Quasigeostrophic Vertical Motions Diagnosed from Along- and Cross-isentrope Components of the Q Vector

Abstract

In a recent paper on the kinematics of frontogenesis, Keyser et al. conjectured that partitioning the Q vector into along- and cross-isentrope components yields vertical-motion patterns that are respectively cellular and banded?the former on the scale of the baroclinic disturbance and the latter on the scale of the embedded frontal zones. This conjecture is examined diagnostically through solution of the quasigeostrophic omega equation, using the output from a nearly adiabatic and frictionless f-plane primitive equation channel model of the evolution of a baroclinic disturbance to finite amplitude. The results of the present study support the proposed conjecture, suggesting the following interpretation of the characteristic comma structure of the vertical-motion field in midlatitude baroclinic disturbances: The comma shape arises from the modification or distortion of a wave-scale dipole pattern by frontal-scale asymmetries. The dipole is associated with the along-isentrope component of the Q vector, reflecting the wavelike pattern in the potential temperature field within the baroclinic disturbance; the asymmetries are associated with the cross-isentrope component of the Q vector, reflecting the presence of frontal zones within the baroclinic disturbance.