Dynamics, radiation, and photochemistry in the mesosphere: Implications for the formation of noctilucent clouds

Abstract

The nature of noctilucent clouds, which occur at very great heights and high latitudes during summer, has remained something of a mystery for over 100 years. The realization that the summer mesopause is the coldest region of the Earth's atmosphere, together with the possibility that transport by atmospheric motions could maintain a substantial mixing ratio of water vapor against very rapid chemical destruction, has led to the present consensus that noctilucent clouds are formed of water ice. A number of recently developed microphysical models have been successful in simulating cloud particle distributions whose characteristics are consistent with satellite radiance observations. However, because of the scarcity of data on temperature, dynamics, and water vapor abundances, these models have had to rely on a number of assumptions about the behavior of these quantities. This paper attempts to illustrate by means of model calculations how various dynamical and photochemical processes interact to produce the unique environment that makes possible the existence of noctilucent clouds. In particular, it focuses on how thermal relaxation influences the altitude and strength of gravity wave breaking and on the effects of such wave breaking on the circulation, temperature distribution, and transport of water vapor near the summer mesopause. It is also shown that, if present understanding of hydrogen chemistry in the mesosphere is even approximately correct, variations in Lyman α radiation should have a significant effect on water vapor abundances near the summer mesopause and, therefore, on the occurrence of noctilucent clouds.