# Wintertime Local Wind Dynamics from Scanning Doppler Lidar and Air Quality in the Arve River Valley
## 2 Passy-2015 Field Experiment
### Objectives and Overview of the Field Experiment  
The Passy-2015 field experiment was designed to give a better understanding of the atmospheric dynamics in the Passy basin under anticyclonic wintertime episodes and improve numerical weather prediction and air quality models in these conditions. More precisely, the objectives were to determine which mechanisms may: \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Episode**} & \multirow{2}{*}{**Time**} & \multicolumn{2}{c}{**Max [PM10]\({}_{Passy}\)**} & **Max [PM10]\({}_{Salanches}\)** & **Max [PM10]\({}_{Chamonis}\)** \\  & & **(Day of the Max/Epis. Duration)** & **(Day of the Max/Epis. Duration)** & **(Day of the Max/Epis. Duration)** \\ \hline
1 & 1–8 January & 115 (6/8) & 60 (6/8) & 60 (1/8) \\
2 & 17–22 January & 98 (4/6) & 48 (4/6) & 37 (5/6) \\
3 & 9–14 February & 88 (3/6) & 73 (3/6) & 48 (1/6) \\
4 & 16–20 February & 66 (2/5) & 54 (2/5) & 61 (3/5) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Pollution episodes during the winter 2015 in the Arve River Valley. The maximum daily [_PM_10] (in \(\upmu\)g:\(\upmu\)m\({}^{-3}\)) recorded during the episode for the stations of Passy, Sallanches and Chamonix are given together with the day during the episode at which this maximum was reached (in parentheses). Figure 2: **(a)** time evolution of PM10 concentrations obtained by applying a 24 h-moving average on raw data recorded at four stations of the Arve river valley: Passy in red, Sallanches in orange, Chamonix in green and Marnaz in cyan. Grey areas represent PM10 pollution episodes based on Passy PM10 measurements; **(b)** time evolution of the sea level pressure measured in the middle of the Passy basin and at Annecy, in black and grey, respectively, and the temperature gradient between Sallanches and Mont-Arbois, in blue. Annecy, Sallanches and Mont-Arbois stations are the closest stations of the METEO-FRANCE operational network for both measurements. Black dots show the days for which albedo at Site 1 was above 0.5, i.e., the days with snow on the ground. * lead to the high PM10 concentrations observed in the Passy basin during winter,
* participate in the spatial variations of PM10 concentrations observed within the Passy basin and its vicinity,
* pilot the time evolution of PM10 concentrations (diurnal cycle and over the whole episode). The field experiment was conducted in winter 2014/2015, from November 2014 to April 2015, with a larger set of instruments in January and February 2015 [29]. Observations were reinforced during two Intensive Observation Periods (IOPs) in February 2015. IOPs were planned according to weather and air quality forecasts. The first one took place from 6 to 14 February and the second from 17 to 20 February, corresponding respectively to episodes 3 and 4 of Table 1. The present study focuses on IOP1, which was the most thermally stable and polluted of the two IOPs as shown in Figure 2. A persistent temperature inversion developed from 9 to 13 February. [PERSON] and [PERSON] [9] have suggested that the use of a simple temperature gradient can be relevant to describe the overall thermal inversion dynamics and the PM10 concentration evolution during a pollution peak. Based on this consideration and on [PERSON] et al. [37], the three phases of the persistent inversion cycle during IOP1 are defined as:  
* the formation stage: an anticyclone formed at the beginning of IOP1 and reached a pressure maximum on the morning of 9 February. The temperature inversion became established during the same day, with a reduction of the synoptic wind and an advection of warm air above the Passy basin (Figure 3). This advection generated a capping inversion, which favoured the decoupling of the atmosphere within the valley from the atmosphere above and thus allowed the development of local dynamics.