Abstract
We investigate possible effects of modulation of the surface layer over Amazonian forest by motions that occur at time scales longer than the usual turbulent time scales (referred to as “low-frequency motions”). We present empirical evidence of their occurrence, possible influences on the turbulence structure in the surface layer and its effects on deviations of Monin-Obukhov Similarity theory predictions. To parameterize these effects, we estimate an ‘outer layer’ ‘fluctuating’ friction velocity (v*) as proposed by McNaughton (2006) to represent the additional energy transported down from the variable motions of the outer layer to the surface layer and study how frequently they could be significantly affecting the structure of surface layer. Based on analyses of scale variability of variances and covariances and relations with the parameterized v*, we propose that the relation between v* and u* could be used as an indicator of the strength of low frequency modulations in the region. We found that the ratio v*/u* is above 2 for roughly 50% of the unstable runs analyzed and it is likely that in these cases the surface layer is different from the textbook descriptions.
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