Far-upstream precursors to early season cold-air outbreaks over the Northeast US

Jason M. Cordeira, Lance F. Bosart, and Daniel Keyser Department of Atmospheric and Environmental Sciences

University at Albany/SUNY, Albany, NY

NROW XI4 November 2009

NSF Support: ATM-0646907cordeira@atmos.albany.edu

Objective:• Investigate far-upstream precursors to early

season cold-air outbreaks (CAOs) over the Northeast US

Motivation:• CAOs can influence energy consumption,

agriculture losses, property damage, loss of life, precipitation, and western North Atlantic cyclogenesis – e.g., Sanders and Davis 1988; Konrad and Colucci 1989, Portis et al. 2006

Objective and Motivation

North American perspective:• Numerous studies have investigated

dynamic, thermodynamic, and kinematic contributions to CAOs over North America– e.g., Konrad and Colucci 1989; Colucci et al. 1999; Colle and Mass 1995;

Konrad 1996; Walsh et al. 2001

• Previous studies agree that common upstream precursors are mid- and upper-level positive height anomalies over Alaska and subsequent anticyclogenesis over North America

Previous Work

Archambault et al. (NROW 2007):• Documented mechanisms linking an early

season CAO in September 1995 to upstream recurving western North Pacific tropical cyclones (TCs)

Previous Work

TC Recurvature

Upper-level Ridge Amplification

Downstream Development Anticyclonic

Wave Breaking

Early-SeasonCold-Air Outbreak

300-hPa streamfunction and non-divergent wind16−23 Sep 1995

Focusing on far-upstream precursors:• Document mechanisms linking early season CAOs to

variability in the structure and evolution of the North Pacific jet stream (NPJ)

• Precursors: physical processes that modify the NPJ and are conducive to downstream flow amplification– e.g., TC recurvature, TCs undergoing ET, extratropical

cyclogenesis, tropical heating anomalies, downstream development

Goal:• Identify linkages between the structure and evolution

of the NPJ to high-impact weather events over the eastern North Pacific and North America

Building On Previous Work

Northeast US CAOs

Part 1. Climatology and identification:• Northeast US CAOs

– domain averaged 850-hPa temperature (35−50°N; 65−85°W)– September, October, and November 1979−2008– daily at 1200 UTC from the 2.5° NCEP−NCAR Reanalysis

– The Northeast US climatology was compared to a similar climatology of 850-hPa temperatures for Albany, NY from the radiosonde archive, but will not be shown

©2009 Google – Map data

35°N

50°N

85°W 65°W

domain

Northeast Climatology and identification:

Northeast US 850-hPa Temperatures

N=900N=931N=900N=2731

850-hPa Temperature (°C)

Fre

qu

ency

SeptemberOctoberNovemberCumulative

N=2731

Anomalies are based on a 21-day centered running mean for 1979−2008

Northeast Climatology and identification:

Northeast US 850-hPa Temperature Anomalies

Standardized 850-hPa Temperature Anomaly

Fre

qu

ency

Cumulative

N=2731

Identifying Cold-Air Outbreaks

Northeast Climatology:

Standardize 850-hPa Temperature Anomaly

Fre

qu

enc

y

CAO was defined as the 2.5% (69) coldest daysSeptember: 9 eventsOctober: 9 eventsNovember: 14 events

Events: 32 events Duration: ~2.2 daysFrequency: ~1 event yr−1

}

Cumulative

Northeast US CAOs

Part 2. Composite structure:• October events only; N=9

• Geography-relative

• Lag day −6 through day 0

• Day 0: “coldest” day of each event– 1000−500-hPa Thickness, SLP, 300-hPa Wind– 850-hPa Geopotential Height and Stand. Anomaly– 300-hPa Geopotential Height and Stand. Anomaly

Day 0Day −2

Day −4Day −6

October Composite Structure: (N=9)

