D z

represents the shearing stress acting on the ground (or more generally on a horizontal unit area). t has the dimension of force per unit area or pressure.

Above the laminar boundary layer, transport usually takes place by turbulent convection. The mass motion may be considered as consisting of overlaying eddies of varying size, eventually superimposed on a laminar flow. The fact that laminar flow is in general horizontal or nearly horizontal near the ground implies that all vertical transport is related to eddy motion.

It is now assumed (Rossby, 1932) that the expressions (2.28)-(2.30), based on molecular diffusion theory, can also be applied in the region of turbulent motion, provided that the coefficient k is given an effective value. This "eddy diffusion parameter" is no longer constant. It will depend on the quantity being transported and on the velocity field of the atmosphere, i.e. k will be a time-dependent function of the height z. The picture of turbulent transport given here is equivalent to the mixing length formulation used in the discussion in section 2.A of convective transport in stellar atmospheres. The eddy diffusion parameter k is related to the mixing length l by k = I ^ . d z

If the temperature profile is approximately adiabatic, dT

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Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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