# Characterization of CYGNSS Ocean Surface Wind Speed Products
## 5 Conclusions  
The performance and properties of several CYGNSS ocean surface wind speed products have been characterized in a variety of ways. The products include the Fully Developed Seas (FDS) and Merged Storm (MRG) winds produced by the project science team and an NOAA product developed independently by a research group at the NOAA Center for Satellite Applications and Research. The FDS product is intended for use globally in conditions other than major organized storms and the MRG product uses a modified retrieval algorithm applied over major storms that is then merged with the FDS product away from storms. The single NOAA product uses a common retrieval algorithm in all conditions. Performance characterization is performed in two ways. One approach compares the CYGNSS product-under-test to a reference wind speed product and assesses performance by the Root Mean Square Difference (RMSD) and mean difference (aka., the bias) between them. A second approach uses the triple colocation method to decompose the RMSD between any two wind speed products into individual estimates of the Root Mean Square Error (RMSE) in each. This approach requires coincident measurements by three independent products. The triple colocation method is advantageous when there is the possibility of significant error in a single reference wind product, in which case, the RMSD between it and the product-under-test may be significantly larger than the RMSE. This is found to be the case with performance assessment in major storms, and triple colocation is the preferred approach in that case. The direct one-on-one comparison between CYGNSS and a reference wind product was performed using a globally distributed population of samples. Two sources for the reference equivalent-neutral winds were considered: the ERA5 reanalysis wind speed at a height of 10 m, and a large network of in situ buoy measurements that have been adjusted to a 10 m reference height. Comparisons were made with the FDS and NOAA products. The bias between CYGNSS and buoy winds was found to be 0.03 m/s and \(-\)0.22 m/s for FDS and NOAA, respectively. The RMSD between CYGNSS and buoy winds was found to be 1.36 m/s for FDS and 1.21 m/s for NOAA. The lower RMSD with NOAA is believed to result primarily from the additional spatial averaging it performs of samples that have significant overlap in their sampling footprints. This reduces the effects of noise while only slightly degrading the spatial resolution. The comparisons between CYGNSS and ERA5 showed similar behavior. The bias is \(-\)0.1 m/s and \(-\)0.03 m/s for FDS and NOAA, and the RMSD is 1.6 m/s for FDS and 1.4 m/s for NOAA. The higher RMSD values with respect to ERA5 may result at least in part from the higher uncertainty associated with ERA5 winds. This hypothesis is supported by the triple colocation results. See Sections 3.1.1 and 3.1.2 for details about the buoy and ERA5 intercomparisons. The triple colocation method of performance assessment was applied using multiple possible triplets of coincident wind speed products for global (away from storms) winds. The consistency of the RMSE estimates between triplets is a measure of the reliability of the error decomposition it performs, and the results were indeed found to be consistent. Averaging across estimates for all possible triplets, the RMSE of the FDS product was found to be 1.35 m/s and the RMSE for the NOAA product was 0.98 m/s. These values were both lower than the RMSD values and this is likely due to the contribution to the RMSD statistic made by uncertainties in the reference wind speed product. For performance assessment at high wind speeds in storms, the triple colocation method is also used. In this case, the availability of suitable reference wind products is more limited and a single triplet (CYGNSS, HWRF, SFMR) is used. The RMSE results are divided into different ranges of wind speed because performance was found to be a strong function of it.