Baroclinic instability in an environment of small stability to slantwise moist convection. Part I: two-dimensional models

Abstract

In the semigeostrophic system, the growth rate of baroclinic waves varies with the inverse square root of the potential vorticity, which acts as the effective static stability. Recent observations in the ascent regions of middle latitude cyclones shown that the effective potential vorticity for saturated air is very near zero. In this paper we examine the structure and rate of growth of baroclinic cyclones when the effective potential vorticity is small for upward (saturated) displacements but large in regions of descent. Analytic solutions for two-dimensional disturbances in a two-layer semigeostrophic model and numerical simulations using a multilevel semigeostrophic model show that when the effective potential vorticity is small in regions of upward motion, growth rates are modestly increased and the region of ascent intensifies and collapses onto a thin ascending sheet. -from Authors