# Wintertime Local Wind Dynamics from Scanning Doppler Lidar and Air Quality in the Arve River Valley
## 4 Results
### Spatio-Temporal Fluctuations of the Along-Valley Wind
#### 4.2.1 Vertical Structure  
Figure 8 presents space and time (z,t) diagrams of \(V_{los}\), extracted for two vertical profiles within the Passy basin (green lines in Figure 7). Each panel represents the time evolution of the profile, extracted at azimuth 295\({}^{\circ}\) in the centre of the basin (panel a) and at azimuth 28\({}^{\circ}\) in the eastern part of the basin (panel b). Red corresponds to up-valley winds (approximately westerly in the basin centre) and blue to down-valley winds (approximately easterly).  
Figure 8: Space and time (z,t) diagrams of \(V_{los}\), extracted for a vertical profile (**a**) in the centre of the basin at azimuth 295\({}^{\circ}\) and (**b**) in the eastern part of the basin at azimuth 28\({}^{\circ}\). Red corresponds to up-valley winds (approximately westerly in the basin centre) and blue to down-valley winds (approximately easterly). Altitude is given in metres above valley level as defined in Section 2.3.  
Figure 7: Location within the Passy basin of the axes used to characterize the along-valley wind. The vertical profiles (green) are extracted from RHI scans 2000 m away from the lidar, the horizontal transect (red) is extracted from the PPI scan at elevation 0\({}^{\circ}\). The cross-valley transects (blacks) are extracted from slanted PPI with elevation angle (ranging from 1\({}^{\circ}\)to 8\({}^{\circ}\)), in the centre (C) and in the east of the basin (_E_). The blue circular area is used to characterize the wind from the Saint-Gervais valley. Complete RHI and PPI scans can be seen in Figure 4.  
The vertical range of the WLS200S (hereafter denoted \(H_{WLS}\)) changes over the period. From 7 to the middle of 8 February, \(H_{WLS}\) remains approximately constant at around 700 m AVL. This height is consistent with the cloud base detected by a ecolimeter at Site 1. The clouds dissipate at noon UTC on 8 February, the time at which \(H_{WLS}\) increases. Then, a diurnal cycle starts, which lasts until 13 February (i.e., during the stagnation stage of the temperature inversion episode). \(H_{WLS}\) starts its increase around 10:00 a.m. UTC, when convection begins in the low-level layers, and reaches its maximum between 4:00 p.m. and 6:00 p.m. UTC everyday. Then, it decreases during the night and reaches its minimum around 8:00 a.m. UTC the next day. The day-to-day comparison shows that \(H_{WLS}\) tends to decrease slightly during the episode, probably following the aerosol layer thickness. Its daily-maximum ranges from 1400 m AVL on 9 February at 4:00 p.m. UTC to 650 m AVL on 12 February. This reduction is first driven by an increasing stability at the beginning of the episode, which confines the pollutant within a thinner layer. Afterwards, it may result from the subsidence of the capping inversion (Figure 3). Finally, \(H_{WLS}\) behaves in a very similar way over the two profiles, which highlights the homogeneity of the aerosol layer structure within the Passy basin. Note that \(H_{WLS}\) may be perturbed by fog episodes in the early morning and is therefore more difficult to interpret.