarctic is such as to introduce into the mesosphere a temperature disturbance whose amplitude and spatial distribution are such as to be able to trigger a sudden warming of the Northern Hemisphere winter variety.