# Wintertime Local Wind Dynamics from Scanning Doppler Lidar and Air Quality in the Arve River Valley
## 1 Introduction  
The cumulative effects of local emissions and specific orography result in urbanized mountainous areas being substantially affected by pollution episodes in wintertime [1; 2; 3; 4]. Pollution episodes of airborne particulate matter with diameters of less than 10 \(\upmu\)m (PM10) have been increasingly documented over the last two decades. These particles can penetrate deeply into the respiratory system and have been associated with increased health issues, such as asthma, cardiovascular risks and lung cancer [5; 6; 7]. PM10 pollution episodes are frequent in winter because of the development of cold air pools resulting from the filling of valley bottoms with cold air [8]. The temperature inversion thus generated drastically reduces the vertical mixing and may trap the pollutants near the ground, especially when it persists over several days [3; 9]. This is favoured by the length of the nights in winter. As a result, the pollutant redistribution becomes essentially driven by horizontal wind dynamics, which is local and mainly thermally driven [10]. A thermo-topographic conceptual model was introduced by [PERSON] [11] and [PERSON] [12] who first linked temperature gradients to the development of valleywind systems. In reality, these systems are much more complex because of their dependence on the thermal stratification structure [13], flux exchanges with tributary valleys [14], and valley dimensions, as shown by several idealized modelling studies [15; 16]. Indeed, the valley depth, width, length and slope inclination drive the temperature range within the valley, leading to more or less intense wind systems [17]. As a result, over complex terrain, the variety of topography can generate a myriad of local meteorological processes that influence pollutant dispersion [18; 19; 20]. The structure of the along-valley wind can be altered by local heterogeneities. For instance, [PERSON] et al. [21] reported a wind acceleration in the up-valley direction as a consequence of volume effect and variations in the heating rate. In the Adige Italian valley, [PERSON] et al. [22] characterised the along-valley wind over 140 km using permanent weather stations. They stated that anomalies in the along-valley wind reversal can be explained by change in the valley geometry that modifies heating and cooling of the atmosphere or by the urban heat island that modifies the pressure field. In recent decades, several field campaigns have been designed and run to understand the influence of local dynamics on pollution. For instance, vertical transport and mixing of pollutants were investigated in October 2000 in Salt Lake Valley during the VTMX campaign (Vertical Transport and Mixing, [23]). [PERSON] et al. [24] and [PERSON] et al. [1] have shown that the Salt Lake basin dynamics can be dominated by local features such as a nocturnal low level jet, which may generate regions of convergence motion. As a result, large differences in tracer dispersion and so in concentrations, were observed depending on the presence or absence of small scale thermally forced flows. In Austria, [PERSON] et al. [25] and [PERSON] et al. [26] investigated the mechanism responsible for pollutant transport within the urbanized Inn valley using airborne observations of aerosol backscatter intensity, temperature and wind. They highlighted three types of wintertime dynamics leading to pollutant concentration heterogeneities: (i) pollutant gradients observed at the valley scale were associated with an advection of polluted air by a density current while (ii) temporal variations at a specific location were explained by multiple flow reversals and (iii) asymmetric pollutant distribution in the cross-valley direction was explained by an advection of the particles by up-slope winds running along the sidewalls warmed by the sun. Complementary idealized modelling studies have shown that the slope circulation is largely dependent on the albedo and thus on the ground coverage of the sidewalls [13].