A Reexamination of the Mechanisms Responsible for Banded Precipitation

Abstract

Abstract The processes responsible for a banded snowfall region during a December 1997 East Coast storm are examined. Conventional data plus a numerical simulation with the fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5) are used. Calculations of slantwise potential area near the bands suggest that the release of conditional symmetric instability played a role in their formation. The location and timing for the appearance of negative moist potential vorticity (MPV) cannot, however, be reconciled with band formation. A balanced MPV model based on the geostrophic momentum approximation is developed. It provided new insights into the mechanisms of MPV generation. A swath of negative balanced MPV now coincides with the snowbands. MPV sources are proposed that are linked to a vigorous mesoscale updraft near the bands. The updraft occurred on the warm, moist side of a zone of midtroposphere frontogenesis. Negative MPV develops through differential ageostrophic transports of geostrophic...