Ensemble Handlingin GrADS

Jennifer M. Adams Brian Doty

IGES/COLA

What is GrADS? • GrADS is an interactive tool that integrates

data access, analysis, and visualization• Handles many data formats:

binary, NetCDF, HDF, GRIB1&2, 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)

A GrADS Graphics Example

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

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

The New Ensemble Dimension in GrADS

• A 5th grid dimension for ensemble members‘set X, Y, Z, T, or E’ or ‘set lon, lat, lev, time, or ens’

• A virtual dimension for forecast time offset‘display temp(ft=2)’‘display temp(ftime=24hr)’

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

GrADS GRIB2 Descriptor File

Wesley’s g2ctl works very well, but doesn’t handle EDEF (yet). DSET /gens/prod/gefs.%iy4%im2%id2/%ih2/pgrb2a/ge%e.t%ih2z.pgrb2af%f2TDEF 17 linear 00z09oct2008 6hrEDEF 23avg 17 00z09oct2008 0spr 17 00z09oct2008 2c00 17 00z09oct2008 1,0p01 17 00z09oct2008 3,1p02 17 00z09oct2008 3,2p03 17 00z09oct2008 3,3p04 17 00z09oct2008 3,4. . .p19 17 00z09oct2008 3,19p20 17 00z09oct2008 3,20ENDEDEF@ ens String avg Unweighted mean of all members@ ens String spr Standard deviation with respect to ensemble mean@ ens String c00 Control forecast@ ens String p01 Positively perturbed forecast

The GRIB2 codes are octets 35 and 36 from Section 4 (PDT # 1, 2, 11, and 12)

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

• Data become more usable and accessible

• Subsets over all dimensions

• Server-side analysis

• File aggregation

• Format translation

• Ensemble metadata standards:

Ensemble Data Sets Behind GDS

Ensemble Forecast Time Series (Longitude, Latitude, and Level are fixed)

Forecast Time --->

Ensemble Forecast Grid (Longitude, Latitude, and Level are fixed)

Forecast Time --->

Ens

embl

e M

embe

r

Ten Ensemble Forecasts (Longitude, Latitude, and Level are fixed)

Forecast Time --->

Ens

embl

e M

embe

r

Time Axis ---->

Ens

embl

e M

embe

r

CFS Daily Hindcast (Longitude, Latitude, and Level are fixed)

Ensemble Forecast Time Series (Longitude, Latitude, and Level are fixed)

Forecast Time --->

Ensemble Mean = tloop(ave(Z,e=2,e=23))Ensemble Min/Max = tloop(min(Z,ens=c00,ens=p20)) +/- StdDev of Ensemble Mean = tloop(sqrt(ave(pow(Z-Zave,2),e=1,e=21)))

Forecast Time --->

+ 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- Int’l agreement requires password protection

TIGGE Data Behind GDS at NCAR

TIGGE Multi-Member Multi-Model Ensemble 500mb Geopotential Height valid August 30, 2008

7-day Lead

3-day Lead

5-day Lead

1-day Lead

TIGGE MME Forecast Error and Ensemble Spread 500mb Geopotential Height valid August 30, 2008

7-day Lead

3-day Lead

5-day Lead

1-day Lead

TIGGE Forecasts of Hurricane Ike valid: 12z 9 Sep - 00z 13 Sep in

it:

00z

8 S

epin

it: 1

2z 8

Sep

init

: 00z

9 S

epin

it: 1

2z 9

Sep