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Abstract

Water vapor is an important climate variable that to date has been poorly
modelled by Atmospheric General Circulation Models. Unphysical, negative
water vapor concentrations are common and simulations suffer from excessive
'noise'. In consequence, model cloud and precipitation fields are poorly defined
and the calculation of the radiation balance of the model atmosphere is
compromised. These effects are particularly severe in the region of the poles.
Negative water vapor concentrations in spectral models result from spectral
approximation in the horizontal, and centred-difference advection in the vertical.
Investigations in the horizontal domain reveal a mismatch between the features
of the water vapor mixing ratio variable and the features of the approximating
functions: namely, surface water vapor mixing ratio varies from about 20 g/kg
at the Equator to about 0.01 g/kg at the poles, but the spherical harmonic
functions of the spectral approximation fit the field in a least-squares sense.
Overshoots and undershoots are produced in the approximation to the field and
undershoots lead to negatives where the field is close to zero, i.e. over extensive
areas of the polar regions.
Investigations in the vertical domain reveal that centred-differencing
overestimates moisture advection where the water vapor mixing ratio gradients
are strongly negative, and this leads to negative water vapor concentrations,
particularly in the tropics in the region below the hygropause.
A review of various numerical options for improving water vapor simulation
reveals the advantages of flux-limited advection in a finite-difference framework,
and the advantages of choosing relative humidity as the prognostic moisture
variable in a spectral framework.
Implementation of relative humidity as the prognostic moisture variable in a
spectral Atmospheric General Circulation Model confirms the conclusions of
these investigations. The resulting water vapor simulations are free of negative
water vapor concentrations, cloud and precipitation fields are significantly more
coherent, and the simulation of the polar climate is vastly improved. Problems
are encountered with the simulation of the stratosphere and the conservation of
moisture due to the specifics of the model formulation. Recommendations for
overcoming these problems are outlined.

Item Type:	Thesis - PhD
Authors/Creators:	Minty, Louise
Keywords:	Water vapor, Atmospheric, Ocean-atmosphere interaction, Climatology
Copyright Holders:	The Author
Copyright Information:	Copyright 1998 the Author - The University is continuing to endeavour to trace the copyright owner(s) and in the meantime this item has been reproduced here in good faith. We would be pleased to hear from the copyright owner(s).
Additional Information:	Thesis (Ph.D.)--University of Tasmania, 1998. Includes bibliographical references
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