SUMMARYMonthly zonal mcan observations of H 2 0 and CH, made by the limb infrared monitor of the stratosphere (LIMS) and the stratospheric and mesospheric sounder (SAMS) instrumcnts on Nimbus 7 have been used to investigate whether the H 2 0 mixing ratios in the stratosphere are consistent with a source via the oxidation of CHI. While both sets of data show considerable seasonally varying structure, total hydrogen (neglecting molecular hydrogen) is relatively featureless with a mean value over the stratosphere of 6.020.35 p.p.m.v. (lo) for the five-month pcriod studied. The uniformity of the total hydrogen ficlds points to the validity of the CH, oxidation hypothesis.The derived fields of total hydrogen are used to deduce a mean H 2 0 mixing ratio for air as it enters the stratosphere of 2.720.35p.p.m.v. (lo) from which a desiccation temperature may be deduced (for example, -87.2"C at 60mb).
This paper presents a report of some aspects of the effect of the dominant greenhouse gas in the earth's atmosphere, water vapour, on the radiative 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 Occultation Experiment, HALOE, on the National Aeronautics and Space Administration Upper Atmosphere Research Satellite, for which retrievals of humidity have been made down to levels of about 150 hPa. These data reveal a number of interesting features in terms of the space and time variability of water vapour in the lower stratospherehpper troposphere. Comparison of HALOE measurements with the water vapour fields calculated in one general circulation climate model reveal substantial differences, which could significantly affect calculations of radiative balance. A second source of global data is the Television Infra-red Observation Satellite's Operational Vertical Sounder, TOVS, which uses infrared vertical soundings. Considerable variability of upper tropospheric humidity is revealed by TOVS measurements, and an interesting relationship between brightness temperature and relative humidity, identified by other workers, is discussed. Sensitivity studies are presented of the effect on the outgoing-radiation spectrum of varying the amount of water vapour throughout the troposphere. It is shown that uncertainties of only a few percent in knowledge of the humidity distribution in the atmosphere could produce changes to the outgoing spectrum of similar magnitude to that caused by doubling carbon dioxide in the atmosphere. Errors in the water vapour amounts generated in model simulations of the climate could, therefore, be significant in climate change calculations.
The natural greenhouse effect of the Earth is strongly influenced by the radiative effects of water vapour and clouds in the atmosphere, which control the energy absorbed from the sun, and that lost through thermal emission to space. Any perturbations to the climate balance, for example through so-called 'radiative forcing' due to increasing CO? amounts, variations in solar constant, or other causes, can be amplified by the feedback processes that involve water in its various phases. The radiative cooling of the Earth in the absence of clouds has recently been shown to be dominated by emission from upper-tropospheric water vapour, in the far infrared portion of the spectrum, and this is illustrated: observations of this radiative flux, and of the distribution of water vapour in the upper troposphere, are urgently needed. The role of clouds is discussed, and it is noted that their response to global warming is not presently unambiguously determined with available models, due to the complexity of competing processes: again, as in the cloud-free case, more accurate global observations are needed. The paper is illustrated by data from satellite experiments, most notably the Earth Radiation Budget Experiment sponsored by NASA.
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