# Wintertime Local Wind Dynamics from Scanning Doppler Lidar and Air Quality in the Arve River Valley
## 4 Results
### Wind Dynamics
#### 4.2.3 Cross-Valley Structure  
This section investigates the structure of the along-valley wind over transects perpendicular to the valley axis, at two different locations, represented by the black lines in Figure 7. Slanted PPI scans were used with elevation angles ranging from 1 to 8\({}^{\circ}\). For each elevation angle, a cross-valley transect was extracted 2000 m away from the lidar, providing horizontal transects from 40 m up to 280 m AVL every 35 m. By means of elevation-by-elevation comparisons, three elevations (75, 150 and 250 m AVL) were selected for analysis and are presented in Figure 10. The axis of ordinates represents the cross-valley distance, centred over the horizontal transect considered. Because of the lower scan acquisition frequency (1 scan per hour), data were smoothed with a Gaussian filter to account for the coarse temporal resolution. In contrast to the previous section, where the wind was blowing along the transect considered, we now look at the along-valley wind blowing through the transect over the valley width.  
Vertical and North-South Structure for Cross-Valley Transects in the Basin Centre  
A comparison of the diagrams obtained from 40 m to 280 m AVL confirms the low wind intensities up to 110 m AVL, with \(V_{los}\) intensities below 1.25 m\(\cdot\)s\({}^{-1}\) on the 75 m elevation diagram in Figure 10a, and the more intense systems aloft. At 150 m AVL, the intensities increase during both day- and night-time. The down-valley wind intensity tends to decrease above 250 m AVL (not shown) and the elevation of its core (jet-like structure) changes during the period, as observed in Figure 8. The new and important feature accessible with the visualization is the north-south wind structure. It can be noted that during the day, the up-valley wind develops over the whole valley width. At night, the down-valley flow does not extend over the valley width but is rather confined to the northern half of the basin. The southern half of the basin is affected by a wind blowing in the opposite up-valley direction.  
Vertical and North-South Structure for the Eastern Cross-Valley Transects  
The same analysis is presented in Figure 11 for cross-valley transects extracted in the eastern part of the Passy basin at elevation 150 m and 250 m AVL, panels a and b, respectively. A similar spatial structure along the \(y\)-axis and over the vertical is observed. At night, the down-valley jet is shifted northward as the altitude increases. This may be linked to the widening of the valley with elevation. When the diagrams shown in Figures 10 and 11 are compared, the down-valley jet appears more intense and narrower in Figure 11. For instance, on 12 February at 8:00 p.m. UTC, 150 m AVL, a broadening of the flow towards the west can be observed with a mean velocity varying from \(-\)2.8 to the east to \(-\)2.1 m\(\cdot\)s\({}^{-1}\) to the basin centre. The sheared structure is more pronounced in this part of the basin, especially at 250 m AVL, where half of the basin width is affected by wind blowing up-valley at night.  
This night-time cross-valley sheared wind structures and its reinforcement in the eastern part of the valley is likely to have very important consequences for pollutant dispersion and will be discussed in more details in Section 5.2.