CO₂ and Heat exchange across the Nocturnal Canopy–Atmosphere interface in the Amazon rainforest

Abstract. We investigated the characteristics of the nocturnal boundary layer (NBL) above and within the Amazon rainforest canopy during the 2022 dry season. The study aims to determine how NBL dynamics influence nocturnal CO₂ and heat exchange across the canopy-atmosphere interface. Utilising observations from the CloudRoots-Amazon22 field campaign conducted at the Amazon Tall Tower Observatory, we distinguished between the strongly and weakly stable regimes to study the effect of radiative cooling and wind shear on CO₂ and heat exchange. Our results reveal a distinct, stable layer above the canopy with an average height of 150 to 188 m, which develops due to strong radiative cooling of the canopy top. Below the canopy, the cooling marks the build-up of a well-mixed layer within the canopy. In the weakly stable regime, increased turbulence at the canopy top was observed, leading to a significant observed CO₂ flux of 3.82 𝜇mol m⁻² s⁻¹ above the canopy. In contrast, in the strongly stable regime, turbulence was almost absent, and the observed flux was only 0.16 𝜇mol m⁻² s⁻¹, suggesting a decoupling of the canopy and the roughness sublayer. The decoupling was confirmed by the 2–3 times decreased vertical heat transport in the strongly stable regime. Even though our method includes typical observational uncertainties, our results show significant differences between CO₂ and heat exchange between the two regimes, stressing the importance of correctly representing the nocturnal dynamics in tall canopies like the Amazon Rainforest.