Coefficient aerodynamic resistance

The flux of trace gases and particles from the atmosphere to the surface is calculated by multiplying concentrations in the lowest model layer by the spatially and temporally varying deposition velocity, which is proportional to the sum of three characteristic resistances (aerodynamic resistance, sublayer resistance, and surface resistance). The surface resistance parametrization developed by Wesely (1989) is used. In this parametrization, the surface resistance is derived from the resistances of the surfaces of the soil and the plants. The properties of the plants are determined using land-use data and the season. The surface resistance also depends on the diffusion coefficient, the reactivity, and water solubility of the reactive trace gas. 

The limitation of the resistance model lies in its application only for sufficient homogeneous surfaces such as forests, lakes and grasslands. Therefore, in dispersion models dry deposition can be described by using partial areas or weighted partial areas within the grid. The aerodynamic resistance through the upper layer can be calculated using Eq. (4.312) and an approach for the turbulent diffusion coefficient (eddy diffusivity) = Ku,z (valid only for neutral conditions) ... 

The aerodynamic resistance grows proportionally with wind speed at the reference height and decreases with increasing roughness of the surface. The quasi-laminar resistance r is based on the idea that close to the surface exists a molecular diffusion sub-layer where the transport only depends on the diffusivity of the molecule and the surface characteristics of the surface but not on atmospheric parameters (such as wind speed). The flux under steady-state conditions is parameterized by B dimensionless transfer coefficient) ... 

The action of an air flow on an adhering particle may also be taken into account by means of the coefficient of aerodynamic resistance O. The conditions for the detachment of the particles will in this case be given by the expression [288] ... 

A three resistance model has been developed to model the deposition of gases to vegetation canopies. These three resistances, which are aligned in series, describe the turbulent transfer from the atmospheric boundary layer into the canopy (the bulk aerodynamic resistance Ra), the transfer from the air in the canopy to the surface of the vegetation (the quasilaminar sublayer resistance Rb), and the transfer from the surface of the vegetation to reservoir or sink for the chemical in the vegetation (the canopy resistance Ra). These resistances are related to the mass transfer coefficient kg according to... 

Aerodynamic measures for optimizing air resistance and their reviewed potentials are described in the literature [21, p. 33], [28, p. 23], [44, p. 675]. A substitution of the main and wide-angle mirror with a camera-monitor system (CMS) is an outstanding measure. The cross-sectional area of the vehicle is reduced and the drag coefficient can be optimized. In addition, wind noise and pollution of the side windows can be significantly reduced by the elimination of the outside mirrors [27, p. 40], [33, p. 536], [44, p.720]. For concept related direct flow of the mirror areas, the proven fuel saving potential of a CMS is estimated at 2.9 % [21, p. 59]. 

---