Subject: Re: water content in atmosphere

From: [email-masked] (Tim Thompson)
Date: 10 Feb 1997 16:55:08 GMT

In article <[email-masked]>,
sven munk <[email-masked]> writes:

> In a project where a refractor supplied with a CCD camera is used to
> determine stellar magnitude and temperature we found that absorption
> of radiation in the near infrared caused by water in the atmosphere
> has the effect of reducing the accuracy of the measurement. We have
> found spectra showing the absorption bands but do not know how much
> the water concentration will vary during the day and the year.
> Anybody knowing where to find the missing information ?

   In the absence of real-time data for your specific site, such as
a radiosonde or water vapor radiometer, then your best bet is to use
assimilation data sets available from the Environmental Modeling Center.
Their appropriate anonymous FTP site is ...

                    ftp://nic.fb4.noaa.gov/pub/fnl/

which you can also access with Netscape. The files you will be looking
for are named ...

                    gdas1.T00Z.PGrbF00
                    gdas1.T06Z.PGrbF00
                    gdas1.T12Z.PGrbF00
                    gdas1.T18Z.PGrbF00

   These files do contain atmospheric profiles for the entire world,
and are updated every 6 hours [T06Z = 06 hours UT and etc.]. Every 24
hours the files are overwritten, so you have to get them yourself; to
the best of my knowledge, the EMC does not archive the data.

   The data are in an the "GRIB" binary format designated by the WMO.
Postscript and Word Perfect (I think) format documents that describe
the data format in detail are on the FTP site, in the directory ...

        ftp://nic.fb4.noaa.gov/pub/nws/nmc/docs/gribed1/

Download all of the files "sect0" through "sect5" of the appropriate format.
Together they make up "Office Note 388", which is thus far the only full
documentation for the data file format.

   Fortran code to read the packed binary files is also available on the
FTP site under the directory ...

        ftp://nic.fb4.noaa.gov/pub/info/

Here you wiill find a number of directories named 'grib[something]', such
as ...

        ftp://nic.fb4.noaa.gov/pub/info/gribsun/

'Gribsun' is the directory that holds code for Sun workstations, but there
is also code for SGI, Cray and etc.

   These profiles come from a large scale data assimilation project
that sucks in all manner of data - radiosonde, satellite, surface weather
observations, anything and everything, and assimilates an atmosphere model.
There is no documentation that I know of that describes the EMC data
assimilation methods.

   The EMC homepage is located at ...

       http://nic.fb4.noaa.gov:8000/

and you should direct any questions relating to all this to them. I just happen
to have already navigated these sites, as I need their data for my own work.
It takes a while to get used to their software, but once you learn how to
extract the profiles it can all be done in pretty automated fashion.

   Unfortunately, you are pretty much on your own dealing with these data.
Despite their obvious value, there just isn't a lot of effort expended at
the EMC to support users.

   There is also a large NASA data assimilation effort, the Data Assimilation
Office, with a home page at ...

       http://hera.gsfc.nasa.gov/dao.home_page.html

Their product does not have the global resolution of the EMC product (yet),
but their data assimilation methods are heavily documented. For now I
recommend the EMC data.

-- 
-----------------------------------------------------------------
Timothy J. Thompson,              [email-masked]

NASA/JPL Terrestrial Science Research element
Advanced Spaceborne Thermal Emission and Reflection Radiometer.
Atmospheric Corrections Team - Scientific Programmer.