The Slope of Moist Symmetric Instability with Water Loading

Idealized numerical experiments, supported by analytic considerations, are performed to determine the preferred direction of symmetric instability when water loading is considered. It is concluded that the most unstable direction is tangent to a surface of neutral buoyancy, which can be defined numerically from the water content of lifted air, and coincides with the tangent to saturated isentropes only when all condensed water is precipitated out, consistent with the thermodynamic approximations made in the definition of equivalent potential temperature. In more common situations, when part or all of the condensed water is retained in the cloud, the orientation of symmetrically unstable normal modes is much more slanted toward the horizontal, to the point that regions of the atmosphere, diagnosed as unstable from the consideration of equivalent potential temperature and vorticity, can in fact be stable.