Abstract
Two large-amplitude gravity waves were observed over the midwestern United States on 5 and 14 January 1989 during the University of Illinois Winter Precipitation Program. On both days, an extensive amount of data was recorded, including data from two radars and a radiosonde facility. The waves originated near Missouri, registered pressure fluctuations 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. The synoptic features at the surface were dissimilar. A deep cyclone was present on 5 January, while a leeside trough was present on 14 January. However, the middle- and upper-tropospheric flow patterns were similar. In both cases, the axis of a trough was immediately upstream of the gravity-wave genesis area and a jet streak had just propagated through the base of the trough toward a downstream ridge. Soundings taken near the 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. The 5 January wave occurred at the back edge of the precipitation associated with a comma cloud, while the wave on 14 January was observed at the leading edge of the synoptic-scale precipitation region.
The gravity 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 present in both the cases. A well-defined duct, formed by an inversion below and critical level above, contributed to the maintenance of waves. Shearing instability and geostrophic adjustment were the likely generation mechanisms, though it was difficult to discount the role of convection.
