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Implementation of Tdry in ropp_fm_bg2ro_1d.f90
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The program 'ropp_fm_bg2ro_1d' takes a BGR file as input
and adds Level 1b (bending angle) and 2a (refractivity)
to the file. The forward modelling from model data to 
bending angle and refractivity is done with the routines 
'ropp_fm_refrac_1d' and 'ropp_fm_bangle_1d'. There
are several versions of both the 'refrac' and the 'bangle'
subroutines.

We have now added a new routine 'ropp_fm_tdry_1d' to ROPP
which maps model data to dry temperatures on observation 
levels. An alternative to a separate 'tdry' routine could 
be to do the tdry calculation in the 'ropp_fm_refrac_1d' 
routine, but since there are several versions of this, it 
quickly gets a bit complicated.

Another constraint is that ropp_fm and ropp_pp should be
kept independent. This means that there must be a Tdry
calculation routine available in both ropp_pp and ropp_fm,
where ropp_pp handles observational data and ropp_fm 
handles model data.

In case one wants to anyway make ropp_pp routines available
in ropp_fm, Ian has provided some instructions. However, 
after a quick look in the automake/configure settings,
I deemed that the risk for introducing errors into the
installation and configuration procedures are too large.  

We have made a stand-alone copy of ropp_pp_tdry called
ropp_fm_tdry, which also contains the required subroutines
from ropp_pp_spline (as purely internal subroutines).
In the future, the ropp_fm_tdry routine could be simplified,
perhaps along the lines suggested by Sean. After all,
computing dry temps for model data is a more well-defined 
problem than computing dry temps for observational data. 

An alternative to separate ropp_fm and ropp_pp versions of
the Tdry calculation could be to make a "ropp_utils_tdry",
available to both the ropp_pp and ropp_fm modules. This
could be discussed for future ROPP versions.

In summary, changes have been made in the following 6 files:

ropp_fm/common/ropp_fm.f90
ropp_fm/tools/ropp_fm_bg2ro_1d.f90

ropp_fm/refrac_1d/ropp_fm_tdry.f90     (new)
ropp_fm/refrac_1d/ropp_fm_tdry_1d.f90  (new)

ropp_fm/build/Makefile.am
ropp_fm/refrac_1d/Makefile.am


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Options for observation levels
------------------------------
There are currently 3 options for defining observation levels: 
a) observed levels from an ATM file using option '-l'; 
b) the 247 standard levels using option '-247L';
c) a regular grid using options '-zmin', '-zmax', '-nz'. 

Default is option c) with zmin=200, zmax=60000, nz=300.

Any pre-existing Lev1b or Lev2a structures in the BGR file is
now over-written.