Observational data from the EUROFLUX tower in Denmark are analyzed. The tower is about 57 m tall, located in a partially closed beech forest with an average canopy height of 25 m. Thin-thermal-couple data were recorded at 4 samples per second at 11 levels, from 0.5m to 57m. In addition, absolute temperature was measured at 10 levels, 4 above the canopy, and 6 within the canopy layer. Sonic anemometers were installed at 5 vertical levels, 3 above the canopy and 2 within the canopy layer. Wind profiles were measured at 13 levels, 7 above the canopy and 6 within. In addition, narrow-band infrared radiometers were installed at several locations to monitor temperature variations over the heterogeneous forest.
The high sampling rate and high vertical resolution of the thermal-couple profile data allow us to investigate eddy structure following each wind gust. This data set indicates that the canopy top is the heat source during the daytime due to the solar heating of the canopy, resulting in unstable stratification above the forest and stable stratification within the canopy. The largest temporal variation of air temperature occurs just above the canopy top due to warm updrafts from the canopy and cool downdrafts. The warm air only randomly penetrates into the canopy layer. Small thermals occasionally form at the forest floor but only penetrate a few meters upward with a lifetime of only 2-3 seconds. Due to the stratification within the canopy layer, the heat from those thermals does not extend high into the canopy layer. The very short Eulerian time scale of the warm thermals suggest horizontal transport of heat within the canopy layer. This horizontal advection may also be important in transporting trace gases, such as carbon dioxide, within the canopy layer.