Mesoscale Structure in the Megalopolitan Snowstorm of 11–12 February 1983. Part III: A Large-Amplitude Gravity Wave

Abstract

A large-amplitude singular gravity wave is studied through the analysis of hourly observations, radar and satellite data and observations of cloud to ground lightning flashes. The wave activity was initiated just ahead of, and to the left of the track of, the major Atlantic Coast cyclone of 11–12 February 1983. The initial wave of depression was followed by a wave of elevation and finally a wake depression. Pressure amplitudes were several millibars. The system propagated northeastward at 15 m s−1 accelerating to more than 25 m s−1 against increasing northeasterly flow in the lower troposphere. Surface northeasterlies increased and became gusty with the approach of the first pressure falls, reaching a maximum at the pressure trough, then abruptly weakening and backing as strong pressure increases were observed. Strong fluctuations in column total condensation rate and radar reflectivity occurred with the passage of the wave. Clustered lightning activity accompanied wave passage, principally near the intersection of the gravity wave and the east–west oriented escarpment of deep, high cloud along the north edge of the dry intrusion. A unique aspect was that lightning from clouds with relatively warm tops in the northern periphery of the dry intrusion lowered primarily positive charge to ground. Disorganized wave activity developed in the presence of modest midtropospheric convection and beneath a shear zone in advance of a propagating jet streak. The singular appearing large amplitude wave organized subsequently in a transient zone of strong frontogenetical forcing. The wave appeared as the northern extension of a coastal front. Overall, the system represented a ducted gravity wave propagating in a layer of large stability beneath an elevated layer of small Richardson number containing a critical level. Whether the accompanying convection and enhancement of condensation heating represented a forcing effect on the wave is unknown.