21st Conf. on Severe Local Storms and 19th Conf. on Weather Analysis and Forecasting/15th Conf. on Numerical Weather Prediction

Wednesday, 14 August 2002: 9:00 AM
A climatological and composite study of cold season banded precipitation in the northeast United States
David Novak, SUNY, Albany, NY; and L. F. Bosart, D. Keyser, and J. S. Waldstreicher
Poster PDF (479.7 kB)
A climatology of banded precipitation events in the northeast U.S. during the cold season (October through April) is presented. Precipitation systems in the northeast U.S. that exhibited greater than 25 mm of rainfall, or 12.5 mm liquid equivalent were identified as cases for study using the Unified Precipitation Dataset (UPD). Composite radar data from these cases were viewed to develop a band classification scheme. This scheme was then applied to cases from November 1996 through April 2001. Out of the 112 cases identified during this period, 89 cases had complete radar coverage.

Examination of these 89 cases revealed that 35 exhibited single banded structure at least once during their evolution, 29 multibanded structure, 30 narrow cold-frontal structure, 32 transitory or undefined structure, and 13 exhibited no defined banding. Note that many cases had more than one type of banded structure during their duration. Further investigation of the single banded events highlighted banded structure in the comma head portion of storms, with nearly 70% of the bands exhibiting some portion of their length in the northwest quadrant of the surface cyclone.

Composites were calculated for the single banded and nonbanded cases to distinguish synoptic flow regimes associated with banded cases from those associated with nonbanded cases. The single banded composite exhibited a closed circulation from the surface to 700 hPa and featured a deformation zone with an identifiable col point northwest of the surface cyclone. Significant frontogenesis northwest of the surface cyclone center coincided with the confluent asymptote of this deformation zone. The nonbanded composite exhibited a much weaker disturbance located in the confluent entrance region of an upper-level jet. The absence of a closed mid-level circulation in the nonbanded composite precludes deformation and subsequent frontogenesis northwest of the surface cyclone. Although mid-level confluence ahead of the surface cyclone contributes to frontogenesis in the nonbanded composite, it is much weaker than in the banded composite. These results suggest that the forecast amplitude and depth of a disturbance can help forecasters discriminate between potential banded and nonbanded cases.

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