North Carolina State University, Raleigh, North Carolina A Climatology of Mesoscale Band Formation and 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 Anantha Ayier North Carolina State University, Raleigh, North Carolina

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?

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?

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

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 that exhibited precipitation amounts greater than 25.4 mm of rain or 12.7 mm liquid equivalent in the case of frozen precipitation, during a 24-h period at a point in the study domain.

Data and Methods 700 hPa low vs. no low: To identify cases with likely comma-head signatures, 3-h North American Regional Reanalysis (NARR; Mesinger et al. 2006) data was examined to identify cases exhibiting a midlevel (700 hPa) closed low at some time during their evolution over the study domain.

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.

Band Environment Evolution Rapid Update Cycle (RUC) analysis and 2 km composite radar data were used to characterize the environmental evolution of the 27 comma-head single band events, and compare with the 19 null events. 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)

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. Values of the 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 was assessed above the frontogenesis maximum in a 200 hPa layer where RH > 70%. 0900 UTC Feb 12 2006

Q1: Is the frontogenesis and stability evolution found in Novak et al Q1: Is the frontogenesis and stability evolution found in Novak et al. (2008) observed in a larger sample of banded events? Case Study Climatology (N=27) Mean evolution consistent with Novak et al. (2008) case study evolution 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.

Q2: Using high-resolution datasets, what is the common evolution of banded cyclones?

Composite Technique All banded events exhibited a 700 hPa trough in the vicinity of the banded event. Given this common feature and that a trough was also a feature observed in many null events, a trough-relative composite framework was utilized. 700-hPa low center serves as anchor point (x=0,y=0) [C /(100 km)/(3h)]

Composite Technique All banded events exhibited a 700 hPa trough in the vicinity of the banded event. Given this common feature and that a trough was also a feature observed in many null events, a trough-relative composite framework was utilized. 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)]

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

Band Composite (trough relative) N =27 [C /(100 km)/(3h)] 2D frontogenesis (shaded) Potential Temperature (Blue, 2 K) Height (solid, 30 m) 15

Band Composite (trough relative) N =27 [C /(100 km)/(3h)] 2D frontogenesis (shaded) Potential Temperature (Blue, 2 K) Height (solid, 30 m) 16

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

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

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

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) Potential Temperature (Blue, 2 K) Height (solid, 30 m) 20

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) Moore et al. (2008)

Composites (T=0) Treble-Cleft (N=15) Diabatic (N=7) 400 hPa PV and winds 700 hPa winds, heights and frontogenesis. 22

Diabatic Composite (trough relative) N =7 Moore et al. (2008) case study T=-4h Composite PV evolution just prior to band formation similar to PV evolution in Moore et al. (2008) East Coast diabatic vortex case study. PVU T= start PVU

Q3: How does the mesoscale forcing and stability evolution differ between cyclones with closed midlevel circulations that develop bands and those that do not? Null Events Band Events 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

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

Trough relative band vs. null composite T=0 N=27 T=0 N=19 700 hPa 400 hPa Wind Speed = shading Height = solid Wind = barbs

Pre-cursor signals? NULL BAND T=-4h N=27 T=-4h N=19 Similar synoptic patterns, HOWEVER Banded events have a stronger temperature gradient and stronger deformation which increases faster than null events.

Summary There is a common band life cycle: FORMATION: Release of conditional instability by increasing frontogenesis MATURITY: Increasing stability offset by increasing frontogenetical forcing DISIPATION: Weakening frontogenesis in presence of large stability Different upper PV distribution can result in band formation, including the treble-cleft, and diabatic Rossby vortex PV distributions The development of a mid-level trough is a common feature of banded cyclones, but is also observed in many null events. 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 are stronger and exhibit a stronger temperature gradient and deformation maximum, resulting in faster growth of frontogenesis.

Backup

Trough relative band vs. null composite T=0 N=27 T=0 N=19 700 hPa 700 hPa PV = shaded Wind = barbs PVU PVU

Trebble-Cleft Composite (trough relative) N =15 400 hPa 700 hPa -Frontogenesis max along trough -Band in left exit region of upper jet -Diabatic PV anomaly associated with trough/band -Classic treble cleft upper PV distribution, with trough/band occurring in PV advection at tip

Composites (T=0) Treble-Cleft (N=15) Diabatic (N=7) 400 hPa PV and winds 700 hPa PV and winds 32

Trough relative band vs. null composite Similarities Banded and Null events both have midlevel trough. Banded and Null events both commonly exhibit a treble-cleft PV structure. Differences Banded cyclone has stronger (and deeper) frontogenesis. Banded cyclones are generally deeper, with stronger upper jet. Are there any precursor signals to these differences?

Diabatic Composite (trough relative) N =7 400 hPa 700 hPa -Not a well defined trough, BUT a strong temperature gradient with confluence. -Band in possible coupled jet region -Two diabatic PV anomalies: strongest one along the coast, and another axis farther inland. -Main trough hangs west of band region, some upper PV advection, but not much.