A Study of Cyclone Mesoscale Structure with Emphasis on a Large-Amplitude Inertia–Gravity Wave
Monthly Weather Review
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) Ϫ1
... 13 hPa (30 min) Ϫ1 ], it also accelerated away from the cyclone, reaching a peak forward speed of 35-40 m s Ϫ1 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 (Ͻ1 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 smallamplitude 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 (Ͻ1 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.