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[IDV #NQA-622760]: IDV v3.1: Problem with "Simple difference over topography" plot

> IDV supporters,
> 
> I'm running IDV v3.1 on a Mac with Mac OS X v.10.6.8 (Snow Leopard).
> 
> Using a 40 km NAM model analysis and forecast GRIB file as a data source, I'm 
> trying to plot contours of 850-500 mb thickness over topography, where the 
> topography is defined by the 700 mb geopotential height field. However, there 
> is a problem. (Guess you wouldn't be reading this if there weren't a problem, 
> right? :-) )
> 
> There is no preexisting formula for 850-500 mb thickness per se, so I try to 
> use "Formulas > Miscellaneous > Simple difference a-b". I specify "Contours 
> Over Topography" as my display type, and click on the "Create display" button.
> 
> I'm prompted first for the field to use for the topography. For this, I 
> select "latest NAM > 3D grid > Geopotential_height @ isobaric", and further 
> specify "700 hPa" as the level to use.
> 
> Then I'm prompted for fields "a" and "b". For these, I specify "latest NAM > 
> 3D grid > Geopotential_height @ isobaric" and select "500 hPa" and "850 hPa", 
> respectively, as the levels to use. I then click "OK" to create the display.
> 
> Knowing that the 700 mb surface relief will be too small to discern using the 
> default vertical scale (0 to 16000 m), I rescale the plot by setting the 
> vertical range to 2500 m (minimum value) to 3500 m (maximum value). 
> (Sometimes I rescale the plot before actually creating a display, but I think 
> the result is the same.)
> 
> The result is the 500-850 mb thickness plotted on a pressure surface.
> 
> Unfortunately, that pressure surface is 500 hPa, not 700 hPa, as requested. 
> The 3D surface on which the contours are plotted float above the plotting 
> domain, which is configured to contain the 700 mb surface. The incorrect 3D 
> pressure surface is confirmed in the legend, which says that the level of the 
> displayed simpleDifference field is 500 mb.
> 
> When I access the Plot Control Window for the simpleDifference (500-850 mb 
> thickness) field, I see that I can reset the level. However, when I select 
> another pressure level (e.g., 700 mb, but 850 mb too), nothing appears to 
> change.
> 
> I've attached a bare-bones bundle to illustrate the point.
> 
> 
Hi Dave,
    This is a very interesting discovery. I played around and researched the 
source codes. You observation is corrected. For whatever reason, there is a 
minor difference in the logic in the creating contour over topo in these two 
different approaches. If you the difference field is not the same as the topo 
field, the result should be the same. I don't quite understand the logic at 
this moment, and would suggest to use the thickness approach if you need to 
difference the geopotential height.



Yuan
> 
> 
> Now, having said all that, I know that I can also plot 500-850 mb thickness 
> using a partially predefined function, namely "latest NAM > 2D > Dervied > 
> Thickness", with "Contours Over Topography" specified as the display type. 
> When I create the plot this way, the thickness is correctly plotted onto the 
> 700 mb surface.
> 
> However, it seems as if it should work doing it both ways. Is there a bug in 
> there somewhere?
> 
> -- Dave
> 
> ************************************************************
> *   Dr. Dave Dempsey                        |       ^   ___       \|/     *
> *   Dept. of Geosciences                    |  ) ^     /||_||\    --0--  *
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> 
> 
> 
> 


Ticket Details
===================
Ticket ID: NQA-622760
Department: Support IDV
Priority: High
Status: Closed