*3.6. Growth of Forecast Errors with Lead Time*

The 0–72 h forecasts of the ensemble groups driven by the GFS, GEOS, and GEM were examined to analyze the growth of the wind forecast errors with the forecast length (Figure 8). The MAE of the wind speed forecasts of the three ensemble member groups increases with the forecast length at a rate of ~0.4 m/s per day for the 3 days (Figure 8a). The MAEs of the wind speed forecasts of the GFS and GEOS groups are rather close to one another, while the GFS group has a slight advantage. The wind speed errors of the GEM group grow faster at nighttime, resulting in MAE of 0.4 m/s and 0.6 m/s more than the other two groups on the first and second days, respectively.

**Figure 8.** Variation in wind speed forecast errors with forecast time for the ensemble groups driven by the three global model forecasts: (**a**) BIAS and (**b**) MAE. The x-axis represents the time (hours).

Comparing Figure 8a,b, we can see that the large MAE of the wind forecasts at night was mainly due to the systematic overestimation of the nighttime wind speeds by the model. It is interesting to note that although the MAE of wind forecasts of the GFS and GEOS groups is relatively close, the bias of the wind speed forecasts of the GEOS group is significantly better than that of the GFS group. Figure 8 shows more clearly that the mean wind forecast error of the GEM group has a large diurnal variation, with a large overestimation of positive wind bias in the nighttime and negative bias in the daytime.
