Dependence of Atmospheric Cooling by Vegetation on Canopy Surface Area During Radiative Cooling at Night: Physical Model Evaluation Using a Polyethylene Chamber

摘要

The plant canopy surface is cooled by radiative cooling at night. Cooling of the canopy surface results in cooling of the surrounding air by convective heat exchange between the canopy surface and the air. Convective heat exchange at the canopy surface is proportional to the convective heat transfer conductance, which depends on the wind speed and the canopy surface area. We conducted physical model experiments using a polyethylene chamber to examine the dependence of the heat exchange at the canopy surface on the canopy surface area during nocturnal radiative cooling. We introduced three canopy models, each having a canopy surface area equivalent to a Leaf Area Index (LAI) of 1,2, or 4. The models were placed in two polyethylene chambers located outdoors and exposed to a nocturnal radiation regime. The canopy surface temperature of the LAI = 1 model was the lowest among the three models, whereas the convective heat exchange between the canopy surface and the air traveling into the chamber was the smallest for the LAI = 1 model. The convective heat exchange did not differ significantly between the LAI = 2 and 4 canopy models. The results showed that the air traveling through the canopy was cooled more at night, when the canopy LAI was large and the canopy surface temperature was relatively high. This air-cooling effect tended to approach an asymptotic value as the LAI became large. We were able to simulate these phenomena with a simple heat balance analysis of the heat exchange between the canopy surface and the air.