Computing the horizontal pressure gradient force in sigma coordinates
Date
1992
Authors
Kozlowski, John
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Abstract
In this thesis we consider the problem of computing the horizontal pressure gradient force under σ coordinates, within a discrete numerical model. This calculation has previously been found to be accompanied by large truncation error when performed in regions of steep (mountainous) terrain. Corby et al.(1972) proposed a limited solution by showing that if one considered an atmosphere where temperature varied linearly with log p, a certain finite difference scheme could be used for which the discretization error vanished when no geostrophic wind existed.
We take this as a starting point, and extend the result of Corby et al.(1972) to a case where a simple wind exists by requiring that isobaric surfaces be tilted rather than horizontal isothermal planes, as was true in his study, as well as to a more complicated case, in which isobaric planes carry a temperature variation , and for which we have derived an accompanying truncation error. In addition we show how the selection of temperature and finite difference scheme by Corby et al.(1972) can be seen to be a consequence of an attempt to transform the continuous hydrostatic equation into a discrete version of it, without loss of accuracy. Finally, we show the importance of employing data which is consistent with both the finite difference and continuous equations by conducting a numerical experiment.
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UN SDG 7: Affordable and Clean Energy