NARCCAP Data Overview

NARCCAP data is stored in CF-compliant NetCDF format. Filenames are of the form VariableName_ModelName_Time.nc.

• VariableName is the IPCC/CF-convention Variable Name as listed in the data tables
• ModelName is the 4-character RCM identifier
• Time is the starting time of the file, in the form YearMonthDayUTC.

So, for example, tas_RCM3_1981010103.nc contains data for variable tas from the RegCM3 regional model starting at 03 UTC on January 1st, 1981 and ending at 00 UTC on January 1st, 1985.

Useful Links

• model-info.html -- Information about the models.
• data-tables.html -- How the variables are grouped
• http://cf-pcmdi.llnl.gov/documents/cf-conventions -- The CF Metadata Standard
• http://cf-pcmdi.llnl.gov/documents/cf-standard-names -- The CF Standard Name Table

NetCDF

NetCDF is a data format for storing array-oriented scientific data, particularly geophysical data. It is highly portable, widely-understood, and has built-in metadata that makes the files self-describing.

The main data array has associated coodinate arrays, xc, yc, and time, for each of its dimensions. Time is an unlimited or record dimension (important for some tools). Each variable in a NetCDF file can have attributes associated with it.

NetCDF Header

pr_WRFP_1979090103.nc.head.txt -- produced by ncdump -h pr_WRFP_1979090103.nc

ncdump

Outputs contents of a netCDF file as ascii. From Unidata, part of the NetCDF libraries. http://www.unidata.ucar.edu/software/netcdf/index.html

Useful options:
ncdump -h -- prints headers
ncdump -v variable -- prints a variable
ncdump -f n1,n2 -- specify precision of output
ncdump -l length -- specify line length of output

Example output: pr-ncdump.csv -- produced using ncdump -p 5,5 -l 2000 pr_WRFP_2001010103_clim.nc

NCO

NCO is a package of command-line utilities for manipulating netCDF files. Each NCO command does one very specific kind of manipulation, but you can combine them to powerful effect.

There are a number of options common to most or all NCO commands. These include -v for specifying the variable and -d for choosing a dimensional slice. Some NCO commands make use of a "record" dimension. This needs to be the first dimension and is unlimited in extent. In NARCCAP data, this is always the time dimension.

• ncap and ncap2: netCDF Arithmetic Processors (examples) -- algebraic manipulation of data
• ncatted: netCDF Attribute Editor (examples) -- modify metadata
• ncbo: netCDF Binary Operator (includes ncadd, ncsubtract, ncmultiply, ncdivide) -- (examples) -- math involving two files
• ncea: netCDF Ensemble Averager (examples) -- average across multiple input files
• ncecat: netCDF Ensemble Concatenator (examples) -- combine files into a single record
• ncflint: netCDF File Interpolator (examples) -- combine inputs via weighted interpolation
• ncks: netCDF Kitchen Sink (examples) -- copies data to ascii or output file
• ncpdq: netCDF Permute Dimensions Quickly, Pack Data Quietly (examples) -- rearrange dimensions or pack data
• ncra: netCDF Record Averager (examples) -- average across time (record dimension)
• ncrcat: netCDF Record Concatenator (examples) -- combine sequential files
• ncrename: netCDF Renamer (examples) -- rename dimensions, variables, or attributes
• ncwa: netCDF Weighted Averager (examples) -- weighted average over one file

Example

seasonal-average -- Shellscript using NCO commands to generate seasonal climatology

NCL

NCL is an interpreted programming language for scientific analysis and visualization. It is designed with NetCDF in mind and allows easy and transparent access to NetCDF data files. It has a large and powerful plotting library oriented toward geophysical data.

NCL's syntax is FORTRAN-like. It supports the idea of coordinate variables and attributes associated with variables. Most parameters are passed to functions via "resource" attributes.

Examples

2d-time-plot.ncl Plot a timeslice from a file. Used in NARCCAP QC process. Example results.

Extensive reference and support materials are available on the NCL website: http://www.ncl.ucar.edu/

• http://www.ncl.ucar.edu/Applications/lcnative.shtml -- Plotting Examples
• http://www.ncl.ucar.edu/Applications/list_dataP.shtml -- Data Processing Examples
• http://www.ncl.ucar.edu/Training -- Materials from NCL training courses
• http://www.ncl.ucar.edu/Document/Language -- Language syntax reference
• http://www.ncl.ucar.edu/Document/Functions -- Documentation for NCL functions
• http://www.ncl.ucar.edu/Document/Graphics/Resources -- Documentation for NCL resources (options and parameters for graphics functions)

Other Tools

There are a number of other tools for working with netCDF data.

GIS

Importing NARCCAP Data into GIS

Downloading