[Met_help] threshold values

John Halley Gotway johnhg at ucar.edu
Tue Apr 20 16:15:36 MDT 2010


Yep, I was just writing you a message about that.  Let me know if that's sufficient for you.  Or if there's an enhancement you'd like to see to "filter" the raw values using a threshold before
computing continuous statistics, we could add that the list of requested upgrades for future development.


Shaw, Michael J CTR USAF AFWA 16 WS/WXE wrote:
> Ah - I'm seeing I can also threshold the masks.  I.e., use the mask for obs, e.g., for values greater than 20mm and compare the model...
> Thanks,
> Mike
> -----Original Message-----
> From: John Halley Gotway [mailto:johnhg at ucar.edu] 
> Sent: Tuesday, April 20, 2010 5:05 PM
> To: Shaw, Michael J CTR USAF AFWA 16 WS/WXE
> Cc: 'met_help'
> Subject: Re: threshold values
> Mike,
> 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
>> 402-232-7690 Comm * 402-272-7690 DSN * 402-294-8230 Fax

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