The Maintenance of the Relative Humidity of the Subtropical Free Troposphere

The relative importance of different processes in the water vapor balance of the troposphere is assessed, using high-resolution hindcast data from the ECMWF Integrated Forecast System (IFS) for December–February 1998/99 interpolated to isentropic coordinates. The focus is on elucidating the processe...

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Bibliographic Details
Published in:Journal of climate 2010-01, Vol.23 (2), p.390-403
Main Authors: Couhert, Alexandre, Schneider, Tapio, Li, Juilin, Waliser, Duane E., Tompkins, Adrian M.
Format: Article
Language:English
Subjects:
Atmosphere
Convection
Data assimilation
Data processing
Drying
Earth, ocean, space
Exact sciences and technology
External geophysics
Heat
Hemispheres
Humidity
Marine
Meteorology
Ocean basins
Radiative cooling
Relative humidity
Subsidence
Subtropics
Temperate regions
Temperature
Transport
Tropical regions
Troposphere
Water in the atmosphere (humidity, clouds, evaporation, precipitation)
Water vapor
Water vapour
Wetting
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Summary:The relative importance of different processes in the water vapor balance of the troposphere is assessed, using high-resolution hindcast data from the ECMWF Integrated Forecast System (IFS) for December–February 1998/99 interpolated to isentropic coordinates. The focus is on elucidating the processes that maintain the relative humidity of the subtropical free troposphere. The dominant drying process in the subtropical free troposphere is cross-isentropic subsidence driven by radiative cooling. In some subtropical regions [e.g., over continents in the Southern (summer) Hemisphere and over western portions of ocean basins in the Northern (winter) Hemisphere], drying by radiative subsidence is partially offset or overcompensated by moistening by cross-isentropic dynamic transport of water vapor from the surface upward (e.g., in convection). Any resultant net drying or moistening of the subtropical free troposphere by cross-isentropic motions is regionally primarily balanced by isentropic mean and eddy transport of water vapor from moister into drier regions. Isentropic transport redistributes water vapor within the subtropics and moderates relative humidity contrasts; however, it does not consistently lead to a substantial net import or export of water vapor into or out of the subtropics.
ISSN:0894-8755
1520-0442
DOI:10.1175/2009JCLI2952.1