GrADS

Important Note

GrADS is still very useful, but it's a little dated given the rise of Python within the atmospheric sciences. Therefore, this tutorial is no longer offered for credit. You are welcome to complete this tutorial for fun, but the MetPy tutorial will instead provide a more valuable experience.

Introduction

As an atmospheric scientist, you have a pretty good chance of running into the Grid Analysis and Display System (GrADS) at some point in your career. Other plotting programs exist (Python's matplotlib is particularly good and MetPy is fantastic for meteorological plots), but GrADS does a fair job of plotting contour and vector data on a base map and can also draw vertical cross sections, line and bar graphs, scatter plots, streamlines, grid boxes, station plots, and Hovmöller diagrams. In fact, GrADS drew the plot that you see on the left of this page. Visit the GrADS Home Page for a nice introduction to the plotting package, including lots of documentation.

GrADS typically reads GRIB (gridded binary) data, but it also handles other common meteorological data formats such as NetCDF and HDF. In general, GrADS reads a data descriptor (or control) file that tells the program which binary data file to open, the initial time, and the coordinate setup; describes the variables; and provides a variety of other setup information. You can run GrADS directly from the command line, but fortunately there is also a GrADS scripting language (which varies a bit from the command line language) for more complicated sets of commands.

NCEP Reanalysis Data

For this assignment, you will plot data from the NCEP/NCAR 40-Year Reanalysis Project (see the CISL Research Data Archive and these frequently asked questions for more information). Through the generous donations of several individuals, the UNCA Atmospheric Sciences department hosts the entire record of Reanalysis data from 1950—2016. For a nice example of the plotting capabilities of GrADS with the Reanalysis data, see my NCEP Reanalysis Plotter.

GrADS From the Command Line

Make sure you have Xming running and then in a suitable directory on Blizzard, start GrADS with grads Just hit 'Enter' when it asks if you want landscape mode and then a blank window should pop up. This is where your plots will appear. Now open a descriptor file at the GrADS prompt and create a plot with ga-> open /REANALYSIS/1974/ctl/at00z12z/hgt.prs
ga-> set lev 500
ga-> set t 187
ga-> d hgt
You should see the 500-mb height field during the 3—4 April 1974 Super Outbreak. You can view the contents of the descriptor file from the Linux prompt (open a new Blizzard window) with more /REANALYSIS/1974/ctl/at00z12z/hgt.prs The descriptor file details the format of the data and you can see on the first line that the GRIB file containing the data is actually located at /REANALYSIS/1974/data/at00z12z/hgt.prs.

While this plot is interesting, you may want to zoom in on the United States. First clear the old plot and then define the latitude and longitude limits: ga-> c
ga-> set lat 20.0 60.0
ga-> set lon -130.0 -60.0
ga-> d hgt
You can also view a rather rudimentary animation sequence by setting mutliple times: ga-> c
ga-> set t 185 195
ga-> d hgt
Notice that previous settings remain until reset. You can also plot a vertical cross section by varying the latitude and level (i.e., height) and holding the longitude constant: ga-> c
ga-> set t 187
ga-> set lat 20.0 60.0
ga-> set lev 1000 200
ga-> set lon -95.0 -95.0
ga-> d hgt
You can find more examples and things to try in this GrADS tutorial. Explore the contents of the /REANALYSIS directory on Blizzard to see what else you can plot. To exit from the GrADS prompt, type quit.

Your job here is to create four different plots showing the 850-mb winds and heights around Hurricane Katrina. Download plots.gs and place it in a suitable directory on Blizzard. Run the script in batch mode with grads -blc "run plots.gs" You could also run the script from the GrADS command line by typing just plots (you would want to comment out the quit line at the end to test GrADS commands after running the script). The script produces three rather ugly PNG images that you can view with eog. Modify the script to produce the following four images:

1. 850-mb heights and wind vectors (colored by magnitude) for 00Z 30 August 2005
2. 850-mb relative vorticity for 00Z 30 August 2005
3. 850-mb divergence for 00Z 30 August 2005
4. 24-hour 850-mb height tendency ( χ = dZ/dt) centered on 30 August 2005

Create all four plots a) on a white background, b) centered on the area of interest, c) on a latitude-longtitude map projection, d) with an appropriate title including units, and e) on a high-resolution base map showing international borders and U.S. states. Your plots will look similar to the following figures.

Peruse the GrADS Documentation for helpful information and useful GrADS commands. The quick reference cards are particularly convenient. Please post your completed plots on your password-protected Web page, along with your modified GrADS script.