From tim@lithos.jpl.nasa.gov Fri Feb 14 08:55:01 1997 Path: solitaire.cv.nrao.edu!hearst.acc.Virginia.EDU!uunet!in1.uu.net!207.22.81.9!europa.clark.net!news.msfc.nasa.gov!pecos.msfc.nasa.gov!not-for-mail From: tim@lithos.jpl.nasa.gov (Tim Thompson) Newsgroups: sci.astro.research Subject: Re: water content in atmosphere Date: 10 Feb 1997 16:55:08 GMT Organization: NASA/JPL, Terrestrial Science Research Element Lines: 89 Sender: astres@pecos.msfc.nasa.gov Approved: astres@pecos.msfc.nasa.gov Distribution: world Message-ID: <5dnju1$649@pecos.msfc.nasa.gov> References: <5dd247$3qc@pecos.msfc.nasa.gov> Reply-To: tim@lithos.jpl.nasa.gov NNTP-Posting-Host: pecos.msfc.nasa.gov Keywords: techniques X-Posting-Tool: modtool v1.1 In article <5dd247$3qc@pecos.msfc.nasa.gov>, sven munk 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, Timothy.J.Thompson@jpl.nasa.gov NASA/JPL Terrestrial Science Research element Advanced Spaceborne Thermal Emission and Reflection Radiometer. Atmospheric Corrections Team - Scientific Programmer.