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

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)

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

Numerical and experimental modelling of the internal tide near a continental shelf

Les processus de mélange sont essentiels au fond de l'océan car ils permettent la remontée des eaux froides abyssales vers la surface. Une grande question de la communauté océanographique concerne la contribution des ondes internes à ces processus car ces ondes, bien que peu énergétiques en regard des courants marins par exemple, sont présentes partout dans l'océan et y déferlent. Les principales sources d'énergie des ondes internes sont le vent et l'interaction de la marée avec la topographie sous-marine. C'est cette dernière configuration que nous considérons ici, au travers d'expériences de laboratoire et numériques, dans le contexte académique d'un talus continental bidimensionnel dans un océan uniformément stratifié. Nous examinons plus particulièrement le processus de génération du champ d'ondes internes et la structure cinématique de ce champ. Nous discutons également de la manifestation des effets non linéaires lorsque le champ d'ondes se réfléchit au fond de l'océan

Valley heat deficit as a bulk measure of wintertime particulate air pollution in the Arve River Valley

© 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).Urbanized valleys are particularly vulnerable to particulate air pollution during the winter, when ground-based stable layers or cold-air pools persist over the valley floor. We examine whether the temporal variability of PM10 concentration in the section of the Arve River Valley between Cluses and Servoz in the French Alps can be explained by the temporal variability of the valley heat deficit, a bulk measure of atmospheric stability within the valley. We do this on the basis of temperature profile and ground-based PM10 concentration data collected during wintertime with a temporal resolution of one hour or finer, as part of the Passy-2015 field campaign conducted around Passy in this section of valley. The valley heat deficit was highly correlated with PM10 concentration on a daily time scale. The hourly variability of PM10 concentrations was more complex and cannot be explained solely by the hourly variability of the valley heat deficit. The interplay of the diurnal cycles of emissions and local dynamics is demonstrated and a drainage mechanism for observed nocturnal dilution of near-surface PM10 concentrations is proposed.Peer reviewe

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

Interactions between the night time valley-wind system and a developing cold-air pool

This is a pre-copyedited, author-produced PDF of an article accepted for publication in Boundary-Layer Meteorology following peer review. The version of record [Arduini, G., Staquet, C & Chemel, C., ‘Interactions between the night time valley-wind system and a developing cold-air pool’, Boundary-Layer Meteorol (2016) 161:1 (49-72), first published online June 2, 2016, is available at Springer online at doi: 10.1007/s10546-016-0155-8The Weather Research and Forecast (WRF) numerical model is used to characterize the influence of a thermally-driven down-valley flow on a developing cold-air pool in an idealized alpine valley decoupled from the atmosphere above. Results for a three-dimensional (3D) valley, which allows for the formation of a down-valley flow, and for a two-dimensional (2D) valley, where the formation of a down-valley flow is inhibited, are analyzed and compared. A key result is that advection leads to a net cooling in the 2D valley and to a warming in the 3D valley, once the down-valley flow is fully developed. This difference stems from the suppression of the slope-flow induced upward motions over the valley centre in the 3D valley. As a result, the downslope flows develop a cross-valley circulation within the cold-air pool, the growth of the cold-air pool is reduced and the valley atmosphere is generally warmer than in the 2D valley. A quasi-steady state is reached for which the divergence of the down-valley flow along the valley is balanced by the convergence of the downslope flows at the top of the cold-air pool, with no net contribution of subsiding motions far from the slope layer. More precisely, the inflow of air at the top of the cold-air pool is found to be driven by an interplay between the return flow from the plain region and subsidence over the plateaux. Finally, the mechanisms that control the structure of the cold-air pool and its evolution are found to be independent of the valley length as soon as the quasi-steady state is reached and the down-valley flow is fully developed.Peer reviewedFinal Accepted Versio

Internal gravity waves: parametric instability and deep ocean mixing

International audienceThe Boussinesq approximation provides a convenient framework to describe the dynamics of stably-stratified fluids. A fundamental motion in these fluids consists of internal gravity waves, whatever the strength of the stratification. These waves may be unstable through parametric instability, which results in turbulence and mixing. After a brief review of the main properties of internal gravity waves, we show how the parametric instability of a monochromatic internal gravity wave organizes itself in space and time, using energetics arguments and a simple kinematic model. We provide an example, in the deep ocean, where such instability is likely to occur, as estimates of mixing from in situ measurements suggest. We eventually discuss the fundamental role of internal gravity wave mixing in the maintenance of the abyssal thermal stratification.The Boussinesq approximation provides a convenient framework to describe the dynamics of stably-stratified fluids. A fundamental motion in these fluids consists of internal gravity waves, whatever the strength of the stratification. These waves may be unstable through parametric instability, which results in turbulence and mixing. After a brief review of the main properties of internal gravity waves, we show how the parametric instability of a monochromatic internal gravity wave organizes itself in space and time, using energetics arguments and a simple kinematic model. We provide an example, in the deep ocean, where such instability is likely to occur, as estimates of mixing from in situ measurements suggest. We eventually discuss the fundamental role of internal gravity wave mixing in the maintenance of the abyssal thermal stratification. Résumé Sur les ondes internes de gravité : de l'instabilité paramétrique au mélange abyssal dans l'océan. L'approximation de Boussinesq constitue un cadre bien adapté à l'étude des fluides stablement stratifiés. Des ondes de gravité internes s'y développent, quel que soit le niveau de stratification, qui peuvent être instables par instabilité paramétrique. Turbulence et mélange en résultent. Après un bref rappel sur les propriétés des ondes de gravité internes, nous montrons comment s'organisent les transferts d'énergie, dans l'espace et dans le temps, lorsqu'une onde interne est paramétriquement instable. Un modèle cinématique simple est employé pour cela. Puis nous illustrons ce processus et ses conséquences par un exemple océanique : l'instabilité paramétique se produit très certainement dans l'océan profond, comme le suggèrent les mesures in situ. Nous discutons finalement du rôle fondamen-tal des ondes de gravité internes dans l'entretien de la stratification abyssale, par le mélange qu'elles induisent