Influence of wave-like structures on scalar fluxes

The nocturnal stable boundary layer dynamics is complicated by the coexistance of turbulent structures with a variety of submeso phenomena: a mix of non-turbulent, non-stationary processes with scales ranging from the ones of the main turbulent eddies to the one of the smallest mesoscale motions. The relative strength of thermal stratification and shear production determines the switching between active, weakly stable turbulent states and passive, very stable non-turbulent states. Wave-like structures, as horizontal meandering and internal gravity waves, are ubiquitous in the nocturnal boundary layer, where they coexist with anisotropic intermittent turbulence. Spectral analysis 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.