# Wintertime Local Wind Dynamics from Scanning Doppler Lidar and Air Quality in the Arve River Valley
## 4 Results
### Tributary Valley Flows
#### 4.3.2 Megeve Valley  
Due to the relative orientation of the Megeve tributary valley with respect to the WLS200S location, the flow that may exit this valley is mainly perpendicular to the lidar beam and cannot be seen on \(V_{los}\) measurements. However, information can be captured using a method based on the CNR signal, which depends among other things on aerosol content (Section 3.1.2). Based on [PERSON] and [PERSON] [40] and [PERSON] et al. [41], a correction was derived to remove instrumental dependence of the CNR (Equation (1)). This gives access to the attenuated backscatter (\(\beta T^{2}\)) defined by:  
\[\beta T^{2}=log_{10}\left(\frac{10^{0.1*CNR(R)}*R^{2}}{I(R)}\right). \tag{3}\]  
The instrumental function I(R) was determined in the same way as in [PERSON] et al. [41], i.e., by fitting the heterodyne efficiency function given by their Equation (12) on cases considered as homogeneous with regard to aerosol distribution. Additional signal processing steps would be needed for PM10 retrieval, but this correction is sufficient for a qualitative analysis of the attenuated backscattered intensity within the basin.  
Horizontal maps of \(\beta T^{2}\) derived from horizontal PPI scans measured around 6:15 p.m. UTC from 9 to 12 February are displayed in Figure 13. \(\beta T^{2}\) increases as the colour moves towards the red. Looking at the western part, in the azimuth range 250 to 310\({}^{\circ}\), a yellow structure crossing the valley can be observed. This structure probably represents an advection of air by the flow coming out of the Megeve tributary valley, which connects with the Passy basin at this location. The color of the structure suggests an air mass that is cleaner than the air within the basin. It is visible from 4:30 p.m. UTC for the four days and was also intermittently present during the whole night (not shown). The similarity between this onset time and that of the Saint-Gervais valley exit flow observed in Figure 12 suggests a similar dynamics between the two tributaries, which share a similar north-south orientation.  
A noteworthy feature was a meandering of the flow, with significant direction changes observed over a 10 min period (not shown). These meanders are often observed under stable conditions and low wind intensities [48]. They may influence pollutant dispersion and may also be one of the sources of the oscillations observed in Figure 9.  
Close to the northern sidewalls and in the eastern part of the basin, red patches are observed that are likely to be associated with more polluted air in these areas than in the southern part of the basin.  
The day-to-day comparison shows a generally lower signal on 12 February (Figure 13d) that is consistent with the PM10 diminution observed within the Passy basin in Figure 2.  
Finally, this method constitutes a complementary approach to the \(V_{los}\) analysis by providing information on a flow perpendicular to the LOS. Moreover, it provides qualitative information on the aerosol content of the air. Its unique ability to retrieve information on the 3D wind structure makes it a very important instrument for studying wind dynamics over complex terrain.