Intermittency and the exchange of scalars in the nocturnal surface layer
The determination of nocturnal surface fluxes in low wind conditions is a majorproblem for micrometeorological studies. The eddy correlation technique, extensively usedin field measurements, becomes inappropriate if not enough turbulent activity exists. Atthe same time, the phenomenon of turbulence intermittency is responsible for the existenceof localized events of short duration within which a large fraction of the total nighttimescalar exchange occurs. The scalar flux within a certain intermittent event varies considerablydepending on the window used for the flux calculation. In many cases, events withvery different time durations occur in the same night, and therefore, the proper determinationof the surface flux would require averaging within data windows of different sizes foreach event. In this work, the surface exchanges of temperature, moisture and carbon dioxideare analysed at a micrometeorological tower in southern Brazil. Intermittent turbulence is acommon occurrence at the location. The analysis shows that the fluxes vary with turbulenceintensity and the estimation technique. A variable-window size method for flux estimationis suggested and shown to produce an increase in the magnitude of the nocturnal surfacefluxes.