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The NCAR Command Language (NCL). Ethan Alpert Visualization and Enabling Technologies Section, SCD, NCAR. What is NCL?. NCL is an interpreted programming language Array based algebraic operators Unique netCDF data model Traditional programming language constructs

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The NCAR Command Language (NCL)

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The ncar command language ncl

The NCAR Command Language (NCL)

Ethan Alpert

Visualization and Enabling Technologies Section, SCD, NCAR

What is ncl

What is NCL?

  • NCL is an interpreted programming language

    • Array based algebraic operators

    • Unique netCDF data model

    • Traditional programming language constructs

    • Wide variety of graphics capabilities:

      • Maps, Contours, XY, Vectors, Streamlines, labelbars, text, tickmarks as well as line, marker and polygon primatives

      • Output to X, NCGM, PostScript

      • Scripts available to convert to image formats

What is ncl1

What is NCL?

  • NCL is available on most UNIX platforms

  • NCL can run in batch or interactive mode

    • Interactive mode has command history and command line editing

  • Many useful functions and procedures

  • Code integration tool (ability to import FORTRAN)

Why use ncl

Why use NCL?

  • Capable data processing environment with many useful functions

  • Strong and easy to use file I/O capability

  • Useful for the development and integration of FORTRAN processing routines

  • Robust publication quality 2D graphics w/ detailed maps

  • Mature product

  • Free!

Downloading ncl

Downloading NCL

  • Go to:


  • Read and agree to GPL license

  • Fill out short registration form

  • Download binaries

    • Precompiled versions exist for:

      • IBM RS6000, DEC Alpha, Sun Solaris, Red Hat Linux and SGI IRIX

Installing ncl

Installing NCL

  • Uncompress tar file in installation directory:

    • cd /usr/local

    • gunzip ncl-4.2.0.Solaris2.7_sun4.tar.gz

  • Untar:

    • tar –xvf ncl-4.2.0.Solaris2.7_sun4.tar

  • Set NCARG_ROOT environment variable:

    • setenv NCARG_ROOT /usr/local/

    • set path = ( /usr/local/bin $PATH)

Important resources for using and learning ncl

Important resources for using and learning NCL

  • Main NCL home page:


  • Contains links to documentation, examples, FAQ, ncl-talk email list, and update information

Reference documentation

Reference Documentation

  • Main reference documentation


    • All syntax and statements defined

    • Links to all procedures and functions

    • Basic overview of graphics

    • Usage tips

    • Information on importing FORTRAN

    • Information on supported data formats

  • Function and Procedure Reference


Getting started using ncl gsun

Getting Started Using NCL (GSUN)

  • Getting Started Using NCL (GSUN)


    • Intended for users with little or no NCL experience

    • Some programming language knowledge is assumed

    • Learning by example concept

    • Starts with basics and builds from there

    • Provides a set of simple functions written in NCL to be used by new users instead of NCL’s object oriented Graphics interface

    • The “Beyond the Basics” section covers incorporating FORTRAN into NCL

Additional resources for using and learning ncl

Additional resources for using and learning NCL

  • NCL users email list


    • Email list devoted to NCL discussion

    • Read by NCL developers and support staff

  • Examples page


  • CCSM NCL page for additional examples


Graphics reference documentation

Graphics Reference Documentation

  • The High Level Utilities Reference Guide


    • Only for users wishing to design their own custom visualization or for whom the default GSUN or WRF example are inadequate

    • NCL reference pages provide brief usage description

    • GSUN “Beyond the Basics” section covers use as well

Data types

Data Types

  • Numeric Types

    double64bits+/- ( 2.22507e-308 ) to (8.98846e+307)

    float32bits+/- ( 1.175494e-38 ) to (1.701411e+38)

    long32bits+/- ( 2.147483e+09 ) (64bit on SGI)

    integer32bits+/- ( 2.147483e+09 )

    short16bits+/- ( 32767 )

    byte8bits ( 0 ) - ( 255 )

  • Non-numeric Types








  • Based on the netCDF data model

    • Values, dimensions

    • Optionally: attributes, coordinate info (meta-data)

  • 4 types of variables

    • Regular variables (in memory)

    • File variables (reference files)

    • Graphic variables (reference plots)

    • Lists (containers for one or more of the above variable types)



  • Descriptive info about a variable or file

    • Any data type but “file” and “list” data types; 1D req.

  • Assigned and referenced using “@” character

    • T@long_name = “temperature”

    • T@_FillValue = -999.0



  • Dimensions may be “named”

    • Provides an alternative way to reference subscripts

  • Assigned with “!” character

    • Example

      T!0 = “time”

      T!1 = “lat”

      T!2 = “lon”

    • Allows for dimension reordering


    • Used to assign coordinate variables

Coordinate variables

Coordinate Variables

  • Allow for more intuitive selection

    • T({12},{35:45},{-95:-105})

    • 1D arrays; monotonically increasing or decreasing

    • Can only be assigned to a named dimension

    • Assigned using “&” character

      T&time = (/0,6,12,18,24/)

Reading data from files

Reading data from files

  • For netCDF, HDF, GRIB

    • Use addfiles function

      A= addfile(“”,”r”)

      K = A->variable

  • For fortran binary files

    • fbindirread, fbinread

  • For text

    • asciiread

Algebraic operations

Algebraic Operations

  • Array based algebra

    Standard operators: -, ^, *, /, %, +

    Unique operators: %, #, <, >

    Logical operators: .eq., .ne., .le., .gt., .ge., .lt.,

    .and., .xor., .or., .not.

  • Dimensionality of operands must either match or be scalar

  • Missing values are filtered out by identifying missing value with _FillValue attribute



  • if - Requires a single scalar logical value (i.e. True or False)

    • multi-dimensional logical values must be reduce to single True or False value

      • any - returns True if any of the input values are true

      • all - returns True if all of the input values are true (excluding missing values)

  • do - Do statements very similar to F77

    do var = startval, endval [, stride]

    . . .

    end do

    do while(expression)

    . . .

    end do



  • assignment (=)

    • If left-hand-side undefined right-hand-side assigned to left-hand-side symbol

    • All dimension names, coordinate variables, and attributes copied or overwritten to left-hand-side

    • UNLESS right-hand-side enclosed in (/ ... /)


      a = b ; dimensions, attributes, and coordinate

      ; variables as well as value copied

      a = (/b/) ; only value of b is copied

    • If left-hand-side is defined data types must match as well as dimensionality

Functions and procedures

Functions and procedures

  • Parameters are “pass by reference”

  • Support for definitions of NCL source based functions and procedures


    function myfunc(x,y[*][*]:numeric)



Incorporating fortran codes

Incorporating FORTRAN codes

  • Create FORTRAN stub text file or add comments to existing FORTRAN codes





  • Call wrapit77

    wrapit77 < wrapper_input >! wrapper_W.c

Incorporating fortran codes1

Incorporating FORTRAN codes

  • Compile FORTRAN and C wrapper to create .o files

  • Link object files to create shared object

    Linux Example:

    nhlcc -c fcode_W.c

    nhlf77 -c fcode.f

    ld -shared -o fcode_W.o fcode.o

Loading and calling fortran shared objects

Loading and Calling FORTRAN shared objects

  • Use external command


    external fcode “path to shared object”

  • Calling FORTRAN routines


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