A preliminary study of storage and transport processes in an idealized valley

An accurate representation of atmospheric boundary layer (ABL) processes is critical for trace gas simulations and inverse modelling on the regional scale. This is particularly challenging over complex terrain such as the Swiss Midlands. Thus, it is not surprising that current operational numerical weather prediction models still provided limited accuracy for the simulation of vertical mixing processes over such regions. Vertical mixing in the ABL induced by local circulations directly impacts the vertical profiles of trace gases emitted at the surface. A bias in these processes may introduce significant errors in the estimate of trace gas concentrations. For example, the accumulation of greenhouse gases (GHG) during nighttime stable boundary layers (SBLs) is significantly underestimated compared to GHG observation sites such as Payerne or tall tower site Beromünster. Therefore, in the present work, high-resolution simulations using Cloud Model 1 (CM1) with idealized topography, representative of the Swiss Midlands, are performed, with the aim to contribute to an improved understanding of the relevant storage and transport processes, e.g. accumulation of passive tracer in nocturnal cold pools, separation of turbulence and local circulation effects, morning depletion and export of tracers to higher levels, entrainment of free-tropospheric air into the boundary layer, and their sensitivity to different environmental conditions, such as  atmospheric stratification, upper-level winds, surface forcings and to the properties of underlying topography. The model setup also includes virtual towers located at valley floor, over the slope and the mountain ridge. First results will be presented, and the current state of the research will be discussed.