Ensemble Handling in GrADS

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Ensemble Handlingin GrADS

• Jennifer M. Adams Brian DotyIGES/COLA

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What is GrADS?

• GrADS is an interactive tool that integrates
  data access, analysis, and visualization
• Handles many data formats binary, NetCDF, HDF,
  GRIB12, BUFR
• Two data models for gridded and in situ data
• Expression handling is flexible, compact,
  recursive
• Programmable interface for scripting
• Written in C code is open source (GPL)

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A GrADS Graphics Example
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What is the GrADS Data Server?

• GDS is a stable, secure, OPeNDAP data server
  that provides subsetting and server-side
  analysis services over the internet
• GDS can serve any GrADS-readable dataset, and
  unifies all data formats into a NetCDF framework
• Open a data set with http//servername/filename
  instead of /disk/filename

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News from GrADS/GDS Team

• GrADS has a 5th grid dimension for ensembles
• GrADS has a GRIB2 interface
• GDS can serve any GrADS data set
• GrADS is a client for all OPeNDAP data sets
• GrADS will support GIS-compatible outputs

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The New Ensemble Dimension in GrADS

• A 5th grid dimension for ensemble membersset X,
  Y, Z, T, or E or set lon, lat, lev, time, or
  ens
• A virtual dimension for forecast time
  offsetdisplay temp(ft2)display
  temp(ftime24hr)

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GrADS Metadata Requirements for Ensemble Members

• Unique name / number
• Initial time
• Length
• If GRIB2, some additional octet values
• One time axis spans all members
• All members must have common X, Y, Z axes

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GrADS GRIB2 Descriptor File

• Wesleys g2ctl works very well, but doesnt
  handle EDEF (yet).
• DSET /gens/prod/gefs.iy4im2id2/ih2/pgrb2a/gee
  .tih2z.pgrb2aff2
• TDEF 17 linear 00z09oct2008 6hr
• EDEF 23
• avg 17 00z09oct2008 0spr 17 00z09oct2008
  2
• c00 17 00z09oct2008 1,0
• p01 17 00z09oct2008 3,1
• p02 17 00z09oct2008 3,2
• p03 17 00z09oct2008 3,3
• p04 17 00z09oct2008 3,4
• . . .
• p19 17 00z09oct2008 3,19
• p20 17 00z09oct2008 3,20
• ENDEDEF
• _at_ ens String avg Unweighted mean of all members_at_
  ens String spr Standard deviation with respect to
  ensemble mean
• _at_ ens String c00 Control forecast_at_ ens String
  p01 Positively perturbed forecast
• The GRIB2 codes are octets 35 and 36 from Section
  4 (PDT 1, 2, 11, and 12)

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Examples of Ensemble Data Sets

• NCEP GFS Ensembles (GENS)
• NCEP Climate Forecast System (CFS)
• NCEP Short Range Ensemble Forecasts (SREF)
• ESRL MRF Reforecasting Experiment
• WCRP CMIP3 Multi-Model Data (IPCC AR4)
• TIGGE

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Ensemble Data Sets Behind GDS

• Data become more usable and accessible
• Subsets over all dimensions
• Server-side analysis
• File aggregation
• Format translation
• Ensemble metadata standards

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Ensemble Forecast Time Series (Longitude,
Latitude, and Level are fixed)
Forecast Time ---gt
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Ensemble Forecast Grid (Longitude, Latitude,
and Level are fixed)
Ensemble Member
Forecast Time ---gt
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Ten Ensemble Forecasts (Longitude, Latitude, and
Level are fixed)
Ensemble Member
Forecast Time ---gt
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CFS Daily Hindcast (Longitude, Latitude, and
Level are fixed)
Ensemble Member
Time Axis ----gt
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Ensemble Forecast Time Series (Longitude,
Latitude, and Level are fixed)
Forecast Time ---gt
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Ensemble Mean tloop(ave(Z,e2,e23))Ensemble
Min/Max tloop(min(Z,ensc00,ensp20)) /-
StdDev of Ensemble Mean tloop(sqrt(ave(pow(Z-Zav
e,2),e1,e21)))
Forecast Time ---gt
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TIGGE Data Behind GDS at NCAR

• Perfect testbed for ensemble handling and
  GRIB2 interface
• Boost to usage of TIGGE data
• Forecasts sorted by date and by provider
• Time series of analyses
• Nearly unbearable load on old hardware
• 48-hour data embargo
• Intl agreement requires password protection

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TIGGE Multi-Member Multi-Model Ensemble 500mb
Geopotential Height valid August 30, 2008
7-day Lead
5-day Lead
3-day Lead
1-day Lead
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TIGGE MME Forecast Error and Ensemble Spread
500mb Geopotential Height valid August 30, 2008
7-day Lead
5-day Lead
3-day Lead
1-day Lead
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TIGGE Forecasts of Hurricane Ike valid 12z 9
Sep - 00z 13 Sep
init 00z 8 Sep
init 12z 8 Sep
init 00z 9 Sep
init 12z 9 Sep