[Met_help] threshold values

John Halley Gotway johnhg at ucar.edu
Tue Apr 20 16:04:58 MDT 2010


I think there's some confusion about how the thresholds you set in the "fcst_thresh" and "obs_thresh" parameters of the configuration file are actually used by MET.  The Grid-Stat and Point-Stat tools
compute a variety of different statistics, and those statistics are grouped together by type.  Two of those group are the "continuous" and "categorical" statistics.  The continuous type statistics are
all grouped together into the CNT lines.  The categorical type statistics are all grouped together into the CTS lines.

Continuous statistics are computed using the raw data values in the fields.  Whereas categorical statistics are computed by applying thresholds to the forecast and observation fields to create fields
of 0's and 1's, defining contingency tables, and deriving statistics.  As you change the threshold values, those contingency table counts and corresponding statistics will change.

However FBAR is a continuous statistic computed over the raw data values.  It's just the mean of the forecast values in the verification region.  There is no thresholding involved.  When you look at
an output CNT line from MET, the "fcst_thresh" and "obs_thresh" values in that line should be set "NA", meaning that thresholds do not apply.

Hope that helps straighten it out.  You can find more information about this in the MET User's Guide.

John Halley Gotway
met_help at ucar.edu

Shaw, Michael J CTR USAF AFWA 16 WS/WXE wrote:
> Hi.
> When I include a forecast thresh of 0 and fcst thresh of 20, masked over a region (EAST poly), I get the same average of precip (accumulated over a week or so) of about 13.5.  As in identical FBAR.  I'm obviously missing something - I wouldn't think a mean of 13.5 would be possible if I'm using only values >20.
> What am I missing?
> Thanks,
> Mike
> ______________________________________________
> Michael Shaw, Contractor
> SAIC/NASA Support Scientist
> michael.shaw.ctr at offutt.af.mil
> 16WS Environmental Characterization
> 101 Nelson Drive
> Offutt AFB, NE 68113-1023
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