Valley floor inclination affecting valley winds and transport of passive tracers in idealised simulations

Abstract

In mountainous regions, diurnal thermally driven winds impact daily weather and air quality. This study investigates how the inclination of idealised valleys affects these winds and the transport of passive tracers using high-resolution numerical simulations with the Weather Research and Forecasting (WRF) model. We explore a range of valley inclinations from 0 to 2.28°, bridging the gap between previous studies on flat and moderately inclined (up to 0.86°) idealised valleys and steeper (2-5°) real Himalayan valleys. We find that during daytime in the inclined valleys, up-valley winds penetrate deeper into the valleys and become stronger, up to a critical inclination beyond which the winds weaken. The flat-floored valley exhibits the strongest night-time down-valley winds overall, but surface-based down-valley winds are more prominent in inclined valleys. Steeper valleys enhance the vertical transport of passive tracers, resulting in ventilation at higher altitudes compared to the flat-floored valley. Despite stronger overall tracer outflow in the flat-floored valley, this occurs at lower altitudes, leading to most of the ventilated tracers being accumulated in the lowest few kilometres of the atmosphere. Consequently, steeper valleys are more efficient at ventilating tracers to the upper troposphere, which would, for example, lead to higher potential for long-range transport. These findings underscore the critical role of valley geometry in shaping wind patterns and pollutant transport, providing valuable insights for improving transport modelling in mountainous regions.