549: Seasonal Statistics

When the Y file contains monthly or daily data, CPT will calculate a seasonal aggregate depending upon the type of variable in the Y files. CPT will calculate a seasonal average from the monthly or daily data if the Y data represent temperatures or precipitation rates (the average is weighted by the lengths of each month), or a seasonal total if the Y data represent counts or precipitation accumulations. Specifically, the method of aggregation is set depending upon the cpt:units tag. If the units are:

• 'K', 'Kelvin_scale', 'C', 'degree_Celsius', 'Celsius_scale', 'F', 'Farenheit'
• 'm/s', 'meters/second', 'm s-1', 'mm/day', 'cm/day'
• 'counts'

then a seasonal average is calculated. If the units are:

• 'm', 'meters', 'cm', 'mm', 'mm/month'

then a seasonal total is calculated. If the units are not listed above, then an average is calculated. However, the seasonal aggregate can be set using the Seasonal Statistics option. It is recommended that the default option be used since selecting a total when an average would be more consistent with the units of the data will result in a slightly different result, but one that is difficult to interpret. For example, if the input data are monthly rainfall in mm, CPT will default to calculating a seasonal total rainfall in mm, but if an average is selected, the months will b weighted by the length in days. The result will be an average rainfall per month, assuming that each month is of the same length (which is most likely not an intended result).

Additional aggregation options are available and will apply if the Y data are daily. These additional options require the setting of a threshold, t:

• counts below the threshold (i.e., the number of days with values lower than t);
• counts above the threshold (i.e., the number of days with values greater than t);
• occurrence below the threshold (i.e., a 1 if any of the days have values lower than t, and a 0 otherwise);
• occurrence above the threshold (i.e., a 1 if any of the days have values greater than t, and a 0 otherwise).