The role of unbalanced dynamics and topography in the generation of mesoscale gravity waves.

摘要

Two well-documented mesoscale gravity wave events, one on 4 January 1994 along the East Coast of the United States and the other on 11–12 July 1981 over eastern Montana during the Cooperative Convective Precipitation Experiment (CCOPE), are investigated with a state-of-art mesoscale numerical model as well as advanced diagnostic tools. The MM5 model simulated reasonably well both cases, providing invaluable high-resolution datasets to evaluate the role of unbalanced dynamics and topography in the gravity wave initiation. This study has presented by far the most systematic and comprehensive inter-comparison between and evaluation of the utility of various unbalanced flow diagnostic tools for the study of mesoscale gravity waves. Composite wavelet analysis also has been employed for the first time to unambiguously track the origin and evolution of mesoscale gravity waves.; Based on the East Coast case simulations, a two-stage conceptual model of wave generation by geostrophic adjustment and frontal occlusion is proposed. The initiation stage is characterized by the generation of an incipient gravity wave in the mid-upper troposphere immediately downstream of the maximum imbalance where the strongest upward motion associated with a tropopause fold passed over an occluding surface front. Downstream of this incipient wave, a slower-moving split front in the mid troposphere (warm occlusion) was developing as air of low equivalent potential temperature surged eastwards above the warm front. During the development stage, the incipient wave merged with the split front. The merger was essential to the rapid amplification and scale contraction of the incipient wave, after which the incipient wave and split front became inseparable. This merger resulted in enhanced vertical motion in a saturated layer of potential instability, which quickly triggered “localized” convection. Thereafter, a large amount of wave energy was transported downward through nonlinear fluxes, resulting in a large-amplitude gravity wave at the surface. A no-terrain simulation revealed that orography was not directly responsible for generating the gravity wave.; For the first wave episode during CCOPE, the model revealed that the wave generation was due largely to lower-tropospheric processes in the presence of topography, and that unbalanced flow played no role. A four-stage wave development conceptual model was proposed in which the blocking of a westward-propagating density current resulting from the remnant of the previous day's mountain-plains circulation (MPS) was responsible for the wave initiation. Topography played two important roles here—not only was it responsible for generating the MPS, but the eastern slopes of the mountains acted to retard the density current and forced the gravity wave.; For the second wave episode during CCOPE, the gravity wave was directly generated by the strong updraft associated with a developing daytime MPS as it impinged upon a stratified shear layer above the deep, well-mixed boundary layer that developed because of strong sensible heating over the Rocky Mountains. Explosive convection developed directly over the weakening gravity wave as an eastward propagating density current produced by a rainband generated within the leeside convergence zone merged with a westward-propagating density current in eastern Montana. The greatly strengthened density current resulting from this new convection then generated a bore wave that—propagated eastward, generating a Mesoscale Convective Complex.; It was confirmed that the Uccellini and Koch (1987) conceptual model for mesoscale gravity wave activity occurred in both events, though the gravity wave generation mechanisms do not consist of “pure geostrophic adjustment”. Wave ducting proposed by Lindzen and Tung (1976) is present in the East Coast case and also the first wave episode