Numerical simulations of a gravity wave event over CCOPE. Part II: Waves generated by an orographic density current

A mesoscale numerical model and detailed observations are used to investiga te the generation and maintenance of a mesoscale gravity wave event observe d in eastern Montana on 11 July 1981 during the Cooperative Convective Prec ipitation Experiment (CCOPE). It is shown that the interaction between an o rographic density current and a mountain barrier leads to the generation of the gravity waves. The simulation results suggest the following four-stage conceptual model. D uring stage I,shortly after sunset, the remnant up-branch of a thermally dr iven upslope Row east of the Rockies was driven back toward the mountain by the pressure gradient force associated with a cool pool over North Dakota. The nocturnal stable layer over eastern Montana was strengthened during pa ssage of this density current. During the 1-2-h transition period of stage II, the advancing density current became blocked as it encountered the high er terrain. An isentropic ridge developed above the original warm lee troug h due to strong adiabatic cooling caused by the sustained upward motion in the presence of orographic blocking. During stage III, an even stronger upw ard 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 d ownward 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 establi shed earlier by the density current. Thus, the mountain-plains circulation may at times generate mesoscale gravity waves (and deep convection) hours a fter diurnal heating has ended.