Frontal Circulations in the Presence of Small Moist Symmetric Stability

Abstract

Observations in saturated frontal regions occasionally show that the flow has become neutral to reversible slantwise displacements along pseudo-angular momentum surfaces so that the effective potential vorticity is nearly zero for further saturated displacements. A strong response to frontogenesis is indicated in these regions, as suggested by the parabolic nature of the Sawyer?Eliassen equation when the potential vorticity vanishes. Using idealized distributions of temperature and geostrophic deformation, we derive solutions of the aforementioned equation for the cross-front circulation in the case where the potential vorticity is vanishingly small for upward displacements but moderate for downward displacements. While the solutions are self-consistent, it is not known whether they are unique. They show that a strong concentrated sloping updraft occurs somewhat to the warm side of the region of maximum geostrophic compression of the isotherms. This circulation closely resembles the flow in a mesoscale precipitation band analyzed by Sanders and Bosart.