WMO/CAS/WWW 6 th INTERNATIONAL WORKSHOP ON TROPICAL CYCLONES Topic 2.4: Observing and Forecasting Extratropical Transition (ET) Rapporteur Jenni Evans.

WMO/CAS/WWW 6WMO/CAS/WWW 6thth INTERNATIONAL WORKSHOP ON TROPICAL CYCLONES INTERNATIONAL WORKSHOP ON TROPICAL CYCLONES

Topic 2.4: Observing and Topic 2.4: Observing and Forecasting Extratropical Forecasting Extratropical

Transition (ET)Transition (ET)RapporteurRapporteur Jenni Evans Jenni Evans

The Pennsylvania State UniversityThe Pennsylvania State University

Working GroupWorking Group Sim Aberson, Jack Sim Aberson, Jack Beven, Andrew Burton, Roger Edwards, Beven, Andrew Burton, Roger Edwards, Chris Fogarty, Bart Hagemeyer, Bob Hart, Chris Fogarty, Bart Hagemeyer, Bob Hart, Naoko Kitibatake, Ron McTaggart-Cowan, Naoko Kitibatake, Ron McTaggart-Cowan, David Roth, Joe Sienkiewicz, Scott Spratt, David Roth, Joe Sienkiewicz, Scott Spratt, Chris Velden Chris Velden


1. From IWTC-V (Cairns)1. From IWTC-V (Cairns)

Research program focusing on ET is needed toResearch program focusing on ET is needed to(i) improve analyses and prediction of the (i) improve analyses and prediction of the

structure changes, significant weather, and structure changes, significant weather, and ocean impacts associated with ETocean impacts associated with ET

(ii) address uncertainties in numerical (ii) address uncertainties in numerical predictions of ET in the region of the stormpredictions of ET in the region of the storm

(iii) understand the far-field impacts of ET(iii) understand the far-field impacts of ET(iv) coordinate with existing programs to obtain (iv) coordinate with existing programs to obtain

detailed observations of the evolutionary detailed observations of the evolutionary structure of extratropical transitionstructure of extratropical transition


2. Towards An Operational 2. Towards An Operational Definition of ETDefinition of ET

• Lack of an ET definition understood by Lack of an ET definition understood by the general public can result in the general public can result in tragedy when tropical cyclone tragedy when tropical cyclone warnings are discontinued (due to ET) warnings are discontinued (due to ET) but the storm makes landfallbut the storm makes landfall

• One solution: CHC labels ET events as One solution: CHC labels ET events as “post-tropical” and continue to use the “post-tropical” and continue to use the tropical storm name tropical storm name

– – e.g. “Post-Tropical Cyclone e.g. “Post-Tropical Cyclone Floyd”Floyd”


2. Towards an Operational 2. Towards an Operational Definition of ETDefinition of ET

• No uniform approach has been agreed No uniform approach has been agreed among all affected nationsamong all affected nations

• Development of consistent descriptors Development of consistent descriptors and definitions of the stages of ET will and definitions of the stages of ET will help progress in operational help progress in operational forecasting techniquesforecasting techniques

• Need additional observations to Need additional observations to develop physically-based ET develop physically-based ET conceptual modelsconceptual models


An effective ET definition An effective ET definition shouldshould

• Be precise enough to satisfy the Be precise enough to satisfy the needs of the operational and needs of the operational and research communitiesresearch communities

• Be accessible to the general publicBe accessible to the general public


3. Recent observational 3. Recent observational studies studies

• Hurricane Michael (2000)Hurricane Michael (2000)– First field project focused solely on ET– Canadian National Research Council Convair 580

aircraft

• Hurricane Ophelia (2005)Hurricane Ophelia (2005)– Joint observational missions by U.S. (NOAA/HRD) and

Canadian Meteorological Center – Storm undergoing ET between North Carolina, US

and Nova Scotia, Canada– 16 September: NOAA P-3 aircraft, U. S. Air Force

C‑130‑J aircraft, and Aerosonde– 17 September: NOAA P-3 aircraft only


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-85 -80 -75 -70 -65 -60 -55 -50 -45 -40 -35 -30 -25 -20 -15 -10 -5 0

Hurricane Ophelia6 - 17 September 2005

Hurricane

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00 UTC Pos/Date

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PPP Min. press (mb)

1098 7

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976 mb

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-80 -75

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1413

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HWIND Aerosonde Plots/Ophelia 16 Aug, 2005

969696

Satellite image at time of second closest approach of aerosonde to wind center and just after WP-3D SFMR penetration across the eye.

