# Evaluation of HY-2 Series Satellites Mapping Capability on Mesoscale Eddies
## 3 Results
### Comparing Identified Mesoscale Eddies  
We applied the eddy identification algorithm to the model and two groups of gridded SSH products to identify mesoscale eddies at daily intervals. Due to the significant distortion of eddy fields at boundaries, we extended inward the eddy datasets with a scale of 2\({}^{\circ}\). The resolution of synthetic data primarily affects eddies' identification properties [43], thus, we ensured the same resolution of 0.08\({}^{\circ}\)\(\times\) 0.08\({}^{\circ}\) in detection. We selected eddy amount, radius and amplitude as the assessment criteria for mesoscale eddy identification. Eddy amount is defined as the number of mesoscale eddies counted. To identify only mesoscale eddies, the area of each individual eddy should be less than the area of a circle with a radius equal to the first baroclinic Rossby radius. Eddy effective radius is defined as \(R=\sqrt{S/\pi}\) and \(S\) is the area enclosed by outlines. Eddy amplitude refers to the level of variation in eddy fluctuations. It is illustrated in Table 3 that the statistics results of the eddies in the evaluation groups are similar to the eddies in the reference groups, with a slight improvement in eddy amount and radius, and poor performance in eddy amplitude. This may be related to the limited mapping improvement space from three to six satellites [8]. It is worth noting that the eddy datasets of the gridded SSH field show a serious incorrect estimation of the amount and radius of eddies at low latitudes. As can be seen from Figure 5c, there are a large number of smaller mesoscale eddies in the gridded products that are not captured correctly but over-identified some larger mesoscale eddies. This explains the incorrect statistical values of eddies at low latitudes. The distribution of the eddy dataset attributes is statistically presented in Figure 5. It shows that smaller mesoscale eddies dominate high latitudes, with the radius distributed around 30 km (Figure 5a), while middle and low latitudes achieve a wider radius range than high latitudes, with the radius up to 140 km and 250 km, respectively (Figure 5b,c). The eddy amplitudes at high latitudes and low latitudes vary little and are distributed around 5 cm (Figure 5d-f). In contrast, the amplitudes at middle latitudes are more dispersed and the maximum value can reach 1.2 m (Figure 5e). The distribution of eddies amount is shown in Figure 6, and the value of each grid represents the times of eddy identification in this location. High latitudes are distributed with smaller mesoscale eddies, which are sparse and discrete. Middle latitudes are dominated by mesoscale eddies throughout the region, which are large in number. While the center of low latitudes is clustered with eddy swarms, the margins are scattered with few mesoscale eddies. From the spatial distribution of eddy amount, HT-2 constellation alleviates eddies overestimation at middle latitudes (36-38\({}^{\circ}\)N, 152-154\({}^{\circ}\)E) and low latitudes (6-9\({}^{\circ}\)S, 84-86\({}^{\circ}\)E), as well as eddy underestimation at high latitudes (53-54\({}^{\circ}\)N, 28-29\({}^{\circ}\)W); middle latitudes (34-35\({}^{\circ}\)N, 152-154\({}^{\circ}\)E); and low latitudes (2-5\({}^{\circ}\)S, 84-87\({}^{\circ}\)E). However, it appears that there are a large number of small radius mesoscale eddies that are underestimated at low latitudes.