# Characterization of CYGNSS Ocean Surface Wind Speed Products
## 3 Results
### Intercomparisons with Reference Wind Speeds
#### 3.1.2 ERA5 Reanalysis Wind Intercomparison  
RMSD and Bias Assessment Using ERA5  
Figure 7a,b show the comparison between FDS and ERA5. The high-density portion of the scatterplot between FDS and ERA5 (Figure 7a) follows the one-to-one line closely, reflecting the low bias and high correlation between the two. For winds below 20 m/s, the RMSD and bias and the correlation between the FDS and ERA5 winds are 1.6 m/s, 0.1 m/s, and 84%, respectively. Figure 7b shows bias (green) and RMSD (black) as a function of ERA5 wind. For winds between 5 and 7 m/s, which is the peak of the wind distribution shown in blue, the RMSD and bias are approximately 1.4 and 0.3 m/s, respectively. For winds above ~10 m/s, both the bias and RMSD increase in magnitude monotonically, with both estimates beginning to show considerable noise above ~25 m/s due to the small number of samples. Note that differences between FDS and ERA5 winds can result from uncertainties in either product. An attribution of uncertainties to each is discussed in Section 3.2. Figure 7c,d show comparisons between NOAA and ERA5. The high-density portion of the scatterplot follows the one-to-one line closely from 0 to ~20 m/s, although some outliers with very high NOAA winds appear near 10 m/s. Above 20 m/s, the highest density of NOAA winds tends to be biased high. For winds below 20 m/s, the RMSD, bias, and correlation of the NOAA winds with respect to ERA5 are 1.4 m/s, 0.03 m/s, and 90%, respectively. Figure 7d shows bias (green) and RMSD between the two as a function of ERA5 wind. For winds between 5 and 7 m/s, which is the peak of the wind distribution shown in blue, the RMSD and bias are approximately 1.2 and 0.02 m/s, respectively. For winds above ~10 m/s, both the RMSD and bias increase monotonically up to a peak in both at around 24 m/s, decreasing above this. Differences between NOAA and ERA5 estimates  
Figure 6: **(a,c)** RMSD between CYGNSS and buoys (indicated by bars) at different rain rate (R; in mm/h) conditions, and (**b,d**) scatterplot of difference between CYGNSS and buoy winds as functions of buoy precipitation rate, with best-fit linear regression lines overlaid in. Panels (a,b) correspond to low wind conditions (\(\leq\)6 m/s), while panels (c,d) depict conditions with buoy winds >6 m/s. The error bars correspond to the 95% confidence limit estimated via the bootstrap method. Matchup samples are illustrated by the number on each bar. Correlation coefficients with a significance level (\(p\)-value < 0.05) are marked with an asterisk (*). result from uncertainties in both. Possible causes for the high NOAA bias at high ERA5 wind speeds are discussed in Section 4. Figure 8 shows a time series of daily FDS vs. ERA5 differences, bias (green) and RMSD (black), from August 2018 to July 2023. Both statistics show roughly periodic oscillations about a consistent mean. For the bias, the time scale of the oscillation is approximately 1 month with a peak-to-peak magnitude of \(\sim\)0.1 m/s about a mean of \(-\)0.2 m/s. For the RMSD, the time scale of the oscillation is approximately 3 months with a peak-to-peak magnitude of \(\sim\)0.2 m/s about a mean of 1.6 m/s. These time scales are consistent with observed thermal variations in the satellite hardware driven by orbital precession. CYGNSS hardware calibration does include corrections for these temperature dependencies [38], but the small residual oscillations observed in Figure 8 suggest that they have not been fully corrected. A downward trend of \(\sim\)0.1 m/s is seen in both the bias and RMSD. This may indicate a long-term drift in CYGNSS receivers due to aging but may also result from a drift in ERA5 with respect to the underlying true wind. With both this and the seasonal cycle, it is not possible to definitively determine the root cause. Figure 7: (**a**) Log-density scatterplot between FDS wind speed and ERA5 matchup reference winds. (**b**) RMSD and bias between the two wind speeds as a function of the reference wind.