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96

9696

NOAA WP-3D SFMR Surface winds in light blue, aerosonde winds in black,buoy winds in dark blue.Aersonde closest approach to wind center was 30 nmsouthwest and 25 nm northeast. Peak winds at2500 ft were 65 kt southeast of center and 75 kt north of center. Excellent agreementwas found between buoy, SFMR and aerosonde windsadjusted to surface values.SFMR winds SW of center were within 10 min of aerosonde.

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AFRC WC-130J 850 MB winds in red, aerosonde winds in black,buoy winds in dark blue.peak 850 mb winds of 55 kt were 35 nm SE and NE of center,associated with major outerband. Aerosonde flew downwind along major band toeast and across major band to north.


4a. Current Methods for 4a. Current Methods for Analysing ETAnalysing ET

• Satellite imagery/analyses, cool SST, Satellite imagery/analyses, cool SST, weakening (at least initially)weakening (at least initially)

• Cyclone Phase Space (CPS; Hart 2003; Cyclone Phase Space (CPS; Hart 2003; Evans and Hart 2003)Evans and Hart 2003) diagnostic of 3D diagnostic of 3D storm structure from operational model storm structure from operational model analyses and forecastsanalyses and forecasts

• Combine with conceptual models – e.g. Combine with conceptual models – e.g. require significant “transformation” to require significant “transformation” to cold-core, not just poleward track and cold-core, not just poleward track and “capture” in westerlies“capture” in westerlies


Wilma (2005)

13h prior to ET40°N, 65°W970hPa, 90kt


Cyclone Phase Space Cyclone Phase Space (CPS)(CPS)


Cyclone Cyclone Phase Phase Space Space (CPS)(CPS)


Cyclone Phase Space Cyclone Phase Space (CPS)(CPS)AdvantagesAdvantages

• Highlights the structural changes of the Highlights the structural changes of the vortexvortex

• Straightforward approach to intercomparison Straightforward approach to intercomparison of numerical model forecasts of storm of numerical model forecasts of storm evolutionevolution

• Accessibility of the CPS has led to its use in Accessibility of the CPS has led to its use in operational centres in the U.S., Canada, and operational centres in the U.S., Canada, and AustraliaAustralia

DisadvantagesDisadvantages• Current dependence on model analyses Current dependence on model analyses

argues against its use in isolation argues against its use in isolation


4b. Current Methods for 4b. Current Methods for Forecasting the Timing of Forecasting the Timing of

ETET• As with ET analysis, many As with ET analysis, many

operational centers indicate that operational centers indicate that they combine the CPS with satellite they combine the CPS with satellite guidance and conceptual models of guidance and conceptual models of ET ET

• Blending of these fairly independent Blending of these fairly independent forms of guidance to create a forms of guidance to create a forecast remains difficult and forecast remains difficult and subjectivesubjective


Forecast Diagnostics of Forecast Diagnostics of Potential Interest to the Potential Interest to the

Forecast CommunityForecast Community• Folks @ SUNYA are generating realtime Folks @ SUNYA are generating realtime diagnostic displays (archived for ~3 diagnostic displays (archived for ~3 months) months) http://www.atmos.albany.edu/facstaff/rmctc/http://www.atmos.albany.edu/facstaff/rmctc/DTmaps/animSelect.phpDTmaps/animSelect.php

• Diagnostics relevant for ET includeDiagnostics relevant for ET include– Streamfunction (midlatitude features)– Velocity potential (regions of large-scale ascent)– Quasi-geostrophic (QG) analyses


5. Current Methods for 5. Current Methods for Forecasting ET ImpactsForecasting ET Impacts


Additional forecasting Additional forecasting challenges associated with ETchallenges associated with ET

I.I. Storm structural changes during and Storm structural changes during and after ETafter ET

a. Potential for reintensification as an extratropical system is of primary concern

b. Modify the surface circulation• Redistributing damaging winds over land• Driving ocean surface wave evolution

c. Coincident modification in the storm precipitation distribution

II.II. Possibility of tornadoes during the Possibility of tornadoes during the tropical phase. tropical phase.


