The problem of air flow over complex terrain is traditionally approached from two somewhat distinct directions, depending primarily on the scale of the orography involved. On one side, the boundary layer view usually considers neutrally stratified turbulent flows over small hills embedded within the boundary layer, with extensions to moderately stable stratified flow. On the other hand, a larger scale point of view of the orography, examines stably stratified flow over hills or mountains that penetrate the boundary layer. In the latter case, the effects of the boundary layer are either assumed negligible or enter through friction at the lower boundary without explicit consideration of the interaction.

In this work, we try to `merge' the two views by considering gravity wave flow coexisting with a turbulent boundary layer. The problem is examined numerically using the Met Office research model BLASIUS, a terrain following, non-linear, non-hydrostatic model that can be used either as an inviscid model or with a choice of either a mixing length closure or a one-and-a half order turbulence scheme that carries the turbulent kinetic energy as a prognostic variable. Orography of various scales, in terms of maximum height and horizontal width is employed. Also, boundary layers of different depths and vertical wind shears are used, to investigate the controlling parameters. The work examines both the behaviour of the flow near the surface as well as the general structure of the gravity waves. The emphasis is placed on the momentum balance and the various drag mechanisms due to gravity waves and turbulence, that coexist in this case. In particular, we want to understand relations of surface pressure drag, momentum flux above the boundary layer and distribution of the total drag within the boundary layer.

The combined effects of gravity waves and turbulent boundary layer need to be properly represented in NWP models, that currently tend to treat these processes separately. Better understanding of the nature of the gravity waves boundary layer interaction, will hopefully result in improved parametrizations for NWP models.