An observational study of water vapor in the mid‐latitude mesosphere using ground‐based microwave techniques

Abstract

Ground‐based spectral line measurements of the 22.2‐GHz water vapor atmospheric emission line are used to deduce the mesospheric water vapor profile. The results generally indicate that the water vapor mixing ratio is independent of height or increases slowly in the 50‐ to 60‐km range with typical values of between 5 and 8 parts per million by volume (ppmv). (Thacker et al. (1981) give an analysis of the 1980 data sets obtained in this experiment, which indicated the existence of a pronounced layer of water vapor near 65 km. The reanalysis of these data contained in this paper, which includes the estimation and removal of spectral baselines, results in marked smoothing of the water vapor mixing ratio profile in the lower mesosphere, and the peak is much less pronounced.) Above 65 km the mixing ratio is seen to decrease rather rapidly to values of near 1 ppmv at 85 km. This decrease is far steeper than that expected from photochemical considerations. There has also been a large amount of variability observed in the water vapor profiles, especially on the day‐to‐day time scale. The water vapor retrievals have been used to estimate the vertical component of the upper mesospheric eddy diffusion coefficient ( K z ). This analysis has indicated either that vertical transport time scales in the mesosphere are perhaps an order of magnitude longer than previous studies have shown or that present understanding of the factors important in controlling the vertical distribution of water vapor in the mesosphere is inadequate.