A STUDY OF CYCLONE MESOSCALE STRUCTURE WITH EMPHASIS ON A LARGE-AMPLITUDE INERTIA-GRAVITY WAVE

analysis is presented of prominent mesoscale structure in a moderately intense cyclone with emphasis on a long-lived, large-amplitude inerti a-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 r ich, time-dependent, three-dimensional structure of the IGW. As the IG W amplified [peak crest-to-trough pressure falls exceeded 13 hPa (30 m in)(-1)], it also accelerated away from the cyclone, reaching a peak f orward 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 o nshore band of heavy snow (''snow bomb'') in which peak hourly snowfal ls 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 i mportant effects of subsynoptic-scale disturbances embedded within the larger-scale cyclonic circulation. The precursor offshore warm-fronta l wave was instrumental in reinforcing the wave duct preceding the IGW . The snow bomb was an indication of vigorous ascent, large upper- (lo wer-) level divergence (convergence), unbalanced flow, and associated large parcel accelerations, environmental conditions known to be favor able for IGW formation. Small-amplitude IGWs (<1 hPa) are first detect ed over the southeastern United States from surface micro-barogram rec ords 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 surfac e frontal boundary where there is a weak wave duct (stable layer) pres ent in the lower troposphere. In the upper troposphere the region of I GW genesis is situated on the forward side of a deep trough where ther e is significant cyclonic vorticity advection by the thermal wind. Dia gnostic evidence supports the importance of shearing instability and/o r unbalanced flow in IGW genesis. The large-amplitude IGW originates o n the downstream edge of the northeastward-advancing packet of small-a mplitude IGWs. Wave amplification occurs near the upshear edge of a hi gh, cold cloud shield that generally marks the warm conveyor belt. Alt hough it is not possible to conclusively state whether the amplifying IGW forms in situ or grows from a predecessor weaker (<1 hPa) disturba nce, rapid amplification occurs 1) as the wave encounters an increasin gly deeper and stronger wave duct, possibly permitting wave overreflec tion, in the cold air damming region east of the Appalachians, and 2) downshear of an area of significantly positive unbalanced divergence a nd parcel divergence tendency. The authors raise the possibility that IGW amplification can be associated with the penetration and perturbat ion of the wave duct by vigorous subsynoptic-scale vertical motions wh ose vigor is increased by wave-induced latent heat release.