This chapter discusses the single most important theory in dynamic meteorology for large-scale flows in the extratropics. It is known as the quasi-geostrophic theory. One example of such flow is briefly presented in Section 6.1 for the uninitiated readers. Section 6.2 discusses a method known as scale analysis for estimating the order of magnitude of each term of a governing equation. Such analysis is applied in Section 6.3 to derive a simplified version of the complete set of governing equations, known as the quasi-geostrophic (QG) system of equations. In Section 6.4, we combine them to get a diagnostic equation that highlights the symbiotic relationship between the rotational and divergent wind components. The QG system of equations is next reduced to one prognostic equation for a single unknown in Section 6.5. Not surprisingly, that is the QG version of the potential vorticity equation. We verify that only Rossby waves could exist in a QG model in Section 6.6. Section 6.7 shows the formulation of a two-layer version of the QG model, which will be used extensively throughout the book. Here we apply it to quantitatively illustrate all concepts encountered so far in the context of a developing baroclinic jet streak. Finally, Section 6.8 discusses how a QG model is cast in a global spherical domain.

Observed features of a synoptic disturbance

Disturbances with a horizontal dimension of the order of a thousand kilometers are ubiquitous in the extratropical atmosphere. They are referred to as synoptic-scale cyclones/ anticyclones.