P'L' Vidale, M' Roberts

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, canno
t represent tropical cyclones properly
K. Trenberth
3
Our 3-D TC tracking algorithm,using 6-hourly,
multi-level data.
Tropical Cyclones in decadal (coarse) climate
simulations

• 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 _at_ T63
• lifetime gt 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)

• TCs are warm-core storms
• While warm core storm,
• vorticity decreases with height

4
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
Had-Hi-NUGAM
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
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
Tropical cyclones intensity in a warmer
world NUGAM and ECHAM5, 20th century and 21s
century
11
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
12
Key questions for our climate modelsexample
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 ?

13
Next a multi-scale GCM TC intercomparison is
neededan 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_at_reading.ac.
uk
14
Where next ?

• Analysis of coupled simulations are the storms
  weaker in the coupled runs ?
• Stratification into Niño/Niña years
• Regional analyses
• Ocean-Atmosphere interactions

15
Cold wake from Katrina and Rita in Gulf of Mexico
SST in Gulf
NASA
16
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

17
What is the role of resolution in climate
research ?

• Matrix of coupled models, with increasing
  resolution
• What is the impact of resolving eddies in the
  ocean ?
• What is the impact of resolving weather ?
• What are the crucial scales for proper coupling ?
• 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

?x 135 km
?x 90 km
?x 60 km
Flux coupler
Completed in 2007
1o - 1/3o ocean model
1/3o ocean model