Modeling and measuring turbulent recirculation bubbles over complex terrain: mixing mechanisms for forest-atmosphere exchanges

A large part (70%) of the earth's land surface is covered with mountains and hills. These rugged surfaces, particularly those covered with forests, distort airflows near the ground and create complexity in understanding land-atmosphere exchanges of mass and energy. One of the most important features in the distorted flows is recirculation formed downwind of forested mountains or hills. These recirculation bubbles (regions) are typical phenomena in complex terrain and play substantial roles in land-atmosphere exchanges. However, studies of these turbulent recirculation bubbles in nature have received little attention and their characteristics and properties are poorly understood. Recently, we have systematically investigated recirculation bubbles by a variety of approaches including analytical modeling, numerical modeling, and experiments. We used the velocity-squared law to close the momentum equation and derived an analytical solution for wind speed over forested hills. These analytical approaches provided insights into the formation conditions of recirculation. To understand the role of recirculation in forest-atmosphere exchanges over complex terrain, we have applied a CFD engineering approach to simulate the recirculation bubbles. We found that recirculation bubbles hold and mix CO2 in a different way from the surroundings. The size and structure of recirculation is closely related to terrain slopes. In 2013, we conducted an experiment using two nearby towers in Black Rock Forest, New York, to investigate the predicted recirculation bubbles. The formation of recirculation bubbles was observed to be dependent on wind speed and direction, terrain features, temperature gradients and prevailing synoptic conditions. We examined the agreements and differences between the model predictions and the observations. In summary, recirculation bubbles are typical phenomena in complex terrain and conduct important mixing mechanisms in forest-atmosphere exchanges of mass and energy. More modeling and measurement studies are needed to investigate these phenomena in depth.

Key Words: recirculation, complex terrain, forest-atmosphere exchange, turbulence

Acknowledgement: This research was supported by NSF Grants ATM-0930015 and PSC-CUNY ENHC-44-83.