Recirculation over complex terrain

This study generated eddy covariance data to investigate atmospheric dynamics leeward of a small, forested hillside in upstate New York. The causes and effects of recirculation eddies were examined to support the larger goal of improving measurement of the exchange of energy, moisture, and trace gases between the terrestrial biosphere and the atmosphere over complex terrain. Sensors operated at five different altitudes on two separate towers—one at the top of the hill and one down the slope to the east—for approximately 8 weeks in the spring of 2013. During the experiment, the vertical potential temperature gradient was found to be the primary factor for determining whether winds interacting with the terrain features caused a recirculating eddy leeward of the hill. The study found evidence that the recirculation influenced carbon dioxide flux and caused the air column to be vertically well mixed. Plain Language Summary The purpose of our overall research project is to improve our understanding of biosphere/atmosphere interactions in nonhomogeneous natural environments. We used the eddy covariance method, which, while imperfect, has been instrumental in furthering knowledge of boundary layer interactions. Seeking additional improvements to such a powerful tool is a worthwhile endeavor, and we hope to advance the technique as we pursue the main project goal. Previous studies in this field have focused on the energy flux balance closure problem, where net radiation measurements often record larger-energy flux than researchers can account for within their observed systems. Eminent scientists have predicted various causes of this imbalance: advection, energy storage, etc. Many proposed “errors” fall into the category of nonturbulent fluxes. We postulate that a large, slow-moving recirculation bubble spinning leeward of a forested hillside may cause a significant amount of flux and other boundary layer changes that would not be captured using the standard eddy covariance method. We know these recirculation bubbles occur due to work by other scientists in hydraulics observations, or through atmospheric modeling. Our two towers were designed to capture this recirculation effect, identify its causes, and begin the process of understanding its impacts.