Atmospheric radiation and atmospheric humidity
J. E. Harries (1)
(1) Blackett Laboratory, Imperial College of Science, Technology,
and Medicine, University of London
ABSTRACT
This paper presents a report of some aspects of the effect of the dominant
greenhouse gas in the ear-th'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 stratosphere/upper 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.
Quart. J. Roy. Meteor. Soc. (1997) Vol.
123, No. 544 Part B, pages 2173-2186
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