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New CDAT v4.1 enables OPeNDAP interface to the
IPCC ESG
climate model simulations data holdings. The OPeNDAP server
enables to authenticate the user access, list files and subdirectories,
display metadata information, allow user to browse
and download the chosen data directly into the VCDAT for
analysis.
OPeNDAP is a software framework that allows
access to remote scientific datasets. The core of OPeNDAP is a
specification of an http-based protocol that describes how clients and
servers should communicate data over the network. There are a variety
of clients and servers available that understand DAP; an adaptation of
the PyDAP server is being used for the IPCC AR4
model output database.
The server provides access to IPCC
datasets. A dataset is an aggregation of a set of related
data files into a single virtual file. In general, a dataset consists
of all data variables for a given combination of model, scenario,
experimental run, temporal frequency, and submodel
(ocean or atmosphere). A dataset is represented by a CDMS
XML file.
Be aware that there is a definite
overhead to accessing data through the OPeNDAP server. Network speed,
HTTP protocol, and server delays combine to limit access speed in
comparison to direct disk reads.
You can
access the IPCC AR4 data in a variety of ways:
- from
VCDAT
- from
CDMS
- through
the web browser
To
access the IPCC data from VCDAT click on the 'Portal Internet Manager'
Button as shown in the image below (click on the image to see the
bigger
picture)
This
will open a new window with the data
holdings directory. For your convenience the IPCC AR4 data portal
catalog has been selected. In the future, more portal catalogs will be
added.
Click
on the 'PyDAP
server' tab (+)
to open it
When you further click on the choosen model
run, first you will be presented with the password authentication
window, and after entering the username and password to the IPCC AR4
data holdings, you will see the metadata for the choosen dataset appear
in
the right window.
Click on the 'Open File' or 'Open File and Exit'
buttons at the bottom of the window to import the data into the VCDAT.
From there on, you can select the variable in the VCDAT in the usual
way and plot it.
You
can access the IPCC data from CDMS module, provided that
CDAT
has been compiled with the opendap client libraries. The form of the
open call is:
f=cdms.open('http://
username:password@climate.llnl.gov/dap/ipcc4/
<scenario>/<model>/<dataset>.xml')
For example:
f=cdms.open('http://username:password@climate.llnl.gov/
dap/ipcc4/20c3m/ncar_ccsm3_0/
pcmdi.ipcc4.ncar_ccsm3_0.20c3m.run6.atm.mo.xml')
Once opened, an OPeNDAP dataset can be
accessed in the normal CDMS fashion. However, since some of the
datasets are very large - 100s of GB - there is a limit on the amount
of data that can be accessed through a single operation. The limit is
currently set to 64MB, but may change in the future. There is no limit
on the number of data accesses that can be made in a single session.
To
access the IPCC AR4 data from the web browser see the PyDAP
server description on the PCMDI software webpage
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CDAT Newsletter, June 2007
