Numerical and experimental study of soil-atmosphere exchange processes across an undulating soil surface influenced by the near-surface atmospheric boundary layer

Abstract

Soil-atmosphere exchange processes are critical to a wide range of applications, such as greenhouse gas release to the atmosphere, 222Rn transport into buildings, geothermal heat production, global water cycle and land management, which are closely related to the environmental health and protection, climate change, and energy supply. Given the importance, this research aims to investigate soil-atmosphere exchange processes with a special focus on bare-soil evaporation, a process of mass, momentum, and heat transfer between the soil and the atmosphere, by interweaving experimental and numerical approaches. A critical feature of bare soil involves undulating surfaces due to either natural or manual processes. The near-surface boundary layer is significantly influenced by the surface geometry besides of atmospheric conditions. The combined influence of soil undulations and the near-surface boundary layer results in distinct exchange behaviors compared to a hydrodynamically smooth surface. This topic has not been studied systematically due to the lack of appropriate models and high-fidelity datasets. Therefore, the overarching goal of this research is to advance our understandings of the mass, momentum, and heat transfer between the soil and the atmosphere by including the combined influence of undulating soil surfaces and the corresponding near-surface atmospheric boundary layer to ultimately improve the representation of such processes in hydrological modeling effort. Accordingly, three phases are defined. First, a fundamental study to investigate the undulating-surface evaporation behaviors under a laminar boundary layer was conducted. A fully coupled model describing the mass, momentum, and heat transfer between the soil and the atmosphere was developed and validated through a laboratory experiment using wind tunnel – soil tank system. This model was then used to investigate the influence of atmospheric conditions, soil properties, and soil surface configurations on evaporation. Results demonstrate that soil undulations affect evaporation by influencing the diffusion in the laminar boundary layer and the capillary flow inside the soil, resulting in a heterogeneous distribution of local evaporative flux along the undulating soil surface. Second, the above model was extended by incorporating turbulence and used to investigate undulating-surface evaporation under turbulent airflow. Hot-wire Anemometry was first employed to measure the velocity profiles above the undulating surface. Results confirmed the presence of recirculation zones in the valleys and the corresponding locally low evaporative flux. Turbulent airflow was found to enhance evaporation and the surface configurations affect local evaporation by influencing the vapor distribution and surface water availability, especially as recirculation zones form. As a joint result of turbulence and undulations, the influence of wind speed on the evaporation was restricted. Third, a reduced model concept was adopted from perspectives of applications, which simplifies the soil-atmosphere exchange via a flux top boundary condition based on Monin-Obukhov similarity theory. The vapor roughness length (z0v) and momentum roughness length (z0m) are two major parameters in this model characterizing the mass and momentum transfer between the soil surface and the atmosphere. The relationship between z0v and z0m, and the subsequent aerodynamic resistance were parameterized through direct measurements of the velocity field above the undulating soil surface. Four laboratory experiments with unique design were conducted and Particle Imaging Velocimetry was employed to collect the velocity field information. Results show that z0v is roughly smaller than z0m by 3 to 7 orders of magnitude, owing to the undulating surface and the ratio of z0v to z0m are significantly influenced by the surface configuration and wind speed. The newly formulized aerodynamic resistance was then used to evaluate the evaporation rate for laboratory and field experiments, demonstrating the efficacy of the approach.