Mechanisms of nocturnal exchange between the rain forest and the atmosphere

Abstract

We have observed a number of events that lead to nocturnal turbulent mixing between the Amazon rain forest and the atmosphere. Two mechanisms that weaken the stable nocturnal thermal inversion at canopy top are identified with nocturnal release of carbon dioxide and concurrent heat flux into and moisture flux out of the Amazon forest canopy. In the first mechanism, abrupt onset of cloudiness interrupts nocturnal radiative cooling, leading to a period with lowered Richardson number and enhanced heat, moisture, and carbon dioxide fluxes. Significant exchange between the forest canopy and the atmosphere can also occur even on totally clear nights. On these nights, occasional nocturnal increases in the wind speed above the canopy stimulate vertical oscillations in the canopy layer that are both amplitude and frequency modulated during the event. A simple model is presented to support the postulate that these oscillations result from a resonant interaction, hypothesized to be a consequence of a vertically fluctuating pressure gradient force operating on a deepening stable layer near canopy top. Turbulent diffusion associated with increased winds deepens and weakens the stable layer at canopy top, lowering the buoyancy frequency in the canopy. Eventually, periods of resonance occur. Model results indicate that the recovery of the layer takes approximately twice as long as the period of enhanced wind that produced the event.