Quantifying Horizontal and Vertical Tracer Mass Fluxes in a Developing Daytime Valley Boundary Layer

Slope winds provide a mechanism for the vertical exchange of air between the valley and the free atmosphere aloft. By this means, heat, moisture and pollutants are exported or imported. However, if the static stability of the valley atmosphere is sufficiently strong, one part of the up-slope flow is redirected towards the valley center and pollutants stay trapped in a circulation within the valley. Besides stability, the strength of the up-slope flow depends also on the surface heating (i.e., forcing). Hence, the venting potential of slope winds is low if the forcing is weak or the static stability is strong. The main objective of this study is to quantify the horizontal and vertical transport of pollutants between the mixed layer at the valley floor, the slope wind layer, the stable valley core, and the free atmosphere above the valley. More specifically, the goal is to determine the dependency of these fluxes on the initial static stability of the atmosphere and the amplitude of the forcing.

For this purpose, we conduct large eddy simulations with the Weather Research and Forecasting (WRF) model for a quasi-two-dimensional valley. The valley geometry consists of two slopes with constant slope angle rising to a crest height of 1500 m and a 4 km wide flat valley floor in between. The valley is 20 km long and homogeneous in along-valley direction. The surface sensible heat flux is prescribed by a sine function with an amplitude of 62.5, 125, 250 and 375 W m^-2. The initial atmospheric conditions are characterized by an atmosphere at rest and by a constant buoyancy frequency which is varied between N=0.006 s^-1 and 0.02 s^-1. A passive tracer is released with an arbitrary but constant rate at the valley floor.

As expected, the atmospheric stability has a strong impact on the vertical and horizontal transport of tracer mass. A horizontal intrusion forms at the top of the mixed layer due to outflow from the slope wind layer. Hence, tracer mass is transported from the slope towards the center of the valley. The efficiency of this mechanism increases with increasing stability. For 125 W m^-2 and the lowest value of N, about 70 % of the tracer mass released at the valley bottom is exported out of the valley. This value drops to about 10 % in the case of the strongest stability. Hence, most of the tracer mass, which enters the slope wind layer at the valley bottom, is leaving it again through horizontal fluxes at the height of the intrusion and therefore remains inside the valley. For the strongest forcing and the weakest stratification, 85 % of the tracer is exported, while for the weakest forcing and the strongest stratification, only 5 % is exported. In general, the weaker the stratification and the stronger the forcing, the higher is the fraction of exported tracer mass. This behaviour can be described using a so-called breakup parameter, which is the ratio of energy required to neutralize the stratification to energy provided by the surface sensible heat flux during daytime.