Coherent Structures and the Dissimilarity of Turbulent Transport of Momentum and Scalars in the Unstable Atmospheric Surface Layer |
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Authors: | Dan Li Elie Bou-Zeid |
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Institution: | 1.Department of Civil and Environmental Engineering,Princeton University,Princeton,USA |
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Abstract: | Atmospheric stability effects on the dissimilarity between the turbulent transport of momentum and scalars (water vapour and
temperature) are investigated in the neutral and unstable atmospheric surface layers over a lake and a vineyard. A decorrelation
of the momentum and scalar fluxes is observed with increasing instability. Moreover, different measures of transport efficiency
(correlation coefficients, efficiencies based on quadrant analysis and bulk transfer coefficients) indicate that, under close
to neutral conditions, momentum and scalars are transported similarly whereas, as the instability of the atmosphere increases,
scalars are transported increasingly more efficiently than momentum. This dissimilarity between the turbulent transport of
momentum and scalars under unstable conditions concurs with, and is likely caused by, a change in the topology of turbulent
coherent structures. Previous laboratory and field studies report that under neutral conditions hairpin vortices and hairpin
packets are present and dominate the vertical fluxes, while under free-convection conditions thermal plumes are expected.
Our results (cross-stream vorticity variation, quadrant analysis and time series analysis) are in very good agreement with
this picture and confirm a change in the structure of the coherent turbulent motions under increasing instability, although
the exact structure of these motions and how they are modified by stability requires further investigation based on three-dimensional
flow data. |
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Keywords: | |
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