# Near Real-Time Freeze Detection over Agricultural Plots Using Sentinel-1 Data
## 4 Results
### Improved Freezing Detection by Applying the Temperature Filter  
The results obtained in the previous section show that our algorithm was successful in detecting frozen plots with a minimum number of anomalies during the freezing period between November and February. These results are consistent across both study sites, and in both descending (morning), and ascending (afternoon) acquisition modes. However, for the period starting from mid-February, and for the month of November, there were many more anomalies detected (detections with temperature \(>\) 3 \({}^{\circ}\)C). To remove these anomalies, the second phase of our algorithm consists of applying a filter that removes all detections when the reported temperature from the nearest weather stations is above 3 \({}^{\circ}\)C. This section also presents the distribution between mid-to-moderate freezing and severe freezing (depending on threshold values as shown in Table 1).  
Figure 11 shows the distribution of the severity of freezing in site A across the land cover LC1, LC2, and LC3 in both VH (Figure 11a-c) and VV (Figure 11d-f), respectively. The results indicate that the severe freezing episodes occurred exclusively when the temperatures were below 0 \({}^{\circ}\)C for both VH and VV polarizations in descending mode (morning). For the VH polarized images, the percentage of severe frozen plots was at its highest when the temperature was at its lowest (\(\sim\)\(-\)2.5 \({}^{\circ}\)C on 26 December 2018), with the percentage of severely frozen plots roughly equal to 40%, 77 % and 75% for LC1, LC2, and LC3, respectively. The second date with the most severe frozen percentages occurred on 14 April 2019 (temperature \(\sim\)\(-\)1.5 \({}^{\circ}\)C) with a percentage of severe freezing of \(\sim\)45% for both LC1 and LC2, and \(\sim\)62% for LC3. In addition, the detection results using VV polarized images also showed that the highest percentage of severely frozen plots also occurred on 26 December 2018, and 14 April 2019. Finally, our algorithm using both VH and VV polarized images did not detect any severely frozen plots in the afternoon period.  
The results for site B (Figure 12) show that the detection of the severe freezing also occurred on the dates with the lowest temperature, and this is in accordance with the results obtained in site A. For VH and VV polarized images in the descending mode (morning), the dates with the most detected severe frozen plots occurred when the reported temperatures were at their lowest (t \(<\)\(-\)3 \({}^{\circ}\)C on 23 and 24 December 2018), and for the three land cover classes. On 23 December 2018, the percentage of severely frozen plots was \(\sim\)60% for LC1 in both VH and VV, \(\sim\)65% and \(\sim\)68% for LC2 in, respectively, VH and VV, and \(\sim\)50% for LC3 in both VV and VH. The percentage of severe frozen plots was slightly higher on 24 December, with \(\sim\)70% of plots detected as severely frozen for LC1 in both VH and VV, \(\sim\)68% and \(\sim\)76% for LC2 in, respectively, VH and VV, and \(\sim\)76% for LC3 in both VH and VV. Finally, for the images acquired in both VH and VV in the ascending mode (afternoon), our algorithm did not show any occurrence of severe freezing across the three land cover classes.  
Figure 11: The distribution of mild-to-moderate and severely frozen plots in site A, for each S1 acquisition, and using VH and VV polarizations after applying the temperature filter. **(a)** VH-LC1, **(b)** VH-LC2, **(c)** VH-LC3, **(d)** VV-LC1, **(e)** VV-LC2, **(f)** VV-LC3. Precipitation (P) and temperature (t) are given in the graph at the top.  
Figure 12: The distribution of mild-to-moderately and severely frozen plots in site B, for each S1 acquisition, and using VH and VV polarizations after applying the temperature filter. **(a)** VH-LC1, **(b)** VH-LC2, **(c)** VH-LC3, **(d)** VV-LC1, **(e)** VV-LC2, **(f)** VV-LC3. Precipitation (P) and temperature (t) are given in the graph at the top.