ATMOSPHERIC RADIATION AND ATMOSPHERIC HUMIDITY

This paper presents a report of some aspects of the effect of the domi nant greenhouse gas in the ear-th's atmosphere, water vapour, on the r adiative balance of the planet. Results from two satellite experiments which provide data on the concentration of water vapour in the upper and middle troposphere are presented. The first is the HALogen Occulta tion Experiment, HALOE, on the National Aeronautics and Space Administ ration Upper Atmosphere Research Satellite, for which retrievals of hu midity have been made down to levels of about 150 hPa. These data reve al a number of interesting features in terms of the space and time var iability of water vapour in the lower stratosphere/upper troposphere. Comparison of HALOE measurements with the water vapour fields calculat ed in one general circulation climate model reveal substantial differe nces, which could significantly affect calculations of radiative balan ce. A second source of global data is the Television Infra-red Observa tion Satellite's Operational Vertical Sounder, TOVS, which uses infrar ed vertical soundings. Considerable variability of upper tropospheric humidity is revealed by TOVS measurements, and an interesting relation ship between brightness temperature and relative humidity, identified by other workers, is discussed. Sensitivity studies are presented of t he effect on the outgoing-radiation spectrum of varying the amount of water vapour throughout the troposphere. It is shown that uncertaintie s of only a few percent in knowledge of the humidity distribution in t he atmosphere could produce changes to the outgoing spectrum of simila r magnitude to that caused by doubling carbon dioxide in the atmospher e. Errors in the water vapour amounts generated in model simulations o f the climate could, therefore, be significant in climate change calcu lations.