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Re: 2004102: vertical derivatives in GEMPAK?



David,

The wmogribX.tbl specifies the scaling that the GRIB parameters will
be stored in by the decoder. 

001 Pressure                          Pa          PRES   10^-2
039 Pressure vertical velocity        Pa s**-1    OMEG   10^-2

In this case, both are being scaled to millibars. So,
you would multiply the pressure values by 100 to get Pa in your formula
(which is what your formula shows).

GEMPAK traditionally uses mb as a unit, since your metar, synop
and upperair observations are using that quantity and calling it PRES
as well. GEMPAK does know to scale the PRES appropriately in
calculations in the grid routines where thermodynamic quantities are
calculated.

Steve Chiswell
Unidata User Support



On Thu, 2004-10-28 at 16:48, David Ovens wrote:
> Steve, 
> 
> I've got one more question for you.  It looks like
>   ldf(pres) 
> is in mb and not Pa and that omeg is in Pa/s.  Thus to get d(thta)/dp
> in Pa instead of my, I really want to do this
> 
>   mul(.01,quo(ldf(thta),ldf(pres)))
> 
> Do you agree?
> 
> David
> 
> On Mon, Oct 25, 2004 at 03:19:56PM -0600, Steve Chiswell wrote:
> > David,
> > 
> > LDF(s) will give you the layer difference where
> > you specify GLEVEL=level1:level2, and as
> > appropriate GVCORD=pres. This is the
> > equivalent of SUB(s1@level1,s2@level2).
> > If your vertical layer "delta-p" is reasonably small,
> > then you would be approaching D()/DP.
> > 
> > Steve Chiswell
> > 
> > 
> > 
> > 
> > 
> > On Mon, 2004-10-25 at 14:38, David Ovens wrote:
> > > Hello Gembuds,
> > > 
> > > I am trying to find out if anyone has written the veritcal equivalent
> > > of the DDX() and DDY() functions, namely, DDP().  A professor is
> > > interested in the tilting terms of the Petterson frontogenesis
> > > equation on constant pressure surfaces and we need the partial
> > > derivative of theta with respect to pressure, dthta/dp.  And, while
> > > we're at it, we may just go ahead and calculate the vertical
> > > deformation terms which require du/dp and dv/dp.
> > > 
> > > I've been hunting around in gemlib/df and gemlib/dv without finding
> > > anything helpful.  I can pretty much see how to do this myself, but am
> > > always interested in not re-inventing the wheel.
> > > 
> > > Thanks for any ideas.