ON THE PARCEL METHOD AND THE BAROCLINIC WEDGE OF INSTABILITY

The heuristic explanation, suggested by the parcel method, for the bar oclinic instability mechanism is reexamined. The parcel method argues that an air parcel displaced within the wedge of instability, that is, between the horizontal and the isentropes, is vertically accelerated by the buoyancy force and hence becomes unstable. However, in the syno ptic scale, the buoyancy is balanced by the vertical pressure gradient force perturbation, which is neglected by the parcel method, and thus the parcel acceleration is essentially horizontal. For the unstable E ady normal modes, the horizontally averaged buoyancy work is found to maximize at the steering level and to vanish at the boundaries, but th e horizontally averaged parcel kinetic energy growth is minimized at t he steering level and maximized at the boundaries. It is shown that th e buoyancy work is vertically redistributed by the pressure gradient f orce perturbation throughout the secondary circulation. The parcel met hod also assumes that a parcel displaced adiabatically within the wedg e of instability finds itself warmer than its new surroundings and thu s contributes toward both vertical and meridional positive heat fluxes . However, since the temperature difference between the parcel and the Environment from which it departed cannot be neglected, the slope of the instantaneous displacement is not a sufficient criterion to determ ine the signs of the heat fluxes. It is shown here that for the Eady n ormal modes solution, the four combinations of ascending or descending of initially colder or warmer parcels make jointly the vertical heat Bur maximize at the steering level and the meridional heat Aux remain constant with height.