Extended Abstract 13.2
An Observational Study Of Cold Season Mesoscale Band Formation In The Northeast United States
David R. Novak, NOAA/NWS, Bohemia, NY; and J. S. Waldstreicher, L. F. Bosart, and D. Keyser
A climatological and composite study of banded precipitation events in the northeast United States (US) during the cold season (October through April) is presented to illustrate how mesoscale band formation is influenced by cyclone evolution. Precipitation systems in the northeast US 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. Mosaic 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 111 cases identified during this period, 88 cases had complete radar coverage. Examination of these 88 cases identified 48 single-banded events, with nearly 80% of these events exhibiting some portion 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 nonbanded cases to distinguish cyclone evolutions associated with banded cases from those associated with nonbanded cases. The banded composite cyclone was marked by cyclogenesis and the development of a closed midlevel circulation. This flow configuration was associated with a deformation zone with an identifiable col point northwest of the surface cyclone. The confluent asymptote of this deformation zone served as a deep-layer focus for frontogenesis, resulting in a band of vigorous ascent within the comma head portion of the composite cyclone. This band of ascent coincided with the composite band position.
The nonbanded composite exhibited a much weaker cyclone located in the confluent entrance region of an upper-level jet. The lack of significant cyclogenesis and associated absence of a closed midlevel circulation in the nonbanded composite precluded significant deformation and subsequent frontogenesis northwest of the surface cyclone, although midlevel confluence ahead of the surface cyclone did contribute to a midlevel frontogenesis maximum found ahead of the surface warm front. Cross-section analysis through composite frontogenesis maxima showed that the nonbanded composite frontal zone exhibited weaker frontogenesis and greater conditional stability than the banded composite frontal zone, resulting in weaker and broader ascent.
Case studies representative of the above composites will be used to provide a mesoscale perspective on band structure and evolution in relation to the synoptic-scale flow signatures identified by the composites. The case studies will extend the findings from the composites to further illustrate the roles of cyclogenesis, deformation and frontogenesis, frontal structure, and environmental stability in band structure and evolution. In particular, Doppler radar signatures from the case studies will be emphasized.
Supplementary URL: http://www.atmos.albany.edu/facstaff/celeste/13.4 Radar.pdf
Session 13, Winter Storms
Tuesday, 12 August 2003, 8:00 AM-10:30 AM
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