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David Novak

A High-Resolution Climatology and Composite Study of Mesoscale Band Evolution within Northeast U. S. Cyclones. David Novak NOAA/ NWS Eastern Region Headquarters, Scientific Services Division, Bohemia, New York

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David Novak

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  1. A High-Resolution Climatology and Composite Study of Mesoscale Band Evolution within Northeast U. S. Cyclones David Novak NOAA/ NWS Eastern Region Headquarters, Scientific Services Division, Bohemia, New York School of Marine and Atmospheric Sciences, Stony Brook University, State University of New York Brian Colle School of Marine and Atmospheric Sciences, Stony Brook University, State University of New York Anantha Ayier North Carolina State University, Raleigh, North Carolina

  2. Motivation • Coarse spatial and temporal resolution analyses have historically limited investigation of band life-cycle • Novak et al. (2008) showed band life-cycle governed by evolution of frontogenesis, stability, and moisture • FORMATION: Release of conditional instability by increasing frontogenesis • MATURITY: Increasing stability offset by increasing frontogenetical forcing • DISSIPATION: Weakening frontogenesis in presence of large stability • Q1: Is this evolution observed in a larger sample of banded events?

  3. Motivation Novak et al. (2004) Conceptual Model Novak et al. (2008) Case Study • Past conceptual models based on coarse spatial and temporal data. Q2: What is the mesoscale evolution of banded cyclones?

  4. Motivation BANDED Feb 12 2006 NON-BANDED Novak et al. (2004) • Q3: How does the mesoscale forcing and stability evolution differ between cyclones with closed midlevel circulations that develop bands and those that do not? Feb 11 2005

  5. Data and Methods Study Domain: Northeast U.S. and portions of southeast Canada Period: October-March 2002-2007 Cases: Daily Weather Maps series used to identify cases with >25.4 mm of rain or >12.7 mm liquid equivalent, during a 24-h period at a point in the study domain.

  6. Data and Methods 700 hPa low vs. no low: 3-h North American Regional Reanalysis (NARR) data to identify cases exhibiting a midlevel (700 hPa) closed low at some time during their evolution over the study domain.

  7. Data and Methods Among the 56 cases exhibiting a 700-hPa low: 2 km composite radar data were examined to identify types of events: -Single-banded :linear reflectivity feature 20–100 km in width and >250 km in length which maintains an intensity of 30 dBZ for at least 2 h. -Transitory null: banding meets all respective criteria in a category except one (usually the lifetime). -Null: neither a single band nor transitory band occurred in comma head.

  8. Q1: Is the frontogenesis and stability evolution found in Novak et al. (2008) observed in a larger sample of banded events?

  9. Band Environment Evolution • Rapid Update Cycle (RUC) analysis and 2 km composite radar data were used to characterize the environmental evolution • RUC (Benjamin et al. 2004) assimilates variety of synoptic and asynoptic datasets • Grids archived with: • 20 km horizontal spatial resolution • 25 mb vertical resolution • Hourly temporal resolution • Analysis was conducted from 4 h prior to band formation (t=-4) to one hour after band dissipation (t=end+1)

  10. Band Environment Evolution • Cross sections of: • 2-D frontogenesis (black) • Saturated equivalent potential temperature (θes) (thin green) • Ascent (dotted) • 70 % RH (thick green) • Cross sections taken following the movement of the band. • Frontogenesis maximum in the 800–500 mb layer within 100 km of the observed radar band were recorded during the band lifecycle (t=-4 h to t=end +1 h) • Conditional stability assessed above the frontogenesis maximum in a 200 hPa layer where RH > 70%. 0900 UTC Feb 12 2006

  11. Case Study Climatology (N=27) • Frontogenesis generally maximizes during band maturity The t=+2 h frontogenesis value is larger than the t=-1 h value at the 85% confidence level. • Stability smallest at time of band formation, and generally increases afterward. Conditional stability at t= 0 is < conditional stability at t= +3 is significant at the 90% confidence level. • There is a common band life-cycle

  12. Q2: What is the mesoscale evolution of banded cyclones?

  13. Synoptic Classification • Review of case database revealed there are a variety of PV evolutions that support banding in the comma-head (i.e., there is no one common type of banded cyclone) PV evolutions subjectively classified as: -Treble-cleft (15 cases): Traditional LC 2 (cyclonic wrap-up)] -Diabatic (7 cases): Band formation to the east of the upper PV trough, with a saturated PV maximum > 1 PVU at the 700 hPa level within 300 km of the band -Cutoff (5 cases) Upper PV maximum isolated from the stratospheric PV reservoir Posselt and Martin (2004) • However, nearly all banded events exhibited a 700 hPa trough near the band.

