## 3 Results  
### Optimal Cloud Probability Threshold  
The output of the NDVI-CPP was the optimal CP threshold value to use in the \"Sentinel-2: Cloud Probability\" mask (representing the best balance between omission and commission errors), i.e., the CP threshold value for which the t-mean FmMVC value was the maximum. As shown in the graphs in Figure 7, after the execution of the procedure, the optimal threshold was found to be equal to 0.66 (66%).  
Figure 5: Start-end months of the 15 variable-sized periods to test for the determination of the optimal reference period for yield forecast.  
Figure 6: Diagram of the method implemented for determining the optimal yield forecast period.  
Then, by applying the 66% CP threshold value to the investigated S2 time-series, were computed all the clear pixel mMVC mosaics. Note that the t-mean FmMVC values reported on the y-axis in Figure 7 are not a reliable reference for evaluating the effectiveness of the CP threshold level, as they represent data averaged over three years, which include mostly clear-sky images that tend to flatten the values. The effectiveness of the CP threshold can only be assessed in cloudy scenes. As an example, in Figure 8, we show the difference in applying three different CP thresholds in March 2018 (which was a particularly cloudy month). The scenes displayed are the monthly RGB composites generated using the B4 (red), B3 (green), and B2 (blue) band values of the same pixels employed for the mMVC mosaics (i.e., the greenest ones).  
The 100% CP represents the case in which only clouds identified for sure are masked; the 66% CP is the cloud probability optimized for the purpose as previously discussed; and CP 40% is the threshold value recommended by the s2 cloudless authors for general applications [68]. The unmasked clouds on the left of the figure are present in all six scenes  
Figure 8: March 2018 RGB composites, over the field under investigation, at 3 different CP threshold percentages: 100% (on the **left**), 66% (in the **center**), and 40% (on the **right**). The scenes are generated using the R = B4, G = B3, and B = B2 band values of the same pixels employed for the MVC (i.e., the pixel having the maximum NDVI).  
Figure 7: “Sentinel-2 Cloud Probability” (CP) threshold variation as a function of the t-mean FmMVC measured over the investigated field. On the left, the variation at 10% intervals and, on the right, the zoom for the variation of a single percentile between 61% and 70% are given, interval in which the maximum value (in red circle) is recorded.  
of the month, so the MVC was unable to mask them. Instead, from the figure on the right, it can be seen that lowering the threshold to 40% meant that non-cloudy pixels were masked as a result. When the optimal threshold of 66% is applied, the FmMVC value is 0.4757, while the thresholds of 100% and 40% correspond to FmMVC values of 0.3369 and 0.3441, respectively. These values would have also significantly distorted the mean FmMVC-yield correlations, which are shown in Figure 9 in the next section, leading to a decrease in the correlations ranging from 1% to 5% across all variable-size periods that include the month of March. In particular, the FmMVC-yield correlation for the month of March alone would have dropped from 95.2% to 90.1% (with a CP threshold of 100%) to 90.3% (with a CP threshold of 40%).