Turbulent Exchange

See Goudriaan 1977, pp 94ff

Exchange Above the Canopy

The transport of heat, water vapor and momentum through the atmosphere is mainly accomplished through the process of turbulent exchange ("forced convection"). Turbulent eddies of wind move up and down through the atmosphere, carrying with them concentrations of heat and vapor and momentum - these concentrations are ultimately deposited elsewhere as the eddies decay. This process of transport is quite efficient (above some minimum wind speed???).

The eddies are generated through many means:

• due to vertical gradients in wind speed - shear instabilities (Kelvin-Helmholtz) develop between adjacent layers moving at different speeds.
• due to interactions with vegetation and other surface features.
• due to variations in air density.

The wanderings of these eddies are guided by the local stability conditions in the atmosphere. There are 3 basic regions in stability space:

• Neutral:
  • Temperature in the atmosphere decreases with height at the adiabatic lapse rate, ie a rising parcel of gas, cooling purely through expansion, stays in temperature equilibrium with its surroundings.
  • An eddy's motion is not influenced by this type of gradient.

• Lapse conditions (unstable):
  • Temperature decreases with height faster than the adiabatic lapse rate - parcels of air raised adiabatically are warmer and lighter than the surrounding air, and so continue to rise (falling parcels continue to drop).
  • Eddies are accelerated along their current path. The turbulent transport rate is enhanced.

• Inversion conditions (stable):
  • The temperature gradient is less negative (or perhaps positive) - parcels of air raised adiabatically are colder and heavier than their surroundings, and therefore fall back down (falling parcels rise).
  • Eddies are decelerated. They oscillate around an equilibrium position, and turbulent transport is greatly suppressed.

And so, the local temperature gradient greatly affects the heat and water vapor and momentum conductances (the "eddy conductivity"). Conductances are corrected for non-neutral conditions by dividing by a diabatic correction factor (see, e.g., the calculations for AKCPY in profl2 [400]). Log wind profiles (derived for neutral stability conditions) can be adapted for non-neutral conditions with this same factor.