Impact of Along-Valley Orographic Variations on the Dispersion of Passive Tracers in a Stable Atmosphere

Julian Quimbayo-Duarte,Gabriele Arduini,Chantal Staquet,Charles Chemel

doi:10.3390/atmos10040225

Abstract

A numerical model is used to investigate the transport of passive tracers in an idealized Alpine valley during stable wintertime conditions after the evening transition. The valley is composed of an upstream-valley section, which opens on a narrower downstream valley section, which opens onto a plain. The ratio between the valley-floor widths of the upstream and downstream sections is either 4 (simulation P1) or 11.5 (P2). The change in the thermal structure of the atmosphere in the along-valley direction and over the plain leads to the development of an along-valley flow. This flow is up-valley in the upstream section during the first three hours of the P1 simulation, reversing to the down-valley direction afterwards, but remains up-valley during the six hours of the P2 simulation. The effect of wind dynamics on the dispersion of passive scalars is identified by tracking areas prone to stagnation, recirculation, and ventilation using the methodology developed by Allwine and Whiteman (1994). Zones identified as prone to stagnation are consistent with those of high tracer concentration in both simulations. The narrowing of the valley is found to significantly reduce ventilation in the upstream section, an observation quantified by a ventilation efficiency.

Highlights

Under wintertime atmospheric conditions, urbanized mountainous areas are affected by air pollution episodes related to particulate matter (PM) due to emission of primary aerosols and precursor gases leading to secondary aerosols
In valleys stable boundary layers are often associated with persistent cold-air pools (PCAPs), which can last for days, leading to an accumulation of air pollutants that can exceed air-quality standards [2]
Let τ be the length of the time series and T the sampling interval of the series

Summary

Introduction

Under wintertime atmospheric conditions, urbanized mountainous areas are affected by air pollution episodes related to particulate matter (PM) due to emission of primary aerosols and precursor gases leading to secondary aerosols. Several studies have addressed the problems of particulate air pollution in mountainous terrain during winter [1,2,3,4]. In the French Alps, PM emissions resulting from wood combustion increase dramatically during wintertime and are a major concern to air quality [5]. In valleys stable boundary layers are often associated with persistent cold-air pools (PCAPs), which can last for days, leading to an accumulation of air pollutants that can exceed air-quality standards [2]. The terrain geometry is responsible for the formation of cold-air pools (CAPs) in complex terrain

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