On Assessing Local Conditional Symmetric Instability from Atmospheric Soundings

Abstract

The standard parcel method of assessing the susceptibility of the atmosphere to moist convection using tephigrams is extended to account for the centrifugal as well as the gravitational potential energy of the displaced air parcel. This leads to a measure of the stability of the moist baroclinic atmosphere to finite slantwise reversible displacements of a two-dimensional air parcel; such a measure differs from previously derived measures of conditional symmetric instability which have considered only infinitesimal displacements in saturated atmospheres. It is demonstrated that the combined gravitational and centrifugal potential of a two-dimensional air parcel or “tube” can be assessed by displacing the tube slantwise along a surface of constant angular momentum, and that this combined potential energy can be estimated using a single atmospheric sounding. Several examples of the application of this technique are presented in the context of a case study of apparent slantwise convection. The results suggest that moist convection in a conditionally unstable baroclinic atmosphere proceeds in such a way as to render the atmosphere neutral to reversible slantwise displacements; such an atmosphere is characterized by moist adiabatic lapse rates along surfaces of constant angular momentum and may be considerably stable to purely vertical displacements. The dynamics of slantwise moist convection are examined in a companion paper.