The impact of embedded valleys on daytime pollution transport over a mountain range

Lang, Moritz N.; Gohm, Alexander; Wagner, Jochen

doi:10.5194/acp-15-11981-2015

Cited by 37 publications

(41 citation statements)

References 41 publications

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“…Compensating lifting and CBL to FT transport are also triggered by along-valley flow convergence (Kossmann and Sturman, 2003;Henne et al, 2004;Weigel et al, 2007), or by terrain-associated convergence zones on the lee sides of isolated mountains or mountain ranges (Edinger and Helvey, 1961). Regional scale plain-to-mountain winds combine with these slope and valley flow mechanisms to lead to a transport of boundary layer air to the FT over the entire mountain range (Henne et al, 2005;Lang et al, 2015), a mechanism that has been termed "alpine pumping" (Lugauer and Winkler, 2005). Vertical transport and mixing by urban heat island-induced circulations is an additional process that can play an important role in the venting of pollutants in urbanized valleys (Rendón et al, 2014(Rendón et al, , 2015.…”

Section: Vertical Transport and Mixing Between The Cbl And The Free Tmentioning

confidence: 99%

Convective Boundary Layer Heights Over Mountainous Terrain—A Review of Concepts

2015

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Mountainous terrain exerts an important influence on the Earth's atmosphere and affects atmospheric transport and mixing at a wide range of temporal and spatial scales. The vertical scale of this transport and mixing is determined by the height of the atmospheric boundary layer, which is therefore an important parameter in air pollution studies, weather forecasting, climate modeling, and many other applications. It is recognized that the spatio-temporal structure of the daytime convective boundary layer (CBL) height is strongly modified and more complex in hilly and mountainous terrain compared to flat terrain. While the CBL over flat terrain is mostly dominated by turbulent convection, advection from multi-scale thermally driven flows plays an important role for the CBL evolution over mountainous terrain. However, detailed observations of the CBL structure and understanding of the underlying processes are still limited. Characteristics of CBL heights in mountainous terrain are reviewed for dry, convective conditions. CBLs in valleys and basins, where hazardous accumulation of pollutants is of particular concern, are relatively well-understood compared to CBLs over slopes, ridges, or mountain peaks. Interests in the initiation of shallow and deep convection, and of budgets and long-range transport of air pollutants and trace gases, have triggered some recent studies on terrain induced exchange processes between the CBL and the overlying atmosphere. These studies have helped to gain more insight into CBL structure over complex mountainous terrain, but also show that the universal definition of CBL height over mountains remains an unresolved issue. The review summarizes the progress that has been made in documenting and understanding spatio-temporal behavior of CBL heights in mountainous terrain and concludes with a discussion of open research questions and opportunities for future research.

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Section: Vertical Transport and Mixing Between The Cbl And The Free Tmentioning

confidence: 99%

Convective Boundary Layer Heights Over Mountainous Terrain—A Review of Concepts

2015

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“…This way, the pollutants are incorporated into the plain-to-mountain circulation and are advected towards the adjacent plain (Wagner et al, 2015a). Valleys embedded in a mountain ridge add another layer of complexity to the system since they develop a slope wind system of their own, which interacts with the plain-tomountain flow Lang et al (2015). However, before pollutants can be incorporated into the complex flow above the mountains, they have to be advected out of the valley atmosphere, a process that is one subject of this study.…”

Section: Introductionmentioning

confidence: 99%

“…Stable conditions, which lead to such recirculations, are common in Alpine valleys throughout the whole year, especially during fall and winter, giving rise to the question of how effective this exchange mechanism really is. The bulk export of pollutants is particularly relevant for the air quality inside the valley, but may also have an impact further away since polluted air is transported towards adjacent plains by the plainto-mountain circulation (Wagner et al, 2015a;Lang et al, 2015). Therefore, there is a need for quantifying the vertical and horizontal fluxes of pollutants for different atmospheric stability conditions and different forcing strengths, which is the general goal of this study.…”

Section: Introductionmentioning

confidence: 99%

Quantifying horizontal and vertical tracer mass fluxes in an idealized valley during daytime

