NCMRWF Global Ensemble Forecast System (NGEFS)
- At NCMRWF the Global Ensemble Forecast System (GEFS) configuration consists of four cycles corresponding to 00Z, 06Z, 12Z 18Z and 10-day forecasts are made using the 00Z initial condition.
- A T190L28 control that is started with T574L64 analysis and run out to 10 days
- 20 perturbed forecasts each run out to 10 days at T190L28 horizontal and vertical resolution.The initial perturbations are generated using Ensemble Transform with Rescaling (ETR) method. (See Wei et al., 2007; Initial perturbations based on the ensemble transform (ET) technique in the NCEP global operational forecast system; Tellus).
The Ensemble Mean and Spread
- The ensemble spread is a measure of the difference between the members and is represented by the standard deviation (Std) with respect to the ensemble mean (EM). On average, small (high) spread indicates a high (low) forecast accuracy.
- The ensemble spread is flow-dependent and varies for different parameters.
- It usually increases with the forecast range, but there can be cases when the spread is larger at shorter forecast ranges than at longer. This might happen when the first days are characterized by strong synoptic systems with complex structures but are followed by largescale “fair weather” high pressure systems.
Probabilistic forecasts of quantitative precipitation
- In these charts, the probability that 24-hour precipitation amounts over a 2.5x2.5 lat-lon grid box will exceed certain threshold values is given. The forecast probability is estimated directly from the 20-member global ensemble.
- At each grid point the number of ensemble members having a 24-hour precipitation amount greater than the limit considered is counted (M) and the probability is expressed as 100*(M/20). In addition to the color shading, the 5, 35, 65 and 95% probability isolines are also drawn.