Momentum transport above forested gentle topography

Much of Earth's surface is covered by forest, agricultural, and urban canopies. Turbulent transport in the atmospheric boundary layer (ABL) over the Amazon rainforest is a key area of study due to the wide array of gases emitted within the forest. The Amazon is characterized by a tall, dense canopy and rivers that run through the area, creating a series of gentle hills and valleys. In this study, a large-eddy simulation dataset is used to focus on the coupling between the two components that make up these topographical configurations, an upslope and a downslope. The streamwise length of the valley floor and hilltop are varied in our idealized configuration, altering the coupling between these two components. When the upslope and downslope are separated by a large enough distance, the flow has many similarities to turbulent flow over a forward and backward facing step. This includes local re-circulations and a growing shear layer across the canopy top. As these length scales shrink, the differences in the valley and hilltop cases become more apparent. An enhancement of turbulence across the shear layer is observed in the hilltop cases, with little change in the mean re-circulations. In the valley cases, mean re-circulations are found to merge and grow while turbulent mixing is reduced.