A high-resolution observational climatology and composite study of mesoscale band evolution within northeast United States cyclones
David R. Novak, NOAA/NWS/HPC, Camp Springs, MD; and B. A. Colle and A. R. Aiyyer
Intense precipitation bands are frequently observed in the comma-head sector of extratropical cyclones. Although previous climatologies over the northeast U.S. have established that these bands are favored during the development of a closed midlevel low, many cyclones with closed midlevel lows fail to produce intense bands. In addition, limited observations and coarse resolution analyses have prohibited the description of the mesoscale forcing and stability evolution during band life cycle. This study utilizes hourly 20-km Rapid Update Cycle analyses and 2-km composite radar data to explore the mesoscale forcing and stability evolution during banded events. The study also explores how the mesoscale forcing and stability evolution differ between cyclones with closed midlevel circulations that develop bands and those that do not.
Seventy-five heavy precipitation cases with a 700-hPa low over the northeast U.S. were identified during six recent cold seasons (Oct–Apr). Thirty-six single-banded events were observed in the comma-head in 30 of the heavy precipitation cases (six cases exhibited multiple events). The remaining 45 cases exhibited a 700-hPa low, but failed to develop a single band in the comma-head. Twelve of these 45 cases were considered pure null events, exhibiting an absence of single-banded or even transitory banded features. Composites were created for the 36 banded and 12 null events, with analysis focused on the frontogenesis and stability evolution.
The results reveal a common cyclone evolution and associated band life cycle. A majority of banded events develop in a region of concentrated upper-Potential Vorticity (PV) advection along the poleward edge of the upper PV “hook”. At midlevels, frontogenesis rapidly increases along a mesoscale trough that extends poleward of the 700-hPa low. This trough is the vertical extension of the surface warm-occlusion, and is a reflection of the development of a midlevel thermal ridge (the trowal). Small conditional and symmetric stability, with embedded shallower layers of conditional and symmetric instability, are often present during band formation. In contrast, band dissipation occurs as the frontogenesis weakens and the conditional and symmetric stability is restored. Many null events also exhibit an upper PV hook, midlevel trough formation, and associated forcing for ascent; however, the null event frontal environments are marked by weaker frontogenesis and larger conditional and symmetric stability. Predictability implications of these results will be presented at the conference.
Session 11B, Winter Weather Part II
Wednesday, 3 June 2009, 4:00 PM-5:30 PM, Grand Ballroom West
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