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NCAR Command Language - NCL

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  1. NCAR Command Language - NCL Juli Rew CISL SOARS, June 4, 2008

  2. Overview of NCL Strategies for Survival Work with Example Programs What We Will Study

  3. NCAR Command Language is a complete programming language for processing data and producing high-quality 2D graphics NCL commands can be run at the command line, or they can be placed in batch scripts What is NCL?

  4. NCL is built on top of NCAR Graphics, which was originally written in Fortran NCAR Graphics was re-written in object-oriented form, and NCL works with graphical “objects” Thanks to Mary Haley of the CISL Visualization and Technology Section for much of the training information given here NCL History

  5. Freely available Easier to use than conventional programming Competes well with expensive products such as IDL Versatile - it can accept and output data in a variety of formats Produces publication-quality graphics Why Use NCL?

  6. NCL has so many features that it can take a lot of time to master NCL has around 800 functions Before writing any new NCL code, look at the downloadable examples to see if one fits GSUN tutorial GSUN line-by-line Strategies for Survival

  7. Use the gsn and csm libraries Documentation is available online at: Help from fellow users by Subscribe at Support: Mary Haley (, ML 35) Strategies for Survival, cont.

  8. begin/end - to start/end scripts (optional) ; - Comment @ - Attribute, e.g.,temperature@units = “Degrees C” Operators like Fortran - + - *, .le., .lt., etc. All parameters are passed by reference (as in Fortran) NCL Syntax

  9. Indexes start at 0 (similar to C) 1D, integer: a = (/1,2,3/) 2D, float: b = (/ (/1.1, .01/), /(.001,.0001/) /) Whole-array syntax is like Fortrana=ba*b (more efficient than looping to copy array) Arrays

  10. Dimensions can be named:temperature!0 = “frtime”temperature!1 = “lat”temperature!2 = “lon” Strings - enclosed in double quotes Syntax, cont.

  11. Coordinate variables- 1D arrays with same name and size as the dimensions they are assigned to- Represent data coordinates for each index of a named dimension- Must be monotonically increasing/decreasingtemperature&frtime = forecast timestemperature&lat = lat_pointstemperature&lon = lon_points Coordinate subscriptstemperature (0,{20:60},{-95:-120}) Syntax, cont.

  12. We will emphasize graphics features here Some examples of input and output to/from ASCII text and netCDF data files Graphics

  13. Load necessary libraries Open “workstation” to send graphics to Change the color map (optional) Set plot resources (optional) Draw the graphics Minimum Steps Needed to Create a Plot

  14. load "$NCARG_ROOT/lib/ncarg/nclscripts/csm/gsn_code.ncl" load "$NCARG_ROOT/lib/ncarg/nclscripts/csm/gsn_csm.ncl” First library contains “generic” interfaces and supplemental routines Second library contains gsn_csm interfaces that use CSM conventions. Load order is important. CSM conventions are accepted ways to label data, e.g., units 1. Load the Necessary Libraries

  15. automatic subtitles contour line labels tickmarks out & lat/lon labels informational label automatic labelbar “basic” interface “metadata aware” interface

  16. Can be PostScript (PS or EPS), PDF, X11 window, or NCAR CGM (NCGM) Has a default color map associated with it, but you will probably want to change this (more later) Can have up to 15 multiple workstations open 2. Open graphics “workstation” wks = gsn_open_wks(“x11”,”test”) ; x11 window wks = gsn_open_wks(“ps”,”test”) ; “” wks = gsn_open_wks(“eps”,”wrf”) ; “wrf.eps” wks = gsn_open_wks(“pdf”,”slp”) ; “slp.pdf”

  17. Do this before drawing anything to the frame. If you use the same color map a lot, can put in “.hluresfile” (more later) Can use one of the other 40+ ones, or create your own. If you don’t change the color map, here’s what you’ll get… 3. Change the color map (opt’l) gsn_define_colormap(wks,”rainbow”)

  18. index 0 is the background color index 1 is the foreground color Pick a better color table… or create your own Default color table (yuck)