Foley and Foley and Hanstrum Hanstrum

(1994)(1994)


I. Storm structural changes I. Storm structural changes during and after ETduring and after ET

• No objective product(s) yet exists for No objective product(s) yet exists for specific guidance on the wind field specific guidance on the wind field redistribution and reintensification during ETredistribution and reintensification during ET

• Operational NWP may be unreliable Operational NWP may be unreliable forecastingforecasting

cyclone structure through ETcyclone structure through ET– wind field– precipitation distribution

downstream cyclone developmentdownstream cyclone developmentpost-ET intensitypost-ET intensitytracktrack



• Use past events to develop operational Use past events to develop operational guidelines for storm changes during ETguidelines for storm changes during ET

• Peak winds generally displaced to the Peak winds generally displaced to the equatorward side (right NH; left SH) of the equatorward side (right NH; left SH) of the storm track storm track

• Surface pressure field spreads near the Surface pressure field spreads near the storm center, resulting in storm size storm center, resulting in storm size increase and enhancement in outer wind increase and enhancement in outer wind speedsspeeds

• Rapid motion enhances the across-track Rapid motion enhances the across-track asymmetry in storm peak windsasymmetry in storm peak winds



• In the later stages of ET, a horseshoe In the later stages of ET, a horseshoe shaped wind maximum in the front half of shaped wind maximum in the front half of the storm domain may be observedthe storm domain may be observed

• Decoupling from the surface over the colder Decoupling from the surface over the colder marine boundary layer marine boundary layer maymay result in weaker result in weaker than expected surface winds; no method yet than expected surface winds; no method yet for discerning these casesfor discerning these cases

• Improved use of ocean surface winds, Improved use of ocean surface winds, particularly from the NASA QuikSCAT particularly from the NASA QuikSCAT satellite, has allowed better monitoring of satellite, has allowed better monitoring of the changes in the cyclone wind field as ET the changes in the cyclone wind field as ET occursoccurs


Evolution of storm size during Evolution of storm size during ETET

• Storm and environmental modulators of Storm and environmental modulators of cyclone surface wind structure differ for cyclone surface wind structure differ for warm season tropical and extratropical warm season tropical and extratropical cyclones.cyclones.

• Two datasets of significant wind speed radii Two datasets of significant wind speed radii in Atlantic storms are available: EBT and in Atlantic storms are available: EBT and H*WindH*Wind– Each is internally consistent– Large differences can exist between these

datasets on a case-by-case basis– Statistically significant differences in the overall

datasets exist– Validation of wind distribution forecast products

is problematic because of this


HWIND17

NH

C17

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EBT (x) vs. H*Wind (y)gale radius

(Moyer et al. 2006)

o >TS intensity

+ no QuikSCAT


Wave field evolutionWave field evolution• Transitioning cyclones can become very efficient Transitioning cyclones can become very efficient

ocean surface wave producersocean surface wave producers• Strong winds (equatorward of track) blowing Strong winds (equatorward of track) blowing

along track can create very high seas due to along track can create very high seas due to “trapped-fetch” resonant wave growth“trapped-fetch” resonant wave growth

• Constant storm movement direction (>18 hours) Constant storm movement direction (>18 hours) and fast motion (>10msand fast motion (>10ms-1-1) needed for trapped ) needed for trapped fetchfetch

• As the storm accelerates and the wind field As the storm accelerates and the wind field expands during ET, the wave maximum becomes expands during ET, the wave maximum becomes displaced farther to right (NH) of storm trackdisplaced farther to right (NH) of storm track

• Arrival of the wave maximum at a location Arrival of the wave maximum at a location typically lags storm passage by 2-3 hourstypically lags storm passage by 2-3 hours


Wave growth time increases Wave growth time increases with trapped fetch (waves stay with trapped fetch (waves stay

with storm)with storm)

Bowyer and Bowyer and MacAfee MacAfee (2005)(2005)


Bowyer and Bowyer and MacAfee MacAfee (2005)(2005)


Wave field evolutionWave field evolution

• An operational modeling tool to An operational modeling tool to compute dominant wave trajectories compute dominant wave trajectories and significant wave heights during ET and significant wave heights during ET has been implemented in Canadahas been implemented in Canada

• Wave forecasts from this model are Wave forecasts from this model are available <1 minute after the forecaster available <1 minute after the forecaster produces or changes the forecast trackproduces or changes the forecast track

• Equivalent techniques not yet available Equivalent techniques not yet available in all regions affected by ET events in all regions affected by ET events


Precipitation distribution Precipitation distribution changeschanges

• Heavy precipitation zone associated with Heavy precipitation zone associated with ET is usuallyET is usually– associated with the tropical cyclone outflow

extending poleward from the storm center– within a few hundred kilometers of center– concentrated along quasi-stationary frontal

zones well ahead of the storm– on the poleward side of the storm track

(“delta” rain region) – substantially less to the equatorward side due

to dry air wrapping around that side of the storm


Precipitation distribution Precipitation distribution changeschanges

• ET import of moist tropical air to ET import of moist tropical air to higher latitudes brings potential for higher latitudes brings potential for extreme precipitationextreme precipitation

