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The NCAR Command Language (NCL)

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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 ... – PowerPoint PPT presentation

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Title: The NCAR Command Language (NCL)


1
The NCAR Command Language (NCL)
  • Ethan Alpert
  • Visualization and Enabling Technologies Section,
    SCD, NCAR

2
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

3
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)

4
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!

5
Downloading NCL
  • Go to
  • http//ngwww.ucar.edu/ncl/download
  • 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

6
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)

7
Important resources for using and learning NCL
  • Main NCL home page
  • http//ngwww.ucar.edu/ncl
  • Contains links to documentation, examples, FAQ,
    ncl-talk email list, and update information

8
Reference Documentation
  • Main reference documentation
  • http//ngwww.ucar.edu/ngdoc/ng/ref/ncl/Overview.ht
    ml
  • 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
  • http//ngwww.ucar.edu/ngdoc/ng/ref/ncl/NclFuncsAnd
    Procs.html

9
Getting Started Using NCL (GSUN)
  • Getting Started Using NCL (GSUN)
  • http//ngwww.ucar.edu/ngdoc/ng/ug/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 NCLs object
    oriented Graphics interface
  • The Beyond the Basics section covers
    incorporating FORTRAN into NCL

10
Additional resources for using and learning NCL
  • NCL users email list
  • http//ngwww.ucar.edu/ncl-talk/
  • Email list devoted to NCL discussion
  • Read by NCL developers and support staff
  • Examples page
  • http//ngwww.ucar.edu/ncl/examples.html
  • CCSM NCL page for additional examples
  • http//www.cgd.ucar.edu/csm/support

11
Graphics Reference Documentation
  • The High Level Utilities Reference Guide
  • http//ngwww.ucar.edu/ngdoc/ng/ref/hlu/HluClasses.
    html
  • 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

12
Data Types
  • Numeric Types
  • double 64bits /- ( 2.22507e-308 ) to
    (8.98846e307)
  • float 32bits /- ( 1.175494e-38 ) to
    (1.701411e38)
  • long 32bits /- ( 2.147483e09 ) (64bit on
    SGI)
  • integer 32bits /- ( 2.147483e09 )
  • short 16bits /- ( 32767 )
  • byte 8bits ( 0 ) - ( 255 )
  • Non-numeric Types
  • string
  • character
  • graphic
  • file
  • logical

13
Variables
  • 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)

14
Attributes
  • Descriptive info about a variable or file
  • Any data type but file and list data types
    1D req.
  • Assigned and referenced using _at_ character
  • T_at_long_name temperature
  • T_at__FillValue -999.0

15
Dimensions
  • 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
  • T(lon,lat,time)
  • Used to assign coordinate variables

16
Coordinate Variables
  • Allow for more intuitive selection
  • T(12,3545,-95-105)
  • 1D arrays monotonically increasing or decreasing
  • Can only be assigned to a named dimension
  • Assigned using character
  • Ttime (/0,6,12,18,24/)

17
Reading data from files
  • For netCDF, HDF, GRIB
  • Use addfiles function
  • A addfile(tmp.nc,r)
  • K A-gtvariable
  • For fortran binary files
  • fbindirread, fbinread
  • For text
  • asciiread

18
Algebraic Operations
  • Array based algebra
  • Standard operators -, , , /, ,
  • Unique operators , , lt, gt
  • 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

19
Statements
  • 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

20
Statements
  • 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 (/ ... /)
  • Example
  • 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

21
Functions and procedures
  • Parameters are pass by reference
  • Support for definitions of NCL source based
    functions and procedures
  • Example
  • function myfunc(x,ynumeric)
  • begin
  • end

22
Incorporating FORTRAN codes
  • Create FORTRAN stub text file or add comments to
    existing FORTRAN codes
  • C NCLFORTSTART
  • FUNCTION ARCLN(NUMPNT, POINTX, POINTY)
  • DIMENSION POINTX(NUMPNT), POINTY(NUMPNT)
  • C NCLEND
  • Call wrapit77
  • wrapit77 lt wrapper_input gt! wrapper_W.c

23
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.so fcode_W.o fcode.o

24
Loading and Calling FORTRAN shared objects
  • Use external command
  • Syntax
  • external fcode path to shared object
  • Calling FORTRAN routines
  • fcodearcln(n,x,y)
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