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Reanalysis in WCRP

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Kevin Trenberth Chair WCRP Observation and
Assimilation Panel WCRP WMO/IOC/ICSU
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• WOAP
• Reanalyses
• US workshop (NASA) on data for reanalyses Sept
  05
• ECMWF workshop atmospheric reanalyses June 06
• ECMWF workshop on ocean data assimilation Aug 06
• JMA, Third Intl Reanalysis Conference Jan 08
• (260 participants, 61 oral, 73 poster
  presentations)
• Report on Reanalysis issues for Eos (Simmons,
  Trenberth and Uppala 2007) -action from
  WOAP-II
• Report on the Japan conference and reanalysis
  issues published in Eos Problems and Prospects
  for Reanalysis (Trenberth, Koike, and Onogi,
  2008)
• Based on
• Statement on Reanalysis from the 3rd WCRP Intl
  Meeting.

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WOAP Reanalyses Letter formally sent to lead
countries and agencies involved in reanalysis
seeking their support for further activities in
reanalysis and pointing out the prospects
conference statement attached. Ccd to program
managers. WG set up jointly by WCRP and GCOS
(WOAP and AOPC) on development of improved
observational data sets for reanalyses. Chair
Russ Vose.
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AR4 IPCC assessment Chapter 3 is an indictment
of many difficulties with continuity of the
climate record, and a testament to the heroic
struggles of those who nonetheless make sense out
of the data. We can surely do better. Chief
outstanding issue in reanalysis

• The underlying data base is not constant, and
  changes disrupt the climate record.
• No baseline reference network to anchor the data
• Radiosondes improve and change type over time
• Satellites mainly after 1979, last order 5 years,
  drift in orbit, change instruments, calibration
• Bias corrections are applied but remain imperfect
• Continuity is a key issue, especially for climate
  change
• Further technological development, change and
  improvement is expected.
• Major challenge is to deal with changing
  observations

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• Some Goals
• 1. Improve and develop input data for climate
  analyses
• For monitoring low frequency variability, we
  need long-term stable homogeneous climate data
  records (of known quality)
• New WOAP/AOPC working group to do this Chair
  Russ Vose

• 2. Improve global estimates of interannual to
  decadal variability and their uncertainty.
  Improve the consistency of the record in the face
  of major changes to the observing system. One
  or more reanalysis must be targeted at the goal
  of producing the most consistent time series
  (rather than the best analysis at any time).
• Retain fairly consistent observation sets (e.g.,
  by excluding special obs, short field campaigns)
• Use OSEs to determine effects of changes
  remaining in observing system
• Focus on bias-corrected observations
• Carry model bias established with recent obs back
  in time
• Suggested time period post 1979.

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• Some other issues
• Coordination of reanalyses proliferation
• Are lessons learned from previous one?
• Is data set development systematic?
• Is there an adequate user base to examine
  results?
• Are resources (computer, people) being used
  wisely?
• Risk of lack of support if not done right?

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• Some other issues
• Nature of archive native model grid data (huge
  volume)
• Much improved pressure level grid (42 levels)
• 25 or 50 mb vertical resolution in troposphere
• 1025, 1000, 975, 950, 925, 900, 875, 850, 825,
  800, 775, 750, 725, 700, 650, 600, 550, 500, 450,
  400, 350, 300, 275, 250, 225, 200, 175, 150, 125,
  100,
• Improved horizontal resolution 1.25? Such is
  planned for MERRA doubles resolution but is
  compatible with 2.5? grid)

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Some general issues regarding future reanalyses

• Use NWP or reanalysis-specific data assimilation
  systems?
• Human resource issue
• NWP community as well as climate community is a
  user of reanalysis
• How best do we meet requirement for estimates of
  uncertainty?
• Will free-of-additional-charge estimates from the
  assimilation system and from comparison of
  different reanalyses be sufficient?
• Desirability to coordinate (stagger) production
  of different centres
• Requires long-term funding of reanalysis
  activities to formalise
• Some things seem to be working do we need to
  fix them?
• There are requirements for data at highest
  resolution on native grids at high frequency
• Will funder/user pressure open up data archives?
• Are there technical/financial issues in meeting
  the diverse needs of users for data services?

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What kind of reanalysis data do we need for
tropical monsoon processes ?

• High spatial-resolution to solve orographic
  effects
• High time-resolution to solve diurnal cycles
• More accurate divergence fields to assess
  tropical/monsoon disturbances
• More accurate atmospheric moisture fields to
  estimate accurate energy water cycling
  processes and atmospheric heating field
• ? downscaling-approach (global and regional)
• ? high-resolution global model for RA
• ? more accurate moisture precipitation field
  from new satellite series (e.g., GPM)
• ? land ocean surface assimilations

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Issues for ocean reanalyses

• Preparation of next IPCC assessment
• Uncertainties of input and output
• Undersampling of the ocean
• QC of historical climate data record
• Truly global models
• Inclusion of IPY data and sustaining the new
  observing capabilities there
• Initialization of coupled models using existing
  syntheses
• Preparation of ocean data assimilation into
  coupled models for decadal predictions
• Requires Data Reanalysis and expansion of proxies
• Requires exploitation of new measurements

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Reanalysis We have gained a lot
The use of a stable data assimilation system has
produced fairly reliable records for monitoring,
research and improved prediction. But there is a
promise of so much more that is possible. That
is the key message for funding agencies.