2016

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Abstract. The transport and mixing of pollution during the daytime evolution of a valley boundary layer is studied in an idealized way. The goal is to quantify horizontal and vertical tracer mass fluxes between four different valley volumes: the convective boundary layer, the slope wind layer, the stable core, and the atmosphere above the valley. For this purpose, large eddy simulations (LES) are conducted with the Weather Research and Forecasting (WRF) model for a quasitwo-dimensional valley. The valley geometry consists of two slopes with constant slope angle and is homogeneous in the along-valley direction. The surface sensible heat flux is horizontally homogeneous and prescribed by a sine function. The initial sounding is characterized by an atmosphere at rest and a constant Brunt-Väisälä frequency. Various experiments are conducted for different combinations of surface heating amplitudes and initial stability conditions. A passive tracer is released with an arbitrary but constant rate at the valley floor and resulting tracer mass fluxes are evaluated between the aforementioned volumes.As a result of the surface heating, a convective boundary layer is established in the lower part of the valley with a stable layer on top -the so-called stable core. The height of the slope wind layer, as well as the wind speed within, decreases with height due to the vertically increasing stability. Hence, the mass flux within the slope wind layer decreases with height as well. Due to mass continuity, this along-slope mass flux convergence leads to a partial redirection of the flow from the slope wind layer towards the valley centre and the formation of a horizontal intrusion above the convective boundary layer. This intrusion is associated with a transport of tracer mass from the slope wind layer towards the valley centre. A strong static stability and/or weak forcing lead to large tracer mass fluxes associated with this phenomenon.The total export of tracer mass out of the valley atmosphere increases with decreasing stability and increasing forcing. The effects of initial stability and forcing can be combined to a single parameter, the breakup parameter B. An analytical function is presented that describes the exponential decrease of the percentage of exported tracer mass with increasing B. This study is limited by the idealization of the terrain shape, stratification, and forcing, but quantifies transport processes for a large range of forcing amplitudes and atmospheric stability.

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“…The present study falls between idealized (e.g. Lehner and Gohm, ; Lang et al ., ; Leukauf et al . ; Quimbayo‐Duarte et al ., ) and real‐case (e.g., Martínez et al ., ; Jiménez and Cuxart, ; Jiménez et al ., ) studies as we consider a realistic orography and a simplified initialization of the atmosphere.…”

Section: Discussionmentioning

confidence: 99%

“…(; ) highlighted the along‐valley flow dependence on topographic characteristics such as valley width and bottom inclination while Lang et al . () discussed the impact of valley floor heights on tracer export. The valley orientation also appears as a key parameter since it governs the short‐wave incoming radiation and may generate an asymmetry in the pollution distribution between sunlit and shadow sidewalls, as reported by Bader and Whiteman (), Chazette et al .…”

Section: Introductionmentioning

confidence: 87%

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

Sabatier

Largeron

Paci

et al. 2020

Quart J Royal Meteoro Soc

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Under wintertime quiescent conditions, thermally driven circulations represent one of the only sources of tracer dispersion over mountainous terrain. Those circulations can be unequally developed at a valley scale since they strongly depend on local morphological arrangement. At the same time, very heterogeneous pollutant distribution can be observed, as for instance in a French Alpine basin located in the Arve River valley. This complex basin regularly shows large variations in pollutant concentrations with certain sectors suffering from poor wintertime air quality. On the other hand, the surrounding tributary valleys appear to be less affected, suggesting that the basin local dynamics may participate in pollutant trapping. The present study intends to classify the pollutant transport mechanisms in terms of efficiency and to identify the most dynamically vulnerable atmospheric volumes regarding pollutant accumulation. This is achieved through a set of semi‐idealized high‐resolution numerical simulations reproducing a full diurnal cycle with passive tracers released continuously at a constant rate. The model is used as a laboratory in order to quantify the influence of several processes on transport mechanism efficiency. This approach underlines the high efficiency of vertical transport by anabatic winds while horizontal transport efficiency by up‐valley wind systems remains weak, leaving the surrounding tributary valleys almost unaffected by the basin pollution during daytime. At night, the efficiency of horizontal transport by the down‐valley wind systems depends on the tracer source location within the basin. In addition, the tracers emitted within the tributary valleys do not reach the basin bottom because of thermal stratification and local morphological arrangement but rather degrade the air quality of mid‐altitude villages lying along the basin sidewalls.

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The impact of embedded valleys on daytime pollution transport over a mountain range

Cited by 37 publications

References 41 publications

Convective Boundary Layer Heights Over Mountainous Terrain—A Review of Concepts

Convective Boundary Layer Heights Over Mountainous Terrain—A Review of Concepts

Quantifying horizontal and vertical tracer mass fluxes in an idealized valley during daytime

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

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