Abstract
Abstract. Idealized large-eddy simulations were performed to investigate the impact of different mountain geometries on daytime pollution transport by thermally driven winds. The main objective was to determine interactions between plain-to-mountain and slope wind systems, and their influence on the pollution distribution over complex terrain. For this purpose, tracer analyses were conducted over a quasi-two-dimensional mountain range with embedded valleys bordered by ridges with different crest heights and a flat foreland in cross-mountain direction. The valley depth was varied systematically. It was found that different flow regimes develop dependent on the valley floor height. In the case of elevated valley floors, the plain-to-mountain wind descends into the potentially warmer valley and replaces the opposing upslope wind. This superimposed plain-to-mountain wind increases the pollution transport towards the main ridge by an additional 20 % compared to the regime with a deep valley. Due to mountain and advective venting, the vertical exchange is 3.6 times higher over complex terrain than over a flat plain. However, the calculated vertical exchange is strongly sensitive to the definition of the convective boundary layer height. In summary, the impact of the terrain geometry on the mechanisms of pollution transport confirms the necessity to account for topographic effects in future boundary layer parameterization schemes.
Highlights
Daytime transport and mixing processes of air pollutants primarily occur within the convective boundary layer (CBL) and are mostly well understood for flat and homogeneous terrain (Steyn et al, 2013)
In this study we performed idealized large-eddy simulations (LES) with the WRF model to investigate the interaction between plain-tomountain and slope wind systems, and their influence on daytime pollution distribution over complex terrain
Simulations over a mountain range with embedded valleys bordered by ridges with different crest heights were compared to simulations with a single ridge and a flat plain by means of tracer analyses
Summary
Daytime transport and mixing processes of air pollutants primarily occur within the convective boundary layer (CBL) and are mostly well understood for flat and homogeneous terrain (Steyn et al, 2013). This paper relates to recent idealized studies (Wagner et al, 2014b, 2015), which investigate the impact of different valley topographies on the CBL structure, and the vertical exchange between the CBL and the free atmosphere under idealized daytime conditions with a constant surface sensible heat flux. Simulations for valleys with different depths are performed and compared to a single-ridge topography to quantify the impact of varying valley floor heights on different transport processes of pollutants over complex terrain, e.g., mountain venting and return flows in the free atmosphere. To compare our results with De Wekker et al (2004), we compute a second CBL height (CBL2) by using the same Richardson-numberbased method following Vogelezang and Holtslag (1996) For this purpose, a modified bulk Richardson number is calculated on every vertical model level starting from the surface. Vertical transport of CBL air beyond this reference height can occur either by turbulent exchange in the EL or by thermally induced circulations
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