Atmospheric radiation and atmospheric humidity

Abstract

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.

Polar Ozone and Aerosol Measurement III measurements of water vapor in the upper troposphere and lowermost stratosphere

“…The precise effect of changes in humidity will depend on the altitude at which these changes occur [Pierrehumbert, 1994], with changes in humidity in the upper troposphere producing a disproportionately large impact on outgoing longwave radiation. It has been estimated that the radiative effect of a 12 -25% change in tropospheric water vapor is comparable to a doubling of CO 2 [Harries, 1996]. Furthermore, water vapor is involved in several potentially important global warming feedback mechanisms.…”

Development of a widely tunable amplified diode laser differential absorption lidar for profiling atmospheric water vapor

“…Water vapor is primarily contained within the lowest 3 km of the troposphere with a high spatial and temporal flux [1][2][3]. Radiosondes are currently used to obtain routine water vapor profiles, but this technique can only provide information at one location at one point in time and cannot easily be used to monitor the spatial and temporal changes of the water vapor concentrations [3].…”

A new technique for estimating outgoing longwave radiation using infrared window and water vapor radiances from Kalpana very high resolution radiometer

“…The OLR originates partly from the surface and partly from the atmosphere. The balance between the incoming SW radiation and the OLR at the top of the atmosphere determines the temperature of the atmosphere and of the Earth's surface [Harries, 1997].…”