A High-Resolution Observational Climatology and Composite Study of Mesoscale Band Evolution within Northeast U.S. Cyclones David Novak NOAA/NWS Hydrometeorological Prediction Center Brian Colle * Anantha Aiyyer # * School of Marine and Atmospheric Sciences, Stony Brook University # Dept. of Marine, Earth and Atmos. Sciences, NCSU © New York Times

The Challenge of QPF Bands make Quantitative Precipitation Forecasts (QPF) difficult –Localized heavy precipitation –Extreme gradients –Evolution 12 Feb 2006 NYC 2

Max = 47 mm Max = 30 mm ObservedNAM Forecast (24 h prior) The Challenge of QPF Bands make Quantitative Precipitation Forecasts (QPF) difficult –Localized heavy precipitation –Extreme gradients –Evolution 3

Objectives Novak et al. (2004) Conceptual Model Novak et al. (2008) Case Study Is there a common banded cyclone and band life cycle?

Novak et al. (2004) BANDED Feb Distinguishing characteristics between cyclones with closed midlevel circulations that develop bands and those that do not? Objectives

Motivation Novak et al. (2004) Feb Distinguishing characteristics between cyclones with closed midlevel circulations that develop bands and those that do not? NONBANDED

Data and Methods Cases: Daily Weather Maps series to identify cases exhibiting > 25.4 mm (rain) or > 12.7 mm liquid equivalent (snow), during a 24-h period at a point in the study domain. Study Domain: Northeast U.S. Period: October-April

Data and Methods 700 hPa low vs. no low: 3-hourly North American Regional Reanalysis (NARR; Mesinger et al. 2006) data examined to identify cases exhibiting a midlevel (700 hPa) closed low 8

Data and Methods Among the 75 cases exhibiting a 700-hPa low: 2 km WSI composite radar data were examined to identify types of events: -Single-banded : meets shape, intensity, and duration criteria. -Transitory null: meets two out of the three criteria (shape, intensity, duration) -Null: meets at most one of the three criteria (shape, intensity, duration) 9

Band Environment Evolution Rapid Update Cycle (RUC) analysis and 2 km composite radar data used to characterize the environmental evolution RUC (Benjamin et al. 2004) assimilates variety of synoptic and asynoptic datasets Archived 20 km horizontal spatial resolution / 25 mb vertical resolution Hourly temporal resolution Band-relative time framework 10

 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 movement of the band UTC 12 Feb 2006 Band Environment Evolution Conditional stability above the frontogenesis maximum in a 200 hPa layer where RH > 70%. Frontogenesis maximum in the 800–500 hPa layer within 100 km of the observed radar band. 11

Frontogenesis generally maximizes during band maturity Stability smallest at time of band formation, and generally increases afterward. Banded event frontogenesis is stronger than null (90% confidence level) Banded event stability is weaker than null (but not statistically significant) *Common band life cycle* Mean Evolution

Stability Types (N=36) Conditional StabilityInertially stableInertially stable and EPV <0 Inertially unstable and EPV <0 >1 K (100 hPa) −1 “Stable”CSIIICSI/II <1 K (100 hPa) −1 but > 0 K (100 hPa) −1 “Neutral”CSIIICSI/II <0 K (100 hPa) −1 CI CI/II 13

Synoptic classification based on PV evolution for cool seasons (72 banded cyclones) Banded Cyclone Types -Treble-clef (53 cases): Traditional LC 2 (cyclonic wrap-up)] Posselt and Martin (2004) -Cutoff (10 cases) Upper PV maximum isolated from the polar PV reservoir -Diabatic (9 cases): Band formation east of the upper PV trough, and within 300 km of a saturated 700-hPa PV maximum (i.e., diabatic PV anomaly) PV evolutions subjectively classified as: 14

Composite Technique All banded events exhibited a 700 hPa trough in the vicinity of the banded event, and trough was also present in many null events. Trough-relative composite framework [C /(100 km)/(3h)] 700-hPa low center serves as anchor point (x=0,y=0) 0900 UTC 12 Feb

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. Composite Technique All banded events exhibited a 700 hPa trough in the vicinity of the banded event, and trough was also present in many null events. Trough-relative composite framework 700-hPa low center serves as anchor point (x=0,y=0) [C /(100 km)/(3h)] 0900 UTC 12 Feb

Treble Clef Cyclone Evolution (N=26) T= − hPa -height -winds -temp 700 hPa -height -winds -temp - frontogenesis 400 hPa -PV -winds -PV adv 400 hPa -PV -isotachs -winds - divergence 17

Treble Clef Cyclone Evolution (N=26) T= hPa -height -winds -temp 400 hPa -PV -PV adv -winds 400 hPa -PV -isotachs -winds - divergence 700 hPa -height -winds -temp - frontogenesis 18

Treble Clef Cyclone Evolution (N=26) T= end 1000 hPa -height -winds -temp 400 hPa -PV -PV adv -winds 400 hPa -PV -isotachs -winds - divergence 700 hPa -height -winds -temp - frontogenesis 19

Box -Average Diagnostics  Frontogenesis (FRNT)  Normalized Frontogenesis (NFRNT)  Magnitude of θ gradient (DELT) Changes in kinematic flow dominate changes in temperature gradient in explaining frontogenesis evolution Differences in normalized frontogenesis appear only 2 h prior to t=0

Cross Sections Treble ClefNull T= − 6 Frontogenesis (shaded) Isentropes (dark green) Ascent (dashed blue) Flow in plane of x-section (vectors) 70% isohume (thick solid) Hakim and Keyser (2001)

A Common Banded Cyclone Evolution and Band Life Cycle 22

Common Banded Cyclone Evolution and Band Life Cycle T= −6 23

T= −6 A B Common Banded Cyclone Evolution and Band Life Cycle

T= −6 A B Common Banded Cyclone Evolution and Band Life Cycle

T= 0 A B Common Banded Cyclone Evolution and Band Life Cycle

T= +6 A B Common Banded Cyclone Evolution and Band Life Cycle

Distinguishing Treble vs. Null events Null and Treble Cleft events synoptically similar Null events were characterized by: -a weaker surface cyclone*, -weaker midlevel frontogenesis*, -larger conditional and symmetric stability, -less jet-front coupling. *statistically significant 28

Summary PRECEEDING FORMATION: Midlevel frontogenesis increases as mesoscale trough develops in region of upper PV advection and left-exit region of upper jet. Conditional stability reduced via differential horizontal temperature advection. FORMATION: Release of instability modes by increasing frontogenesis along mesoscale midlevel trough, along poleward edge of upper-PV “hook”, and in the left-exit region of upper jet. MATURITY: Increasing stability offset by increasing frontogenetical forcing. LHR from band contributes to increasing frontogenesis. DISSIPATION: Kinematic flow changes associated with new diabatic PV anomaly weakens frontogenesis in presence of large stability. Jet-front coupling disrupted by increasing separation. 29 The primary discriminator between banded and null events is the strength of the frontogenesis. The synoptic flow is similar between PV hook and null cases, although the PV hook cyclones are deeper, and exhibit a smaller separation distance between the upper jet and midlevel frontogenesis. There is a common banded cyclone evolution and band life cycle:

Back up 30

Null Cyclone Evolution T= hPa -height -winds -temp 700 hPa 400 hPa -PV -PV adv -winds 400 hPa -PV -isotachs -winds - divergence 31

Band vs. Null Scatterplot 32