Tropical cyclones in global climate models: the role of resolution

1. 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) L. Shaffrey, I. Stevens, W. Norton, J. Slingo (UK-HiGEM) *NCAS-Climate, Walker Institute, University of Reading, UK

2. 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

3. Tropical Cyclones in decadal (coarse) climate simulations Our 3-D TC tracking algorithm,using 6-hourly, multi-level data. TCs are warm-core storms While warm core storm, vorticity decreases with height • Compute 850hPa vorticity • truncate to T42 • identify and track, using (weak) vorticity threshold • Compute multi-level vorticity at T63 • Reference tracks back onto T63 grid, at all vertical levels • Further identify/filter TCs using: • intensity at 850hPa @ T63 • lifetime > 2 days • vertical gradient of vorticity • TC centre must be present at all levels • Reference full-resolution winds, precip onto tracks • Build storm composite climatologies • Finally, find Extra-tropical Transition by core reversal (to cold core)

4. T106 resolution T213 135km resolution 60km 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 Had-Hi-NUGAM MIROC Hurricane Mitch

5. 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

6. 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

7. 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

8. But model formulation matters ! Statistics of Tropical Cyclones in AMIP2 integrations

9. Impact of resolution on TC Intensity 135km 135km 135km T106 T106 T106 90km 90km 90km T213 T213 T213 60km 60km 60km MIROC Had-Hi-NUGAM I P 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. ECHAM5 W

10. L. Bengtsson, K. Hodges, ESSC, Reading ECHAM_T213 ECHAM_T319 135km 135km 135km 90km 90km 90km 60km 60km 60km 60km+4K 60km+4K 60km+4K ← Changes in precipitation ← Changes in wind speed Tropical cyclones intensity in a warmer world: NUGAM and ECHAM5, 20th century and 21s century

11. Hurricane Felix, cat. 5, 2 Sept 2007 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 ?

12. Next: a multi-scale GCM TC intercomparison is needed;an opportunity to derive proxies for studying TCs in low-resolution GCMs Exchange of 6-hourly: U,V,W,T, at multiple vertical levels; sfc. Prec.; MSLP To upload data to UJCC server in Yokohama: p.l.vidale@reading.ac.uk

13. Where next ? Analysis of coupled simulations: are the storms weaker in the coupled runs ? Stratification into Niño/Niña years Regional analyses Ocean-Atmosphere interactions

14. Cold wake from Katrina and Rita in Gulf of Mexico SST in Gulf NASA

15. Summary and future work In 2008 In 2007 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 • 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;