P.L. Vidale*, M. RobertsK. Hodges, ESSC

A. Clayton, M.-E. Demory, J. Donners

with big thanks to:S. Emori, A. Hasegawa (NIES)

T. Davies and many others at UKMO L. Bengtsson (ESSC)

L. Shaffrey, I. Stevens, W. Norton, J. Slingo (UK-HiGEM)

*NCAS-Climate, Walker Institute, University of Reading, UK

Tropical cyclones in global climate models: the role of resolution

Evolution of N. Atlantic hurricane frequency in past 100+ years.2005 was a true record year: 15 hurricanes (incl. Katrina), 27 named storms … and some of the most intense storms in US history. Katrina damage = 200 bn U$. In the same region, in the last 2 weeks, Dean (cat 5), Felix (cat 5)

Yet, most GCMs, especially the coarse resolution ones used for long (e.g. IPCC) integrations,

cannot represent tropical cyclones properly

K. Trenberth

Our 3-D TC tracking algorithm,using 6-hourly, multi-level data.

1. Compute 850hPa vorticity

– truncate to T42

– identify and track, using (weak) vorticity threshold

2. Compute multi-level vorticity at T63

3. Reference tracks back onto T63 grid, at all vertical levels

4. Further identify/filter TCs using:

1. intensity at 850hPa @ T63

2. lifetime > 2 days

3. vertical gradient of vorticity

4. TC centre must be present at all levels

5. Reference full-resolution winds, precip onto tracks

6. Build storm composite climatologies

7. Finally, find Extra-tropical Transition by core reversal (to cold core)

TCs are warm-core storms

While warm core storm,vorticity decreases with height

Tropical Cyclones in decadal (coarse) climate simulations

Are the modelled TCs really warm core storms ?Is the near-core circulation correct; do we see changes with resolution ?

Composite of 100 most intense TCs in 25 yrs

MIROCHurricane Mitch

Had-Hi-NUGAM

135km 60kmresolution T106 T213resolution

Statistics of Tropical Cyclones in AMIP2 integrations: role of resolution

Once we can trust that we are targeting the right phenomena, we want to ask:can we realistically compute where they originate, where they reach maximum intensity and where they eventually die ? Do models agree on location ?

Statistics of Tropical Cyclones in AMIP2 integrations: role of resolution

Once we can trust that we are targeting the right phenomena, we want to ask:can we realistically compute where they originate, where they reach maximum intensity and where they eventually die ? Do models agree on location ?

Statistics of Tropical Cyclones in AMIP2 integrations: role of resolution

Once we can trust that we are targeting the right phenomena, we want to ask:can we realistically compute where they originate, where they reach maximum intensity and where they eventually die ? Do models agree on location ?

Statistics of Tropical Cyclones in AMIP2 integrations

But model formulation matters !

Impact of resolution on TC Intensity

135km

90km60km

MIROC

T106

T213

T106

T213

T106

T213

135km

90km60km

135km

90km60km

Had-Hi-NUGAM

P

W

I

ECHAM5

So, despite inter-model differences, our main result is quite robust: a larger number of the more intense Tropical Cyclones are simulated as the model resolution is increased.

Tropical cyclones intensity in a warmer world:

NUGAM and ECHAM5, 20th century and 21s century

L. Bengtsson, K. Hodges, ESSC, Reading

135km

90km60km 60km+4K

135km

90km60km 60km+4K

135km

90km60km 60km+4K

ECHAM_T213 ECHAM_T319

← Changes in wind speed

← Changes in precipitation

Key questions for our climate models:example from high-impact weather in climate models

• General aim in the community: regionalisation of impacts and their prediction under climate change

• With a 3-model intercomparison, no general consensus on Tropical Cyclones, but some issues for studying small scale processes in climate:– do any of our GCMs converge ? – how much resolution is enough for treating each problem ?– is there any verification data, e.g. from satellites ?– can we learn from the process-resolving models ?

• With this knowledge, to which degree can we use partially resolved phenomena as proxies for what happens in nature and use top supercomputers to study, e.g. for tropical cyclones:– globally, in decadal and centennial simulations– with large ensemble simulations (significant sample size) →

meaningfully study the local impact of extreme events, e.g. probability of landfall in Miami or Tokyo ?

– in transient and stabilised climate change simulations ?Hurricane Felix, cat. 5, 2 Sept 2007

Next: a multi-scale GCM TC intercomparison is needed;an opportunity to derive proxies for studying TCs in low-resolution GCMs

Model resolution /

type

1-5 km

Case studies

10-20 km

Climate runs

Case studies

40-60 km

Climate runs

Ensemble runs

100-300 km

IPCC-type climate runs

Participants MSSG-A (MSRG)

NICAM

(CCSR/FCRGC)

UK-CASCADE

AFES/CFES (AOSG)MRIIFS T799(ECMWF)

MIROC T213NUGAMECHAM5 T319IFS T319(ECMWF)

MIROC T63HadGAMECHAM5 T63IFS T159(ECMWF)

Benefit to other models

Realistic processes: resolved convection

Resolved flow Partially resolved storms

Sample size

Variability

Multiple scenarios

Observations TRMM, CALYPSO

Obs. case studies

Best Track

ECMWF analysis

ERA-interim ERA40/JRA25

IPCC

Impacts

WillisRe

Wind gusts, radius of max wind, storm surge, forward TC speed, max. precip.

Winds, intensity, fwd. TC speed, storm surge

Precipitation, landfall stats.

Exchange of 6-hourly: U,V,W,T, at multiple vertical levels; sfc. Prec.; MSLPTo upload data to UJCC server in Yokohama: p.l.vidale@reading.ac.uk

Where next ?

Analysis of coupled simulations: are the storms weaker in the coupled runs ?

Stratification into Niño/Niña years

Regional analyses

Ocean-Atmosphere interactions

Cold wake from Katrina and Rita in Gulf of Mexico

SST in Gulf

NASA

Summary and future work

• Coupled ultra-high resolution model, to study interactions (e.g. tropical cyclones on ocean mixing, ENSO interactions) and extremes;

• Used AMIP2 simulations to perform 3-model intercomparison of TC characteristics and their resolution dependence;

• Resolution seems to mostly affect storm intensity;

• Model formulation seems highly relevant, especially for geographical distribution;

• There is value in resolution, but … it is still unclear where the models at various resolution converge on simulating key processes: value of model intercomparison;

In 2007 In 2008UJCC-HiGEM:• Tropical Cyclones intercomparison including

CRMs;• Explore role of high-resolution SSTs, used

to force NUGAM model;• Extend coupled climate integrations at ultra-

high resolution;

Collaborative:• CCSR/FRCGC/NIES/ESC/Univ. of Tokyo:

tropical cyclones• WillisRe: impacts of tropical cyclones• Analysis of climate change simulations (with

Reading North-Atlantic group);• Weather and climate variability, with HiGEM

and ESSC;• Extremes, with HC, Oxford, Edinburgh