# Wintertime Local Wind Dynamics from Scanning Doppler Lidar and Air Quality in the Arve River Valley
## 4 Results
### Tributary Valley Flows
#### 4.3.1 Saint-Gervais Valley  
Since the WLS200S was located at the Saint-Gervais valley exit, the flow penetrating into the basin could be detected using horizontal PPI scans. At low elevation (40 m AVL), a wind intrusion coming from the south is observed at around 5:00 p.m. UTC on WLS200S horizontal and vertical scans (not shown). This intrusion penetrates into the valley over a restricted area (on average 1000-1500 m from the WLS200S in the LOS direction) and then vanishes. Figure 12 synthesizes this information by showing the direction of the prevailing flow blowing through a restricted zone close to the WLS200S. This zone is defined between two circular arcs, one of radius 200 m and the other 1000 m, in the sector from azimuth 250\({}^{\circ}\) to 60\({}^{\circ}\) (blue area in Figure 7). For each azimuth of the horizontal PPI scan, a spatial average of \(V_{los}\) (\(\langle V_{los}\rangle\)) is computed. The azimuth of the maximum \(\langle V_{los}\rangle\) is considered as the direction of the dominant away-from-lidar wind component blowing through the arc. Conversely, the azimuth of the minimum \(\langle V_{los}\rangle\) represents the direction of the dominant toward-lidar wind component. Red (blue) arrows represent the away-from-lidar (toward-lidar) wind components in terms of direction and mean intensity. It should be noted that these components may be the result of a flow superimposition.  
From Figure 12, it can be seen that the low-level southern intrusion is observed between 4:00 p.m. and 5:00 p.m. UTC, with the dominant red away-from-lidar component blowing from the south, every day from 9 to 12 February (grey areas on panels a to d). This direction is consistent with the relative orientation of the Saint-Gervais valley compared to the WLS200S location. Vertical scans show that the thickness of this wind layer does not exceed 100 m meaning that it affects only the lowest level of the basin atmosphere (not shown). The only day during which the intrusion signature is not observed is the 13 February, but it may have been masked by the down-valley wind, which was more intense during the destruction stage of the temperature inversion episode (Figures 8 and 9).  
Figure 12: Representation of the dominant away-from-lidar (red arrows) and toward-lidar (blue arrows) wind components blowing through the area delimited by an arc of a circle of radius 1000 m, 40 m AVL, represented in blue in Figure 7. Wind directions are given by the arrow orientations (a northerly wind points downwards) and wind force by the vector length. Each panel is associated with one day and grey areas indicate the time period during which southerly flows are observed.  
Looking at the diurnal evolution, before 4:00 p.m. UTC, it can be seen that the domain is mainly affected by the up-valley wind with directions of the away-from- and toward-lidar components consistent with the valley axis direction. From 5:00 p.m. UTC, the circulation reverses to down-valley wind. The early morning presents a similar pattern, but the winds are more fluctuating.  
Wind intrusions were also visible on vertical lidar scans at higher elevation, in the range 200-540 m AVL (not shown). These flows may also affect the basin dynamics by interacting with the down-valley flow presented in Section 4.2.