Wind : Biophysics

  Properties of Wind


Wind is a major environmental rather meteorological factore that affects the biophysical process

Properties of Wind

  • It exerts a force on objects against which it blows
  • It is effective in transporting heat
  • It is highly variable in space and time
  • It is effective in mixing of the atmospheric boundary layer of the earth (less known fact)

  Planetary boundary layer


Planetary boundary layer is also called Atmospheric Boundary Layer

This depth of influence of the surface on the atmosphere is called the planetary boundary layer

It can extend from hundreds of meters at night to several thousand meters during days when surface heating is strong (approximately 1 km from the surface of Earth (1-3km))

This is a zone of transition from the distant flow near the surface to the frictionless or smooth flow in free atmosphere

Surface layer

The lowest 50 m of this planetary boundary layer is referred to as the surface layer. In this region fluxes of momentum, heat, and mass are virtually constant with height and profiles of wind speed, temperature, and concentration are logarithmic.

In biophysics we are interested in studing force of the wind on, or the rate of heat transfer from living organisms in their microenvironments, need to study:

  • wind speed in the vicinity of the organism
  • average wind in the surface boundary layer of the earth

Depends on :

rates of turbulent transfer in the surface boundary layer

 

  Internal Boundary Layer (IBL)


The internal boundary layer is the layer of air in immediate contact with the surface. It is often called the constant flux layers, since the transfer of heat, momentum and mass is invariant with height.

The thickness of the layer depends on :

  • surface roughness
  • distance from the edge

The thickness of IBL increases with the fetch (distance of uniform crop cover). Only 10% of the IBL is fully adjusted.

If travels 200m fetch then only 2m is IBL where equation prevails

  Turbulences/Eddy Movement


  • The mixing in the atmosphere occurs due to turbulence/disturbances
  • Except for a thin layer of air close to surfaces, the atmosphere is essentially always turbulent
  • This is characterized by random fluctuations in wind speed and direction caused by a swirling or eddy motion of the air
  • These swirls or eddies are generated in two ways

These swirls or eddies are generated in two ways:

Mechanical Turbulence - As wind moves over natural surfaces, the friction with the surface generates turbulence

Thermal or convective turbulence – Turbulence generated when air is heated at a surface and moves upward due to buoyancy

The fluctuations from mechanical turbulence tend to be smaller and more rapid than thermal fluctuations

Example : Looping (small scale mechanical turbulence that tears the plume apart and spreads it with distance, the thermal updrafts and downdrafts cause the entire plume to be transported upward or downward)

Large eddies, which are produced either mechanically or thermally, are unstable and decay into smaller and smaller eddies until their energy is converted into heat

The size of the smallest eddies produced by mechanical and convective motion (rather than breakdown of larger eddies) is called the outer scale of turbulence. The eddy size at which significant molecular interaction (viscous dissipation) begins is called the inner scale of turbulence

The outer scale is generally taken to be a few meters and the inner scale a few millimeters

  Wind as Vector


Wind is a vector quantity involving both magnitude and direction, while the other environmental variables are scalars, where only magnitude is specified (x, y and z coordinates)

 

Wind Rose

Wind rose is defined as graphical representation of wind speed and direction of a particular location. Wind rose diagram are used to study the wind direction and speed over a period of time. (details discussed in Air Pollution Dispersion)

 

 

 

 

  Air flow over cropped surface


Air flow over cropped surface is estimated by Wind Profile Equation

Frictional Velocity

Zero Plane Displacement 

Wind speed and canopy

Wind speed and canopy

Wind Speed Depends on:

  • Leaf Area Index - Leaf area index (LAI) is a dimensionless quantity that characterizes plant canopies. It is defined as the one-sided green leaf area per unit ground surface area (LAI = leaf area / ground area, m2 / m2)
  • Height
  • Mean distance between leaves in canopy

 

 

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