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

Motivation Coarse spatial and temporal resolution analyses have historically limited investigation of band life-cycle Novak et al. (2008) showed b and 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 Past conceptual models based on coarse spatial and temporal data. Novak et al. (2004) Conceptual ModelNovak et al. (2008) Case Study Q2: What is the mesoscale evolution of banded cyclones?

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

Data and Methods 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. Study Domain: Northeast U.S. and portions of southeast Canada Period: October-March

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.

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.

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

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)

 Cross sections of: 2-D frontogenesis (black) Saturated equivalent potential temperature (θ es ) (thin green) Ascent (dotted) 70 % RH (thick green)  Cross sections taken following the movement of the band UTC Feb Band Environment Evolution Conditional stability assessed above the frontogenesis maximum in a 200 hPa layer where RH > 70%. 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)

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 Case StudyClimatology (N=27)

Q2: What is the mesoscale evolution of banded cyclones?

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) Synoptic Classification 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.

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)]

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

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

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

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

T = start [C /(100 km)/(3h)] 2D frontogenesis (shaded) Potential Temperature (Blue, 2 K) Height (solid, 30 m)

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

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

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

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 NON-BANDED Feb

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 Band Events (N=27)Null Events (N=19)

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

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

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 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. The primary discriminator between banded and null events is the strength of the frontogenesis, rather than the 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