WHAT IS TURBULENCE?
Turbulence is random motion associated with fluid flow. It is the most fundamental property of fluid flow at high Reynolds number, which is the case for almost all flows of geophysical interest. Turbulence is experienced in the atmosphere most noticeably as wind gustiness, but can also take on less obvious forms.
Turbulence is a key mechanism by which momentum (i.e. wind), heat and moisture are mixed to and from the earth's surface and atmosphere. Since changes from one location to another of these meteorological variables drive local-scale regional winds as well as larger scale weather and climate systems, accurate representation of turbulent mixing is key to making accurate predictions of such phenomena.
Turbulence is also the primary process that disperses air pollutants. Accurate representation of turbulent mixing is therefore crucial to accuracy of air pollution concentration predictions.
WHAT IS TURBULENCE MODELING?
Turbulence modeling is the process of generating mathematical representations for turbulent mixing. The representations themselves are called "turbulence models", however atmospheric scientists tend to call them turbulence "parameterizations" or "schemes" to distinguish them from, for example, weather forecast "models", which are the large computer codes comprising the basic equations and various parameterizations (turbulence being one of which) that generate weather predictions.
Turbulence parameterizations in meteorology are primarily based on the "eddy viscosity" and "eddy diffusivity" concept, in which turbulent mixing is assumed to act more or less the same way as molecular viscosity and molecular diffusivity. The analogy of turbulent mixing to molecular viscosity and diffusion is one the primary hypothesis historically guiding turbulence modeling, and is discussed in many textbooks. It is also one of the most controversial, inspiring great debate and basic research on the nature of turbulence.