Resolution

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Transcript Resolution

Resolution and Athena –
some introductory comments
Tim Palmer
ECMWF and Oxford
General Comments
• Better resolution = better representation of
the laws of physics and of relevant
boundary forcings (eg topography)
• There are two dominant modes of
instability in the atmosphere: baroclinic
and convective. We should aspire to be
able to simulate them both. Shame they
occur on such disparate scales!
Goals
• 1. To get to the minimum scale that will still allow
parametrised convection. Benefits include:
– Better topography
– Better representation of nonlinear stages of
baroclinic instability (important for blocking)
• 2. To get to convectively resolved scales
Benefits include:
– MJO, ENSO, monsoons etc….?
Possible Nomenclature
• Low resolution (everything so far)
• Intermediate resolution (highest resolutions
that will still allow parametrised deep
convection)
• High resolution (resolved convection)
ECMWF
• Increasing resolution has been an important
element in ECMWF’s strategy for improving the
skill of medium range forecasts.
• By extrapolation, there should also be an
important element for monthly, seasonal and
decadal forecasts.
• Increasing resolution in seasonal forecast
systems is a key element of the strategic goals
of some of ECMWF’s “competitors”.
Oxford
• Impact on climate change signals
– Global mean temperature depends critically
on feedbacks with cloud. The better we can
represent cloud systems, the more confidence
we can have in the representation of these
cloud feedbacks
– Simulating regional climate change accurately
is also essential for guiding mitigation,
adaptation and geoengineering strategies
Impact of intermediate resolution
JJA Precip Climate Change.
From AR4
…increased probability of warm dry summers
over Europe. A robust signal from CMIP3 multimodel ensemble. Reliable?
Timeslice
estimates of JJA
precip climate
change using
MRI climate
model at three
resolutions
(Matsueda and
Palmer, in
progress)
Much weaker
drying signal at
high resolution
180km
(climate)
60km
20km
(NWP)
observed
Climate change,
blocking frequency
and model resolution
180km (climate)
20km (NWP)
Surface pressure
Potential Vorticity
Impact of high resolution
Velocity Potential Spectra
0.625
1.0
0.875
0.75
0.5
0.375
0.25
0.1
In preparation by Tompkins, Jung and Vitart
motivated by Tompkins and Jung (2003)
Large-scale vs Convective Precipitation (DJF)
4
LSP/CP
3
2
1
0
1.0
0.8
0.6
0.4
Scaling Factor
0.2
0.0
Conclusions
• The Athena Project is an amazing outcome of the
Summit (please tell Guy Brasseur)
• Resolution is a key issue for
monthly/seasonal/decadal and for the climate
change problem (given its widespread implications
for mitigation, adaptation and geoengineering
policy)
• Resolutions within which convection (a dominant
mode of instability) is parametrised, should be
considered “intermediate”, not “high”.