A COMPARATIVE-STUDY OF LARGE-AMPLITUDE GRAVITY-WAVE EVENTS

Two large-amplitude gravity waves were observed over the midwestern Un ited States on 5 and 14 January 1989 during the University of Illinois Winter Precipitation Program. On both days, an extensive amount of da ta was recorded, including data from two radars and a radiosonde facil ity. The waves originated near Missouri, registered pressure fluctuati ons as large as 10 mb, and produced distinct precipitation bands along their updraft regions. The waves were long-lived and maintained their identity over 1000 km, a distance several times their wave-lengths. T he synoptic features at the surface were dissimilar. A deep cyclone wa s present on 5 January, while a trough was present on 14 January. Howe ver, the middle- and upper-tropospheric flow patterns were similar. In both cases, the axis of a trough was immediately upstream of the grav ity-wave genesis area and a jet streak had just propagated through the base of the trough, toward a downstream ridge. Soundings taken near t he gravity waves were remarkably similar, with both soundings showing a surface inversion capped by a deep layer of near-neutral stability. However, the relationship between the location of the gravity wave and the region of large-scale precipitation differed in the two cases. Th e 5 January wave occurred at the back edge of the precipitation associ ated with a comma cloud, while the wave on 14 January was observed at the leading edge of the synoptic-scale precipitation region. The gravi ty wave had the structure of a solitary wave of elevation on 5 January , while it appeared as an undular bore with an embedded pressure jump on 14 January. A critical level, with small Richardson numbers, was pr esent in both the cases. A well-defined duct, formed by an inversion b elow and critical level above, contributed to the maintenance of waves . Shearing instability and geostrophic adjustment were the likely gene ration mechanisms, though it was difficult to discount the role of con vection.