How Variables are Geo-Located in RTOFS NetCDF Files

RTOFS data is modeled on a curvilinear orthogonal grid. This means that while the data does form a grid, the rows and columns of that grid appear as curved lines on the surface of the earth, as shown in the image to the left. Because each row is not a constant latitude and each column is not a constant longitude, the data cannot be located with 'latitude' and 'longitude' axis arrays as is done in many NetCDF files containing grid data. Each pixel of the grid must have its own latitude and longitude. The latitudes and longitudes for data pixels are stored in six extra variables, LOPP, LAPP, LOUV, LAUV, LOVV, and LAVV. LOPP and LAPP are the latitudes and longitudes of all variables other than the currents, while the others geo-locate the current data pixels. More information on the RTOFS grid structure can be found on the NOAA NCEP website (http://polar.ncep.noaa.gov/ofs/index.shtml). The following table associates the location variables with the variables that they locate.