Parametrization of SL Diffusion Processes Accounting for Surface Source Action

Last years, a great number of dispersion models were developed. They possess different features and need different computer resources. Recently, the PC-oriented Eulerian multi-layer model EMAP (Eulerian Model for Air Pollution) was created in Bulgaria and applied to different pollution problems. The vertical diffusion block of the model uses a 2nd order implicit scheme including dry deposition as a bottom boundary condition, realised on a non-homogeneous (log-linear) staggered grid. The experiments with EMAP show that, if the concentration at the first computational level is used for calculation of the dry deposition flux, the deposited quantity changes when the height of this level is changed. It is obviously that the roughness level concentration is necessary as to calculate properly dry deposition. It is not possible to have a model level at this height, because roughness usually changes from one grid point to another. On the other hand, because of the steep gradients in the surface layer (SL), many levels must be introduced near the ground for adequate description of pollution profiles. This would increase memory and time requirements without any practical need. Usually, the first computational level is placed high enough above roughness. As a result, a good estimate for the roughness level concentration is necessary, determined on the base of the calculated concentrations. The problem becomes much more complex when surface sources of pollution have to be treated. Such are the processes of evaporation and re-emission of tracer under consideration. A proper parametrization of the diffusion processes in the surface layer can avoid these difficulties. An effective SL parametrization, based on similarity theory, was elaborated and tested by Syrakov and Yordanov (1996a, b). It allows to have the first computational level at the top of this layer. Here, an upgrade of this parametrization is presented, taking into account the presence of a continuous surface source.