  14. Composite Technique • A trough-relative composite framework was utilized, since: • Nearly all banded events exhibited a 700 hPa trough near the band. • Many null events also exhibited a 700 hPa trough • 700-hPa low center serves as anchor point (x=0,y=0) [C /(100 km)/(3h)]

  15. Composite Technique • A trough-relative composite framework was utilized, since: • Nearly all banded events exhibited a 700 hPa trough near the band. • Many null events also exhibited a 700 hPa trough • 700-hPa low center serves as anchor point (x=0,y=0) • RUC field moved such that anchor point is at mean low position (40 N/75 W). • RUC field rotated about this point such that the 700 hPa height trough is aligned N-S. • Fields averaged (composited) for selected cases. [C /(100 km)/(3h)]

  16. 700 hPa Band Composite (trough relative) N =27 T = -6 Box average of diagnostics: • Frontogenesis (FRNT) Mean Band Location • Normalized Frontogenesis (NFRNT) • Magnitude potential temperature gradient (DELT) [C /(100 km)/(3h)] • 2D frontogenesis (shaded) • Height • (solid, 30 m) • Potential Temperature (Blue, 2 K)

  17. Band Composite (trough relative) N =27 T = -6 [C /(100 km)/(3h)] • 2D frontogenesis (shaded) • Height • (solid, 30 m) • Potential Temperature (Blue, 2 K)

  18. Band Composite (trough relative) N =27 T = -4 [C /(100 km)/(3h)] • 2D frontogenesis (shaded) • Height • (solid, 30 m) • Potential Temperature (Blue, 2 K)

  19. Band Composite (trough relative) N =27 T = -2 [C /(100 km)/(3h)] • 2D frontogenesis (shaded) • Height • (solid, 30 m) • Potential Temperature (Blue, 2 K)

  20. Band Composite (trough relative) N =27 T = start [C /(100 km)/(3h)] • 2D frontogenesis (shaded) • Height • (solid, 30 m) • Potential Temperature (Blue, 2 K)

  21. Band Composite (trough relative) N =27 T = maturity [C /(100 km)/(3h)] • 2D frontogenesis (shaded) • Height • (solid, 30 m) • Potential Temperature (Blue, 2 K)

  22. Band Composite (trough relative) N =27 T = end [C /(100 km)/(3h)] • 2D frontogenesis (shaded) • Height • (solid, 30 m) • Potential Temperature (Blue, 2 K)

  23. Band Composite (trough relative) N =27 T = end+2 Frontogenesis evolution dominated by changes in kinematic flow (NFRNT) rather than increase in temperature gradient (DELT) [C /(100 km)/(3h)] • 2D frontogenesis (shaded) • Height • (solid, 30 m) • Potential Temperature (Blue, 2 K)

  24. Q3: How does the mesoscale forcing and stability evolution differ between cyclones with closed midlevel circulations that develop bands and those that do not? BANDED Feb 12 2006 NON-BANDED Feb 11 2005

  25. Band Events (N=27) Null Events (N=19) • The primary discriminator between banded and null events is the strength of the frontogenesis, rather than the stability. • The t=0 h band event frontogenesis value is larger than the t=0 null event frontogenesis value at the 99% confidence level. • Banded event stability (averaged over t=0 to+3) is greater than the null event stability (averaged over t=0 to +5 ) is only significant at the 75% level

  26. Trough relative band vs. null composite BAND NULL T=0 N=27 T=0 N=19 F = shaded θ=thin green EPV = 0.25 (thick black)

  27. Pre-cursor signals? NULL BAND T=-6h T=-6h Similar synoptic patterns, HOWEVER Banded events have a stronger temperature gradient and stronger deformation which increases faster than null events.

  28. Summary • There is a common band life cycle: • FORMATION: Release of conditional/symmetric instability by increasing frontogenesis • MATURITY: Increasing stability offset by increasing frontogenetical forcing • DISIPATION: Weakening frontogenesis in presence of large stability • There is no one common type of banded cyclone, however, the development of a mid-level trough is a common feature of banded cyclones • The primary discriminator between banded and null events is the strength of the frontogenesis, rather than the stability. • The synoptic flow is similar between banded and null cases, however, the banded cyclones exhibit a stronger temperature gradient and deformation maximum, resulting in faster growth of frontogenesis.

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