# Wintertime Local Wind Dynamics from Scanning Doppler Lidar and Air Quality in the Arve River Valley
## 4 Results
### Wind Dynamics
#### 4.2.2 Horizontal Structure along the Valley Axis  
The wind structure along the valley axis is investigated here by looking at the \(V_{los}\) field along the red axis marked in Figure 7. Figure 9a shows a space and time (x,t) diagram of \(V_{los}\) with the axis of ordinates representing the horizontal LOS distance from the lidar, located at the origin. The colour convention is the same as in Figure 8.  
Unlike the vertical range of the lidar, the horizontal range does not follow a diurnal variation. Because the lidar laser beam remains in the aerosol layer close to the ground, the horizontal range remains constant at around 5 km, except in the early morning periods probably because of fog episodes.  
The morning and evening transitions of the along-valley flows at 40 m AVL are consistent with those presented in the previous section. Oscillations can be observed in the wind pattern behaviour, especially during the night-time. These oscillations may explain the asymmetric distribution observed in Figure 6, which shows a slight predominance of up-valley winds during the stagnation stage of the temperature inversion (from 10 to 12 February). Up-valley winds are more continuous than down-valley winds that are largely affected by oscillations and do not achieve a stationary state. The 10 min time resolution allows a visual estimation of the wind reversal period close to 1 h. A Fast Fourier Transform was applied to quantify these oscillations more accurately, but the resulting power spectrum did not permit a clear period determination (not shown). We suspect that this may have been the result of an over-complex signal, including oscillations caused by several mechanisms such as the emission of internal gravity waves in the valley by the intrusion of katabatic winds [44] or flow from tributary valleys [45], or the along-slope oscillations of katabatic winds [46].  
A remarkable feature is that flow reversal often occurs simultaneously over the whole valley, at least along the red line of Figure 7. To quantify the horizontal homogeneity of \(V_{los}\) along this line, a criterion is applied for each time step. It is classified as non-homogeneous when its standard deviation is above the threshold of 0.5 m\(\cdot\)s\({}^{-1}\), and a least 10% of the LOS dataset is of the opposite sign (to account for the change in direction).  
Figure 9: **(a)** space and time (x,t) diagrams of \(V_{los}\) along the horizontal valley axis. Red corresponds to up-valley winds (approximately westerly in the basin centre) and blue to down-valley winds (approximately easterly). **(b)** mean and standard deviation of \(V_{los}\) computed over the LOS distance, between 0 and 5 km, for each time step. Red stars represent the time step at which the wind in the along-valley direction is considered as heterogeneous based on the criterion defined below.  
Means and standard deviations of \(V_{los}\) are displayed in Figure 9b, where heterogeneous cases are identified with red stars. It can be seen that most of the heterogeneities occur during the daytime and more precisely in the late afternoon, during the evening transition. A recurrent heterogeneity is the one occurring systematically at 4:00 p.m. UTC. At that time, the wind first reverses close to the lidar (i.e., from 0 to 1000 m LOS). Then, after 1.5 to 2 h the wind reversal spreads over the whole valley. This 4:00 p.m. UTC wind reversal may be explained by considering the lidar position, which is close to both the southern sidewalls of the Passy basin and to the St-Gervais tributary valley mouth. The former favours the impact of down-slope winds and theory states that these winds usually reverse before valley winds that have more inertia [10; 47]. The tributary valley mouth favours the impact of down-valley wind of the St-Gervais valley, which is in the shadow before the Passy Basin (more detail in Section 4.3.1).