The Nongeostrophic Structure of Baroclinic Waves and Its Relation to Fronts and Jet Streaks

Abstract

Some fundamental properties of nongeostrophic baroclinic waves are examined by solving the equations of motion linearized about Eady's basic state at the next order of balance beyond quasi-geostrophic (QG) theory. The study fits into a general effort to broaden the view of “slow-manifold” behavior. The specific motivation is to identify balanced properties of surface and upper-tropospheric frontal regions that are filtered from both the QG and the semigeostrophic models. Among the questions to be answered are whether alongfront flow is subgeostrophic or supergeostrophic, and whether the ageostrophy is realized primarily as a velocity or pressure correction of the QG solution. A rudimentary model of jet streaks is constructed from nongeostrophic neutral waves in an attempt to reproduce along-jet ageostrophic velocities and propagation speeds more realistically than in existing models. Past work has concentrated on correcting phase speeds and growth rates for higher-order balanced effects. These results are extended by using a more appropriate solvability condition near the short-wave cutoff and by considering the detailed structure of the nongeostrophic modes. The eigenvalue corrections are interpreted physically in the framework of a generalized potential vorticity inversion problem with sources determined at the QG level. It is shown that ageostrophic shear in the nondivergent (alongfront) wind affects the time dependence primarily indirectly, by tilting the basic isentropes in the meridional direction and setting up an anomaly pattern in the QG potential vorticity field. This has some of the same consequences as shortening the horizontal scale in the QG model.