Mixing efficiency and entrainment at an atmospheric inversion layer

The context is that of the convectively-driven atmospheric boundary layer capped by an inversion layer (i.e. a stably-stratified interface) and we focus on the regime of equilibrium entrainment, i.e. when the boundary-layer evolution is in a quasi steady state. The parameterization of the entrainment process across the interfacial layer is usually based on the entrainment ratio, namely the ratio of the negative of the heat flux at the interface to the heat flux at the ground surface. Hence the issue is to relate the entrainment ratio to measurable parameters. In this study, we rely on a formulation of convective entrainment in terms of mixing efficiency, which can be computed directly for instance from high-resolution vertical profiles of virtual potential temperature. We discuss the applicability of this parameterization for an explicit treatment of the entrainment process in classical boundary-layer parameterization schemes implemented in meso-scale models

Evaluation of local ozone production of Chamonix valley (France) during a regional smog episode

Proceedings of the 27 NATO/CCMS International Technical Meeting on Air Pollution Modeling and Its Application held in Banff, Canada, 24-29 October 2004.During the summer 2003 a POVA Intensive Observation Period (IOP) aimed at determining the sources of airborne pollutants and monitoring their concentrations in two French Alpine valleys: the Chamonix and the Maurienne valleys (see figure 1 for geographic location). The Pollution of Alpine Valleys (POVA) program was launched in 2000 after the traffic interruption under the Mont-Blanc that followed the tragic accident in the tunnel. The Mont-Blanc tunnel was reopened at the end of 2002 and caused the high duty vehicle traffic (about 1100 trucks per day) to be back in the Chamonix valley. The summer 2003 IOP took place from 5 to 12 July in the Chamonix valley. A high ozone event occurred from 5 to 14 July at regional scales and was well characterised by measurements at rural monitoring sites. To better understand the particular atmospheric circulation, and to study the chemical reactions of airborne pollutants within the valleys, mesoscale modelling is applied. For meteorological calculation, the fifth generation PSU/NCAR Mesoscale Model (MM5) was used at scales ranging from 27 to 1 km. MM5 was coupled with the Chemistry Transport Model (CTM) CHIMERE at regional scales and with the CTM TAPOM at a one-kilometre resolution. Simulations were performed for the period 5-12 July 2003 with different emission sets aiming at studying the impact of the international road traffic in the valley on airborne pollutant concentrations

Drivers of severe air pollution events in a deep valley during wintertime: a case study from the Arve river valley, France

© 2020 Elsevier Ltd. All rights reserved. This manuscript is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International Licence (http://creativecommons.org/licenses/by-nc-nd/4.0/).The Arve river valley airshed in the French Alps experiences particularly severe air pollution during wintertime stable atmospheric conditions associated with persistent cold-air pools. PM10 data recorded in the region indicate that the urbanised area of the central basin-shape section of the valley is generally the most polluted, with a harmful impact on the health of inhabitants. In the present work, we examine the air pollution transport potential of the Arve river valley airshed using results from high-resolution numerical simulations of a cold-air pool documented as part of the Passy-2015 field campaign. Passive tracers were used to model PM10 with emissions provided by a detailed inventory developed by the local air-quality agency. The observed differential in PM10 levels between valley sections was well captured by the numerical model and could not be explained solely by the differential in emissions. The stagnation, recirculation and ventilation potential of the airshed was evaluated spatially and temporally using integral quantities. The analysis indicated that the central basin-shape section of the valley is poorly ventilated and hence air pollution there would originate mostly from local emission sources. This stagnation zone appears to be almost decoupled from the rest of the airshed. The airshed was decomposed in separate valley sections so as to quantify the fate of the pollutants emitted within each section. Air pollution apportioned according to the contribution of emissions from the different valley sections shows that indeed the central basin-shape section is dominated by local sources. The situation was found more complex in the valley sections further downstream, where the contribution from the sum of the non-local sources can be as large as that from local sources. This study allows to identify the origin of the strong pollution in the Arve river valley, through the link between the local topography, emission sources and pollutant transport.Peer reviewedFinal Accepted Versio

