The Generation of Propagating Gravity Wave by an Orographic Density Current

A mesoscale numerical model is used to understand how the interaction between an orographic density current and a mountain barrier can lead to the generation of observed propagating gravity waves. This study shows that the mountain-plains circulation can generate mesoscale gravity waves (and deep convection) hours after diurnal heating has ended.

The simulation results suggest the following four stage conceptual model. During stage I, shortly after sunset, the remnant up-branch of the thermally-driven upslope flow east of the Rockies was driven back towards the mountain by the pressure gradient force associated with a cool pool to its east. The nocturnal stable layer was strengthened during passage of this density current. During the 1-2 h transition period of stage II, the advancing density current became blocked as it encountered the higher terrain. An isentropic ridge developed above the original warm lee trough due to strong adiabatic cooling caused by the sustained upward motion in the presence of orographic blocking. During stage III, an even stronger upward motion center formed to the east of the density current head updraft in response to an eastward horizontal pressure gradient force produced by the isentropic ridge. In stage IV, as the density current head collapsed and downward motion developed to the west of the original updraft in quadrature phase with the isentropic perturbation, a gravity wave was generated. This wave propagated eastward with the mean wind (opposite to the motion of the earlier density current) and was maintained by the strong wave duct established earlier by the density current.