Synoptic composites of the ET lifecycle of North Atlantic TCs:

Factors determining post-transition evolution

Contributions from:

Clark Evans

Jenni Evans

Motivation: TCs never lose their diapers…

TCs that intensify after extratropical transition

TCs that weaken after extratropical transition

Motivation: Hurricanes cannot be considered passive in midlatitude flow

Ivan (2004)

Karl (2004)

Motivation: The Impact of Hurricane Karl on 72hr Ensemble Forecasts

Cyclone Predictability is a function of its structure

• Predictability is a function of cyclone structure

• Model interpretation/trust is a function of structure

• MPI is a function of cyclone structure

The role of the downstream and upstream long-wave patternhas been shown to play a critical role in determining TC evolution after ET (McTaggart-Cowan et al. 2003)

Tropical cyclones aren’t passive features upon entering the midlatitude flow

The details of tropical development and extratropical transition can dramatically influence the hemispheric long wave pattern

NWP models often obtain their worst forecast skill when a hurricane is about to enter the midlatitude flow (Harr and Jones 2003).

Understanding the reasoning behind these requires improved understanding of extratropical transition.

Motivation: Previous Research

Some questions raised:

• What are the structural aspects of tropical cyclones that favor constructive interaction What are the structural aspects of tropical cyclones that favor constructive interaction with a midlatitude trough?with a midlatitude trough?

• What are the structural aspects of the midlatitude trough that favor constructive What are the structural aspects of the midlatitude trough that favor constructive interaction with a TC?interaction with a TC?

• What factors determine the rapid or slow transition of a TC?What factors determine the rapid or slow transition of a TC?

• What factors determine intensification or decay after extratropical transition of the What factors determine intensification or decay after extratropical transition of the cyclone?cyclone?

• What are the determining factors for post-ET structure of the cyclone: cold core vs. What are the determining factors for post-ET structure of the cyclone: cold core vs. warm-seclusion? warm-seclusion?

34 North Atlantic Transitioning Cyclones Examined

Bonnie (1998) Gordon (2000)

Danielle (1998) Isaac (2000)

Earl (1998) Michael (2000)

Ivan (1998) Nadine (2000)

Jeanne (1998) Allison (2001)

Karl (1998) Erin (2001)

Mitch (1998) Gabrielle (2001)

Nicole (1998) Humberto (2001)

Cindy (1999) Karen (2001)

Dennis (1999) Michelle (2001)

Floyd (1999) Cristobal (2002)

Gert (1999) Gustav (2002)

Harvey (1999) Isidore (2002)

Irene (1999) Josephine (2002)

Jose (1999) Fabian (2003)

Alberto (2000) Isabel (2003)

Florence (2000) Kate (2003)

Cyclone Phase Space• Unifies the fundamental structural description of Unifies the fundamental structural description of

cyclones into a multi-dimensional continuum cyclones into a multi-dimensional continuum ((MWR 2003a,bMWR 2003a,b) : ) :

B: B: 900-600hPa: Storm-relative thermal asymmetry900-600hPa: Storm-relative thermal asymmetry

-V-VTTLL:: 900-600hPa: Thermal wind (cold vs. warm core)900-600hPa: Thermal wind (cold vs. warm core)

-V-VTTUU:: 600-300hPa: Thermal wind (cold vs. warm core)600-300hPa: Thermal wind (cold vs. warm core)

• Will be focusing on a cross section of B vs –VWill be focusing on a cross section of B vs –VTTLL here. here.

Cyclone Phase Space

Cyclone Phase Space: ET Example [Floyd]

Transition begins (time=TB) when B > 10m => significant thermal gradient.

Transition ends (time=TE) when cyclone is cold-core

TE

TE+24h

TE+48h

TB

TMID

TB-24h

TB-48h

34-Cyclone Composite Mean Phase NOGAPS-analysis based Trajectory with key milestones labeled

Composite Mean ET Structural Evolution Summary

TB-72h

TE+72h

Compositing Method• NOGAPS 1°x1° operational analyses from 34 storms

1998-2003

• Storm-center-relative composites. No coordinate rotation for storm motion.

• ±40° longitude

• -20° to +30° latitude (never extending outside 0°-90°N)

• Raw field and anomaly from NCAR/NCEP Reanalysis2 30-year monthly-mean are both composited.

