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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: [email protected]
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