# Characterization of CYGNSS Ocean Surface Wind Speed Products
## 3 Results
### Triple Colocation Validation
#### 3.2.2 CYGNSS Storm-Specific Data Products  
The storm-specific version of the triple collocation analysis uses a similar approach as the non-storm case above, except that the population of wind speeds is divided into bins of different wind speed ranges in order to characterize the performance as a function of wind speed. This is done because the dependence of performance on wind speed becomes significant at higher wind speeds. For consistency of the comparison between the performance of the two CYGNSS products (MRG and NOAA), a matchup dataset simultaneously satisfying the coincidence criteria for all four of MRG, NOAA, SFMR, and HWRF is assembled. This results in a total matchup dataset of 1.87 million samples. Density scatter plots between various pairs of products in the matchup dataset are shown in Figure 11. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & & & \multicolumn{4}{c}{**RMSE (m/s)**} \\ \cline{3-6}  & & **FDS** & **NOAA** & **ERA5** & **ASCAT** & **GMI** \\ \hline \multirow{5}{*}{**CryGNSS**} & FDS, ERA5, ASCAT & 1.39 & - & 0.71 & 0.42 & - \\ \cline{2-6}  & FDS, ERA5, GMI & 1.32 & - & 0.85 & - & 0.41 \\ \cline{2-6}  & FDS, ASCAT, GMI & 1.34 & - & - & 0.57 & 0.31 \\ \cline{2-6}  & NOAA, ERA5, ASCAT & - & 1.00 & 0.66 & 0.49 & - \\ \cline{2-6}  & NOAA, ERA5, GMI & - & 0.93 & 0.74 & - & 0.59 \\ \cline{2-6}  & NOAA, ASCAT, GMI & - & 1.02 & - & 0.46 & 0.45 \\ \cline{2-6}  & ERA5, ASCAT, GMI & - & - & 0.76 & 0.31 & 0.57 \\ \hline \multicolumn{6}{c}{Average RMSE over all triplets} & 1.35 & 0.98 & 0.74 & 0.45 & 0.47 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Triple colocations results for observations away from storms. Two CYGNSS products and three non-CYGNSS products are used, and various combinations of triplets are considered. The Root Mean Square Error (RMSE) in individual products is estimated for each triplet by the triple colocation analysis procedure. Figure 11 highlights the significant increase in scatter that is present when comparisons are made between the HWRF product and any of the other three. This suggests that HWRF has a significantly larger number of outliers than the other products. To address and mitigate this issue, we apply an additional filter to the dataset which requires that matchups are only included if HWRF and SFMR wind speeds differ by less than 10 m/s. The 10 m/s threshold used is an ad hoc value. This reduces the number of samples from ~1.87 million to ~1.64 million in the matchup dataset. The density scatterplots after this filter is applied are shown in Figure 12.  
Figure 12a compares HWRF and SFMR and confirms the action of the additional quality filter (compared with Figure 11a). Figure 12b,e plot the MRG and NOAA products against HWRF and show that the most egregious outliers have been removed by the filter. Figure 12c,f plot the MRG and NOAA products against SFMR and demonstrate that their relationships are largely unaffected by the filter--the highest density portion of the dataset is similar to those in Figure 11c,f and the outliers have been largely eliminated. This supports the assertion that HWRF outliers have been largely removed while the relationship of the MRG and NOAA products to SFMR has not been significantly affected. Triple colocation analysis is performed on this filtered matchup dataset. Table 2 lists the RMSE values derived by triple colocation analysis for the two reference wind speed products. The RMSE values for SFMR are consistent whether MRG or NOAA is used with HWRF to form the triplet of matchups. SFMR's RMSE values are 2.0-2.3 m/s for wind speeds below 30 m/s and 3-4 m/s above 30 m/s, which is consistent with the results reported in [32]. As expected, HWRF has much larger RMSE values (even after applying the additional filter to remove its most egregious outliers). The RMSE estimates for HWRF are also consistent between the two sets of triplets (HWRF, SFMR, MRG and HWRF, SFMR, NOAA) and they grow with increasing wind speed, from ~3-6 m/s below 30 m/s to ~10 m/s above 30 m/s.