# Characterization of CYGNSS Ocean Surface Wind Speed Products
## 3 Results
### Triple Colocation Validation
#### 3.2.2 CYGNSS Storm-Specific Data Products
Figure 11: Log density scatterplots for each pair of storm-specific matchup datasets. (**a**) SFMR vs. HWRF, (**b**) HWRF vs. MRG (**c**) SFMR vs. MRG, (**d**) MRG vs. NOAA, (**e**) HWRF vs. NOAA, and (**f**) SFMR vs. NOAA. The RMSE of the CYGNSS MRG and NOAA products is estimated in a similar manner using triple colocation except that a finer partitioning of the wind speed is applied in order to better characterize the dependence on wind speed. The results are shown in Figure 13. Also included in the figure is the RMSD between SFMR and each CYGNSS product. The RMSE values for NOAA and MRG are both significantly lower than their RMSD counterparts, with the difference resulting from the error source decomposition performed by the triple colocation analysis. The MRG product has a slightly lower RMSE than the NOAA product, although that difference decreases and they become more comparable at the highest wind speeds. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Reference Wind Speed Product**} & \multirow{2}{*}{**Triplet of Matchups**} & \multicolumn{4}{c}{**Wind Speed Range (m/s)**} \\ \cline{3-6}  & & **0–10** & **10–20** & **20–30** & **30–75** \\ \cline{3-6}  & & \multicolumn{4}{c}{**RMSE (m/s)**} \\ \hline \multirow{2}{*}{SFMR} & MRG, HWRF, SFMR & 2.31 & 2.20 & 2.30 & 2.93 \\ \cline{2-6}  & NOAA, HWRF, SFMR & 2.32 & 1.97 & 2.19 & 4.26 \\ \hline \multirow{2}{*}{HWRF} & MRG, HWRF, SFMR & 3.77 & 5.13 & 6.24 & 10.17 \\ \cline{2-6}  & NOAA, HWRF, SFMR & 3.35 & 5.37 & 6.55 & 10.33 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Wind speed bin-wise RMSE of SFMR and HWRF from triple colocation analysis using both CYGNSS high wind speed products, MRG and NOAA. Figure 12: Log density scatterplots after a data-quality filter has been applied which requires coincident HWRF and SFMR values to differ by no more than 10 m/s. (**a**) SFMR vs. HWRF, (**b**) HWRF vs. MRG (**c**) SFMR vs. MRG, (**d**) MRG vs. NOAA, (**e**) HWRF vs. NOAA, and (**f**) SFMR vs. NOAA. These are the samples used for triple colocation analysis. 34-Knot Significant Wind Radius, R34, in Storms  
The CYGNSS MRG wind speed product also produces a secondary product derived from it--the 34-knot wind radius of the storm, R34 [13]. R34 is defined as the minimum distance from the storm center beyond which all wind speeds in a given quadrant are less than 34 knots (~17.5 m/s). It is a metric commonly used by operational agencies to characterize the size of a storm. Uncertainty in the CYGNSS R34 product is also characterized using the triple colocation approach. In this case, the near coincident matchups are between CYGNSS and three other R34 products: (1) estimated directly from wind field measurements made by the spaceborne SMAP passive microwave radiometer; (2) estimated directly from wind field measurements made by the spaceborne ASCAT-B radar scatterometer; and (3) provided by the NOAA Best Track reanalysis model. The triple colocation results are shown in Table 3. As with previous applications of the method, multiple choices are available to use as triplets and the consistency of the estimated RMSE values for different triplets is an indication of the reliability of the results. The average RMSE value across all three possible triplets is also shown in Table 3. These values, in order from lowest to highest, are 38 km for ASCAT-B, 44 km for Best Track, 49 km for SMAP, and 54 km for CYGNSS. The higher error for CYGNSS, in spite of its relatively low wind speed retrieval error in the vicinity of 34 knots, may be due to the fact that it does not provide a continuous, gap-free image of the storm's wind field. Rather, its measurements are made along narrow tracks as the GPS specular reflection point transits the storm. This spatial undersampling likely results in some gaps in sampling at locations where the wind speed is above and below 34 knots.