Re: GEMPAK helicity calculation?

David,

Helicity is computed within the NSHARP GUI. The storm relative motion
can be adjusted through the hodograph pane by moving the circle icons to
the desired
magnitude and direction.

The sndiag program I package with the Unidata distribution of GEMPAK
will
compute helicity from sounding data (as well as many other parameters).
This
program requires input for storm (SYSTEM) motion as well.

You can look at the source code for NSHARP in
$GEMPAK/source/programs/gui/nsharp/winds.c

I provided an example of a bulk computation using 1000mb and 700mb
levels in
the tutorial (example #4) here:
http://www.unidata.ucar.edu/software/gempak/tutorial/diagnostic_exercises.html

The example above uses the 850mb winds as a substitute for the mean
storm
motion, and the 1000:700 layer as a rough 0 to 3km layer. In reality
(and as in
sndiag and nsharp), you would integrate the speed shear (eg the terms in
the square
brakets) at all levels rather than just the 1000:700 levels as in the
example, but it
is a first order demonstration of the formula capabilities. Likely you
would want to use something for input of storm motion vector such as
what is provided with the
radar data (eg the storm relative wind product).

Steve Chiswell
Unidata User Support 


On Wed, 2008-01-09 at 14:34 -0600, David Stensrud wrote:
> Hi Steve,
> 
> I was searching through past GEMPAK postings and noticed your name
> associated with a discussion of using GEMPAK to calculate helicity from
> sounding data.  From what I can tell, this calculation is not available with
> the present GEMPAK software.  I am willing to spend the time to write code
> to accomplish this if I can get an example and some guidance on how to put
> it all together.  Your guidance on how best to calculate helicity from
> GEMPAK sounding data files would be most appreciated.
> 
> Thank you in advance for any suggestions.
> 
> Best regards,
> 
> Dave
> ___________________________________________________________________
> David J. Stensrud
> NSSL/FRDD, Room 4368
> National Weather Center
> 120 David L. Boren Blvd.    Norman, OK  73072-7304
> (405) 325-6170        address@hidden
> 
> "When you grab all you can get, that's what happens:
>  the more you get, the less you are."
>  - Eugene Petersen
> ___________________________________________________________________
> Now Available, Parameterization Schemes: Keys to Understanding Numerical
> Weather Prediction Models, David J. Stensrud
> For more information see www.cambridge.org/9780521865401
> 
> 
> 
-- 
Steve Chiswell <address@hidden>
Unidata