# Evaluation of HY-2 Series Satellites Mapping Capability on Mesoscale Eddies
## 4 Discussion  
The results demonstrate the differences in the ability of altimetry satellites to improve the detection of eddies at different latitudes. On the one hand, although high latitudes have the greatest density of orbital coverage among three regions, it results in some eddies being underestimated because smaller mesoscale eddies (\(\sim\)30 km) are not easily captured. On the other hand, low and middle latitudes, where the orbital coverage is similar, produce quite different results. Specifically, the eddy fields are well identified at middle latitudes, while serious misidentification occurs at low latitudes, where the number of eddies is halved and their radius doubled. To further investigate this problem, we selected several representative examples for transient observations. Figure 9 shows the eddies detected in three missions at different latitudes. We used eddies in the models as real eddies to compare the performance of eddies detected from synthetic SSH fields. A real eddy is considered to be captured if there is a similar shape and overlapping area to the eddies in the model. Figure 8: Distribution of eddies amplitude generated from models, reference groups and evaluation groups at (**a–c**) high latitudes, (**d–f**) middle latitudes, and (**g–i**) low latitudes. The resolution of the grid is 0.25\({}^{\circ}\)\(\times\) 0.25\({}^{\circ}\). Unit of amplitude in cm. The first feature illustrated in the figure is that the differences in orbital coverage could affect eddy detection. Specifically, both high latitudes and low latitudes have low sea surface variability and simpler eddy fields, while the orbital coverage at high latitudes is twice as high as orbital coverage at low latitudes. However, the result is that more than half of the eddies are correctly identified at high latitudes, with the eddy contours matching well with the model, but the area of eddies at low latitudes is generally exaggerated, with some smaller mesoscale eddies being identified as larger structures. The second feature illustrated in the figure is that the complexity of the eddy fields also affects the eddy identification results. To be specific, the orbital coverage is similar at middle latitudes and low latitudes, but due to the complexity of the eddy fields at middle latitudes, which have a higher number and larger radius of eddies, it instead produces better eddy matching results. To better reflect the ability to identify real eddies, we analyzed the results of all real eddies captured from synthetic SSH fields and calculated eddy recognition rates and accuracy, as shown in Table 4. It can be seen that the eddy recognition rates are similar to the results obtained in Figure 9, with the highest values at middle latitudes, followed by high latitudes and finally, low latitudes. Specifically, eddy recognition rates and accuracy at  
Figure 9: Examples of eddies identified on models, reference groups and evaluation groups at (**a–c**) high latitudes, (**d–f**) middle latitudes, and (**g–i**) low latitudes. The background color is from SSH fields of different datasets on the current day. The centers of eddies detected in model datasets are shown as black crosses and their contours as thin black lines. The centers of eddies detected in the reference groups and evaluation groups are black circles and their contours are bold black lines. Unit of SSH in cm. middle latitudes are all around 50%, at high latitudes around 30%, while eddy recognition rates at low latitudes are only around 20% but can reach nearly half in accuracy. Then, we compared the differences in eddy recognition between the reference groups and evaluation groups in all regions. We found that under the background of an increase in orbital coverage of around 40% at three latitudes, the eddy recognition rates of the evaluation groups improved by 14% and 14% at high and low latitudes, and the accuracy rates of the evaluation groups improved by 16% and 7% at high and low latitudes.