Decomposing the Atmospheric Flow Using Potential Vorticity Framework

Eero Holopainen Department of Meteorology, University of Helsinki, Helsinki, Finland

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Jussi Kaurola Department of Meteorology, University of Helsinki, Helsinki, Finland

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Abstract

The invertibility principle of potential vorticity (PV) is used, within a quasigeostrophic framework, to formally partition the observed atmospheric flow in various ways. The first method separates the contributions of the interior PV and temperature at the lower boundary. In the literature, this approach has mainly been applied to some idealized flows. In real situations, however, this partitioning appears questionable because any nonzero temperature anomaly at the boundary automatically implies a nonzero stability part of the interior PV and, hence, these two contributions never occur independent of each other in nature. At large horizontal scales, the two contributions have a large amplitude at every level and almost cancel each other. The second method decomposes the 3D flow into contributions arising from the vorticity and the stability parts of the PV. This formal partitioning appears essentially to decompose the flow into its barotropic and baroclinic part. The third method explores the role of PV of various layers in inducing the flow. This approach shows that the upper-level flow is very little influenced by PV at the lower levels. In the lower troposphere, however, the large-scale, flow is dictated by PV in the upper troposphere and lower stratosphere, whereas the smaller-scale features are induced by the local PV, the latter including the effect of the boundary temperature. The third method may be useful as means of verification of atmospheric general circulation models.

Abstract

The invertibility principle of potential vorticity (PV) is used, within a quasigeostrophic framework, to formally partition the observed atmospheric flow in various ways. The first method separates the contributions of the interior PV and temperature at the lower boundary. In the literature, this approach has mainly been applied to some idealized flows. In real situations, however, this partitioning appears questionable because any nonzero temperature anomaly at the boundary automatically implies a nonzero stability part of the interior PV and, hence, these two contributions never occur independent of each other in nature. At large horizontal scales, the two contributions have a large amplitude at every level and almost cancel each other. The second method decomposes the 3D flow into contributions arising from the vorticity and the stability parts of the PV. This formal partitioning appears essentially to decompose the flow into its barotropic and baroclinic part. The third method explores the role of PV of various layers in inducing the flow. This approach shows that the upper-level flow is very little influenced by PV at the lower levels. In the lower troposphere, however, the large-scale, flow is dictated by PV in the upper troposphere and lower stratosphere, whereas the smaller-scale features are induced by the local PV, the latter including the effect of the boundary temperature. The third method may be useful as means of verification of atmospheric general circulation models.

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