The Frontogenetical Forcing of Secondary Circulations. Part I: The Duality and Generalization of the Q Vector

Abstract

Hoskins, Draghici, and others have shown that the Q vector is an important quantity in quasi-geostrophic (QG) theory for the diagnosis of ageostrophic circulations. In this paper, a vorticity dynamics perspective is used to develop a generalized Q vector, Q*. An equation that relates the material derivative of the thermal-wind imbalance vector to Q* and ageostrophic terms in isentropic flow is obtained from the primitive equations (PE). The generalized Q vector is the vector mean of two terms: a frontogenetical vector that, for horizontal isentropic flow, is equal to the vector frontogenesis function F, and a vortex stretching term proportional to the stretching and reorientation of vorticity by the horizontal wind. Invoking just one QG assumption, the substitution of geostrophic for total velocity gradients, reduces Q* to Q and leads directly to the omega equation of QG theory. The frontogenetical and vortex stretching parts of the generalized Q vector of PE theory become equal in the QG limit. Thus, the conventional Q vector has dual physical interpretations in terms of vorticity and thermodynamic properties. The divergence of the Q vector in its vortex stretching form is equal to the forcing term in the Sutcliffe, Wiin-Nielsen, and Trenberth version of the omega equation. The self-destruction of balanced flow and its restoration by vertical secondary circulations is explained in terms of entropy and vorticity properties. A new assumption (Alternative Balance or AB) consists of omitting the material derivative of the thermal-wind imbalance vector, so that thermal-wind balance is restored instantaneously by secondary circulations. This approximation reduces the PE omega and secondary circulation equations to diagnostic forms resembling their QG equivalents, except for the replacement of Q by Q*. Under the AB assumption, the Q* vector points toward rising motion and inertial gravity waves are excluded.