Numerical modeling techniques are sought when the formulated problem equation and model cannot be solved analytically. Numerical modeling involves solving a system of partial differential equations. A system of equations is derived comprising of mass/energy conservation equations, the CD equation, Navier-Stokes equation or turbulence transport equations. The equations mathematically represent the fate of a pollutant's downwind concentration as a function of wind velocity, time and turbulence parameters to name a few. Also, the equation more accurately captures the physics of the atmospheric dispersion of a dense gas. In order to obtain a solution to this system of equations, the number of unknown parameters must equal the number of equations. This is known as "closure" to the system of equations. Typically, "closure equations" are those that contain the turbulence parameters.