Numerical experiments of barotropic flow interaction with a 3D obstacle

In this paper the results of numerical simulation experiments on the interaction between a neutrally stratified atmospheric flow and a symmetric 3D obstacle are presented and discussed. These experiments provide evidence of the presence of an ageostrophic (AGG) vorticity structure, interacting with the obstacle and hidden beneath the upper cyclone-anticyclone dipole predicted by the quasigeostrophic (QG) theory. The AGG cyclone was disclosed when the numerical simulations were run with velocities of the mean flow exceeding a critical threshold, which allowed the upper QG cyclone to be advected downstream of the obstacle. The numerical circulation model, resolving the primitive flow equations and used in our simulations, was applied to a square domain of 2000 km x 2000 km. A 3000-m-high Gaussian-shaped obstacle, characterized by a half-height width of 400 km. was placed at the center of the domain. This obstacle provided the topographic forcing, which was the source of the disturbances on the flow. The simulations were characterized by different values of the mean-flow Rossby number: an analysis of the relative role played by the different forces involved in such a process permitted the isolation of the main mechanism originating the ageostrophic structure, which was not clearly identified in previous works.