Northeast US CAOs

1000−500-hPa Thickness (dam; dashed), 300-hPa Wind (m s−1; solid), and SLP (hPa)

hPa

LL

HL

L

H

H

LL

HL

H

LH

L HL

Northeast US CAOs

850-hPa Temperature (°C) and Standardized Anomaly

Day 0Day −2

Day −4Day −6

Approximate centroid of minimum 850-hPa T anomaly

October Composite Structure: (N=9)

LL

HL

L

H

H

L

HL

H

LH

L HL

Day 0Day −2

Day −4Day −6

Northeast US CAOs

300-hPa Geopotential Height (dam) and Standardized Anomaly

LL

HL

L

H

H

LL

HL

H

LH

L HL

81

87

87

84 9

2 92

86

97 9

7

87

87

88

October Composite Structure: (N=9)

Locations of TCs, if present, during each event at day −6

300-hPa Geopotential Height (dam) and Standardized Anomaly

Northeast US CAOs

+

−+−

Day −2 October Composite Structure: (N=9)

Barotropic response in association with anomalous tropical heating?

Northeast US CAOs

Summary of October composite:• Cold air originates over northern Canada and

is displaced equatorward on synoptic time scales

• Equatorward displacement coincides with– anomalous upper-level subtropical ridge over the

tropical western Pacific– persistent Aleutian cyclones in the NPJ poleward-

exit region– anomalous upper-level ridge development over

Alaska and the Yukon– surface anticyclogenesis in the lee of the Rockies

Far-upstream precursors

Part 3. Individual examples• All composite members featured the

equatorward displacement of cold air in response to upstream upper-level ridging

• The antecedent structure and evolution of the North Pacific flow (NPJ) was grouped into three types:1. Strong zonal flow; N=3

2. Amplifying flow; N=3

3. Amplifying flow in association with North Atlantic blocking; N=2

23 October 1981

300-hPa Geopotential Height (dam), Precipitable Water (mm), and MSLP (hPa)

mm

Day −6: 1200 UTC 17 October

L1L1

Key point:Downstream response influenced by extratropical cyclone evolution in the presence of a strong (60−70 m s−1), zonal NPJ

Similar to events on 1 October 1984 and 7 October 1986

X

Day −3: 1200 UTC 20 October

Type 1: Strong zonal flow

23 October 1981

300-hPa Geopotential Height (dam), Precipitable Water (mm), and MSLP (hPa)

mm

Type 1: Strong zonal flow

GMS-5 Infrared SatelliteVelocity Potential (106 m2 s−1) and

Irrotational Wind (m s−1)

L1

L1

Day −3: 1200 UTC 20 October

10 m s−1

13 October 1988

300-hPa Geopotential Height (dam), Precipitable Water (mm), and MSLP (hPa)

mm

Day −6: 1200 UTC 7 October

Key point:Downstream response influenced by TC recurvature/ET and Aleutian cyclogenesis as a result of downstream development

Similar to events on 8 October 1987 and 23 October 1987Similar to the September event documented by Archambault at NROW in 2007

Day −3: 1200 UTC 10 October

Type 2: Amplifying flow

L1L2L1

19 October 1992

300-hPa Geopotential Height (dam) and Anomaly (dam)

Key point:Downstream response influenced by Alaskan ridging and North Atlantic blocking in a process similar to that described by Konrad and Colucci (1989)

Similar to an event on 23 October 1997

Type 3: Amplifying flow in association with North Atlantic blocking

+ +

−

L1 L2

−40 −30 −20 −10 10 20 30 40 dam0

Northeast US CAOs

Conclusions:• Early season CAOs over the Northeast US

result from the equatorward displacement of cold air from northern Canada on synoptic time scales

• Composite imagery suggests the structure of the NPJ, Aleutian cyclogenesis, and ridging over northwest Canada are important precursors of CAOs– tropical heating anomalies are worth considering

Northeast US CAOs

Conclusions:• Three types of North Pacific flow patterns

were identified prior to October CAOs:– A zonal NPJ strengthened in association with TC

outflow in the equatorward entrance region

– Downstream development initiated by TC recurvature and subsequent ET

– A displacement of the circumpolar vortex in association with upper-level ridging over Alaska and North Atlantic blocking