1. Transport

The transport consists of advection in 3 dimensions, horizontal and vertical diffusion, and entrainment. The advection is driven by meteorological fields (u,v) which are input every 3 hours. The two horizontal wind component u and v are derived from observations according to the Optimal Interpolation method (Kerschbaumer and Reimer, 2003). The wind components are “terrain following”. Terrain following means practically that the ground level wind patterns follow the orography of Europe. The inclusion of the orography is “ensured” in the process of making the meteorological fields. In the LOTOS model the wind components, as well as other meteorological components are input into the model. The vertical wind speed w is calculated by the model as a result of the divergence/convergence of the horizontal wind fields. The recently improved and highly-accurate, monotonic advection scheme developed by Walcek (2000) is used to solve the system. The number of steps within the advection scheme is controlled by the Courant number. The number of steps is chosen such that the Courant restriction is fulfilled everywhere.

Entrainment is caused by the growth of the mixing layer during the day. Each hour the vertical structure of the model is adjusted to the new mixing layer depth. After the new structure is set the pollutant concentrations are redistributed using linear interpolation.

Horizontal and vertical diffusion

The horizontal eddy diffusion coefficient K_h is defined as the product of an empirical constant η and a velocity deformation tensor Def .

K_h = η |Def|

The empirical constant η has a value of 9000 m² (Liu and Durran, 1977). The K_h value is constraint between 10 m²s^-1 and an upper limit of 10⁵ m²s^-1.

Vertical diffusion is described using the standard K_z-theory. The K_z values are calculated within the stability parameterisation and are described in the Chapter on meteorology. Vertical exchange is calculated employing the new integral scheme by Yamartino et al. (2005).