Caractérisation des mouvements oscillants dans l'atmosphère stable d'une vallée encaissée

In a valley sheltered from strong synoptic effects, the dynamics of the valley atmosphere at night are dominated by katabatic winds. In a stably stratified atmosphere, these winds undergo temporal oscillations, whose frequency is given by Nsinθ for an infinitely long slope of constant slope angle θ, N being the buoyancy frequency. Such an unsteady flow in a stably stratified atmosphere may also generate internal gravity waves (IGWs)

Modéles d'atmosphère locale et qualité de l'air en région montagneuse, étude de scénarios

La qualité de l'air dans les régions montagneuses dépend d'une dynamique atmosphérique locale aux vallées qui interagit avec les échelles synoptiques de météorologie. La modélisation associe un calcul tri dimenionnel instationnaire de type simulation des grands échelles à des schémas de réaction chimique. Ces modèles sont évalués par comparaison de leurs résultats avec des mesures collectées sur le terrain et par la bonne reproduction de phénomènes typiques de ces régions tels que les vents de vallée et l'évolution diurne de la hauteur de couche mélangée. Les modèles sont utilisés dans le cadre de scénarios pour évaluer des impacts d'émission sur la qualité de l'air. L'imbrication de domaines de calcul permet de prendre en compte la très large gamme d'échelles de mouvement. On s'appuie sur l'étude de deux sites réels : la vallée de Chamonix encaissée entre de fortes pentes, l'agglomération grenobloise avec des émissions de pollution typique d'une ville importante

Energetics of Deep Alpine Valleys in Pooling and Draining Configurations

This is an Open Access article licensed under a Creative Commons Attribution 4.0 license (http://creativecommons.org/licenses/by/4.0/).The Weather Research and Forecast numerical model is used to investigate the nocturnal atmospheric boundary layer in a valley that opens either on a wider valley (draining configuration) or on a narrower valley (pooling configuration). One draining case and three weak to strong pooling cases are considered. Results show that the structure of the nocturnal boundary layer is substantially different for the draining and pooling configurations. The greater the pooling, the deeper and colder is the boundary layer. Down-valley winds are weaker for pooling and draining configurations than in an equivalent valley opening directly on a plain. For the strong pooling case, an up-valley flow develops from the narrower to the wider valley during the evening transition, affecting the mass budget of the wider valley during that period. Considering the heat budget of the valley system, the contribution of the diabatic processes, when appropriately weighted, hardly varies along the valley axis. Conversely, the contribution of advection varies along the valley axis: it decreases for a pooling configuration and increases for a draining configuration. Consequently, for a pooling configuration, the heat transfer between the valley and the plain is reduced, thereby increasing the temperature difference between them. For the strong pooling case, this temperature difference can be explained by the valley-volume effect once the down-valley flow has developed. This occurs in a valley when the `extra' heat loss within the valley due to the surface sensible heat flux balances the heat input due to advection.Peer reviewedFinal Published versio

Analysis of UK and European NOx and VOC emission scenarios in the Defra model intercomparison exercise

This is a PDF file of an unedited manuscript that has been accepted for publication. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertainSimple emission scenarios have been implemented in eight United Kingdom air quality models with the aim of assessing how these models compared when addressing whether photochemical ozone formation in southern England was NOx- or VOC-sensitive and whether ozone precursor sources in the UK or in the Rest of Europe (RoE) were the most important during July 2006. The suite of models included three Eulerian-grid models (three implementations of one of these models), a Lagrangian atmospheric dispersion model and two moving box air parcel models. The assignments as to NOx- or VOC-sensitive and to UK- versus RoE-dominant, turned out to be highly variable and often contradictory between the individual models. However, when the assignments were filtered by model performance on each day, many of the contradictions could be eliminated. Nevertheless, no one model was found to be the 'best' model on all days, indicating that no single air quality model could currently be relied upon to inform policymakers robustly in terms of NOx- versus VOC-sensitivity and UK- versus RoE-dominance on each day. It is important to maintain a diversity in model approaches.Peer reviewedFinal Accepted Versio

