# [gradsusr] Question about why the wind vectors barbs not showing up for the 0-6 KM Shear difference

Nimrod Micael nmicael at aggies.ncat.edu
Sun Sep 1 20:00:52 EDT 2013

```I dont understand how not taking a vector difference if that is not the
correct way get's to getting to a wind barb like in your example.

On Sun, Sep 1, 2013 at 7:58 PM, Nimrod Micael <nmicael at aggies.ncat.edu>wrote:

> Yes that's exactly what I want...what you have in your example on your
> site. I understand that I have to do this:
>
>
> ***although height is in km though in my script**
>
> "define mag1000="zinterp(mag(u,v),height,1000)
>
> "define mag6000="zinterp(mag(u,v),height,6000)
>
> "06shear = mag6000-mag1000?
>
> "d 06shear"
>
>
> Would I then be able to do just do that?
>
>
> function zinterp(field,zgrid,zlev)
>
> *----------------------------------------------------------------------
> *
> * Bob Hart (hart at ems.psu.edu) /  PSU Meteorology
> * 3/4/1999
> *
> * GrADS function to interpolate within a 3-D grid to a specified
> * height level.  Can also be used on non-pressure level data, such
> * as sigma or eta-coordinate output where height is a function
> * of time and grid level.
> *
> * Advantages:  Easy to use, no UDFs.  Disadvantages:  Can take 3-10 secs.
> *
> * Arguments:
> *    field = name of 3-D grid to interpolate
> *
> *    zgrid = name of 3-D grid holding height values at each gridpoint
> *            Units of measurement are arbitrary.
> *
> *    zlev  = height level at which to interpolate (having same units as zgrid)
> *
> * Function returns:  defined grid interp holding interpolated values
> *
> * NOTE:  Areas having zlev below bottom level or above upper level
> *        in output will be undefined.
> *
> * NOTE:  No distinction in the function is made between height above
> *        sea level, height above ground surface, or geopotential. The
> *        function will give output regardless of which is sent.
> *        It is up to the user to be aware which height variable is
> *        being passed to the function and treat the output accordingly.
> *
> * Example function calls:
> *
> *      "d "zinterp(temp,height,5000)
> *
> * Would display a temperature field interpolated to 5000.
> *
> *      "define t1000="zinterp(temp,height,1000)
> *
> * Would define a new variable, t1000, as a temp field at 1000.
> *
> *      "d p1000="zinterp(lev,height,1000)
> *
> * Would display a field of the pressure at a height of 1000.
> *
>
>
>
>
> On Sun, Sep 1, 2013 at 7:30 PM, Jeff Duda <jeffduda319 at gmail.com> wrote:
>
>> The reason you are only approximating the 0-6 km shear is because you
>> are substituting the 6 km AGL winds for the 500 mb flow.  Since most
>> datasets are isobaric and don't offer data on a constant height AGL
>> surface, it's common to use the 500 mb flow since 500 mb is reasonably
>> close to 6 km AGL for most locations over the central and eastern US.
>> It does remain an approximation since it may be up to a few thousand
>> feet off.  In my experience viewing observed soundings, during the
>> warm season the 6 km AGL level is typically a little above 500 mb (in
>> the 450-500 mb layer).  There is a Grads script out there that can use
>> linear interpolation in the vertical to interpolate data on isobaric
>> surfaces to constant height surfaces if you need very accurate
>> estimates.  Look for zinterp.gs. It might even be in the grads
>> scripting library on the web site.  I use that script to display 0-6
>> km shear on my forecasting web site.  Here is an example:
>> http://www.meteor.iastate.edu/~jdduda/forecast/NAM/18Z/0-6_shear_f24.gif
>>
>> Jeff Duda
>>
>> On Sun, Sep 1, 2013 at 6:12 PM, Nimrod Micael <nmicael at aggies.ncat.edu>
>> wrote:
>> > @Jeff,
>> >
>> > I see but is it approximation? That's my understanding from Rich
>> Thompson's
>> > post. http://www.stormtrack.org/forum/archive/index.php/t-23323.html
>> >
>> >
>> > On Sun, Sep 1, 2013 at 6:55 PM, Jeff Duda <jeffduda319 at gmail.com>
>> wrote:
>> >>
>> >> The contours are just the magnitude of the shear vector, but that's
>> >> not the same as the difference in wind speed at two levels.
>> >>
>> >> On Sun, Sep 1, 2013 at 4:58 PM, Nimrod Micael <nmicael at aggies.ncat.edu
>> >
>> >> wrote:
>> >> > I see what your saying Jeff. But what if you are interested in the
>> >> > magnitude
>> >> > only of u and v and not the two separate u and v vectors. I am
>> trying to
>> >> > get
>> >> > a map like this....
>> >> >
>> >> > http://www4.ncsu.edu/~nwsfo/storage/cases/20100425/shr6_10042600.gif
>> >> >
>> >> > This doesnt have two separate vectors: u and v.
