An Observational Study of Cold Season–Banded Precipitation in Northeast U.S. Cyclones

Abstract

A climatological and composite study of banded precipitation in the northeast United States during the cold season (October?April) is presented. Precipitation systems in the northeast United States in April 1995 and from October 1996 to April 2001 that exhibited greater than 25.4 mm (1 in.) of rainfall, or 12.7 mm (0.5 in.) liquid equivalent, were identified as cases for study. A total of 111 cases were identified during this period, of which 88 had available radar data. Of these cases, 75 exhibited banded structure whereas 13 did not. A band classification scheme was developed from a subset of study cases. Application of the classification scheme to the 88 cases revealed that banded cases can exhibit a variety of banded events during their evolution. Single-banded events were the most common (48), followed by transitory (40), narrow cold frontal (36), multi (29), and undefined (9). Further investigation of the single-banded events highlighted banded structure in the comma-head portion of storms, with 81% of these events exhibiting a majority of their length in the northwest quadrant of the surface cyclone. Composites were calculated for cases exhibiting single-banded events in the northwest quadrant of the surface cyclone and for nonbanded cases to distinguish synoptic and mesoscale flow regimes associated with banded events and nonbanded cases. The banded composite was marked by cyclogenesis and the development of a closed midlevel circulation. This flow configuration was associated with deformation and strong midlevel frontogenesis northwest of the surface cyclone center, which coincided with the mean band position. The nonbanded composite exhibited a much weaker cyclone located in the confluent entrance region of an upper-level jet. The absence of a closed midlevel circulation in the nonbanded composite limited deformation and associated frontogenesis northwest of the surface cyclone. Cross-section analysis through the respective composite frontogenesis maxima showed that the banded composite frontal zone exhibited stronger and deeper frontogenesis and weaker conditional stability than the nonbanded composite frontal zone. Case studies from the northeast United States confirm the composite results, highlighting the importance of deep-layer frontogenesis coincident with weak conditional stability. These results are in qualitative agreement with the Sawyer?Eliassen equation, which predicts that the frontogenetical response will be enhanced (reduced) in the presence of small (large) moist symmetric stability.