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WHAT ARE THE EQUATIONS FOR MOVING BETWEEN GEOGRAPHIC AND ROTATED LAT/LON ON THE NMMs E-grid?

April 26, 2017equations geographic LAT lon moving nmm rotated

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WHAT ARE THE EQUATIONS FOR MOVING BETWEEN GEOGRAPHIC AND ROTATED LAT/LON ON THE NMMs E-grid?

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(This answer applies to all non-WRF NMM and all Eta MOdel E-grids too.) Let all longitude be reckoned positive east. Let lat_g and lon_g be the geographic lat/lon and lat_r and lon_r be the rotated lat/lon. Let lat_0 and lon_0 be the geographic lat/lon of the E-grid’s central point. This is where the grid’s rotated equator and rotated prime meridian cross. First find the rotated lat/lon of any point on the grid for which the geographic lat/lon is known.