Nocturnal Boundary Layer Height Estimated from Direct Turbulence Measurements at the ATTO Tower

The height of the Boundary Layer (BL) varies dynamically throughout the diurnal cycle, influencing weather patterns, air quality, and surface-atmosphere energy exchanges. Although important, estimating BL height is extremely challenging. While in the daytime the BL height is well-defined by the inversion layer, the height of the Nocturnal Boundary Layer (NBL) is more complex to identify due to weak vertical mixing, low turbulence, and stable atmospheric conditions. Based on turbulence-based relationships, the NBL height (h_N) can be determined by the level where turbulence intensity sharply decreases from its surface value. However, it requires measurements of turbulent quantities with high vertical resolution, which are usually difficult to obtain. In the absence of direct turbulence data, various indirect methods have been employed, using parameters such as temperature, humidity, and wind speed profiles from radiosondes, tethered balloons, and ground-based remote sensing instruments like ceilometers, Sodar, and radiometers. These methods are considered indirect because they utilize mean profile characteristics influenced by the vertical extent of turbulence to estimate h_N. This study provides direct estimates of h_N from measurements of turbulent fluxes of momentum and sensible heat. For this, one year of data collected by sonic anemometers installed at 11 heights above the canopy on the ATTO towers in the Central Amazon were analyzed. Our findings highlighted the forest topography dynamic effect: under northeast winds, associated with lower roughness, h_N varied between 81 m during very stable stratification and 172 m in neutral conditions. Conversely, under southeast winds, where roughness is higher, h_N ranged between 81 and 223 m. These estimates reveal the significant control exerted by stability and topography (that drives the roughness length) on the h_N variability. We emphasized the practical significance of estimating the NBL height. In the context of a tropical rainforest, a shallower NBL and reduced mixing conditions facilitate the accumulation of high concentrations of gases and organic compounds above the canopy. This highlights the relationship between boundary layer characteristics and their potential impact on atmospheric composition in the Amazon rainforest.