# The RUNE Experiment--A Database of Remote-Sensing Observations of Near-Shore Winds
## 4 Results
### Preliminary Intercomparisons
#### 4.3.3 Dual Setup and PPIs  
There are only a limited amount of positions where the scanning patterns from the concurrent dual and PPI setups intersect, and we expect differences in the estimated 10 min wind speed because of the difference in the size of the measuring volume, and the inherent spatial and temporal variability of the wind speed. The scanning pattern of both setups was chosen such that at 5 km west of position 2, and they both measure at 50, 100 and 150 m ASL. Furthermore, there are intersecting points at 50 m ASL, \(\approx\)0.9 and 1.7 km west of position 2. Figure 8: Scatter plots of 10 min mean wind speeds at \(\approx\)126 m ASL for WLS66 and Alíze (**a**) and at 150 m ASL for Bura and 3E (**b**). A line through the origin with a slope of 1 is shown in black. Figure 9: Scatter plot of the 10 min mean wind speeds for Bura and those reconstructed by the dual setup (for the position \(x=447,052\) m) at \(\approx\)100 m ASL. A line through the origin with a slope of 1 is shown in black. Figure 10 shows scatter plots of the 10 min reconstructed mean wind speeds from the PPI and dual setup scans during phases 2 and 3. We only use data with 50% or higher actual availability within a 10 min interval to increase the number of 10 min intervals that can be compared. The number of matching 10 min intervals is 4964, 4605, 1325, 1218, 1204 for Figure 10 panels a-e, respectively. Panels a and b show the wind speed at 50 m ASL at \(\approx\)900 and 1700 m west of position 2, respectively. The agreement is good (\(R^{2}=0.97\) for both panels) but a cluster of points where the PPI scan shows higher wind speeds is clearly seen. The mean bias is 0.1% and 0.8% for panels a and b, respectively. Figure 10 (panels c-e) shows comparisons at position 7 at 50, 100 and 150 m ASL. The agreement is also good (\(R^{2}=0.86\), 0.88, and 0.83 for 50, 100, and 150 m ASL, respectively) and the mean bias is 2% for all heights. The lower agreement found compared to that of the positions closer to position 2 (in panels a and b) are most probably related to the assumption of flow homogeneity within the PPI scan that is needed for wind reconstruction, which is less valid when the PPI scan covers a larger area. At further distances, there will be larger differences between the LOS velocities in a scan, due to the higher spatial separation. This makes it more difficult to reconstruct the wind speed. Furthermore, there can also be a bias due to a physical mean wind speed gradient present in the scanning area. The wind reconstruction from these PPI scans is less accurate for southerly and northerly winds as the LOS velocities are low and change less within the scan. Figure 11 shows the mean and standard deviation of the difference between the reconstructed wind speed from the dual and the PPI setups at 50 m ASL as a function of wind direction. The wind direction was determined from WLS66 at 76 m AGL due to its high actual availability and sorted into 30\({}^{\circ}\) bins. Figure 10: Scatter plots of the 10 min mean wind speeds for the PPI and dual setups (see text for details). A line through the origin with a slope of 1 is shown in black. The \(x\)-position is relative to position 2 of Vara with positive values going eastward and the \(z\)-position is ASL. Scatter plots at positions of \(\approx\)900, 1700 and 5000 m west of position 2 of Vara are shown in panel (**a,b**) and (**c–e**), respectively. From here on, we assume that there is no bias in the dual setup because it needs fewer assumptions for reconstructing the wind speed. For the positions closest to the coast (\(x=-904\) and \(x=-1722\) m), the bias and standard deviation are highest for southerly winds (a mean bias of \(\approx\)0.3 m\(\cdot\)s\({}^{-1}\) for the 180\({}^{\circ}\) sector). For the point near position 7, there is a bias in mean wind speed for the southerly and southwesterly winds of more than 1 m\(\cdot\)s\({}^{-1}\). Very few samples are available for northerly winds, but a clear bias is observed for the furthest westward position.