A Case Study of an Unusually Intense Atmospheric Gravity Wave

Abstract

A remarkable long-lived, large-amplitude gravity wave in the Carolinas and Virginia on 27 February 1984 is investigated by means of a subsynoptic-scale case study. The wave was characterized by a minor-wave of elevation followed by a sharp wave of depression with a period of 2?3 h, a horizontal wavelength of 100?150 km and surface pressure perturbation amplitudes of 3?14 mb. The wave propagated toward the east-southeast at 1 5 m s?1, accelerating to more than 20 m s?1 after it crossed the Atlantic coast. Wave passage was accompanied by gusty surface easterly winds reaching 30 m s?1 and an abrupt termination of precipitation with the rapid surface pressure fall. The synoptic criteria identified by Uccellini and Koch as common to many cases of large amplitude gravity waves were present in this case. Gravity waves were first detected across western Tennessee and northern Mississippi in a wake depression region to the rear of an advancing squall line. An amplifying wave emerged out of the wave packet across the southern Appalachians as the downstream squall line was most intense. The wave, once organized, amplified still further in the cold air damming region east of the Appalachian mountains and followed the back edge of the precipitation shield to the coast. Meanwhile, a second gravity wave formed just to the north of the primary wave in southeastern Kentucky around 1300 UTC 27 February. It propagated rapidly northeastward at ?30 m s?1 as a zone of enhanced pressure fails superimposed on a broader region of synoptic pressure falls. Geostrophic adjustment appeared to play a prominent role in the organization and intensification of the primary gravity wave, whereas both shearing instability and geostrophic adjustment contributed to the genesis of the second wave. A particularly important aspect of this case was the juxtaposition of prominent jets in the upper and lower troposphere in the region of wave formation and amplification. The low-level jet carded a plume of warm, moist unstable air northward over cooler, stable boundary layer air and helped to trigger a line of active convection in northern Georgia. The gravity wave, which organized and intensified to the rear of the convective line, appeared as a prominent wake depression in the mean sea level isobaric pattern. Forced subsidence to the rear of the convective line in the presence of a deep, cold and stable boundary layer may have contributed to wave amplification. East of the mountains the prominent stable layer or wave duct was capped by a deep layer of weak stability and strong vertical wind shear containing a critical layer, conditions favorable for wave trapping and wave reflectance. Wave propagation and maintenance was in excellent agreement with the Lindzen and Tung ducted gravity wave model. Dissipation occurred as the wave approached and crossed the coastal front boundary.