LOW FREQUENCY MODULATION OF ATMOSPHERIC SURFACE LAYER IN AMAZONIA

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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Random Errors in Turbulence Measurements in the Atmospheric Surface Layer: Implications for Monin–Obukhov Similarity Theory

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-12-096.1 ◽

2012 ◽

Vol 69 (12) ◽

pp. 3700-3714 ◽

Cited By ~ 31

Author(s):

Scott T. Salesky ◽

Marcelo Chamecki

Keyword(s):

Surface Layer ◽

Diurnal Variation ◽

Atmospheric Surface Layer ◽

Friction Velocity ◽

Similarity Theory ◽

Statistical Hypothesis ◽

Random Errors ◽

Propagation Analysis ◽

Error Propagation Analysis ◽

First Time

Abstract An error propagation analysis is conducted to estimate random errors in the friction velocity u* and the Monin–Obukhov similarity theory (MOST) stability variable z/L from estimated random errors in the turbulent fluxes. Errors in the dimensionless mean wind shear φm and mean temperature gradient φh are also estimated. To the authors’ knowledge, this is the first time that errors in calculated values of z/L, φm, and φh have been systematically analyzed. Random errors in z/L are found to be large for unstable conditions, reaching values of 40% or greater. It is shown through statistical hypothesis tests that random errors cannot explain departures of calculated values of φm and φh from theory. The deviation of calculated values of φm from empirical curves is found to have a strong diurnal variation that increases with height; deviations of φh from theory are not found to have clear diurnal variation. These results support the findings of previous studies, which have suggested that additional dimensionless parameters representing additional physical processes need to be included in the set of governing parameters for surface layer similarity. Implications for atmospheric surface layer turbulence are also discussed.

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