20000309: gdvint interpolation

>From: "Matthew G. Fearon" <address@hidden>
>Organization: Desert Research Institute
>Keywords: 200003092327.QAA01340

>Steve,
>
>I was wondering the methodology or scheme that is used within
>the gdvint program to interpolate from one vertical coord. to the
>other. For example, say I want to convert from pressure to sigma
>and I use the following specification with the gdvint program
>
>   glevel   = 8000-4500-500;10000-8050-50
>   gvcord   = pres/sgma:1
>
>this will give me back sgma levels of 10000, 9950, 9900, 9850,
>9800 etc.. to 8000  and levels of 8000, 7500, 7000 etc.. to 4500
>
>I realize sigma is p/psfc but there are only so may pressure levels
>available in model output, e.g. 1000 950 900 etc every 50mb, to
>use for interpolation of the sigma levels. I was wondering kind
>of vertical interpolation method is being used besides p/psfc. Is it
>linear or is some other scheme being used ?
>
>Sorry my question is a bit wordy. Hopefully you know what
>I am getting at. Please reply when it is convenient.
>
>Thank you,
>Matt Fearon
>address@hidden
>
>


Matt,

The interpolation routines are called from: 
$GEMPAKHOME/source/programs/gd.gdvint/vivint.f (calls to vc_quad, vc_lnin)

Where possible, the quadratic interpolation using log pressure are performed,
otherwise, a linear interpolation using lon pressure is performed.

Steve Chiswell