Wind in the Boundary Layer

• Within the boundary layer, the observed wind can usually be decomposed into three main categories:

  • the mean wind

  • waves
  • turbulence
• Each of these three wind properties can exist individually
• Mean Wind:
  • The mean wind is important for horizontal transport of quantities such as moisture, heat, momentum, and pollutants - i.e., advection
  • typical speeds are 2-10 m/s
  • friction slows the winds near the surface, the wind velocity is 0 m/s right at ground level
• Waves:
  • occur mostly at night in the nocturnal boundary layer.
  • transport little heat, moisture and other scalar variables like pollutants
  • are effective at transporting energy and momentum, however.
• Turbulence:
• the vertical transport of moisture, heat, momentum, and pollutants is dominated by turbulence.. this is a very important process . Q:  What is turbulence?
• Answer
• can be visualized as consisting of irregular swirls of motion called eddies - see example to the right ->
• eddies can vary in size - anywhere from 100-3000 m, however, eddies exist in size as small as a few millimeters
• Q:  what creates turbulence?
  • solar heating generating thermals - nothing more than larger eddies
  • wind shear
  • deflected flow around obstacles such as trees and buildings, creating turbulent wakes downstream of the obstacle
• in summary, turbulence allows the boundary layer to respond to changes in surface forcings (daytime heating, for example). 
• This does not occur in the free troposphere, the free troposphere acts like the earth's surface does not exist.