Boundary-layer dynamics in extra-tropical cyclones

Extra-tropical cyclones contribute high impact weather including strong winds and extreme precipitation. The atmospheric boundary layer plays a key role in the dynamics of these systems, providing significant forcing to both the momentum and thermodynamic budgets. However, our basic understanding of the interaction of boundary layers with extra-tropical cyclones still needs development. In this study, I move from the typical Ekman pumping formulation to one that includes a comprehensive momentum and thermodynamic budget. Such a model is based on Ekman balance in the boundary layer and is compared against a primitive equation model.

The new model, based on semi-geostrophic theory, enables the calculation of circulations induced by the boundary layer, and also the larger-scale frontal environment. I will outline some of the background to this new type of “Sawyer-Eliassen” circulation equation and then give example solutions including around low-level frontal jets and an Eady-wave cyclone development.