• Can lead to flash floodsCan lead to flash floods• In the vicinity of mountainous In the vicinity of mountainous

regions, upslope flow and regions, upslope flow and precipitation enhancement can precipitation enhancement can exacerbate flooding potentialexacerbate flooding potential


SubjectiveSubjective approach to forecasting approach to forecasting precipitation distribution with ETprecipitation distribution with ET1.1. When ET is likely and/or fronts are relatively close to the When ET is likely and/or fronts are relatively close to the

storm center, the forecast region of heavy precipitation is storm center, the forecast region of heavy precipitation is shifted from right to left of trackshifted from right to left of track

2.2. QPF guidance on more refined spatial scale is derived from QPF guidance on more refined spatial scale is derived from the operational model track forecast most similar to the the operational model track forecast most similar to the official NHC track forecastofficial NHC track forecast

3.3. Regions of strongest flow are identified that areRegions of strongest flow are identified that are> 35 ktperpendicular to a frontal or coastal boundary or local terrain

Heavy precipitation is expected in these regions due to strong forced Heavy precipitation is expected in these regions due to strong forced ascentascent

4.4. Other favorable (Other favorable (not ET-specificnot ET-specific) indicators of local QPF ) indicators of local QPF maxima: around upper-level jet streaks, frontogenesis maxima: around upper-level jet streaks, frontogenesis zones and PV anomalieszones and PV anomalies

5.5. Reasonable spatial continuity in the heavy precipitation Reasonable spatial continuity in the heavy precipitation region ensured throughregion ensured throughconceptual modelscurrent precipitation structure (from radar and satellite)incorporation of diurnal cycle of precipitationuse of storm analogs


SubjectiveSubjective approach to forecasting approach to forecasting precipitation distribution with ETprecipitation distribution with ET6.6. Selection of an analog depends on satisfying a number of Selection of an analog depends on satisfying a number of

common criteriacommon criteria(i) size of the current rain shield(ii) vertical wind shear(iii) similar storm track with similar proximity to topography(iv) fronts in the vicinity of the storm(v) susceptibility of location to topographic rainfall enhancement

Storms such as Wilma (2005) and Alberto (2006) are Storms such as Wilma (2005) and Alberto (2006) are exceptions that remain to be explained.exceptions that remain to be explained.

7.7. QPF calibrationQPF calibrationpast events (15-25 year dataset) are used to calibrate likely

average or extreme QPFschecks are applied on the upper limits of QPF amounts (areal

average amounts in the QPF graphics and the text QPF statement to NHC)

example: soft cap of 2.5 inches per six-hour period is usually enforced for the 28-km forecast grid

Roth (2006, pers. Roth (2006, pers. comm.)comm.)


Tornadoes associated Tornadoes associated with ETwith ET• Studies of Florida tornadoes reveal an Studies of Florida tornadoes reveal an

increased likelihood of tornadoes in increased likelihood of tornadoes in hurricanes with hybrid characteristics hurricanes with hybrid characteristics interacting with the mid-latitudesinteracting with the mid-latitudes

• Compared with purely tropical storms, Compared with purely tropical storms, tornadoes associated with hybrid storm tornadoes associated with hybrid storm passage arepassage are– consistently the most dangerous– result in longer and more active tornado

outbreaks– rarest form of warm-season cyclone-related

Florida tornadoes


Cyclone Phase Space Cyclone Phase Space (CPS)(CPS)Hybrid storms in these Hybrid storms in these

studies werestudies were•interacting with a midlatitude trough to the northwest•accelerating to the north or northeast•at least potentially beginning ET•may provide a useful model for tornado forecasting in other ET events


Tornadoes associated Tornadoes associated with ETwith ET

• Most tornadoes occurred in the Most tornadoes occurred in the overrunning zone near or north of a strong overrunning zone near or north of a strong low-level jet (>17.5 mslow-level jet (>17.5 ms-1-1) intersecting a ) intersecting a surface warm frontsurface warm front

• This maximizes moisture convergence in a This maximizes moisture convergence in a highly sheared, moist environment with highly sheared, moist environment with diffluence aloftdiffluence aloft

