Correcting Surface Pressure to Sea Level

[Mark Hess]

Perfect. Thanks. My experience with meteorological data is quite pedestrian, so please excuse my ignorance.  That is, however,  what I had suspected, but when I try to download the data using the NOMADS GRIB server and their "Grib Filter" tool, selecting the "mean sea level" level parameter and the "PRES" variable, the resulting file is empty.  When I retrieve the file using GrADSdap and your advice, it plots as expected.   Perhaps I am selecting incorrect parameters/variable when using the Grib Filter tool.  If you have any insight on this, all suggestions are welcome.  

Mark
  ----- Original Message ----- 
  From: Kevin M Levey 
  To: GRADSUSR at LIST.CINECA.IT 
  Sent: Thursday, March 11, 2010 1:32 PM
  Subject: Re: Correcting Surface Pressure to Sea Level


  Hi Mark,


  If you are plotting from the GFS grib data, then use the mean sea-level pressure variable instead of geopotential height, i.e. 


  'define mslp = prmslmsl/100' 


  prmslmsl is in Pascals, hence divide by 100 to get hPa (or mb).


  Regards,


  Kevin M Levey, MSc in Oceans and Atmospheric Sciences (University of Cape Town)
  Director of Meteorological Operations
  CustomWeather, Inc.
  San Francisco, California, USA


  "Taking the World by Storm!"


  http://www.customweather.com
  http://www.myforecast.com
  http://www.1stweather.com


  cell: 415-794-0411
  work: 415-777-3566
  email: klevey at customweather.com


  On Mar 11, 2010, at 03/11/10 - 12:16 PM, Mark Hess wrote:


    I tried to access the GrADS User archives for this answer, but the page would not allow me to search.Perhaps someone can point me in the right direction.  When plotting surface pressure (isobars)  from the GFS GRIB files, my data show very low pressure values over land.  While not sure, I am guessing this is because the data need to be corrected to sea level.  Is this correct? If so, can someone provide a resource which explains how to 'correct' the data? If I am incorrect, Can you please explain what I am doing wrong? Thanks.

    Mark


-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://gradsusr.org/pipermail/gradsusr/attachments/20100311/1a949365/attachment.html