Environmental Fluid Mechanics 1:
Mass Transfer and Diffusion

Course Script > Ch 6

Chapter 6:  Atmospheric mixing
(PDF)

Previous chapters dealt solely with transport in various water bodies and presented examples of one-dimensional solutions to the transport equations.  We now turn our attention to transport and mixing in the atmosphere, and by necessity, we will have to give more attention to three-dimensional solutions.  Because of the atmosphere's unique composition and boundary and forcing conditions, atmospheric turbulence is more complicated than the idealized homogeneous, stationary, isotropic case.  Moreover, these complications impact transport and mixing because they determine the values of the turbulent diffusion and dispersion coefficients.  Hence, a concise discussion of atmospheric mixing requires studying atmospheric turbulence and the resulting modifications in the behaviour of mixing coefficients from the idealized case. 

This chapter begins with an introduction to atmospheric turbulence and a review of turbulent boundary layer structure.  The log-velocity profile for a turbulent shear flow is introduced, and the behaviour of turbulence throughout a neutrally stable atmospheric boundary layer is described.  Because of their importance to turbulence characteristics, the buoyancy effects of heating and cooling within the boundary layer are discussed qualitatively.  The discussion on mixing begins with a review of turbulent mixing in three-dimensional, homogeneous, stationary turbulence.  The solution for a continuous point source is derived and used to illustrate mixing in the remaining section.  The chapter closes by adapting the idealized solution in homogeneous, stationary turbulence to the turbulence in the atmosphere.

See Lecture 11.

 

 

 

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