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The NCAR Command Language (NCL) is a programming language designed
specifically for the analysis and visualization of data. NCL can be run
in interactive mode, where each line is interpreted as it is entered at your
workstation, or it can be run in batch mode as an
interpreter of complete
scripts. The power
and utility of the language are evident in three areas:
Each of above areas is discussed below.
The file handling capabilities of NCL represent one of its greatest strengths.
In addition to being able to read and write the usual
binary
and ASCII files,
NCL can import and export the
self-describing and
network-transparent
data formats
netCDF and
HDF.
NCL can also import the proprietary data formats
GRIB
and CCM History Tape.
A single NCL function is used
to import any of the "supported
data formats" (netCDF,
HDF,
GRIB, or
CCM History Tape).
This same function
can also be used to create HDF and netCDF files.
NCL does not create GRIB files, since GRIB files require
identifiers for the meteorological center where they were
written and for the model used to generate the data.
Nor does NCL create CCM History Tape files, since
this format will be replaced by netCDF in the future.
File types are identified by appropriate
file name suffixes.
The legal suffixes are: .nc [with .cdf allowed as an alternate], .hdf, .grb,
and .ccm.
Details
on the supported formats are
provided in the NCL
Reference Manual.
Once a dataset with a supported format is imported, all of its contents
are immediately available to NCL.
If you are not familiar with such a data format, see
"The
netCDF User Guide."
Once a file has been imported, NCL can print a summary of its contents.
NCL contains a suite of functions for reading and writing ASCII
and binary files. Many of these are illustrated in the
"Learning NCL by example" chapter of this document.
If you already have Fortran or C functions to read or write
complicated ASCII or
binary files, then it may be easier for you to incorporate those
functions into NCL rather than write NCL versions of them using built-in
NCL functions.
See the section
"Incorporating your own Fortran
or C code" in the "Going beyond the basics" chapter of this
document.
Many examples in this document illustrate NCL's ability
to deal simply and effectively with its supported datasets.
For a full description of NCL's file I/O capabilities, see the
"Files and
file variables" section of the
NCL Reference Manual.
NCL contains many facilities for manipulating data that, taken together,
surpass those available in most traditional programming languages.
The following headings summarize the features of NCL that make it such a
valuable tool for data processing.
NCL has many of the features of programming languages such as
Fortran and C.
It has variables,
data types, and the usual
arithmetic,
relational,
and logical operators.
It also has constructs for looping
and decision making. All
of these language elements are illustrated in the
"Learning NCL by example" chapter of this document.
Much of NCL's usefulness for data processing derives from its
strong array processing capabilities. The
arithmetic
operators apply to arrays
as well as scalars so
that arrays can be efficiently
added, multiplied, compared, and so forth.
NCL also automatically handles
missing values.
For details on the powerful array subscripting syntax of NCL,
see the section on
array subscripting in the
"Basics" chapter.
NCL has functions that are analogous to Fortran and C functions -- they
return a value to the calling script; NCL has procedures that are
analogous to Fortran subroutines and to C functions that do not return a
value. One very useful feature
of functions is that they can return any data type. For example,
if you have a function that performs the same
operation on every element of a linear
array, you can have the function return a linear array as its result.
Space for any returned value or values is created automatically by NCL.
NCL has a rich set of built-in
functions and procedures, including
interpolation and curve-fitting functions, functions for spherical harmonics,
and functions for basic statistics.
Many of NCL's functions are illustrated in the
"Learning NCL by example" chapter of this document.
You should also browse the
list of built-in functions
and procedures and
the section on "NCL source
code functions and procedures" in the NCL Reference Manual.
Incorporating your own C or Fortran functions or subroutines
into NCL can be accomplished by using a
"wrapper
generator" process. For Fortran this process has been partially automated --
see the section
"Incorporating your own Fortran
or C code" in the "Going beyond
the basics" chapter of this document.
For details on the general procedure, see
"Extending
the NCL function and procedure set" in the
NCL Reference Manual.
Another useful NCL feature is the capability of defining and using
missing values.
Variables, or array elements,
may be set to a user-specified missing value to indicate missing or invalid
data; these missing values are
propagated in arithmetic calculations according to well-specified rules.
The third area where NCL excels is graphic display.
Publication-quality plots can be produced in black and white, grayscale,
or color.
These plots can be drawn on a workstation screen, saved as an
NCGM file,
or saved in PostScript.
Interfaces exist for the rapid display of:
- Contour plots
- XY plots
- World map projections
- Vector plots
- Streamline plots
- Output
primitives (lines, markers, areas, text)
Combinations of these plot types can also be produced. For
example, you can easily transform and overlay a contour plot on any
of the
ten
supported world map projections.
Plot details, like the number of contour levels, the contour label fonts
to be used, map area masks, and so forth can be controlled by setting
values for resources before
invoking the plotting
interface calls. These resource values can be set using NCL directly,
or they can be set in an external ASCII file that NCL can read.
All of the plot types listed, as well as how to set
resource values, are illustrated in the
"Learning NCL by example" chapter of this document.
The interfaces for rapid plot display do not
encompass the full flexibility of the plotting capabilities available
in NCL. To find out more about plotting, see the
"Going beyond
the basics" chapter in this document.
home |
toc |
about doc |
intro |
examples |
basics |
beyond basics |
hints and tips |
appendix |
glossary
