Part 4:
Microscale
The microscale is the smallest scale in which we classify weather phenomena. Microscale swirls of air have widths of a few meters and last less than a few minutes. Like mesoscale flow, microscale flow is forced by heating and obstructions to flow. These cause tiny motions in the air to form. These motions are known as eddies, and they twist and turn the air, mixing it with the air molecules that surround the eddies. Eddies caused by heating are grouped and called thermal turbulence. Those caused by obstructions are called mechanical turbulence. Though both thermal turbulence and mechanical turbulence interact with each other, we will first look at each influence separately.
Differential heating of a surface will cause some air molecules to become excited, while others have less energy. The interactions between excited molecules and slower ones cause tiny eddies to form. These little whirlwinds add to turbulence in the friction layer and to instability. The development of friction near the surface involves both molecular viscosity and eddy viscosity. Molecular viscosity is the slowing of motion in a fluid due to the random movements of molecules and their interaction with each other at different speeds. This effect is primarily very near the surface of the earth (within a few millimeters). Eddy viscosity is the friction, or drag, created by turbulent eddies and their interaction. Molecular viscosity has only a slight effect on air flow, but does aid in the formation of eddies, which induce larger magnitudes of drag force higher above the earth's surface. Eddy viscosity is the primary influence on friction in the lower atmosphere. Friction increases as the eddy viscosity increases. Eddy viscosity is affected by the roughness of the surface and the obstructions present, which leads us into mechanical turbulence.
Mechanical turbulence is determined by both the speed of the prevailing wind and the roughness of the surface over which the air flows. As wind moves through trees or over rough surfaces, the air is broken up into eddies that make the wind flow irregular. We feel these irregularities at the surface as abrupt changes in wind speed and direction -- gusts. The eddies can either combine to form larger eddies, or cancel each other out and lessen the effect.
Thermal influences interact with mechanical influences. If there is surface heating, an eddy formed by flow obstructions may be lifted up because the air is unstable. Or the eddy created could cause instability by mixing air of different temperatures. Each influence affects the other. Next we will look at some specific examples of microscale turbulence and flow.
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