• Tornadic phase likely also associated withTornadic phase likely also associated with– Approach of an upper short wave– An increase of the 850-500 hPa wind speeds– Dry intrusion at mid-levels may be a factor


Tornadoes associated Tornadoes associated with ETwith ET

• Significant tornadoes in tropical cyclones Significant tornadoes in tropical cyclones with hybrid characteristics most likely inwith hybrid characteristics most likely in– Fast-moving cyclones– Outer rainbands – June and October (October peak ET month)

• Heavy rain phase begins before severe Heavy rain phase begins before severe weather phase and continues during, and weather phase and continues during, and well after, the tornado outbreak phasewell after, the tornado outbreak phase

• No link between TC and tornado intensityNo link between TC and tornado intensity


Climatological studies of Climatological studies of ETET

• Regional climatologies documenting Regional climatologies documenting wind, wave and precipitation wind, wave and precipitation distributions are needed fordistributions are needed for– conceptual model development– forecast validation studies

• Combine with synoptic composites Combine with synoptic composites and numerical studiesand numerical studies


Role of Recurving Storms Role of Recurving Storms (ET?) in Interannual (ET?) in Interannual

VariabilityVariability• Explore relationship between Explore relationship between

recurving tropical cyclonesrecurving tropical cyclones and the and the subsequent winter climatesubsequent winter climate– anomalous TC recurvature may lead to

anomalous snowcover at the start of winter

– does not return to normal by winter's peak

– thereby alters the albedo of the hemisphere and the radiative balance


7. Crucial needs for 7. Crucial needs for advancement in ET advancement in ET

forecastingforecasting• Forecasts of the timing of ETForecasts of the timing of ET• Analyses of storm intensity during ETAnalyses of storm intensity during ET• Forecasts Forecasts and verificationand verification of cyclone of cyclone

intensity during ETintensity during ET


7. Crucial needs for 7. Crucial needs for advancement in ET advancement in ET

forecastingforecasting• Size forecasts through ETSize forecasts through ET• Verifications of these size forecastsVerifications of these size forecasts• Precipitation distribution forecastsPrecipitation distribution forecasts• Forecasts of the likelihood of Forecasts of the likelihood of

tornadoes in early stages of ETtornadoes in early stages of ET

This list is very similar to that This list is very similar to that identified by Jones et al. (2003)!identified by Jones et al. (2003)!


8. Proposals for moving 8. Proposals for moving forwardforward


Improve understanding of the Improve understanding of the structural changes and structural changes and

impacts during ET to impacts during ET to improveimprove• conceptual modelsconceptual models• numerical modelsnumerical models• forecasting paradigms: enable forecasting paradigms: enable

forecasters to better anticipate forecasters to better anticipate changes, and give more detailed changes, and give more detailed warnings associated with ETwarnings associated with ET


Ongoing modeling Ongoing modeling improvements benefiting ET improvements benefiting ET

forecastingforecasting• Higher resolution global analyses assimilating an Higher resolution global analyses assimilating an

increasing number of observationsincreasing number of observations– improve understanding and aid refinement of conceptual

models of the impacts of ET on the larger-scale flow

• Evaluations of ensemble-based ET forecastsEvaluations of ensemble-based ET forecasts– map the range of model uncertainty during the ET

process

• Diagnostics (observational, simulation) of the Diagnostics (observational, simulation) of the extent and multi-scale nature of the interactions extent and multi-scale nature of the interactions between the remnant TC and its environmentbetween the remnant TC and its environment– promising avenues for forecast improvement through

both conceptual models and modeling advances


Focused field Focused field experimentsexperiments

• Field experiments of opportunityField experiments of opportunity– e.g. US Canada collaboration in Ophelia

• THORPEX Pacific Asian Regional THORPEX Pacific Asian Regional Campaign (T-PARC)Campaign (T-PARC)– in planning stage – presently slated for 2008


1. Improved availability of 1. Improved availability of forecast tools and analyses to forecast tools and analyses to forecasters across all affected forecasters across all affected regions, andregions, and2. Continued communication 2. Continued communication of recent results between of recent results between operational and research operational and research communitiescommunities

...remain vital to the ...remain vital to the advancement of ET advancement of ET forecastingforecasting

QuestionsQuestions


Possible role of ET in Possible role of ET in Interannual VariabilityInterannual Variability

WMO/CAS/WWW 6 th INTERNATIONAL WORKSHOP ON TROPICAL CYCLONES Topic 2.4: Observing and Forecasting Extratropical Transition (ET) Rapporteur Jenni Evans.

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