Grid-Stat: Neighborhood Methods

Another way to analyze forecasts in a spatial context is to use neighborhood methods. These methods are implemented in the Grid-Stat tool. Two main parameters are used for neighborhood methods: the size of the neighborhood around each point and a raw threshold value to be applied to the forecast and observation fields.

The Grid-Stat configuration file contains several options for neighborhood methods:

• The fcst_thresh and obs_thresh parameters specify the raw threshold(s) used to define the 0/1 bitmap fields on which the neighborhood method is run
  
• The nbr_width parameter specifies the size(s) of the neighborhoods (e.g. a width of 5 means a 5x5 square)
• The nbr_thresh parameter specifies the ratio of valid points a neighborhood must contain to derive a coverage value for it - typically applies along the edges of the domain
• The cov_thresh parameter specifies threshold(s) for the coverage fields themselves
• The output_flag parameter contains entries for the NBRCTC, NBRCTS, and NBRCNT line types

Look in the following file to see contingency table counts for neighborhoods of size 9 (3x3 square) and 25 (5x5 square):

out/grid_stat/grid_stat_APCP_24_240000L_20050808_000000V_nbrctc.txt

The statistics are calculated using the neighborhood coverage value for both forecasts and observations. A coverage value is calculated for a target point using the 9 neighbor points around it. So in this example, if 5 of the 9 neighboring points have values above the threshold, the forecast value at the target point is 5/9. If 3 of the 9 points have observed values above that threshold, then the observed value at the target is 3/9.  From this step, all statistics are calculated as they would be on standard forecast and observation values.

As a comparison, you may want to compare the forecast verified with a neighborhood method to one verified with the regular grid matching method.

head -2 out/grid_stat/grid_stat_APCP_24_240000L_20050808_000000V_nbrctc.txt
head -2 out/grid_stat/grid_stat_APCP_24_240000L_20050808_000000V_ctc.txt