The Lagrangian Parcel Dynamics of Moist Symmetric Instability

A simple model describing the slantwise ascent of a two-dimensional horizontal air tube Subject to moist symmetric instability is developed under the assumptions that the Froude number is small and that mixing is absent. It is shown that, in general, the horizontal velocities attained by the tube are comparable to those of the mean flow and that vertical velocities of up to a few meters per second are possible. The tube ascends slantwise in such a way that its buoyancy remains nearly zero, unless the environment is my very nearly moist adiabatic, in which case ascent at an angle of 45° to the vertical is preferred. Results of the analysis support the contentions of Bennetts and Hoskins and Emanual that moist symmetric instability is the cause of some mesoscale rainbands. In a companion paper, it is demonstrated that the stability of the moist baroclinic atmosphere to two-dimensional slantwise displacements of arbitrary magnitude can be approximately assessed by reversibly lifting parcel along surfaces of constant angular momentum and comparing their density with that of their environment.