Unbalanced Frontogenesis. Part I: Zero Potential Vorticity

Abstract

Unbalanced frontogenesis is studied in a two-dimensional, Boussinesq, rotating fluid that is constrained between two rigid, level surfaces. The potential vorticity is zero. The initial state is unbalanced because there is no motion and the potential temperature is given by the error function of x. An analytic solution is derived based on the neglect of the barotropic pressure gradient. The solution procedure uses momentum coordinates to obtain nonlinear solutions. When the initial Rossby number (Ro) is less than 1.435 the horizontal wind components display an inertial oscillation. During the first part of the inertial period (0 < ft < π) the isentropes develop a tilt and frontogenesis occurs, while in the second part (π < ft < 2π) the isentropes return to a vertical orientation and frontolysis brings the temperature gradient back to its original value at ft = 2π. For larger values of Ro a frontal discontinuity forms before ft = π. The importance of the barotropic pressure gradient is determined in a scale collapse problem with a constant potential temperature and no rotation. In this case the inclusion of the barotropic pressure gradient increases the time before the discontinuity forms. Numerical solutions of the original problem with rotation show that the presence of the barotropic pressure gradient term increases the critical Rossby number from 1.435 to about 1.55. Otherwise the complete solutions are very similar to the analytic solutions, except that the isentropes are no longer straight and the vorticity shows evidence of strong vertical advection by a small-scale vertical jet. Further, shorter timescales are expected with unbalanced fronts as compared with balanced fronts.