Is Low-Level Jet height a good approximation for the top of the nocturnal boundary-layer?

Low-Level Jets (LLJs) influence the dynamics of the Nocturnal Boundary Layer (NBL) by enhancing mechanical turbulence below the jet nose through vertical wind shear. In this study, we have evaluated whether the jet nose height ( h Njet ) can serve as a reliable proxy for estimating the NBL height ( h N ), defined as the level where turbulence diminishes to negligible magnitudes ( h Nflux ). We have used unique high-resolution measurements from sonic anemometers installed at 11 heights above the canopy at the Amazon Tall Tower Observatory (ATTO) in the central Amazon during 2022–2023. Nights with LLJ were selected based on the presence of a wind speed maximum (jet nose), along with a decrease in speed for at least two levels above and below. Given that both the height and intensity of LLJs vary over time, h Njet and h Nflux were determined from mean profiles grouped by jet nose heights: 81–100 m, 151–172 m, and 223–247 m. We have also considered the predominant wind directions associated with LLJs at the ATTO region: N–NE and S–SE. Our results show that, for the same jet nose height, N–NE jets are generally weaker and occur under more turbulent and less stable conditions, with h Njet being comparable to h Nflux . In contrast, S–SE jets are stronger and associated with enhanced stable thermal stratification, which likely suppresses turbulence and results in h Njet values higher than h Nflux . These differences are likely influenced by radiative cooling above the canopy, as S–SE jets occurred under larger net radiative loss. We conclude that the LLJ nose identified from wind speed profiles does not provide a good approximation of the NBL height under strongly stratified conditions, when turbulence above the canopy is significantly suppressed.