| Conditional symmetric instability (CSI), i.e., the stability of a conditionally saturated moist basic flow with vertical and horizontal shear to transverse perturbations, is investigated. It is shown that there exists a similarity among SI (Symmetrical Instability), BI (Buoyancy or Convective Instability), and II (Inertial Instability). Local dynamical considerations suggest that CSI circulations will be slantwise and lie between the most unstable direction of the moist basic state and least stable direction of the dry basic state.;When generalized energy considerations are made, some of the above results are extended to linear CSI with a nonuniform basic state in which the stratification and shear of the basic flow are functions of space. The generalized energetics are also studied for nonlinear inviscid SI. The results indicate: (a) the linear theory fails to predict the stability in certain cases where the basic state is transitional between stability and instability; (b) the initial growth of the SI perturbations can be fairly well approximated by linear theory, but the long time nonlinear evolutions will be bounded energetically if the SI region is finite. However, a further extension of the energetics to CSI shows that the nonlinear evolution of a CSI circulation will energetically depend much more on the precipitation in a complicated way.;By treating the latent heat as a source which is implicitly related to the motion field, the existence, uniqueness and stability of steady viscous CSI circulations are studied. Viscous CSI circulations are proved to be unique and asymptotically stable when the heat sources are weak and less sensitive to the motion perturbations. By considering the fact that moist updrafts are narrow and using eddy viscosity of O(10('3) m('2)/s), the stability criterion suggests that some frontal rainbands were probably dominated by the CSI mechanism even in their mature quasi-steady stage.;For a uniform basic flow in which the wind shear, dry and moist stratifications are constant, linear solutions are found for different CSI modes, e.g., isolated, periodic, and hybrid modes. Inviscid growth rates, spatial structures including the formulae for the width and slope angle of moist updraft, and stability criteria are found and discussed. When a bulk viscosity is accounted for, the most rapidly growing CSI circulations bear a close resemblance to some observed rainbands. The stability criterion of viscous CSI also shows a better comparison with observational data than inviscid CSI. |
