An observational case study of a continental mesoscale vortex

An observational case study of a continental mesoscale vortex embedded in a polar air stream has been conducted, with emphasis on observed vortex features and their possible relationship to the polar-low phenomena and accompanying synoptic scale features at the 850 mb, 700 mb and 500 mb levels. The path of this 14–16 December 1983 event (determined from GOES satellite imagery) was over the relatively data-rich region of the midwestern United States, with the mesoscale vortex rotating cyclonically around and to the south of an occluded synoptic-scale surface low pressure system (and 500 mb low). This case study exhibited features common to the polar low with its convective cloud mass, although a distinct surface signature never developed. The mesoscale vortex formed and remained within a strong cyclonic shear region (4 times 10^-5 s^-1) at the 850 mb level, but was supported by the evolution of a baroclinic pattern as well. Spatial and temporal cross-sections (both horizontal and vertical) revealed a weak low-level warm core capped by a strong cold dome, most pronounced at the 700 mb level and distinctly separate from the synoptic scale low in the surface map and 500 mb height field. This low static stability mesoscale system occurred entirely beneath the 500 mb level with cloud tops near 600 mb, and was associated with a weakly defined 600 km wave at the 700 mb level. The pure CISK model results for wavelength growth by Bratseth and the combined moist baroclinicity and CISK studies by Sardie and Warner correspond best to this continental event. It is suggested that both barotropic and baroclinic instability mechanisms (combined with CISK) can exist cooperatively in support of the development and maintenance of both continental and maritime polar low events.