20040218: contour plot aesthetics

Mike,

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Also, our general GEMPAK support address is
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Since you did not mention what the grid spacing of the data set you
are looking at, my first question is why do you want to use SKIP 
to thin the data set? The GEMPAK help states:

SKIP is a variable which determines the contour points or plot
      points to skip.  Input is entered as:
  
        skip_contour / skip_plot_x ; skip_plot_y
  
      The defaults for skip_contour and skip_plot are 0.
  
      Skip_contour thins the input grid before computing the
      contours to be drawn.efore contouring.

When you change the GAREA, you will change the grid points
being displayed, and therefore which points are skipped.
If you are skipping a relative maxima or minima, then you will notice
a shift in location of the feature. If you reduce the data points being
used, and your data set is a highly varying field (eg, your divergence 
field will sharply change from + to - values) then you will effect the 
max/min locations. A smoothly varying field of 500mb hght would be less 
sensitive and therefore SKIP can be more useful. 

If your grid is larger than the number of grid points you can view
within your area, then you would want to use IJSKIP to subset your grid
rather than SKIP. If you want to smooth the shape of the contours,
then look at the CONTUR parameter. Using SKIP will speed up the display
process since you are considering fewer grid points in the contour 
algorithm, but you should consider how noisy the resultant data field 
is, and do not under sample the data in highly varying quantities. 

For example, using the ETA 212 grid, today at F000, the quantity above 
veries from -19**-13 to 34**-13 over the Sierras. Here is a GDLIST 
output of a section of a row: 
SCALE = 13
ROW 59     
-1.80    
-0.47     
0.89    
-2.48    
-0.68    
-2.66    
-3.40    
14.34
19.46   
-11.62   
-13.53     
0.60     
0.40    
-1.40    
-0.50    
-0.95
1.87    
-0.51    
-2.99     
0.30

If you skip every other point, EG skip=1, values remainin are:
-1.8, .89, -.68, -3.4, 19.46, -13.53, .4, -.5, 1.87, -2.99
you will still have both the greatest value (19.46) and the least
(-13.53) in your list. This is still useful.
If you set SKIP=2, values remaining are:
-1.8, -2.48, -3.4,  -11.62,  .4,  -.95,  -2.99
then you will miss both 14.34 and 19.46. In fact, you will lose 
almost all positive values- and the minimum will shift to
the -11.62 location instead of the -13.53 location.

The fact that the field varies from 19.46 to -11.62 in adjacent
grid points illustrates how sensitive the field is (some q vector
references can be found from Fred Sanders and/or Mel Shapiro).
Also beware of using auto SCALE=999 since in boring conditions,
you might have lots of contours, but the SCALE several orders
of magnitude greater (If you are generating a time series, you
most likely want to fix the SCALE appropiately). This also changes the
perception/look of the plot.


Steve Chiswell


 



On Wed, 2004-02-18 at 13:17, Michael Evans wrote:
> Steve and Robbert,
> 
> I have a question about GEMPAK and was told by Dave Novak at NWS eastern
> region headquarters that you two are often helpful when it comes to
> answering questions about gempak on a gempak mailing list.
> 
> My question: I am looking at divergence of Q vectors using gempak, using
> the following:
> 
> GLEVEL=500:700
> GFUNC=div(sm5v(qvec(ldf(hght),vlav(geo)))).
> 
> I have been varying the SKIP and GAREA parameters, and have been seeing
> some fairly large differences in my resulting Q vector convergence
> patterns.  I have mostly used SKIP=4, occasionally SKIP=3, and I have
> been specifying my GAREA by listing a station, for example: GAREA=avp,
> GAREA=avp-, GAREA=abe ect.  Obviously I would like to see not much
> change in my div Q result when I change these parameters, but I have
> been seeing some big variations.  For example, changing GAREA from avp
> to mdt (keeping everything else the same, including SKIP=4) has been
> enough to move my convergence maximum from southwest WV, to northern
> Virginia.
> 
> So, I guess what my question boils down to is: is there a recommended
> size for my GAREA, and recommended value for SKIP to use when
> calculating Q vector divergence.  The calculation seems to be very
> sensitive to how those two parameters are set.
> 
> Any help appreciated!
> 
> - Mike