Extreme Temperature Regimes during the Cool Season

Robert X. BlackRebecca Westby

School of Earth and Atmospheric SciencesGeorgia Institute of Technology, Atlanta, Georgia

DOE/BER Regional and Global

Climate Modeling Program

Presentation Overview

General project objectives & research approach

Preliminary statistical results for cold air outbreaks: Focus on Atlanta

Illustrative synoptic & dynamic analyses: Jan 2004 Case Study

Considerations of recent cold air outbreak behavior: Winters of 2009/2010 & 2010/2011

Summary & future research directions

Project Overview

General project objectives & research approach:

Quantify the modulation of extreme temperature regimes (ETRs) by low frequency modes (LFMs)

Assess the representation of ETRs and ETR-LFM linkages in global coupled climate models (CMIP5)

Assess likely future changes in regional ETR behavior and ETR-LFM linkages (CMIP5)

General Approach and Datasets

Identify extreme temperature regimes (ETRs) in terms of local anomalies in either temperature or wind chill index (e.g., Walsh et al 2001; Osczevski and Bluestein 2005)

Basic data: Daily averaged reanalysis data (NCEP/NCAR, ERA-40 & NASA-GMAO MERRA)

E vent Date W ind-C hill R ank

Temperature R ank

D ifferenc e in R ank ing

15-F eb-79 1 1* 010-J an-68 2 2 018-F eb-58 3 7 429-F eb-80 4 4 07-J an-59 5 20 15

18-J an-94 6 8 223-Dec -89 7 3 -4 1-F eb-80 8 11* 3

25-Dec -83 9 13* 41-J an-63 10 16 66-J an-96 11 5 -6

10-J an-81 12 6* -6 18-J an-71 13 12 -1 7-F eb-95 14 28 14 16-J an-00 15 18* 310-Dec -77 16 19 3

Top 15 winter wind-chill events for Albany, NY (1948-2006) and relative ranking for temperature- only criterion. Asterisks denote events in which the peak wind-chill amplitude occurred on a slightly different day than peak temperature amplitude.

Interannual Variability in Cold Air Events in AtlantaRelationship with the Arctic Oscillation

Downward trend in cold air events until last 2-3 winters Greatest number of cold days occurred in 2009/2010 (!) Significant negative correlation with the AO (r = -0.55)

Cold front passes through Atlanta ~12Z January 5, 2004 Highs in the 70s Jan 5 -> Lows in the 10s on Jan 7

1/05/2004 1/07/2004

Cold Air Outbreak: 12Z Jan 5, 2004 (NOAA/HPC)

Cold Air Outbreak: 12Z Jan 5, 2004 (Winds/EPV)

1/05/2004 1/07/2004

p

Remote Influence of Local PV Anomalies (‘Charges’)

Poisson-like PV balance condition indicates nonlocal effects analogous to induction of electric field by localized charges

x,y

1q

2q

3q

Spheroids of constant Z’ associated with isolated q anomalies

[e.g., Hoskins et al. 1985]

4 q

Vertical extentrelated to L/N;Large scales & weak N favor a downwardinfluence

Piecewise PV Inversion: Quasi-Geostrophic Form

q( )Z } mq

( )mZ}( )nmZ

(

(

'( ') )

' )m m

m

Z qq ZZ Z

Cold Air Outbreak: Jan 5, 2004 (QGPV Anomalies)

Diagnose contributions of PV anomalies within different vertical layers to the northerly flow in lower troposphere

Anomalies defined as deviations from monthly mean flow Divide PV anomaly field into three parts:

1) Upper tropospheric PV (500-300 hPa)2) Lower tropospheric PV (600-975 hPa)3) Surface theta at lower boundary (975-1000 hPa)

QGPV Inversions: Invert Entire PV Anomaly Field

Generally excellent quantitative correspondence over most regions

Notable errors near base of trough where strong curvature exists

Actual wind is subgeostrophic due to locally large Rossby number

Supergeostrophic flow in ridge

300 hPa vector wind anomalies

QGPV Inversions: Invert Entire PV Anomaly Field

Generally excellent quantitative correspondence over most regions (including over midwest US)

