Passive tracer under stable conditions: impact of background wind and valley geometry in an idealized setup

Accurate representation of atmospheric boundary layer (ABL) processes is crucial for trace gas simulations and inverse modeling on a regional scale, particularly in areas with complex terrain such as the Swiss Midlands. Challenges arise from uncertainties in local turbulent transport, advection, and diffusion. Local circulations in the ABL, influenced by complex terrain, directly impact the vertical profiles of trace gases emitted at the surface. Biases in these processes can lead to significant errors, especially during nighttime stable conditions, affecting the estimation of trace gas concentrations, notably greenhouse gases accumulation. By conducting high-resolution idealized simulations using Cloud Model 1 in Large Eddy Simulation configuration, we aim to provide insights into the trace gas dispersion over the Swiss Midlands, characterised by rolling terrain. This study investigates sensitivities of trace gas simulations to large-scale background wind and valley depth, within stable boundary layers. We investigate how these factors influence the storage and transport processes of passive tracers in nocturnal cold pools, as well as their morning depletion. The main focus of our study is on the temporal evolution of the passive tracer concentration at virtual towers located at the valley center, eastern and western slopes, and the ridge top. The results show that at the valley center the stable regime leads to the build-up of the passive tracer near the valley floor. Furthermore, it is found that the peak values are reduced and the timing of the peak is delayed with increasing inlet height. This means that despite the low vertical turbulent mixing, the local re-circulation within the valley transports the tracer to higher elevations, resulting in stratified tracer accumulation.