# Wintertime Local Wind Dynamics from Scanning Doppler Lidar and Air Quality in the Arve River Valley
## 1 Introduction
More recently, the KASCADE experiment (KAtabatic winds and Stability over CAdarache for Dispersion of Effluents) took place in the French pre-Alps to characterize the local dynamics at the intersection of two valleys under stable conditions. [PERSON] et al. [27] have shown that the onset, speed and depth of the down-valley winds depend on the valley dimensions: the larger valley was influenced by large scale circulation whereas the narrower one was primarily affected by thermally driven winds. All these studies finally revealed the need for dedicated local field experiments since the valley wind dynamics are complex, strongly influenced by the local terrain characteristics, and sometimes decoupled from the large scale wind. The understanding of the wind structure complexity requires high frequency measurements with large spatial coverage. This was pointed out by [PERSON] et al. [24] and [PERSON] et al. [21], who both used scanning Doppler wind lidar. A recent field experiment (Passy-2015) dealing with air pollution and wind dynamics took place in the vicinity of the town of Passy in the Arve river valley in 2015. This location in the French Alps close to Mont-Blanc is one of the worst places in France regarding air quality. European standards state that the daily-averaged concentration of PM10 of 50 \(\upmu\)g\(\cdot\)m\({}^{-3}\) must not be exceeded, with a tolerance for 35 exceedances per year (directive 2008/50/EC). These limits are regularly exceeded near the town of Passy. For instance, during the winter of 2016-2017, the local air quality agency reported that 30 consecutive days were over the limit during a single pollution episode [28]. Besides the large PM10 concentration recorded in winter, strong spatial heterogeneities are observed over a few tens of kilometres within this narrow steep-sided valley. These heterogeneities may be due to variations in emission rate and/or to local dynamics. To evaluate the role of the local dynamics, a better understanding is needed. This was the main motivation for the Passy-2015 field experiment that took place during winter 2014-2015 in the Arve river valley [29]. The present study focuses on an analysis of the local wind dynamics in the section of the Arve river valley close to the town of Passy, during a persistent temperature inversion episode. [PERSON] et al. (2015) have shown a good overall correlation between the daily PM10 concentration and the heat deficit, with a correlation coefficient ranging between 0.44 and 0.69 over January and February 2015. Using temperature profiles acquired from a radiometer at a given site within the valley, they revealed that the hourly PM10 evolution cannot be solely explained by the temperature inversion dynamics and is probably influenced by the local dynamics. The aim of the present study is to characterise the fine-scale wind dynamics within the valley on the basis of the field experiment dataset and to identify which features of this local dynamics may participate in the observed high concentrations and spatial heterogeneities in PM10. This study mainly relies on data from a scanning Doppler Wind Lidar (DWL), which offers the advantage of mapping out the velocity field in the horizontal and vertical and thus providing 3D wind data. The paper is organized as follows: Section 2 gives an overview of the Passy-2015 field experiment and Section 3 introduces the scanning DWL and the associated database. Local wind characteristics retrieved from the scanning DWL are presented in the Section 4 based on a spatial and temporal analysis. A discussion of the results and their relevance to an understanding of the PM10 concentrations observed within the valley is finally proposed in Section 5.