ESRL Global Systems Division

Probabilistic Forecasting Software Application Information

CalcEnsembleStats

The CalcEnsembleStats procedure computes up to six statistics over one of the three weather elements: Temperature, 3-hr total precipitation, and wind speed. The figure below (click on image for full resolution) shows the dialog that appears when the procedure is invoked.

When the tool is invoked, the user may select one of the Weather Elements on the left side of the dialog. Then the user may select any of the six statistics in any combination. After selecting "Run" or "Run/Dismiss" the tool calculates each statistic as a separate grid and displays the result as a temporary weather element in the GFE.

The statistics are defined as follows:

Mean - Arithmetic Mean. Values at each grid point are summed and divided by the number of models (usually 21).
Mode - Most frequent value. The most frequent value for each grid point over all ensemble members is calculated. The gridded display for mode tends to have a speckled appearance.
Median - The median is the value at each there are as many values on the left of the distribution as on the right. When Median is selected this value is displayed at each grid point.
StdDev - The standard deviation represents an average departure from the mean value. The gridded display of standard deviation gives the forecaster a measure of where ensemble members agree or disagree. Smaller value indicate agreement while larger values indicate disagreement.
10 Pct - Calculates a grid that represents the 10th percentile for that grid point.
50 Pct - Calculates a grid that represents the 50th percentile for that grid point. This is also called the median.
90 Pct - Calculates a grid that represents the 90th percentile for that grid point.

Tutorial

The following tutorial will demonstrate the features of the Calc Ensemble Stats tool.
  1. Make sure you have the ensemble members loaded into the GFE by using the WeatherElements->WeatherElementGroups menu. There are several options to load all 21 SREF members at once for each weather element.
  2. Select one of the grids that you loaded into the GFE to define the time range for the tool.
  3. From the ProbTools menu, select CalcEnsembleStats.
  4. When the dialog appears, select the weather element of your choice and any or all of the statistical methods listed in the second column.
  5. Select Run or Run/Dismiss.
  6. The tool will take a few seconds to load the data and generate the statistics. They will display automatically in the Grid Manager. Select any of them with Mouse Button-1 (MB1) to display them in the Spatial Editor.
  7. Change the time in the Grid Manager and run the tool again with a different weather element.