projection-truelat1,2(namelist.wps)

[skyfive]

Hello,
I don't understand about the projection settings in namelist.wps (WPS).
For 'mercator', the truelat1 should be 20N, right?
I see many people are setting truelat1 as 0, 30, or other specific values?

How should we decide on this value?

[dcvz]

It depends on your application, your domain size, etc.

See http://www.mmm.ucar.edu/wrf/users/tutor ... eneral.pdf

You can change the settings in namelist.wps and run plotgrids.exe to see what it produces. Once you're happy with it, then you can run geogrid.

[skyfive]

Thanks very much. It's very helpful.
I am still wondering, for mercator projection, is it a rule which could help us to choose a best truelat1?
For example, when the center of our domain is near equator, we should choose a small truelat1; when the center is near 15N, we should choose 30N; when the center is near 30N, we should choose truelat1>30?


Thanks.

[dcvz]

There are no "rules", but for a mercator projection I would set truelat1 to be the same as ref_lat (so the projection is true at the center of the domain). Keep in mind though that the mercator projection has more distortion as you get farther from the equator. There are better projections to use when the domain goes poleward of 30 degrees or so.

[skyfive]

Thanks very much, dcvz, it's very helpful.

[alainaketh]

Hi,

Just to let you know that I ran a simulation using Mercator projection in a place where latitude was ~ 54.0 N. It's way too high for this projection, however I had to use it because:

1) Lambert outputs were examined Matlab, the cell size varied along the domain so the .nc files could not be read by ArcGIS or processed with QGIS
2) I have no clue on how to set up a simulation with lat-lon.
3) I understand that Polar stereographic is used only to model either North or South pole domains.

I set truelat1 to be equal to ref_lat. In ArcMap, I set the projection of the workspace to WGS84, then imported the WRF output file, and then reprojected it to WGS 1984 (see note below on how to do that).

The result? A very, very nice indeed precipitation map.


* * * * * *
How to change projection of WRF output files.
1) Go to ArcMap
2) In the Search Box, search "netcdf". Select "Make NetCDF raster layer"
3) Select your *.nc file, the variable to display, and the name of the layer. For example "RAINC_Layer".
4) Click ok
5) In the ArcToolbox, go to Data Management > Projections and Transformations > Raster > Project Raster
6) Input your raster, in this example "RAINC_Layer"
7) The name of the reprojected raster
8) In "Output Coordinate System" click on the little button and a new window will be displayed.
9) Select > Geographic Coordinates > World > WGS 1984

Bob's your uncle!
* * * *

Cheers,

Alaina.