Regimes de turbulência noturna e a relação com a altura da camada limite noturna, jatos de baixos níveis e mortalidade de árvores na Amazônia

Abstract

Turbulence plays a fundamental role in energy and mass exchanges between the forest and the atmosphere. However, during nighttime, reduced turbulence and increased atmospheric stability can lead to decoupling between the canopy and the atmosphere, making the structure of the Nocturnal Boundary Layer (NBL) particularly complex. In the Amazon, our understanding of how turbulence organization determines NBL height and how larger-scale phenomena, such as Low-Level Jets (LLJs) and convective systems, influence these dynamics remains limited, in part due to the scarcity of turbulence measurements with high vertical resolution. In this work, we investigated the organization of nocturnal turbulence above the forest canopy in Central Amazonia, its relationship with NBL height variability and LLJs, as well as the influence of downdrafts and extreme winds associated with tree mortality. We used high-frequency measurements collected at different heights above the canopy, including those obtained along the 325-m tower at the Amazon Tall Tower Observatory (ATTO), this unique observational setup allowed us to directly investigate the vertical structure of turbulence throughout the NBL, which had previously been studied using indirect approaches. The results revealed two distinct turbulence regimes, a very stable regime and a weakly stable regime. Transitions between these regimes were modulated by proximity to the canopy, seasonality, and the occurrence of downdrafts, which promoted coupling between the forest and the atmosphere. NBL height (hN), determined from vertical turbulence profiles, varied with atmospheric stability and surface roughness. In addition, hN was lower than previous estimates based on thermal inversion, indicating that indirect methods may overestimate hN. During nights with LLJs, the jet nose height did not universally represent hN. Under weakly stable conditions, it was consistent with hN, whereas under very stable conditions it was located above hN, showing that the use of LLJs as indicators of hN depends on the coupling state between the canopy and the atmosphere. The influence of downdrafts associated with convective systems was also investigated using ERA5 reanalysis data. The analysis of an event associated with a previously documented blowdown showed that the reanalysis reproduced the main thermal and dynamical patterns associated with the passage of the storm, although its resolution limited the representation of local gust magnitudes. The analysis revealed an intensification of extreme winds between 2005 and 2020, particularly during the wet season. Overall, the results show that the structure of the nocturnal atmosphere over the Amazon forest results from the interaction between local turbulent processes and larger-scale atmospheric forcing. These findings advance the understanding of the physical mechanisms controlling forest–atmosphere coupling and establish a connection between boundary-layer dynamics and atmospheric processes associated with tree mortality in the Amazon.