Development of a High-Resolution Nested Air Pollution Model: The Numerical Approach

A new 3-D model REGINA (REGIonal high resolutioN Air pollution model) is under development at the National Environmental Research Institute (NERI). The model is based on models developed over the past decades at NERI. The goal is to obtain a nested model capable of high-resolution operation. To reach this goal it is necessary to implement sufficiently accurate numerical methods. The model will be applied to studying air pollution phenomena (monitoring, forecasting, and scenarios) over Denmark. In the present paper, the outline of the new model is presented. The numerical methods for transport and chemistry are described. The horizontal transport in the model is solved using an accurate space derivatives algorithm. This method traditionally requires periodic boundary conditions, which are not applicable for nested modeling. Therefore a new method for calculating nonperiodic boundary conditions has been developed. The numerical solution to the chemistry part of the model is obtained from an implementation of a new combination of two existing numerical methods. The results from extensive testing of the numerical solution of the advection and the coupling of the solution of advection and chemistry in the model using Molenkamp–Crowley rotation tests are presented. The same tests have been applied to the model with and without nesting. The results show that the numerical methods are suitable for modeling air pollution levels at high resolution.