[tgcmgroup] TIE-GCM coordinates

[Stan Solomon]

Two good questions were recently posted to the mailing list:

 > I want to compare the neutral densities derived from satellite missions
 > (CHAMP, Grace) with those computed with TIEGCM. Thus, the density
 > information has to be given in the same reference system. In which
 > reference system are the  coordinates (lat, lon, ZG/ZGMID) defined? Is
 > the time given in UTC+0?

TIE-GCM calculations are performed on a spherical grid, so latitude and 
longitude are really spherical latitude and longitude, i.e., they are 
not corrected for the ellipsoid.  For typical model resolutions, the 
difference between spherical latitude and geodetic latitude is not 
important.

Altitude is more difficult, because the model operates on pressure 
surfaces, so a reference pressure height must be assumed, which is based 
on the underlying tidal/atmosphere model.  Geometric height is relative 
to the surface of the ellipsoid, and is intended to be used for 
comparison with, e.g., satellite measurements, with the caveat that 
there are significant approximations involved in registering the lower 
boundary of the model, and, since it is an estimate of the height of a 
pressure surface, it will depend on the entire temperature profile.  See 
the document at:
https://download.hao.ucar.edu/pub/stans/tgcm/ZCoordinates.pdf
for more details.


 > I have computed realistic results with the default setup for a quiet
 > day. However, I was not able to  calculate good results for the
 > Halloween storm (28–29 Oct 2003, doy:301-302 ). What is the correct
 > setup for this scenario.  With which setup was the storm benachmark
 > 'tiegcm_res2.5_nov2003_heelis_gpi_prim.nc ' created?

The "Halloween" storm is difficult, because it was the most severe storm 
this century, and because there are gaps in the OMNI data.  That's why 
we used the November 2003 storm as an example.  That particular run was 
done, several years ago, with the "Heelis" forcing, as indicated in the 
file name.

Stan

-- 

Stanley C. Solomon
High Altitude Observatory
National Center for Atmospheric Research
Boulder, Colorado, USA