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20 Grid Projections & Interpolation
We've already seen that GEMPAK can use multiple files for grid diagnostics. GEMPAK can perform grid calculations which combine grids with differing projection and grid point spacing (introduced in GEMPAK5.8.1). If the grids are not of the same projection and grid point spacing, the first grid file in the list defines the internal navigation used for the computation of the diagnostic. So, an entry of "GDFILE = eta + gfs" is remapped to the eta file navigation, while "gfs + eta" is remapped to the gfs file navigation.

Many model data sets are transmitted on widely used grids. Some examples are:

Grid Name

Region of Coverage

Resolution

Models Commonly Available
Grid 211 CONUS 80 Km NAM, NGM, GFS, RUC
Grid 212 CONUS 40 Km NAM, GFS
Grid 202 CONUS 190.5 Km GFS, NGM
Grid 213 CONUS 95.25 Km GFS, NGM
WAFS/Thinned Global 1.25 degree GFS, UKMet

If we want to use model data with objective analyses of surface or upperair data sets, we can grid our data onto the same grid as the model data we have, although this is not required. Luckily, this is quite easy using the CPYFIL parameter with GDCFIL.

If we have two gridded data sets that are not of the same projection or resolution, we can interpolate one grid to the projection used by the other data set. The program GDBIINT can interpolate gridded data from one projection to another.

The following example shows how to use RUC boundary layer temperature grids to estimate the precipitation type in gridded NEXRAD data using masking functions. The RUC grids are interpolated to the same projection and resolution as the national radar mosaic grids. The temperature grids are then used to create MASK values for the radar reflectivities for rain, frozen precip, and snow.

 


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