>> >> >
>> >> >
>> >> >
>> >> >
>> >> >
>> >> > On Sun, Sep 1, 2013 at 5:52 PM, Jeff Duda <jeffduda319 at gmail.com>
>> wrote:
>> >> >>
>> >> >> The way you're doing it, you're only computing the difference in
>> wind
>> >> >> speed between two layers.  If you're trying to calculate vertical
>> wind
>> >> >> shear, you should be calculating the vector difference, which is the
>> >> >> difference in the components.  So you should have a delta-U wind
>> >> >> component of the shear (u500-usfc) and a delta-V component of the
>> >> >> shear (v500-vsfc).  Those are the two quantities you would display
>> to
>> >> >> get the vectors.
>> >> >>
>> >> >> Jeff
>> >> >>
>> >> >> On Sun, Sep 1, 2013 at 3:39 PM, Nimrod Micael <
>> nmicael at aggies.ncat.edu>
>> >> >> wrote:
>> >> >> > I see Jeff. So then how would i make it to display the magnitude
>> of
>> >> >> > u,v
>> >> >> > for
>> >> >> > 2 different levels then? Since I am taking the difference between
>> the
>> >> >> > 500 mb
>> >> >> > wind and the sfc wind?
>> >> >> >
>> >> >> >
>> >> >> >
>> >> >> > On Sun, Sep 1, 2013 at 4:33 PM, Jeff Duda <jeffduda319 at gmail.com>
>> >> >> > wrote:
>> >> >> >>
>> >> >> >> displaying wind barbs requires two inputs in the display command
>> >> >> >> (i.e., 'd u;v').  You are only supplying one.
>> >> >> >>
>> >> >> >> Jeff Duda
>> >> >> >>
>> >> >> >> On Sun, Sep 1, 2013 at 2:53 PM, Nimrod Micael
>> >> >> >> <nmicael at aggies.ncat.edu>
>> >> >> >> wrote:
>> >> >> >> > The shaded is plotting fine but the wind barb is not showing
>> up.
>> >> >> >> > Can
>> >> >> >> > anyone
>> >> >> >> > help me out why?
>> >> >> >> >
>> >> >> >> > 'set gxout shaded'
>> >> >> >> > 'set lev 1000'
>> >> >> >> > 's= mag(u,v)'
>> >> >> >> > 'set lev 500'
>> >> >> >> > 'a = mag (u,v)'
>> >> >> >> > 'set cmin 20'
>> >> >> >> > 'bulk = (a-s)*1.92'
>> >> >> >> > 'd bulk'
>> >> >> >> >
>> >> >> >> > 'set gxout barb'
>> >> >> >> > 'set lev 1000'
>> >> >> >> > 'barbs= mag(u,v)'
>> >> >> >> > 'set lev 500'
>> >> >> >> > 'barba = mag (u,v)'
>> >> >> >> > 'set cmin 20'
>> >> >> >> > 'bulkb = (barba-barbs)*1.92'
>> >> >> >> > 'd bulkb'
>> >> >> >> >
>> >> >> >> >
>> >> >> >> > Wind vectors are not showing up. Why???
>> >> >> >> >
>> >> >> >> > Thanks.
>> >> >> >> >
>> >> >> >> > --
>> >> >> >> > Nimrod Micael
>> >> >> >> >
>> >> >> >> > _______________________________________________
>> >> >> >> > gradsusr mailing list
>> >> >> >> >
>> >> >> >>
>> >> >> >>
>> >> >> >>
>> >> >> >> --
>> >> >> >> Jeff Duda
>> >> >> >> Graduate research assistant
>> >> >> >> University of Oklahoma School of Meteorology
>> >> >> >> Center for Analysis and Prediction of Storms
>> >> >> >> _______________________________________________
>> >> >> >> gradsusr mailing list
>> >> >> >
>> >> >> >
>> >> >> >
>> >> >> >
>> >> >> > --
>> >> >> > Nimrod Micael
>> >> >> >
>> >> >> > _______________________________________________
>> >> >> > gradsusr mailing list
>> >> >> >
>> >> >>
>> >> >>
>> >> >>
>> >> >> --
>> >> >> Jeff Duda
>> >> >> Graduate research assistant
>> >> >> University of Oklahoma School of Meteorology
>> >> >> Center for Analysis and Prediction of Storms
>> >> >> _______________________________________________
>> >> >> gradsusr mailing list
>> >> >
>> >> >
>> >> >
>> >> >
>> >> > --
>> >> > Nimrod Micael
>> >> >
>> >> > _______________________________________________
>> >> > gradsusr mailing list
>> >> >
>> >>
>> >>
>> >>
>> >> --
>> >> Jeff Duda
>> >> University of Oklahoma School of Meteorology
>> >> Center for Analysis and Prediction of Storms
>> >> _______________________________________________
>> >
>> >
>> >
>> >
>> > --
>> > Nimrod Micael
>> >
>> > _______________________________________________
>> >
>>
>>
>>
>> --
>> Jeff Duda
>> University of Oklahoma School of Meteorology
>> Center for Analysis and Prediction of Storms
>> _______________________________________________
>>
>
>
>
> --
> Nimrod Micael
>

--
Nimrod Micael
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