Potential Vorticity as Meridional Coordinate

The dynamical equations of atmospheric flow are written using potential vorticity as the meridional coordinate and potential temperature as the vertical coordinate. Within this system, the atmosphere is divided into undulating tubes bounded by isentropic and constant potential vorticity surfaces, and, under adiabatic and frictionless conditions, the air moves through the tubes without penetrating through the walls. A model that uses this system of coordinates incorporates as built-in both the dry convective adjustment and barotropic adjustment processes, which prevent the folding of isentropic and potential vorticity surfaces but crudely represent the effects of interactions with the mean flow. The Eliassen–Palm flux vector in this frame of reference yields a new interpretation of the eddy momentum transport. Eddy momentum exchange is through the form drag created by pressure forces exerted on the potential vorticity–potential temperature tubes.