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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.

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Presentation on theme: "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."— Presentation transcript:

1 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 XI 4 November 2009 NSF Support: ATM

2 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 Objective and Motivation

3 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 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

4 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-Season Cold-Air Outbreak 300-hPa streamfunction and non- divergent wind 1623 Sep 1995

5 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

6 Northeast US CAOs Part 1. Climatology and identification: Northeast US CAOs –domain averaged 850-hPa temperature (3550°N; 6585°W) –September, October, and November –daily at 1200 UTC from the 2.5° NCEPNCAR 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°W65°W domain

7 Northeast Climatology and identification: Northeast US 850-hPa Temperatures N=900 N=931 N=900 N= hPa Temperature (°C) Frequency September October November Cumulative

8 N=2731 Anomalies are based on a 21-day centered running mean for Northeast Climatology and identification: Northeast US 850-hPa Temperature Anomalies Standardized 850-hPa Temperature Anomaly Frequency Cumulative

9 N=2731 Identifying Cold-Air Outbreaks Northeast Climatology: Standardize 850-hPa Temperature Anomaly Frequency CAO was defined as the 2.5% (69) coldest days September: 9 events October:9 events November: 14 events Events: 32 events Duration: ~2.2 days Frequency: ~1 event yr 1 } Cumulative

10 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 – hPa Thickness, SLP, 300-hPa Wind –850-hPa Geopotential Height and Stand. Anomaly –300-hPa Geopotential Height and Stand. Anomaly

11 Day 0Day 2 Day 4Day 6 October Composite Structure: (N=9) Northeast US CAOs hPa Thickness (dam; dashed), 300-hPa Wind (m s 1 ; solid), and SLP (hPa) hPa

12 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)

13 Day 0Day 2 Day 4Day 6 Northeast US CAOs 300-hPa Geopotential Height (dam) and Standardized Anomaly October Composite Structure: (N=9) Locations of TCs, if present, during each event at day 6

14 300-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?

15 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

16 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

17 23 October hPa Geopotential Height (dam), Precipitable Water (mm), and MSLP (hPa) mm Day 6: 1200 UTC 17 October Key point: Downstream response influenced by extratropical cyclone evolution in the presence of a strong (6070 m s 1 ), zonal NPJ Similar to events on 1 October 1984 and 7 October 1986 Day 3: 1200 UTC 20 October Type 1: Strong zonal flow

18 23 October hPa Geopotential Height (dam), Precipitable Water (mm), and MSLP (hPa) mm Type 1: Strong zonal flow GMS-5 Infrared Satellite Velocity Potential (10 6 m 2 s 1 ) and Irrotational Wind (m s 1 ) Day 3: 1200 UTC 20 October 10 m s 1

19 13 October 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 1987 Similar to the September event documented by Archambault at NROW in 2007 Day 3: 1200 UTC 10 October Type 2: Amplifying flow L1L1

20 19 October 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 dam0

21 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

22 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


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