Abstract
The Hoskins-Bretherton (HB) model is adopted to study two-dimensional frontogenesis in unsteady basic shear flows. The solutions exhibit the nonlinear evolution of an unstable Eady wave up to the formation of a frontal discontinuity. This development is described by the HR solution for a steady shear flow with time reinterpreted by means of a coordinate transformation. The computations are carried out by means of a relatively simple but hightly accurate approximation to the exact solution. The results show that typical wintertime variability of midlatitude zonal flows may either retard or accelerate frontal development compared to frontogenesis in a steady basic flow. Rapid frontal development is also associated with relatively rapid frontal movement and a more intense ageostrophic circulation. In contrast, a prolonged period of development is associated with relatively slow movement and a weaker ageostrophic circulation. The effect of different time-varying basic flows is examined and the results interpreted in relation to atmospheric frontogenesis.
