THIS DIRECTORY CONTAINS:
========================

README        This file
compile       File containing a command to compile testpres0
              (an example for any other program calling pres0)
pres0.F       the pres0 subroutine 
testpres0.F   the test program which illustrates how 'pres0' should be called
              (the path to datafiles 'dset' is set to 'MCD_DATA/' edit or
              change it to suit your needs)

DESCRIPTION
===========

Pres0 si a subroutine designed to determine surface pressure on Mars at
any given point in latitude, longitude, local time and solar longitude. It
uses data issued from the Mars Climate Database but is
independant from it, so you don't need the full MCD to be installed to use
pres0. Note however that pres0 uses the following data files:
VL1.ls        Data file of solar longitude and daily mean surface pressure
              recorded by Viking Lander 1
mola32.nc     Mola topography at a resolution of 32 pixels/degree
ps_clim.nc    Subset of data from the Mars Climate Database

If you don't install the full database, you obviously will have to make
sure these files can be accessed by pres0.


HOW IT WORKS
============

1- Calculation of the daily averaged pressure at the exact coordinate
(x,y,z,t) of Viking Lander 1 (z known from MOLA) (horizontal bilinear
interpolation for x,y, hydrostatic correction for z, linear interpolation
on months and time of day) as deduced from the MCD.

2- Calculation of a correction factor (coef) between the "MCD" surface
pressure at viking 1 and the smoothed measurements (in VL1.ls file) to
correct atmospheric mass discrepancy.

3- Calculation of the surface pressure at the selected point (x,y,t) 
(z known from MOLA) as deduced from the MCD (horizontal bilinear
interpolation for x,y, hydrostatic correction for z, linear interpolation
on months and time of day).

4- Application of the correction factor coef, to the MCD surface pressure
at the selected point.

Note that the Mean temperature used for the hydrostatic correction is the
temperature in the 7th atmospheric layer, i.e. an altitude of ~1km.


RUNNING PRES0
=============

Prerequisites:
--------------

To run pres0 you need also:

- a Fortran compiler
- the netCDF library

NetCDF library was tested on various systems (AIX,HP-UX,IRIX and
IRIX64,Linux,MacOSX,OSF1,Solaris, SunOS4,UNICOS). If you have not
installed it yet, you'll find installation instructions on this web page:

  http://www.unidata.ucar.edu/software/netcdf

Pres0 Input/output:
------------------

Pres0 input arguments:
    - dset   string containing path to datafiles VL1.ls, mola32.nc and 
             ps_clim.nc. e.g.: "path/to/files/"  (must end with a trailing '/')
    - lon    Longitude coordinate of the point (East degrees)
    - lat    Latitude coordinate of the point (North degrees)
    - solar  Ls Solar longitude (degrees)
    - loctim Local time at longitude lon (hours)

And you have 3 output values:
    - pres   surface pressure (Pa) at given space and time coordinates.
    - alt    surface's above areoid altitude (m) at given space coordinates
    - ierr   control variable (0 if all went well) (see file pres0.F)