Intermittency and the surface exchange of scalars at the nocturnal boundary layer

The determination of nocturnal surface fluxes in low wind conditions is a major problem for micrometeorological studies. The eddy correlation technique, extensively used in field measurements becomes inappropriate if not enough turbulent activity exists. At the same time, the phenomenon of turbulence intermittency is responsible for the existence of localized events of short duration for which a large fraction of the total nighttime scalar exchange occurs. Even though appreciable progress has been made towards a good understanding of this phenomenon in recent years, the near-random character of intermittent exchange makes the determination of surface fluxes under these conditions a difficult task. The scalar flux within a certain intermittent event varies largely depending on the window used for the flux calculation. In many cases, events with very different time durations occur at the same night, and therefore, the proper determination of the surface flux would require averaging within data windows of different sizes for each event. In this work, the surface exchange of temperature, moisture and carbon dioxide is analyzed at a micrometeorological tower at southern Brazil. Intermittent turbulence is a common occurrence at the location. The analysis shows how the determined fluxes vary with turbulence intensity and the estimation technique. Furthermore, it is shown that the exchange of scalars depends not only on turbulence intensity, but also on the turbulence history at a given night. It means that intermittent events preceded by long calm periods tend to transfer a larger portion of scalars than events of similar intensity preceded by well-mixed conditions. Finally, the effects of intermittent mixing on regional flux estimates from a single point, for which calm or intermittent conditions are common, are analyzed.