  19. Resources are the heart of your NCL graphics code. There are over 1,400 resources! Resources are grouped by object type. There are 11 “graphical” objects: contours, labelbars, legends, maps, primitives, streamlines, text strings, tickmarks, titles, vectors, XY plots 4. Set optional resources

  20. Starts with 2 or 3 lower-case letters based on object it is associated with. Some examples: “xy” - XY Plots “cn” - Contour plots “vc” - Vector plots “ti” - Titles “tm” - Tickmarks “gsn” - special resources not associated with any object Made up of full words with first letter of word capitalized: “xyLineColor”, “cnFillOn”, “tiMainString”, “vcRefMagnitudeF”, “gsnMaximize” Some have an “F” on the end to indicate a floating point resource: “xyLineThicknessF” Detour: Anatomy of a resource

  21. Resources are set by attaching them as attributes to an NCL logical variable: res = True res@mpMinLatF = 30 ; decimal not necessary Most have default values. There are many types: res@tiMainString = “This is a title” res@tmXBLabelFontHeightF = 0.01 res@cnLineLabelsOn = True res@xyLineColors = (/5,7,11/) res@xyLineColors = (/“red”, “green”, “blue”/) Anatomy of a resource (cont’d)

  22. Call one of the gsn_csm_xxxxx functions from the second library we loaded. The general format is: plot = gsn_csm_contour(wks,data,res) plot = gsn_csm_vector(wks,u,v,res) plot = gsn_csm_contour_map(wks,data,res) plot = gsn_csm_pres_hgt(wks,data,res) 5. Draw the graphics

  23. NCL scripts that follow can be downloaded from the web: Now for Some NCL Graphics Code Examples Scripts have names like xy1a.ncl, xy1b.ncl, … The first one is usually one with no resources set, and each subsequent script adds a few more resources.

  24. setenv NCARG_ROOT ~/ncl-4.2.0.a034 Go to example NCL scripts directory:cd SOARS/NCL To run these scripts:ncl xy1a.ncl Let’s look at what’s in our first script using vi:vi xy1a.ncl Running NCL

  25. Line color changed (using color index values) Default color map used Resource introduced: xyLineColor - sets curve color Example xy1b.nclgsn_csm_y

  26. X values added Line color changed (using named color) Line thickness increased “long_name” attributes set Resource introduced: xyLineThicknessF - sets line thickness Example xy1c.nclgsn_csm_xy

  27. Axes limits changed Resources introduced: trYMinF, trYMaxF, trXMinF, trXMaxF - sets mins and maxes for X and Y axes (transformation resources) These resources can also apply to contour, vector, and streamline plots Example xy1d.nclgsn_csm_xy

  28. Dash pattern set for curve Title resources set Resources introduced: xyDashPattern - sets dash pattern for curve (17 available) tiMainString, tiXAxisString, tiYAxisString - sets strings for axes and main title, can also be used for contour, vector, etc, plots If have both long_name attributes and tiX/YString set, ti* resources will take effect Example xy1e.nclgsn_csm_xy

  29. Explicitly set the bottom tickmark labels Resources introduced: tmXBMode, tmXBValues, tmXBLabels - sets strings for bottom tickmarks Note about tickmark resources: all four sides of the axes have their own set of tickmarks, for example: tmYLMode (Y left), tmYRMode (Y right), tmXTMode (X top) Example xy1f.nclgsn_csm_xy

  30. Generating some bogus wind data Each array entry has two components: wind speed and wind direction There is a special function for creating wind roses, called WindRoseBasic Data will be written to an ASCII file This example online at Example rose_1.ncl

  31. Now we are reading our data from an ASCII file called “Bogus.dat” that we created previously Recall that there are two components to each array entry Example rose_1a.ncl

  32. This example reads from a netCDF file and contours the temperature fields Fourth plot uses a predefined color table; Original example online Example contour.ncl

  33. Animate NCL image files using idt (part of NCL) Movies: Save series of images to Postscript files. Convert images to mpg using “convert” utility of ImageMagick Or use utility such as ncview to view netCDF data file Example movie: Animation

  34. NCL Workshop this summer July 8-11 Register by July 1 Info: More Training Available