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Improving the simulation of water vapor [sic] in spectral climate models
thesisposted on 2023-05-26, 16:37 authored by Minty, Louise
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.
Rights statementCopyright 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). Thesis (Ph.D.)--University of Tasmania, 1998. Includes bibliographical references