AN INTENSE SMALL-SCALE WINTERTIME VORTEX IN THE MIDWEST UNITED-STATES

An intense small-scale low pressure system that moved across portions of the midwest United States is examined. The system produced a contin uous band of significant snowfall, typically only 50 km wide but exten ding over 1500 km in length. The system traveled across the Iowa Depar tment of Transportation surface mesonetwork, allowing high-resolution surface analyses that show a closed circulation and intense pressure g radients around the mesolow, comparable to those occurring in warm sea son MCS events. Radar and satellite images also revealed the small-sca le low-level circulation, which apparently was confined below about 80 0 mb. Although the strong vorticity advection aloft and baroclinicity at lower levels present in this system are typical of baroclinic cyclo nes, the unusually small scale and short lifetime of the surface syste m are more reminiscent of polar lows. Mesoscale simulations of the sys tem using the Pennsylvania State University-National Center for Atmosp heric Research Mesoscale Model Version 5 with 20-km horizontal grid sp acing and initialized with standard synoptic-scale data were unable to capture the closed circulation and significantly underestimated the s trength of the mesolow. The inclusion of mesonet surface data in an in itialization significantly improved the initial pressure field but did not significantly change the simulation. The simulation was also not strongly sensitive to variations in horizontal and vertical resolution , surface characteristics, convective parameterizations, and the use o f nudging toward observations. However, an adjustment of upper-level f ields to support the surface mesoscale low did result in a significant ly improved simulation of the event, apparently due to better simulati on of forcing from warm advection in low levels. A simulation neglecti ng latent heating produced a surface low that was at least 1 mb weaker than the full-physics run and had much weaker and disorganized upward vertical motion. The mesoscale low was apparently the result of upper -tropospheric forcing, which eliminated a capping inversion in a small region, permitting precipitating convection and latent heat release.