# Characterization of CYGNSS Ocean Surface Wind Speed Products
## 4 Discussion of Sensitivity to Persistence
Such high winds in the past would have introduced energy into the surface roughness spectrum at smaller wavelengths that cascaded over the subsequent 1-2 h to longer waves, which would then have anomalously high amplitudes relative to an amplitude consistent with the lower wind speed in the present (when the CYGNSS measurement was made). That measurement would be of a rougher surface and have a lower scattering cross section, and hence the retrieved wind speed would be too high and thus have a positive bias observed. Again, as with the previous case at lower wind speeds, the condition of anomalously large negative persistence rarely occurs (~3% of the time), so while useful for diagnosing GNSS-R sensitivity to sea state, it is not a significant contributor to overall performance. The behavior of the NOAA wind speed bias at high (>20 m/s) wind speeds noted in Table 6 is quite different from that of the FDS product. The bias with moderate wind speed persistence is 10.8 m/s, it grows to 11.9 m/s with large positive persistence, and grows further to 15.3 m/s with large negative persistence. The overall bias, averaged across all wind speeds, is not significantly affected because high winds happen so infrequently. The most likely cause of the large bias lies in the approach followed to create the GMF used by the NOAA wind speed retrieval algorithm. Because it was constructed empirically from a composite population of global CYGNSS and ERA5 matchups at low wind speeds and CYGNSS and HWRF matchups in storms at high wind speeds, the higher wind speed portion of its GMF has been tuned to the underdeveloped sea state conditions typical of major storms. When Fully Developed Seas at high wind speeds are encountered (which would generally be the case with moderate to low persistence values), the retrieval algorithm will tend to overestimate the wind speed. As Table 6 indicates, moderate to low \begin{table}
\begin{tabular}{l c c c c c c} \hline \hline
**Wind Speed Range (m/s)** & \multicolumn{3}{c}{**5-10 m/s**} & \multicolumn{3}{c}{**>20 m/s**} \\ \hline
**\% of Samples** & \multicolumn{3}{c}{**62\%**} & \multicolumn{3}{c}{**0.02\%**} \\ \hline
**Persistence Range** & \(\boldsymbol{p<-25\%}\) & \(\boldsymbol{p>-25\%}\) & \(\boldsymbol{p>25\%}\) & \(\boldsymbol{p<-25\%}\) & \(\boldsymbol{p>-25\%}\) & \(\boldsymbol{p>25\%}\) \\ \hline \% Samples @Wind Speed 1 (FDS/ERA5 matchup) & 3.27\% & 94.88\% & 1.85\% & 2.93\% & 95.38\% & 1.69\% \\ \hline FDS—ERA5 Bias (m/s) & 2.32 & 0.00 & 0.40 & 9.48 & 2.05 & 1.34 \\ \hline \% Samples @Wind Speed 1 (NOAA/ERA5 matchup) & 2.66\% & 95.81\% & 1.54\% & 4.15\% & 93.83\% & 2.02\% \\ \hline NOAA—ERA5 Bias (m/s) & 1.92 & \(-\)0.03 & 0.13 & 15.33 & 10.83 & 11.9 \\ \hline \hline \end{tabular}  
* Percentage of all ERA5 samples within the specified range of wind speed values that have the specified range of persistence values.  
\end{table} 
Table 6: Dependence of wind speed bias (CYGNSS–ERA5) on wind persistence at typical and high wind speeds. wind speed persistence tends to occur \(\sim\)95% of the time, and the high NOAA retrieval bias will be present then as well as with larger values of wind speed persistence.