Semi‐idealized simulations of wintertime flows and pollutant transport in an Alpine valley: Origins of local circulations (Part I)

Abstract

Mountainous terrains are known for driving their own dynamics which respond to the local morphological arrangement of the area. Thermally driven flows in particular develop at slope and valley scales and ensure a certain degree of pollutant dispersion under quiescent wintertime synoptic conditions. The present study focuses on a section of the Arve River valley situated close to Mont Blanc which frequently suffers from severe pollution episodes under stable wintertime conditions whilst surrounding valleys appear to be less affected. The particular shape of this basin‐like section and its location at the confluence of several tributary valleys raises the question of the extent to which local circulations participate in pollutant trapping over restricted sectors. A set of high‐resolution numerical simulations are designed in order to improve our understanding of the local flow structure and their sensitivity to thermal stratification, radiative forcing and snow cover. The tributary valleys play a major role in both daytime and night‐time dynamics by deflecting the entering daytime flux and constraining the night‐time flow trajectories. In addition, the basin morphology greatly influences the circulations. During daytime a two‐layer wind structure is developed and driven by spatial variations in sun exposure which is particularly heterogeneous under wintertime forcing. Early spring radiative forcing or the presence of snow allow both of them to develop more homogeneous circulations through the reduction of sun exposure variations, except when snow cover is restricted to shaded basin sidewalls. At night, a three‐layer wind structure is developed. It favours air mass ventilation in the western linear branch of the basin whilst stagnation and recirculation prevail in the curved part of the valley which is also the most polluted. This study therefore highlights spatial variations in circulation patterns consistent with the accumulation of observed pollutants and their heterogeneous distribution.

Fine-Resolution WRF Simulation of Stably Stratified Flows in Shallow Pre-Alpine Valleys: A Case Study of the KASCADE-2017 Campaign

“…In cold pool systems, the stagnation can be extreme with the consequence of very low temperatures sometimes observed on the ground (Lehner et al [2] mentioned a recorded minimum temperature of −52.6 • C observed in the Grünloch basin in 1932). In the French Alps, the Arve valley has recently received a great deal of attention due to dedicated observations and high-resolution numerical simulations [3][4][5]. For a localized potential pollution source, a question arises regarding the behavior of the particle plume, that is, how it is transported and dispersed in a complex terrain, especially during stable stratification conditions with suppressed turbulent vertical mixing.…”

“…In this paper, we present an attempt to simulate the flows by running a weather model with a fine (~100 m) horizontal resolution. Recent studies have demonstrated the potential of such simulations in complex areas (e.g., [4,5] for the Arve valley with the Meso-NH model; [26] for the large Inn valley in Alpine Austria with the COSMO model), though stable conditions often remain more difficult to simulate than convective boundary layers [27,28]. Our study is based on the Weather Research and Forecasting (WRF) model [29,30].…”

Improvement of the vertical mixing in chemistry transport modeling based on a 1.5-order turbulence kinetic energy-based eddy diffusivity closure scheme

“…Pollutants dispersion processes over the valley-basin city are much more complicated than over flat areas. Therefore, pollution episodes have been frequently witnessed over complex terrain, especially in wintertime (Chen et al, 2019;Sabatier et al, 2020).…”

Semi‐idealized simulations of wintertime flows and pollutant transport in an Alpine valley. Part II: Passive tracer tracking

“…(b) Vertical separation of the basin atmosphere along the south-north cross-section drawn in red in (a) F I G U R E 2 Hourly PM10 concentrations measured at three stations located in the Arve River valley ( Figure 1a shows the station locations) during (a) the IOP1 of the Passy-2015 field experiment in February 2015 and (b) a second anticyclonic episode documented during the field experiment in March 2015 Doppler Lidar observations by Sabatier et al (2018). Then, a numerical study was performed in a companion paper (Sabatier et al, 2019) whicht will be referred to as Part I hereafter. In Part I, a high-resolution numerical simulation was run in a semi-idealized framework (i.e.…”

“…(). Then, a numerical study was performed in a companion paper (Sabatier et al ., ) whicht will be referred to as Part I hereafter. In Part I, a high‐resolution numerical simulation was run in a semi‐idealized framework (i.e.…”