[Met_help] Clarification on data ingest by the MODE tool (METv2.0)

[John Halley Gotway]

Matthew,

It sounds to me like there's a few questions here...

(1) How does MODE handle distances between grid points?

MODE does all it's computations in the grid units, not actual physical distances.  So the area of an object is just a count of the grid boxes which comprise it.  And the distance between two objects
is just a count of grid units between the centers of the objects.  The parameter you're seeing in the configuration file "grid_res" actually isn't used all that much.  Actually, the only place it's
used is further down in the configuration file to set reasonable defaults for other parameters.  But you could read about those other parameters and set them how you'd like.

(2) What type of projections can the MET tools read?

MODE (as well of the other MET tools) can read data that's on lat/lon, lambert conformal, polar stereographic, or mercator projections.  It sounds like your data is on a global lat/lon projection,
which may work.  There are a couple of big issues with this though.  First, the size of each grid box on a global lat/lon grid will vary considerably.  However, MODE will count them all the same.
Depending on your data, if you want to treat objects near the poles the same as objects nears the equator, this will be problematic.  Second, you'll need to choose a longitude at which to cut the grid
in half - perhaps the prime meridian for example.  But if you have an object that straddles that line, MODE will treat it as two separate objects.  It isn't set up to handle true global data in that way.

I'm guessing the NCEP/NCAR reanalysis 1.0x1.0 degree GFS data is on NCEP Grid 3: http://www.nco.ncep.noaa.gov/pmb/docs/on388/grids/grid003.gif
If that's the case, MODE should be able to read data on that grid.

(3) What data format should I use?

You should use GRIB.  That'd be easiest.  And I'm guessing you'd be able to get this data in GRIB format.

Hope that helps.  Please let us know if more questions come up, and if you're having difficulty, feel free to send some sample data files and your MODE config file, and we'd be happy to take a look.

Thanks,
John Halley Gotway
johnhg at ucar.edu

Matthew Souders wrote:
> I am seeking a bit of clarification about how exactly the MODE tool ingests
> gridded data and accounts for distances between grid points. My data sets
> are both regular lat/lon grids (not regular dx/dy with constant spacing) and
> have global coverage (not a region like the WRF might)...I am working with
> NCEP/NCAR Reanalysis and GFS forecast fields from the archived 1.0X1.0
> degree grids. I need to know whether MODE can correctly interpret real
> distances from GRIB files in regular lat/lon format or whether I'd somehow
> need to convert regular lat/lon to regular dx/dy (a process which would be
> impossible if I were covering the entire globe since dx changes with
> increasing latitude).
> 
>>From what I can tell from the User's Guide for METv2.0, I can use any
> gridded data, but I have to make some kind of rough estimate as to the
> typical grid spacing. But lat/lon grids aren't regularly spaced enough for
> that to make sense. Any clarification you might be able to provide on how
> best to work with globally-covering regular lat/lon grids would be
> appreciated.
> 
> Matthew Souders
> Stony Brook University
> School of Marine and Atmospheric Sciences
> 
> 
> 
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