The role of frontogenesis in cyclogenesis at the tropopause

Meridional temperature gradients at the “break” in the tropopause in mid-latitudes are quite strong and, moreover, of opposite sign above and below the core of the jet, respectively. This has some interesting consequences for cyclogenesis at the tropopause and attendant mixing of air between the stratosphere and the troposphere.

Frontogenesis may be associated with the change of the absolute value of the temperature gradient and/or the result of the change in direction of the temperature gradient. The latter effect is referred to as “deformation frontogenesis”.

In this presentation the role of different frontogenetic mechanisms in the dynamics of a simple baroclinic wave is first elucidated with the help of a three layer primitive equation model and compared with linear theory based on the quasi-geostrophic two-layer model. All forms of diffusion and dissipation (numerical and explicit) are eliminated from the numerical model so that the inviscid process of frontogenesis and associated adjustment to thermal wind balance can be evaluated as accurately as possible. Particular attention is paid to deformation frontogenesis. It is concluded that a relatively small deformation of a front is enough to start the process of cyclogenesis. Deformation frontogenesis due to horizontal shear is the principal effect that is responsible for initial cyclogenesis. By increasing the vertical resolution of the model, it is shown how deformation frontogenesis and cyclogenesisis are linked at the break in the tropopause.

Since the concept of frontogenesis is related to air parcels, i.e. is a Lagrangian concept, the differences between the linear (Eulerian) perspective and the non-linear (Lagrangian) perspective are investigated by monitoring the deformation of air parcels as they move through a baroclinic wave in the model simulation. This is done by introducing three passive scalars into the model with initial distributions set equal to the three space coordinates, respectively.

The implication of the facts distilled from the numerical experiments and linear theory for understanding cyclogenesis at the tropopause in the real atmosphere is illustrated with the results of a case study using ECMWF-analyses (van Delden and Neggers, 2003). We show that cyclogenesis at the tropopause is connected to the vertical divergence of the horizontal convergence of the Q-vector (the vector frontogenetic function) at the tropopause, inducing vortex stretching at the tropopause on the west side of a trough.

Van Delden, A., and R. Neggers, 2003: A case study of tropopause cyclogenesis. Meteorol. Appl., 10, 187-199.