The role of balance in coupling the boundary layer and large scale dynamics

Semi-geostrophic theory has proved a powerful framework for understanding the dynamics of mid-latitude weather systems. However, one limitation is the lack of a realistic boundary layer representation. Semi-geostrophic theory can be modified to include an atmospheric boundary layer by replacing the geostrophic wind with the 'geotriptic' (or Ekman-balanced) value in the substantive derivative and appropriately approximating the momentum diffusion term- the so-called semi-geotriptic theory.

In this talk I will outline the balanced semi-geotriptic theory, and show new numerical solutions which include a well-mixed boundary layer. The revised semi-geotriptic model is compared with a hydrostatic primitive equation model for a test case of a two-dimensional idealised sea breeze diurnal cycle. The hydrostatic primitive equation model is shown to produce inertial oscillations which persist beyond the evening decay of the boundary layer until the following morning. In contrast, the semi-geotriptic model decays following the boundary layer state in a more realistic way. The semi-geotriptic model thus demonstrates utility as a critical tool in understanding boundary-layer-dynamics coupling issues in operational models.