Turbulent Transport and Turbulence Structure
Understanding the turbulent transport of momentum and scalars such as
temperature, water vapor, and trace gases in the atmospheric boundary layer
plays important roles in many disciplines such as meteorology, hydrology, agriculture and air quality control.
It is often assumed that turbulence transports all scalars (temperature, water vapor, and CO2) similarly;
this similarity assumption is often also extended to include momentum, which is usually referred to as the Reynolds analogy.
The Reynolds analogy has numerous applications in atmospheric boundary layer parameterizations and measurements;
however, it is generally recognized that the Reynolds analogy is not correct in many settings which motivates us to examine how the turbulent transport of momentum is
different from that of scalars and what is the role of atmospheric stability in modulating the transport efficiencies measured by, for example, correlation coefficients.
A new transport efficiency is defined based on quadrant analysis(See Li and Bou-Zeid, 2011). We observed that the transport efficiencies for scalars increase as instability
increases,as shown below by the color dots, while the transport efficiency for momentum decreases as instability increases, indicatd by the black dots.
A decorrelation of momentum and scalar fluxes is observed as the atmosphere becomes more unstable.
A change in the dynamics of coherent structures as the ABL transitions from close to neutral conditions to unstable conditions is also observed (see cross stream vorticity below),
and is probably responsible for the different trends of transport efficiencies of momentum and scalars. Under close to neutral conditions, vortical structures
(resembling hairpin vortices) are observed while under very unstable conditions, thermal plumes become dominant which contributes substantially to transport of scalars but not momentum.
The dissimilarity between two scalars, temperature and humidity (one active and the other passive), is also investigated with a specific focus on the flux-structure parameter relationships.
The Monin-Obukhov Similarity Functions for normalized structure parameters of temperature and humidity are
proposed based on data sets collected over a lake surface and a glacier surface.
It is found that the two functions for temperature and humidity are the same under
stable conditions and very unstable conditions, which implies that the two scalars
are transported similarly. However, the flux-structure parameter relationships differ under
unstable but close to neutral conditions, corresponding to a decorrelation between the two scalars.
As mentioned earlier, a decorrection is observed to occur with the dissimilarity between the flux-structure parameter relationships for temperature and humidity.
A simple analytical model is thus proposed to link the difference between the two flux-structure parameter relationships to the correlation coefficients (See Li et al.2012 for more details).
The decorrelation is mostly likely due to entrainment or advection from the top of the internal boundary layer.
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