Colored vectors are often used to represent [magnitude, direction] vector data such as ocean currents and surface winds. To display vector data, the cwrender tool has a second mode for the --enhance option that was covered previously. To summarize, the two modes are:

Color enhancement — When passed a single variable name, it creates a color enhancement image of that one variable. This mode was demonstrated in the Color Enhancement Images section.
Colored vector enhancement — When passed two variable names, it creates an image of color-enhanced arrows or wind barbs on a solid background. This mode is covered in this section.

We'll look at rendering two types of data in the next exercises.

Ocean Surface Winds

Let's look at the surface wind data file example_ascat_wind_mar_2023.nc which contains data from the MetOp-C satellite Advanced Scatterometer (ASCAT) instrument. The instrument uses radar to measure backscatter to determine speed and direction of winds over the surface of the oceans. Take a look at the file contents with cwinfo:

Terminal window showing output of cwinfo command on MetOp-3 ASCAT wind data file

You can see that the data file contains both windspeed in m/s and direction in degrees. As shown previously, we can pass the cwrender tool just a single variable — the windspeed — and enhance with a purpose-built wind palette as follows:

cwrender --enhance windspeed --palette Wind-0-50 --range 0/50 --units knots --coast black/gray40 --grid white example_ascat_wind_mar_2023.nc windspeed.png

Note the use of --units knots because wind is often displayed in knots. You should see this resulting image:

A color enhanced image of global windspeed data from the MetOp-C ASCAT instrument

Now to create a vector image, we name both the magnitude and direction variables and add an option to draw proper weather map wind barbs rather than the default arrows:

--enhance windspeed/direction
--enhancevector magdir/barb

We also magnify some area of interest to show the vectors properly, and specify a black background:

--magnify 35/-125/12
--background black

Putting this all together:

cwrender --enhance windspeed/direction --enhancevector magdir/barb --magnify 35/-125/12 --background black --palette Wind-0-50 --range 0/50 --units knots --coast black/gray40 --grid white example_ascat_wind_mar_2023.nc windbarbs.png

You should see this wind barb plot off the coast of California:

Rendering of color enhanced wind barbs off the California coast from MetOp-C ASCAT data

Wind barbs follow a convention as described by the Wikipedia page on meteorology station data diagrams:

https://en.wikipedia.org/wiki/Station_model#Plotted_winds

Ocean Surface Currents

Now's look at the ocean currents data file example_altim_surface_curr_feb_2023.nc and render the vector data as color enhanced arrows. As before, look at the contents of the file:

Terminal window showing the output of the cwinfo command on the ocean surface currents data file

The file contains sla (sea level anomaly) in meters, and ocean current data as two vector components:

ugos — Zonal (eastward) component of the geostrophic current in m/s
vgos — Meridional (northward) component of the geostrophic current in m/s

We can render the sea level anomaly data as a color enhancement as follows:

cwrender --enhance sla --palette Ocean-balance --range -1/1 --coast black/gray40 --grid white example_altim_surface_curr_feb_2023.nc global_sla.png

which results in this image:

Color enhanced image of global sea level anomaly from satellite altimetry measurements

Using a command similar to the one above for surface winds, we can render the ocean current vectors:

cwrender --enhance ugos/vgos --magnify 35/-75/10 --background black --palette Blue-Red --range 0/2 --function step10 --coast black/gray40 --grid white example_altim_surface_curr_feb_2023.nc nw_atlantic_currents.png

You should see the following image:

Ocean surface currents in the northwest Atlantic rendered as color enhanced vector arrows

Note the use in the command of these options:

--magnify 35/-75/10 — This centers the image on [35ºN, 75ºW], with a magnification factor of 10 to allow space for each arrow.
--function step10 — This specifies a linear step function with 10 steps for the color bar.

Bonus exercises:

pFad - (p)hone/(F)rame/(a)nonymizer/(d)eclutterfier! Saves Data!