Parameter Studies of Canopy Aerodynamic Resistance by means of Large Eddy Simulations

The ‘canopy convector effect' was introduced recently by Rotenberg and Yakir, (2010, 2011) to explain a substantially large sensible heat flux over a semi-arid ecosystem (Yatir forest, Israel) in spite of a moderate canopy skin temperature. Being located at the edge of the Negev desert, the Yatir forest is exposed to net radiation intensities larger than 700 Wm^-2, while experiencing distinct dry summer periods. Besides the incoming radiation, the turbulent fluxes above the forest are amplified by the albedo difference between the forest and the surrounding shrubland. To allow for moderate air temperatures (indispensable to ensure the observed growth of the forest), the incoming energy load has to be carried away by enhanced turbulent transport inside and above the forest. The combination of the large sensible heat flux above the forest and moderate air temperatures can be explained by a small canopy aerodynamic resistance (CAR) for heat transfer. To quantify the ‘canopy convector effect' and to investigate its influence on the forest's microclimate, (atmospheric) boundary layer dynamics and energy balance closure, the present work aims at analyzing the CAR by means of large eddy simulations (LES). Multiple simulations were performed to study the dependency of the CAR on several parameters. The parameters taken into account are canopy attributes (tree density, leaf area density of single trees and forest shape), forest size, ratio of desert to forest albedo and wind strengths. An empirical parametrization of the CAR was developed based on the LES results and compared with field observations. Employing this empirical parameterization of the CAR may help understanding effects of afforestation on surface-atmosphere-exchange energy fluxes and on regional climatic conditions in dry-land areas.