Design and field campaign validation of a multi-rotor unmanned aerial vehicle and optical particle counter

© Author(s) 2020. This work is distributed under the Creative Commons Attribution 4.0 License (https://creativecommons.org/licenses/by/4.0/).Small unmanned aircraft (SUA) have the potential to be used as platforms for the measurement of atmospheric particulates. The use of an SUA platform for these measurements provides benefits such as high manoeuvrability, re-usability, and low-cost when compared with traditional techniques. However, the complex aerodynamics of an SUA (particularly for multirotor airframes), combined with the miniaturisation of particle instruments poses difficulties for accurate and representative sampling of particulates. The work presented here relies on computational fluid dynamics with Lagrangian particle tracking (CFD-LPT) simulations to influence the design of a bespoke meteorological sampling system: the UH-AeroSAM. This consists of a custom built airframe, designed to reduce sampling artefacts due to the propellers, and a purpose built open-path optical particle counter–the Ruggedised Cloud and Aerosol Sounding System (RCASS). OPC size distribution measurements from the UH-AeroSAM are compared with the Cloud and Aerosol Precipitation Spectrometer (CAPS) for measurements of Stratus cloud during the Pallas Cloud Experiment (PaCE) in 2019. Good agreement is demonstrated between the two instruments. The integrated dN/dlog(Dp) is shown to have a coefficient of determination of 0.8, and a regression slope of 0.9 when plotted 1:1.Peer reviewe

Interactions between downslope flows and a developing cold-air pool

A numerical model has been used to characterize the development of a region of enhanced cooling in an alpine valley with a width of order (Formula presented.) km, under decoupled stable conditions. The region of enhanced cooling develops largely as a region of relatively dry air which partitions the valley atmosphere dynamics into two volumes, with airflow partially trapped within the valley by a developing elevated inversion. Complex interactions between the region of enhanced cooling and the downslope flows are quantified. The cooling within the region of enhanced cooling and the elevated inversion is almost equally partitioned between radiative and dynamic effects. By the end of the simulation, the different valley atmospheric regions approach a state of thermal equilibrium with one another, though this cannot be said of the valley atmosphere and its external environment.Peer reviewe

Trace gas/aerosol boundary concentrations and their impacts on continental-scale AQMEII modeling domains

Copyright 2011 Elsevier B.V., All rights reserved.Over twenty modeling groups are participating in the Air Quality Model Evaluation International Initiative (AQMEII) in which a variety of mesoscale photochemical and aerosol air quality modeling systems are being applied to continental-scale domains in North America and Europe for 2006 full-year simulations for model inter-comparisons and evaluations. To better understand the reasons for differences in model results among these participating groups, each group was asked to use the same source of emissions and boundary concentration data for their simulations. This paper describes the development and application of the boundary concentration data for this AQMEII modeling exercise. The European project known as GEMS (Global and regional Earth-system Monitoring using Satellite and in-situ data) has produced global-scale re-analyses of air quality for several years, including 2006 (http://gems.ecmwf.int). The GEMS trace gas and aerosol data were made available at 3-hourly intervals on a regular latitude/longitude grid of approximately 1.9° resolution within 2 "cut-outs" from the global model domain. One cut-out was centered over North America and the other over Europe, covering sufficient spatial domain for each modeling group to extract the necessary time- and space-varying (horizontal and vertical) concentrations for their mesoscale model boundaries. Examples of the impact of these boundary concentrations on the AQMEII continental simulations are presented to quantify the sensitivity of the simulations to boundary concentrations. In addition, some participating groups were not able to use the GEMS data and instead relied upon other sources for their boundary concentration specifications. These are noted, and the contrasting impacts of other data sources for boundary data are presented. How one specifies four-dimensional boundary concentrations for mesoscale air quality simulations can have a profound impact on the model results, and hence, this aspect of data preparation must be performed with considerable care.Peer reviewedFinal Accepted Versio