19990510: circ vectors in gdcross

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Date: Mon, 10 May 1999 14:29:32 -0600
From: Steve Chiswell <address@hidden>
To: John Egentowich <address@hidden>
Subject: 19990510: gdcross circ function
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John,

The OMEG values (V component of circ) will be in mb/sec.
The U component will be the tangential component of AGE in m/s.
So, the two sides of the vector aren't the same units to begin with.


Circ returns a vector whose U component is the tangential
component of V (in your case age()) and the V component is the
vertical motion. The cross sectional representation of the vectors
will be scaled by the aspect ratio of the plot due to the scale
exaggeration between the horizontal and vertical distances on
a plot. For example, a cross section with cxstns=den>bna
has a horizontal distance on the order of 2000km, while the
vertical scale is on the order of 10km...so the length of
the vertical vector is also exaggerated around 200:1.

To see what the values of the V component (aka omeg) are,
GFUNC    = vr(circ(age,omeg))
SCALE    = 
for todays' eta running CXSTNS   = den>bna,

shows:
 Grid file:                 $HDS/1999051012_eta_grid211.gem
 Date/time:                 f000
 Vertical coordinate:       pres
 Grid function:             vr(circ(age,omeg))
 Vector function:           circ(age,omeg)
 Endpoints:                 den>bna           Number of points:    21
 Scaling factor:        10** 3      Panel:  0                                   
        


 LEVELS:        -4.00    -2.00     0.00     2.00     4.00
 COLORS:            3        3        3        3        3
 LINTYP:            1        1        1        1        1
 LINWID:            1        1        1        1        1
 LABEL:             1        1        1        1        1


When scale is set to "blank" so the program picks its own, you
see that the omeg values range from -4 x10-3 to 4x10-3 mb/sec
(or roughly -4 to 4 microbars per second).

If you set SCALE=0, you see that the range is -.004 to .004 mb/sec
which is equivalent to the above -4 to 4 microbars.
To see the U component of circ use gfunc=ur(circ(age,omeg)).

As far as what symbol to use in the WIND= parameter, it really makes
no since to use wind barbs- since they are a horizontal representation
of wind speed- and thats not what we have here.
Most likely you want WIND=an.

So what would a plot look like if the omega values weren't being scaled
so much?
Try:
GFUNC=mag(circ(age,omeg))
GVECT=vecr(ur(circ(age,omeg)),mul(vr(circ(age,omeg),.001)))

My only suggestion as to how to quantify for the reader the relative
magnitude of the omega and tangential ageostrophic wind is to
use contours for both quantities with the vectors.

Steve Chiswell
Unidata User Support
address@hidden


On Mon, 10 May 1999, John Egentowich wrote:

> Steve,
> I have a quick gempak question.  When I create cross sections to include      
> ageostrophic winds using "circ(age,omeg)" and displaying wind barbs in m/s -- 
>   
> are the vertical speeds scaled up?  I get cross sections that have great 
> looking
> ageo circulations however the vertical wind speeds are unreasonable.  In 
> "quiet"
> environments (without convection) I have vertical ageo wind speeds of 10-20 
> m/s.
> That can't be right unless the vertical component is scale up.  Please 
> enlighten
>  me.  Thanks for your help.
> John
> 
> ----------------------------------
> E-Mail: John Egentowich <address@hidden>
> Date: 10-May-99    Time: 07:29:37
> Phone: 919-515-1437
> ---------------------------------
>