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Lance F. Bosart and Anton Seimon

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.

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Lance F. Bosart, W. Edward Bracken, and Anton Seimon

Abstract

An analysis is presented of prominent mesoscale structure in a moderately intense cyclone with emphasis on a long-lived, large-amplitude inertia–gravity wave (IGW) that moved through the northeastern United States on 4 January 1994. Available National Weather Service WSR-88D Doppler radar and wind profiler observations are employed to illustrate the rich, time-dependent, three-dimensional structure of the IGW. As the IGW amplified [peak crest-to-trough pressure falls exceeded 13 hPa (30 min)−1], it also accelerated away from the cyclone, reaching a peak forward speed of 35–40 m s−1 across eastern New England. The IGW was one of three prominent mesoscale features associated with the cyclone, the others being a weak offshore precursor warm-frontal wave and an onshore band of heavy snow (“snow bomb”) in which peak hourly snowfalls of 10–15 cm were observed. None of these three prominent mesoscale features were well forecast by existing operational prediction models, particularly with regard to precipitation amount, onset, and duration. The observed precipitation discrepancies illustrate the subtle but important effects of subsynoptic-scale disturbances embedded within the larger-scale cyclonic circulation. The precursor offshore warm-frontal wave was instrumental in reinforcing the wave duct preceding the IGW. The snow bomb was an indication of vigorous ascent, large upper- (lower-) level divergence (convergence), unbalanced flow, and associated large parcel accelerations, environmental conditions known to be favorable for IGW formation.

Small-amplitude IGWs (<1 hPa) are first detected over the southeastern United States from surface microbarogram records and are confirmed independently by the presence of organized and persistent mesoscale cloud bands oriented approximately along the wave fronts. The area of IGW genesis is situated poleward of a weak surface frontal boundary where there is a weak wave duct (stable layer) present in the lower troposphere. In the upper troposphere the region of IGW genesis is situated on the forward side of a deep trough where there is significant cyclonic vorticity advection by the thermal wind. Diagnostic evidence supports the importance of shearing instability and/or unbalanced flow in IGW genesis.

The large-amplitude IGW originates on the downstream edge of the northeastward-advancing packet of small-amplitude IGWs. Wave amplification occurs near the upshear edge of a high, cold cloud shield that generally marks the warm conveyor belt. Although it is not possible to conclusively state whether the amplifying IGW forms in situ or grows from a predecessor weaker (<1 hPa) disturbance, rapid amplification occurs 1) as the wave encounters an increasingly deeper and stronger wave duct, possibly permitting wave overreflection, in the cold air damming region east of the Appalachians, and 2) downshear of an area of significantly positive unbalanced divergence and parcel divergence tendency. The authors raise the possibility that IGW amplification can be associated with the penetration and perturbation of the wave duct by vigorous subsynoptic-scale vertical motions whose vigor is increased by wave-induced latent heat release.

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