Demo of conversion of spherical harmonics data to a regular lon-lat grid
Creation date: 11 December 2001

Compare gridded lon-lat data (1x1 degree) directly from MARS (ECMWF) (left plot) to gridded lon-lat data derived with ecll from spherical harmonic (T159) data (right plot). In this case temperature (K) at model level 30 for the first guess (FC+6) valid on 1 february 1993 (1993013118+006) is shown. Only part of the globe has been plotted. I used for the spherical harmonics conversion to a regional grid:
ecll -v RHT_T159_06_930131_ERA40 T930131_ERA40.gb -40 1 70 75 -1 40
and plotted T930131_ERA40.gb with MAGICS (plotmapec.sc).

For conversion to global grid (TM3 preprocessing) use:
ecll -v RHT_T159_06_930131_ERA40 TGLOBE930131_ERA40.gb -180 1 360 90 -1 181

Note that

• GHT files (lon-lat) have a size of 31.420.800 bytes
• the interpolated TGLOBE930131_ERA40.gb file (lon-lat) has a size of 31.419.840 bytes (960 bytes less?)
• the spherical harmonics RHT_T159_06_930131_ERA40 file has a size of 12.585.600 (less than half of the gridded size)

Spherical harmonics (T159 from ERA40)	Lon-lat (1x1 degree) directly from MARS at ECMWF