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) 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 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: – intensity at T63 – lifetime > 2 days – vertical gradient of vorticity – 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 MIROC Hurricane Mitch Had-Hi-NUGAM 135km60kmresolution T106T213resolution

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 90km 60km MIROC T106 T213 T106 T213 T106 T km 90km 60km 135km 90km 60km 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 90km 60km 60km+4K 135km 90km 60km 60km+4K 135km 90km 60km 60km+4K ECHAM_T213ECHAM_T319 ← Changes in wind speed ← Changes in precipitation

Are the modelled TCs really warm core storms ? Is the near-core circulation correct; do we see changes with resolution ? Had-Hi-NUGAM, MIROC, ECHAM: 100 most intense TCs in 25 yrs HadGAM-HiGAM-NUGAM tangential wind Hurricane Mitch 850hPa 500hPa 200hPa 850hPa 500hPa 200hPa 850hPa 500hPa 200hPa 135km60kmresolution

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 km Climate runs Case studies km Climate runs Ensemble runs km IPCC-type climate runs ParticipantsMSSG-A (MSRG) NICAM (CCSR/FCRGC) UK-CASCADE AFES/CFES (AOSG) MRI IFS T799 (ECMWF) MIROC T213 NUGAM ECHAM5 T319 IFS T319 (ECMWF) MIROC T63 HadGAM ECHAM5 T63 IFS T159 (ECMWF) Benefit to other models Realistic processes: resolved convection Resolved flowPartially resolved storms Sample size Variability Multiple scenarios ObservationsTRMM, CALYPSO Obs. case studies Best Track ECMWF analysis ERA-interimERA40/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.; MSLP To upload data to UJCC server in Yokohama:

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 2008 UJCC-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

What is the role of resolution in climate research ? 1 o - 1/3 o ocean model ∆x = 135 km∆x = 90 km ∆x = 60 km 1/3 o ocean model Flux coupler Completed in 2007 Matrix of coupled models, with increasing resolution: 1.What is the impact of resolving eddies in the ocean ? 2.What is the impact of resolving weather ? 3.What are the crucial scales for proper coupling ? 4.What are the emerging processes ? Vertical resolution is fixed: 38 atmospheric levels, 40 oceanic levels Hundreds of years of simulation have been completed on L-H experiments, including AMIP2