Wavelet analysis and the governing dynamics of a large-amplitude mesoscalegravity-wave event along the East Coast of the United States

Detailed diagnostic analyses are performed upon a mesoscale numerical simul ation of a well-observed gravity-wave event that occurred on 4 January 1994 along the East Coast of the United States. The value of using wavelet anal ysis to investigate the evolving gravity-wave structure and of using potent ial vorticity (PV) inversion to study the nature of the flow imbalance in t he wave generation region is demonstrated. The cross-stream Lagrangian Ross by number, the residual in the nonlinear balance equation, and the unbalanc ed geopotential-height field obtained from PV inversion are each evaluated for their usefulness in diagnosing the flow imbalance. All of these fields showed clear evidence of strong imbalance associated with a middle-to-upper tropospheric jet streak, and tropopause fold upstream of the Large-amplitu de gravity wave several hours before the wave became apparent at the surfac e. Analysis indicates that a train of gravity waves was continuously generated by geostrophic adjustment in the exit region of the unbalanced upper-level jet streak as it approached the inflection axis in the height field immedi ately downstream of the maximum imbalance associated with the tropopause fo ld. A split front in the middle troposphere, characterized by the advance o f the dry conveyor belt above the warm front, was overtaken by one of these propagating waves. During this merger process, a resonant interaction resu lted, which promoted the rapid amplification and scale contraction of both the incipient wave (nonlinear wave development) and the split front (fronto genesis). The gravity wave and front aloft became inse arable following thi s merger. p The situation became even more complex within a few hours as th e vertical motion enhanced by this front-wave interaction acted upon a satu rated, potentially unstable layer to produce elevated moist convection. An analysis of the temporal changes in the vertical profile of wave energy flu x suggests that moist convective downdraughts efficiently transported the w ave energy from the midlevels downward beneath the warm-front surface, wher e the wave became ducted. However, pure ducting was not sufficient for main taining and amplifying the waves; rather, wave-CISK (Conditional Instabilit y of the Second Kind) was crucial. This complex sequence of nonlinear interactions produced a long-lived, larg e-amplitude gravity wave that created hazardous winter weather and disrupte d society over a broad and highly populated area. Although gravity waves wi th similar appearance to this large-amplitude wave of depression occasional ly have been seen in other strong cyclogenesis cases involving a jet streak ahead of the upper-level trough axis, it is unknown whether other such eve nts share this same sequence of interactions.