Some errors near cold front

No differences where 925 hPa surface dips below ground

Proceed to piecewise PV inversion

925 hPa vector wind anomalies

QGPV Inversions: Invert PV “Pieces”

Upper tropospheric PV induces southwesterly flow over midwest

Lower tropospheric PV inducesnortheasterly flow over midwest

Strong cancellation among the contributions of interior PV

Surface theta induces northerlies

925 hPa vector wind anomalies

QGPV Inversions: Invert Surface PV “Pieces”

Isolate cold surface theta anomalies over the western US/Canada

Invert cold surface theta anomalies

Provides a large contribution to northerly flow over midwest US

Cold anomalies east of Rockies promote northerlies to the east

925 hPa vector wind anomalies

Average surface air temperature anomalies 12/15 – 01/14

1/07/2004

Winters of 2009/10 & 2010/11: Unusual Behavior!

AO index →(NOAA/CPC)

Composite T Anomalies →

(NOAA/ESRL)

Winters of 09/10 & 10/11: North Atlantic Jet Structure

Climo characterized by two jets: Subtropical jet & eddy-driven jet

North-South jet anomaly dipole found during 2009/10 with strong westerly anomalies near 30N

Net impact: Effective merger of the climatological jet features

Similar behavior during 2010/11

Zonal wind averaged from 300W-360W

(12/15 – 1/14)

2010/11: Nov 1 to Jan 202009/10: Nov 1 to Jan 20

300 hPa Zonal Wind Evolution over North Atlantic

Eddy driven jet strengthens during Fall and early winter Subtropical jet develops beginning in January Eddy driven jet abruptly collapses during Spring onset

Climo: July 1 to June 30 Climo: Nov 1 to Jan 20

Composite 500 hPa Geopotential Height Field

Total heights:Left: climo

Right: 2009/10(12/15 – 1/14)

Stationary eddies:

Left: climo Right: 2009/10(12/15 – 1/14)

Composite 500 hPa Geopotential Height Field

Total heights:Left: climo

Right: 2010/11(12/15 – 1/14)

Stationary eddies:

Left: climo Right: 2010/11(12/15 – 1/14)

Summary and Future Research Directions Cold Air Outbreaks are evidently alive and well

Cold Air Outbreaks strongly modulated by AO/NAO

January 2004 case study: Southward surge of cold air through the midwest is primarily effected by cold

surface theta anomalies positioned east of Rocky Mountains

Recent winter behavior: Possible alterations in the seasonal cycle of the North Atlantic jetstream?

Future work: More fully explore the low frequency modulation of ETRs in different geographical regions

Future work: Examine the behavior of ETRs and their low frequency modulation in coupled climate models

PV Balance Condition: Large-scale atmospheric disturbances are

governed by the linear balance condition:

Poisson-like => nonlocal response in Z’

22

2 2 2

1 cos 1' '( cos ) cos

'

g fq f Zf a a f p p

Z

‹

[e.g., Black 2002]

1' 'Z q ‹

Boundary Conditions Polar Continuity Longitudinally cyclic Z’ = 0 at low latitude boundary (100N) Upper and lower boundaries:

a) Boundary q’ not included:

b) If boundary q’ is included: 'R Cp

o

Z R pp gp p

' 0Zp

[Black 2002]

EAS 6502 - Quasi-Geostrophic Theory

Given a 3-D distribution of q’ and boundary conditions for ’, one can invert the QG balance to infer the 3-D ’ distribution. (From which the temperature and horizontal windfields can be deduced via hydrostatic & geostrophic balance, respectively)

Note: Laplacian-like operator localized q anomalies are associated with a anomaly distribution that may extend horizontally and vertically away into the far field (from q’).

Permits dynamic interaction of spatially separated q anomalies

Quasi-Geostrophic Potential Vorticity

2 0

0

1 ( )fqf p p

L