The Strong Effects of Non-Quasigeostrophic Dynamic Processes on Atmospheric Energy Spectra

Abstract

Ageostrophic effects on geostrophic motions are examined for f-plane flows characterized by small Rossby numbers. For small Rossby numbers, the ageostrophic field is determined by the geostrophic field through the familiar quasi-geostrophic relationships. Although the ageostrophic field is relatively weak, the two fields can interact nonlinearly to produce relatively large effects since the two are spatially well correlated. If the geostrophic field is very turbulent, energy exchanges by ageostrophic processes may significantly affect the geostrophic energy spectra. The ageostrophic effects examined here are those neglected by quasi-geostrophic theory. It is hypothesized that time-mean statistics of some of these ageostrophic effects may be significant compared with similar quasi-geostrophic effects under atmospheric-like conditions. The hypothesis is tested with a low-order model which is described in terms of geostrophic and ageostrophic normal modes. Results suggest that energy exchanges between geostrophic modes due to ageostrophic processes can dominate at smaller synoptic scales for the reason suggested. The geostrophic energy at these scales is decreased if these mechanisms are excluded. Implications regarding studies of climate and geostrophic turbulence are presented.