NCL: WRF plotting - zooming in on maps

This page illustrates different ways to zoom in on WRF-ARW plots drawn over maps.

Currently, three methods are illustrated below:

Using wrf_contour or wrf_vector with wrf_map_overlays to plot the data, which uses the native WRF map projection defined on the file.
Using gsn_csm_contour_map with wrf_map_resources to plot the data in the native WRF map projection defined on the file.
Using gsn_csm_contour_map to plot the data, but in a different projection than what is defined on the file.

The first two methods require setting the special ZoomIn option to True, and setting the Xstart, Xend, Ystart, and Yend options as described below.

The second method requires that you read the latitude / longitude variables off the WRF file (they usually have names like "XLAT" and "XLONG").

wrf_zoom_1.ncl: This script creates two plots using wrf_xxxx plotting functions:

A simple filled contour plot of the HGT variable read off a WRF output file.
The same data, but zoomed in on the upper left corner of the map.

In order to zoom in on WRF-ARW data and keep the same map projection that is defined on the file, you must:

Calculate the indexes of the zoomed in area of interest.
Use the indexes to subset the variable(s) you are interested in plotting, before calling wrf_contour or wrf_vector.
Use the same indexes for setting the special Xstart, Xend, Ystart, and Yend resources, and set the special ZoomIn resource to True before calling wrf_map_overlays.

A Python version of the wrf_zoom_1_2 projection is available here.

wrf_zoom_2.ncl: This example plots the same data as the previous example, except this time gsn_csm_contour_map is used to plot the data.

The data is still being plotted in the native WRF map projection, so you must call wrf_map_resources in order to set the correct map resources. You must also call this function after setting the ZoomIn / Xstart / Xend / / YStart / YEnd resources, or the map won't be properly zoomed.

The reason for using gsn_csm_contour_map instead of wrf_contour and wrf_map_overlays is simply if you don't want all the extra titles and layout that the wrf_xxxx functions give you.

wrf_zoom_3.ncl: This example plots the same data as the previous two examples, except the data is overlaid on a standard lat/lon map projection.

In order to plot data in a non-native projection, you must:

Read the 2D lat/lon arrays off the file and attach them as special lat2d / lon2d attributes to the data being plotted.
Zoom in on the area of interest by setting desired NCL map resources.

You no longer need to call wrf_map_resources or set the special ZoomIn/X/YStart/End resources when zooming in.

Note that if you plot WRF-ARW data in a different map projection than what's defined on the WRF file, you may see the grid is slightly skewed. The results are still correct, but the image will be oriented a little differently.

The reason for plotting WRF-ARW data in a non-native projection may be if you need to compare WRF data to another data variable that is defined on a different map projection.

dataonmap_zoom_10.ncl
Curvilinear grid

Data: wrfout_d01_2003-07-15_00:00:00 (NetCDF WRF output file)

This example plots the Q2 variable read off a WRF output file and illustrates yet another way to zoom in on the data, by actually selecting a lat/lon area of interest, instead of "the upper left domain" like the previous examples.

This example shows two ways to zoom in on WRF-ARW data: 1) using the same native WRF map projection, or 2) plotting the data in a different map projection.

The first plot shows how to plot the data in the native map projection by first using the wrf_user_ll_to_xy function to calculate the x,y index locations into the 'q2' array that represent the approximate lat/lon area of interest. These index values are used with the ZoomIn resource to set the special Xstart, Xend, Ystart, and Yend resources before you call wrf_map_resources to set the correct NEW native map projection parameters.

The second plot shows how to plot the data using the 2D lat/lon arrays read off the file. In this case, you don't need the special index values, because NCL will use the lat/lon arrays to figure out the correct subset of the data to plot.

You may notice that the two plots are not plotted in the exact same lat/lon area. This is because the lat/lon corners of the first plot are only *approximately* equal to the requested lat/lon area of interest. The wrf_user_ll_to_xy function gives you the x,y indexes that are closest to the actual lat/lon area of interest.

The second plot is the exact area requested, because NCL now has the actual lat/lon arrays, and can use this to interpolate the data as needed, in the area of interest.

For another example of subsetting a WRF map domain, see the "wrf_Zoom.ncl" example at http://www2.mmm.ucar.edu/wrf/OnLineTutorial/Graphics/NCL/Examples/SPECIAL/wrf_Zoom.htm