The presence of wave‐like structures in the planetary boundary layer and their influence on the scalar fluxes and on the surface energy balance were investigated analyzing one year of continuous measurements collected in southern Brazil. Submeso oscillating patterns in the wind velocity components, temperature and scalar (CO₂, H₂O) concentrations were isolated using their auto‐correlation functions. The analysis showed that low‐wind speeds are necessary to trigger wavy motions. During nighttime, in presence of large vertical temperature gradient, horizontal meandering and internal gravity waves are dominant features of the stable boundary layer. Furthermore, a significant number of meandering cases was identified also during daytime in neutral conditions associated with low values of net radiation. One case study showed how, during daytime, the wave‐like patterns may be triggered by variations in the net radiation. Spectral analysis on the whole dataset showed that oscillations in the wind velocity and temperature field are frequently associated with CO₂ and H₂O wavy patterns with similar timescales. These non‐turbulent oscillations produce unpredictable large scale contributions to vertical fluxes of temperature and scalar concentrations. The energy budget analysis showed how the choice of a proper averaging time filters out these contributions and improves the energy budget closure, as well as the estimation of the net ecosystem exchange. The results confirm the influence of submeso motions in scalar dispersion, flux patterns and surface energy balance during low wind speed conditions and stable stratification.

Methane (CH4) atmospheric mixing ratio measurements are analyzed for the period between June 2013 and November 2018 at the Amazon Tall Tower Observatory (ATTO). We describe the seasonal and diurnal patterns of nighttime events in which CH4 mixing ratios at the uppermost (79 m a.g.l.) inlet are significantly higher than the lowermost inlet (4 m a.g.l.) by 8 ppb or more. These nighttime events were found to be associated with a wind direction originating from the southeast and wind speeds between 2 and 5 m s−1. We found that these events happen under specific nighttime atmospheric conditions when compared to other nights, exhibiting less variable sensible heat flux, low net radiation and a strong thermal stratification above the canopy. Our analysis indicates that even at wind speeds of 5.8 m s−1 the turbulence intensity, given by the standard deviation of the vertical velocity, is suppressed to values lower than 0.3 m s−1. Given these findings, we suggest that these nighttime CH4 enhancements are advected from their source location by horizontal nonturbulent motions. The most likely source location is the Uatumã River, possibly influenced by dead stands of flooded forest trees that may be enhancing CH4 emissions from those areas. Finally, biomass burning and the Amazon River were discarded as potential CH4 sources.

Eulerian low‐wind temperature statistics are investigated through the analysis of sonic anemometer observations gathered in two experimental campaigns, the Urban Turbulent Project in Northern Italy and the Large‐Scale Biosphere‐Atmosphere Experiment in Amazonia (Brazil). The observed auto‐correlations and spectra functions are tested with the theoretical relationships previously proposed for the horizontal velocity components in low wind speed conditions. The comparison showed that the temperature field, similarly to the horizontal velocity field, presents a characteristic oscillatory behaviour with a distinct isolated spectrum peak frequency due to the wind meandering. The ratio between this frequency and the one associated to the horizontal velocity components is close to one. This, together with the similarity between the temperature and velocity spectra and auto‐correlation functions, suggests that a dynamical link between temperature and velocity oscillations exists.