TURBULENT AND MOLECULAR FLUXES NEAR THE GROUND

• Recall from Section 1 that we had identified some of the important external forcings (e.g., radiation, frictional drag, transpiration, etc.) that drive fluxes of heat, moisture, and momentum through the boundary layer.
• It is important to remember, that without a bottom boundary on the atmosphere (e.g., land and ocean), there would be no boundary layer, and therefore, no fluxes of heat, moisture, and momentum through the lower parts of the atmosphere.
• So, let's now focus on the fluxes of heat, moisture and momentum near the ground.  What physical processes are responsible for the transport of these variables?

Molecular Flux

• For the air in direct contact with the ground, it is not turbulence that is the dominant process for transporting heat, moisture, and momentum. 
  • Rather, heat is transported by molecular conduction
  • momentum is transported by molecular viscous transfer
  • moisture is tranported by evaporation/transpiration or condensation.
• The molecular processes dominate within the lowest few millimeters of air.  This layer is often called the microlayer.
• Within the microlayer, molecular transport, such as the conduction of heat is described by:

  (1)

where n_q is the molecular thermal diffusivity (on order of 2x10^-5 m²s^-1 for air). 

Q:  Observations have shown that a typical value for the kinematic heat flux is about 0.2 Kms^-1.  What is the corresponding temperature difference between the ground and air over the lowest 1 mm of air? Answer    Is this a reasonable answer?  Explain.

• Once the heat, moisture and momentum are transported up through the microlayer, then turbulence takes over and turbulent fluxes dominate.
• Within the lowest few centimeters of air, the sum of the molecular and turbulent transport yields a nearly contant flux that is often called the total effective turbulent flux.
• When people refer to a surface flux of some variable, they are usually referring to the effective turbulent flux.