# Validating CYGNSS Wind Speeds with Surface-Based Observations and Triple Collocation Analysis
## 3 Results
### Surface-Based Analysis
#### 3.1.2 Extreme Events  
Focusing on extreme storm events (anemometer readings of wind speeds greater than 17 m/s for 3 h), a more nuanced picture emerges. Figure 3 shows an example timeseries for TC Jasper, which passed over a weather station based on Holmes Reef. To increase the number of measurements in this visualisation, the size of the grid square of valid measurements is increased from the initial 0.25\({}^{\circ}\) to 0.5\({}^{\circ}\) distance from the weather station. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Name**} & **Height Above** & \multirow{2}{*}{**Lat**} & \multirow{2}{*}{**Lon**} & **No. of** & \multirow{2}{*}{**R \({}^{*}\)**} & **RMSD (m/s)** & **Bias (m/s)** \\  & **MSL (m)** & & & & **Matchups** & & \\ \hline Flinders Reef & 6.5 & \(-\)17.72 & 148.45 & 8303 & 0.81 & 2.13 & 1.03 \\ Browse Island & 4.6 & \(-\)14.11 & 123.55 & 8625 & 0.83 & 1.60 & 0.70 \\ Cape Wessel & 18 & \(-\)11.00 & 136.76 & 8387 & 0.78 & 1.77 & 0.45 \\ Arlington Reef & 8.5 & \(-\)16.72 & 146.11 & 8367 & 0.81 & 2.18 & 1.09 \\ Lihou Reef Lighthouse & 9.8 & \(-\)17.13 & 152.15 & 30,730 & 0.83 & 1.62 & \(-\)0.07 \\ Rottnest Island & 43.1 & \(-\)32.01 & 115.50 & 6058 & 0.64 & 2.48 & 0.56 \\ Mcluer Island & 2.4 & \(-\)11.05 & 132.98 & 1045 & 0.81 & 1.74 & 0.73 \\ \hline \hline \end{tabular} \({}^{*}\) All Pearson R values are significant \(p<0.001\).  
\end{table} 
Table 2: _Cont._Figure 3: TC Jasper passing anemometer located in the Great Barrier Reef at Holmes Reef [\(-\)16.47, 147.87] during 11–13 December 2023 (blue), compared to CYGNSS NOAA 1.2 (red). The dot’s opacity shows the distance as calculated by the inverse geodesic problem from the weather station, meaning brighter red is closer to gauge. Figure 2: Logarithmic density plots of matchups between CYGNSS algorithms and coastal anemometer readings in Australia and the Pacific. **(a,b)** compare FDS and NOAA against the Australian hourly average winds, and **(c,d)** for the Pacific max 1 min gust. Black dashed line is 1:1, grey solid line represents best fit. Different colour bars are used for the two regions. Holmes Reef is one of the few anemometers that is away from any significant landmass, resulting in it having a higher number of CYGNSS passes (12,604 CYGNSS matchups, compared to an average of 8177, from Table 1). Most of the anemometer storm events were closer to land and resulted in very few CYGNSS passes. By averaging the time between CYGNSS measurements in this event, we approximate a return rate of 16 h within the total 1\({}^{\circ}\) box, and other events from coastal anemometers have approximately 12 hourly revisit times. There was no TC overpass of an anemometer that coincided with a Merged measurement. Figure 3 demonstrates that the peak intensity of \(\sim\)23 m/s was captured by CYGNSS approximately 6 h before the storm was nearest to the anemometer. The maximum wind speed as the storm passed the anemometer was 27.8 m/s 0.1\({}^{\circ}\) away according to TC best track dataset IBTrACS. The pass at 10 a.m. on the 11 th December underestimated winds at a key time as TC Jasper approached, with the fading colour gradient at the top suggesting that getting nearer did not improve accuracy. CYGNSS measurements are quite varied over short timescales, which may be attributed to small spatial variations within the 1-degree cell around the anemometer. Measurements from all extreme events form the scatterplots shown in Figure 4.  
NOAA had less negative bias than FDS and Merged, particularly for the blue storm events. NOAA captured only 22% the number of FDS measurements. FDS has higher correlations and lower errors than Merged in both datasets. The storm values in Merged are lower because only FDS values are used. The limited coverage of storm-centred gridded product missed these storm events. Correlations over Australia are lower than over the  
Figure 4: Extreme wind events, defined as more than three consecutive hours above 17 m/s, compared to anemometer readings at the time.