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Sensitivity of a multi‐layer quasi‐geostrophic β‐channel to the vertical structure of the equilibrium meridional temperature gradient
Author(s) -
Pavan Valentina
Publication year - 1996
Publication title -
quarterly journal of the royal meteorological society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.744
H-Index - 143
eISSN - 1477-870X
pISSN - 0035-9009
DOI - 10.1002/qj.49712252904
Subject(s) - geostrophic wind , zonal and meridional , sensitivity (control systems) , temperature gradient , channel (broadcasting) , baroclinity , atmospheric sciences , geology , climatology , meteorology , physics , computer science , engineering , computer network , electronic engineering
A multi‐layer, quasi‐geostrophic, Boussinesq model on a β‐channel is used to study the sensitivity of the mid‐latitude storm‐track to changes in the vertical structure of the zonal mean equilibrium meridional temperature gradient. Motivation for this study is taken from the observation that the doubled CO 2 climatology of general‐circulation models presents a weaker gradient in the lower troposphere and a stronger gradient in the upper troposphere than in a ‘control’ climatology. It is observed that the dynamics of the quasi‐geostrophic model are generally more sensitive to the lower than to the upper tropospheric temperature gradient as a consequence of the importance of shallow eddies. In order to give a good representation of the eddy dynamics it is necessary to use a sufficiently high resolution, which, in the case chosen, proved to be between 10 and 20 tropospheric layers. If only three layers are used the general features of the response are still reproduced, but the model does not represent the sensitivities of the eddy meridional momentum‐flux convergence adequately. It is argued that this is connected with the particular choice taken when parametrizing the surface Ekman friction. When a higher‐resolution model is used, the eddy momentum‐flux convergence proves to be more sensitive to changes in the upper than in the lower meridional equilibrium temperature gradient but is generally much less responsive to these changes than other quantities like the surface eddy potential‐vorticity flux or eddy kinetic energy. For each variable the two sensitivities become close to each other as the baroclinicity of the equilibrium flow is increased because of the prevalence of deep eddies in the dynamics, sensitive only to the total radiative equilibrium wind vertical gradient. For very high values of the baroclinicity of the equilibrium flow the sensitivities approach a constant value in accordance with a simple scaling argument.