Abstract: The first part of the present work investigates instability of a baroclinic basic flow against the mesoscale disturbances parallel to it. The focus of this study is to search for the possibility for a mesoscale disturbance of band structure to occur in a stably stratified atmosphere and its effect on triggering and organizing deep convections,The two- and three-dimensional perturbation equations, suitable to solving eigenvalue problems by use of the generalized matrix method and shooting method, are formed under anelastic approximation. The criteria for symmetric instability to set in are discussed for a linear, a hyperbolic-tangential and a real wind profile sounded before occurrence of a squall line event in SESAME 1979. The effect of baroclinicity of the basic flow on occurrence of this kind inertial convection is also analyzed in terms of energetic calculation.