[Met_help] Question regarding parameter units

[John Halley Gotway]

Jeff,

The MODE tool in MET, which does the object-based verification, does use grid squares to characterize the size of and distance between objects.  Before running MODE, the user must ensure that the
forecast and observed datasets are on the same grid.  And MODE resolves forecast and observed objects on that grid, and all the computations and comparisons are done on that grid.  An object in MODE
is really just the union of a contiguous set of grid squares.

So the interpretation of the MODE output should always be done relative to the grid spacing.  For example, on a 4-km grid, an object with an area of 100 grid squares is roughly equal to 1600 sq km (4
* 4 * 100).  For regional domains where the area of each grid box doesn't vary much over the grid, this should work fine.  But for larger grids, where the area of each grid box does vary greatly, it
may not work as well.

The reason we do the math on the grid rather than converting back and forth between grid-relative units and earth-relative units is speed.  Using grid-relative values is much quicker.  In the ASCII
MODE output, we do dump out the centroid of each object both in grid units (x, y) and in (lat, lon).  But the areas and the distances are all in grid units.

Hopefully that clears it up.  Feel free to write with any more questions.

Thanks,
John Halley Gotway
johnhg at ucar.edu


Hamilton, Jeffrey A Mr USAF AFWA 2 WXG/ wrote:
> After running MET, I have noticed that a unit known as a grid square is
> very widely used to characterize the objects size and distance from
> forecast to observation. Those of us here at AFWA are assuming that
> there is some relationship between model grid resolution and the
> specific value of the grid square, but we wanted to check to be sure as
> to the exact characterization of the value.
> 
>  
> 
> Thanks for your input,
> 
>  
> 
> // SIGNED //
> 
> JEFF HAMILTON, Civ SCEP
> 
> Model Verification
> 
> 2 WXG/ WEA
> 
> Air Force Weather Agency
> 
> Offutt AFB, NE 68113
> 
> (402) 232-7207
> 
>  
> 
> 
> 
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