# How to transform WRF Spherical Earth to WGS 84?

Weather Research Forcasting Model assumes a spherical earth model with a radius of 6370km. I'm looking to display the data onto the WGS-84 Ellipsoid model as that is the model my system uses for its simulations. We are looking to incorporate real-world changing weather so we are hoping to use WRF's output as we don't want to write our own weather simulator.

While I'm primarily concerned with the continental US I will eventually want to do this for the entire globe.

I've found information, most notably here: http://computing.io/wp/2016/01/wrf-lati ... nd-datums/ That suggests that WPS and WRF may just be treating the WGS-84 information in the land-use data as spherical earth information and this suggest using the XLAT and XLONG output directly from WRF and putting it onto the WGS-84 Ellipsoid. This doesn't sound 100% correct however, and certainly wind information would need to be transformed slightly differently as well well some other information, in particular I would expect the geopotential height may need to be adjusted for the change in surface shape.

I know that to do transforms between Ellipsoids I'd need to use one of the Helmert, Molodensky-Badekas, or Molodensky methods. But it seems like I need reference points on both which I don't actually have for WRF. Has anyone done such a conversion before or could you provide some insight into the steps to take?

• You probably don't need actual tfm parameters for an equation-based method. The WRF sphere is probably coincident at the center with WGS84 (or at least close enough to make no difference). That means that longitudes are the same, and only the latitude values would differ which would be handled by the LL-XYZ, XYZ-LL changes within the tfm method. Commented Feb 9, 2016 at 17:55

Proj.4 is a very flexible tool and you can define your own projection stuff, like ellipsoids / shperes via the parameter +a and +b (the semimajor radius of the ellipsoid axis). I'm not a specialist WRF but found this page,

https://www.cmascenter.org/sa-tools/documentation/4.2/html/grids_ellipsoids_map_proj.html

if I' ask the WEB for "proj.4 WRF to WGS84". The page looks promising if you follow the paragraph "3.5.2.Map Projection and Ellipsoid Specification". It's part of the documentation for a "Community Modeling and Analysis System" located at the University North Carolina - Institute for the Environment.

FAX:

...
A list of Datum Specifications which can be used in datum transformation and a list of Named Ellipses are available. You may specify the ellipsoid parameters as shown in the following examples:

For the Weather Research and Forecasting Model (WRF) sphere:
setenv WEIGHT_FILE_WLLIPSOID "+a=6370000.0,+b=6370000.0"

For the Fifth Generation Mesoscale Model (MM5) sphere:
setenv WEIGHT_FILE_ELLIPSOID "+a=6370993.0,+b=6370993.0"

...