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[IDV #ZXW-221688]: IDV v3.1: Simple difference formula

Greetings Dave!

I've checked into this, and I am not sure what's happening here. However,
I was able to get thing to work *if* I do the following.

1) Create a simple contour plot of the analysis Geopotential heights, but
make sure to select "Set As Time Driver" in the dropdown box in the time
subset panel of the Field Selector.

2) Do the simple difference formula, but this time when you select the
forecast heights for the 'a' field, make sure to select "Match Time
Driver" in the dropdown box of the time subset panel. Also, don't
forget to select the level as 500 mb, or you will get an error.

3) For the 'b' field, select your analysis geopotential height field
and leave the dropdown box in the time subset panel as Use Default.

By forcing the Analysis to be a time driver and the forecast to use
those times, we ensure that 5 fields are used. While this is a workaround,
it does not address the real issue. I'll look into this more. Did you used
to be able to do this kind of analysis successfully in the IDV? It's possible
a bug was introduced when the time driver stuff was implemented.

Cheers,

Sean

> IDV supporters,
> 
> I'm running IDV v3.1 on a Mac (OS X v10.6.8).
> 
> I have a NAM 40 km forecast file with an analysis time, forecasts every 6 
> hours out to 2 days, then another forecast 12 hours beyond that (to 2.5 days).
> 
> I also have separate files containing NAM 40 km analyses of 500 mb height 
> only, every 12 hours.
> 
> In other words, I have the following:
> 
> First row below: Analysis and forecast times available in NAM 40 km forecast 
> file (at least 2.5 days old)
> Second row: NAM analysis times available in separate files for 500 mb height
> 
> 12Z 18Z 00Z 06Z 12Z 18Z 00Z 06Z 12Z 00Z  (note the 12-hour jump between the 
> last two)
> --  --  00Z --  12Z --  00Z --  12Z 00Z
> 
> I want to compute the difference between the forecast and analyzed 500 mb 
> heights at the times for which I have both available.
> 
> So, I load the NAM forecast file, then I load five NAM 500 mb height analysis 
> files while specifying that the Data Source Type is "Aggregate Grids by Time".
> 
> [As a little detour, I select Data Sources: > 12112612 nam (the NAM forecast 
> file) > 3D grid > Geopotential_height @ isobaric, and I note that under 
> "Times", the 10 times appearing in the first row of available times above are 
> listed, as I expect.]
> 
> I then select Dashboard > Data Sources > Formulas > Fields > Miscellaneous > 
> Simple difference a-b (and also ... > Displays > Plan Views > Color-Filled 
> Contour Plan View), and click "Create Display".
> 
> So far so good.
> 
> For "Field: a", I select the NAM forecast file (12112612 nam) > 3D grid > 
> Geopotential_height @ isobaric, and specify Level > 500 hPa. It shows times 
> from the 12Z analysis at 6 hour intervals all the way to the 00Z forecast 2.5 
> days later, a total of 11 different times. Curiously, this includes a 
> forecast time at 2 days, 6 hours, which is not actually present in the 
> file--a first warning that something bad will soon happen?
> 
> For "Field: b", I select the time-aggregated NAM analyses of 500 mb height  
> (12112812 HQA5 nam) > Geopotential_height @ isobaric. Five times are listed, 
> as I expect.
> 
> Wanting to match common times in the two files, for "Field a:" I click on the 
> "Times" tab, pull down the menu and select "Use Selected", and highlight only 
> the five times for which I also have NAM analyses of 500 mb heights. In 
> "Field b:", I use the default times (the five time-aggregated NAM 500 mb 
> height analysis file times).
> 
> Then I click "OK".
> 
> The result is an animation containing four frames, not five. (Based on the 
> times appearing in the display labels for each plot, the last one of the five 
> common times is not included.)
> 
> What happened to the last of the five plots in the animation? (And are the 
> four plots that are part of the animation difference fields of the 500 hPa 
> height fields at the times that I actually tried to specify? it's not obvious 
> that they should be, though they might.) And why did that spurious time show 
> up in the "Field: a" default time listing?
> 
> I've included a bundle for the plot. The data that the bundle accesses should 
> be around through about December 5 or 6 on our server.
> 
> -- Dave
> 
> 
> ************************************************************
> *   Dr. Dave Dempsey                        |       ^   ___       \|/     *
> *   Dept. of Geosciences                    |  ) ^     /||_||\    --0--  *
> *   San Francisco State University   | )  )    /  ||_||  \    /|\     *
> *   1600 Holloway Ave.                      |  )  )  /   ||_||    \          *
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> 
> 
> 
> 
> 


Ticket Details
===================
Ticket ID: ZXW-221688
Department: Support IDV
Priority: Normal
Status: Open