The effect of land-surface heterogeneity on the mixing in the Stable Boundary Layer

Abstract

This study aims to look into the effect of small-scale surface heterogeneity on the mixing properties within the stable boundary layer (SBL) and the impact of different configurations of the heterogeneity on this mixing. The motivation for this research is to better understand the turbulent mixing phenomena in the SBL and to be able to represent the mixing more accurate in weather prediction models. In the current models, this mixing is highly uncertain or even missing. It is likely that this mixing is driven by thermal memories that are spatially distributed over the land-surface in the form of surface heterogeneity. These surface heterogeneities (e.g. ditches or roads) effectively store heat during the day and then turn into heat sources, driving turbulent mixing, during the night. A simulation with surface heat sources in an idealized SBL is setup and four case studies are performed. The simulations show that the heat sources clearly influence the mixing in the SBL. A buoyancy plume is formed above a heat source. The plumes of two heat sources are attracted towards each other. At smaller distances the plumes even merge and form a single plume. In the three-dimensional simulations, the distance between the heat sources has an influence on their effect on the SBL. At small distances, the kinetic energy in the system decreases with increasing distances between two heat sources. At larger distances this decrease appears to level off. Apparently, the bending and merging of the plumes allows turbulent length scales to increase, causing the effect of the buoyancy sources to be larger. To verify these simulation results, a field experiment is conducted. An infrared camera pointing at a vertical high-emissivity cloth was able to capture the two-dimensional (vertical and horizontal) air temperature pattern above heat sources. The merging effect between two plumes is also observed in this field experiment. At small distances a single merged heat plume was visible above two heat sources and at larger distances two separated heat plumes are formed.