Estimation of nocturnal boundary layer height in the central Amazon, supported by gas concentration profiles

Abstract. The height of the nocturnal boundary layer (hn) is a fundamental parameter for weather and climate prediction. However, because turbulent processes weaken at night, estimating hn remains challenging. In addition, our understanding of its variability is limited, especially due to the predominant use of indirect methods that do not always accurately reflect the physical definition of the boundary layer. In this study we used micrometeorological measurements collected at the Amazon Tall Tower Observatory, in central Amazon. These measurements enable the study of turbulent sensible heat flux (H) profiles from the canopy top up to 300 m above ground, from which hn can be defined. Our analysis focused on the seasonal differences between dry and wet periods for a La Niña year and an El Niño year. Also, we explore how variations in hn affect the vertical distribution of CO and CH4 concentrations. The results revealed significant variations, such as: largest values of hn were observed during the wet season of a year marked by the La Niña phenomenon (∼ 270 m ± 40 m), while smallest values of hn occurred in the dry season associated with El Niño (∼100 m ± 27 m). It was also observed that hn can act as a “barrier” to the entry or exit of air masses with high concentrations of CO and CH4. This study provides important insights into the variability of hn above the Amazon forest, with implications for improving parameterizations in atmospheric models.