Wintertime Local Wind Dynamics from Scanning Doppler Lidar and Air Quality in the Arve River Valley

Abstract

Air quality issues are frequent in urbanized valleys, particularly in wintertime when a temperature inversion forms and the air within the valley is stably stratified over several days. In addition to pollutant sources, local winds can have a significant impact on the spatial distribution and temporal evolution of pollutant concentrations. They can be very complex and difficult to represent in numerical weather prediction models, particularly under stable conditions. Better knowledge of these local winds from observations is also a prerequisite to improving air quality prediction capability. This paper analyses local winds during the Passy-2015 field experiment that took place in a section of the Arve river valley, near Chamonix-Mont-Blanc. This location is one of the worst places in France regarding air quality. The wind analysis, which is mainly based on scanning Doppler lidar data sampling a persistent temperature inversion episode, reveals features consistent with the higher pollutant concentrations observed in this section of the valley as well as their spatial heterogeneities. In particular, an elevated down-valley jet is observed at night in the northern half of the valley, which, combined with a weak daytime up-valley wind, leads to very poor ventilation of the lowest layers. A northeast-southwest gradient in ventilation is observed on a daily-average, and is consistent with the PM10 heterogeneities observed within the valley.

Synoptic‐flow interaction with valley cold‐air pools and effects on cold‐air pool persistence: Influence of valley size and atmospheric stability

“…Lareau and Horel, ; Sabatier et al . ) or the formation of wakes and vertically oriented lee vortices which alter the manner of the ambient flow interaction with the CAP (Jeglum et al . ) may further be influential.…”

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

“…These observations, along with those presented in Sabatier et al . (), are also important for supporting the relevance of the numerical simulations for studying the situations behind these two pollution episodes.…”

“…The wind roses are determined across the 1000–1700 UTC interval corresponding to the period of UVW establishment during the IOP1 as reported by Sabatier et al . ().…”

“…In addition, a scanning Doppler wind lidar deployed at SITE2 provides spatialized measurements of line-of-sight velocity (V los ) with an atmospheric volume documented every hour. This database has been investigated in depth by Sabatier et al (2018) who provided an observation-based analysis of the wind structure during the persistent stage of the IOP1 of the Passy-2015 field experiment. Potential temperature and humidity profiles were measured at SITE1 by radiosoundings launched every 3 hr with a 10 m interpolated resolution during IOPs or retrieved from microwave radiometer (MWR) measurements available every 6 min with a 50 m vertical resolution.…”

“…Circulations and transport processes developed from meso-(mountain-valley) to micro-(slope) scales, according to the terminology of Lehner and Rotach (2018), must therefore be considered for this area halfway between a valley and a basin, the latter being known to inhibit the along-valley circulation (Clements et al, 2003;Whiteman et al, 2004;Arduini et al, 2017). A first analysis based on scanning Doppler lidar measurements acquired during the Passy-2015 field experiment has allowed the characterization of an along-valley wind system and has raised several questions about the complex flow layout (Sabatier et al, 2018). At this point, numerical models represent a complementary tool for observations for the determination of mechanisms governing the circulation structure in this highly complex terrain.…”