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Tropical cyclones in global climate models: the role of resolution. P.L. Vidale *, M. Roberts K. 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) - PowerPoint PPT Presentation
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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: [email protected]
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