Gravity waves generated by the dry convective boundary layer: Two-dimensional scale selection and boundary-layer feedback

This study examines the generation of gravity waves by the dry convective boundary layer in two dimensions. In particular, a number of cases are considered which range from zero background-flow conditions to conditions with strong wind shear. These experiments agree with previous studies that identified a distinct horizontal-scale selection of the gravity waves due to changes in wind speed. Detailed linear analysis is completed which attributes the horizontal-scale selection to two processes. The first process is the dissipation of downstream-propagating gravity waves by critical levels, and the second process is the trapping of upstream-propagating waves. The result is a narrow region of the generated spectrum of gravity waves that are able to propagate vertically. The experiments also agree with previous studies that have observed a resonant interaction between the boundary-layer thermals and the gravity waves. This resonance is described as a result of the trapped gravity waves, which form a resonant cavity near the top of the mixed layer. Finally, the experiments are used to evaluate the role of shear in the gravity-wave generation. In cases with little resonance, shear is shown to play a minor role in the wave generation. In these cases, changes in the wave's amplitude spectrum are mostly due to linear wave refraction and the aforementioned selection mechanism.