34-Cyclone Composite Mean Evolution 320K Potential Vorticity

TB-24h TB

TE TE+24h

34-Cyclone Composite Mean Evolution:320K PV Cross Sections

TB-24h TB

TETE+24h

Boxes represent the calculated one standard deviation spread about the 34-cyclone consensus mean trajectory for each time

Variability About the Composite Mean

Considerable variability about mean once transition completed=> posttropical phase can take many forms….

Floyd (1999): Non-intensifying cold-core development

Hugo (1989): Explosive cold-core development

Charley (1986): Schizophrenia

Dennis (1999): “ET-Interruptus”.Cindy (1999): Absorption.

Keith (1988): Explosive warm-seclusion development

Three key subcomposites

• Fast [<=12hr] vs. Slow [>=48hr] Transitioning

• Post-ET Intensification (N=6) vs. Weakening (N=11)

• Post-ET Cold-Core (N=15) vs. Warm-Seclusion (N=6)

TB: Fast (left) vs. Slow (right) Transitioning

Strengthen (N=6)

500mb Height

& Anom.

SST & Anom.

Strengthen (N=6)

TB: Post-ET Weakening (left) vs. Intensification (right)

Strengthen (N=6)

500mb Height

& Anom.

SST & Anom.

Strengthen (N=6)

Post-ET Weakeners (Solid) vs. Intensifiers (Dotted): T-Test: 75%, 90%, 95%, 99%

Two post-ET intensifiers:What determines whether the storm re-acquires warm-core structure?

Gustav (2002) Irene (1999)

TE: Post-ET Cold-core (left) vs. warm-seclusion (right)

Strengthen (N=6)

320K PV

320K PV

Post-ET Cold-core (Solid) vs. Warm-seclusion (Dotted): T-Test: 75%, 90%, 95%, 99%

Strengthen (N=6)

Post-ET Cold-core vs. Warm-Seclusion: Statistics

# Cases Latitude

at TB

Longitude at TB

Best-Track Intensity at

TB (hPa)

Mean radius of gale force winds at TB(km)

Post-ET Warm-

seclusion

6 32.5 -72.7 982.0 259.8

Post-ET Cold-core

15 36.2 -57.9 986.5 171.1

T-test: 90% T-test: 95%

Eliassen-Palm (EP) Flux and its Divergence

Outward eddy momen-tum flux

Outward eddy heat flux

Analyses performed by Clark Evans

EP- Flux

All Strengthening Weakening Cold-Core Warm-Seclusion

Outward eddy momen-tum flux

Outward eddy heat flux

Summary• A well-defined 34-member ensemble mean trajectory through

cyclone phase space is calculated for ET in the North Atlantic.

• TC diabatic PV destruction aloft leads to a lifting of the mid-latitude tropopause, erosion and narrowing of the approaching trough

• TC advects an environment into the trough that has static stability 10-20% lower than prior to TC arrival, enhancing Eady growth rate.

• Variability from this mean trajectory is small in the tropical phase, and then increases dramatically once extratropical transition has completed:– cold-core intensifying/decay, warm-seclusion, merger, tropical.

Summary• Post-ET Weakening:

– Positively tilted UL trough, SSTs near normal and below 26.5C on average

• Post-ET Strengthening:– Negatively tilted UL trough, SSTs above normal and above 26.5C on average

• Post-ET Cold-core evolution:– Broad UL trough, considerably smaller than average TC size

• Post-ET Warm-seclusion evolution:– Narrowing UL trough, considerably larger than average TC size, scale

matching (Molinari et al. 1995; Hanley et al. 2001).

Summary• Introduction of trough into TC leads to eddy PV forcing. The adiabatic

secondary circulation that results (Molinari et al. 1995) attempts to restore thermal wind balance.

• The momentum component of the EP forcing far precedes the thermal flux component.

• Thus, it appears that the development of frontal structure (increase in ‘B’) within the TC during ET may be initially a consequence of the TC’s adiabatic response to the eddy momentum forcing

• This eddy forcing is over a deeper layer and at lower isentropic level than with Elena-type rapid intensification.

Summary• Post-ET intensifiers have a marked increase in the magnitude of eddy PV flux

compared to weakeners

• Post-ET warm seclusion is associated with a narrower depth of cyclonic eddy PV flux at a level comparable to Molinari et al. (1995)

• Speculation: Whether a trough interaction with a TC leads to RI or ET and warm-seclusion is a matter of timing of the interaction during the cyc. phase trajectory

• With an average track error of 300-500km at 3-5 days, and the subtle sensitivities just shown, it is evident why the long-wave pattern decreases markedly during ET