E4. Why Does Turbulence Transport Momentum and Scalars Differently?
Li, Dan; Katul, Gabriel; Bou-Zeid, Elie
The exchanges of momentum, water vapor, and heat between the earth surface and the atmosphere play a critical role in meteorology, hydrology, ecology, and other fields. To model these exchanges, which are dominated by turbulent transport, it is often assumed that momentum and all scalars are transported similarly; this is known as the Reynolds analogy. This study examines the breakdown of this analogy and the dissimilarity between turbulent transports of momentum and scalars (i.e., temperature and water vapor) in the atmospheric surface layer. Frist, observations indicate the dissimilarity mainly appears under unstable conditions and concurs with a change in the turbulence structure topology. The increase in the transport efficiencies of scalar fluxes under unstable conditions is then shown to be caused by a ‘scale-resonance’ between the turnover mixing eddy and the vertical temperature profiles. Based on this evidence, a recently-proposed phenomenological theory that explains the observed scaling laws in the stability correction function for momentum is modified to include the dissimilarity between momentum and scalar transfer, and extended to explain the behavior of the stability correction function for temperature across a wide range of atmospheric stability conditions.
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