Air quality forecasting using a numerical weather prediction model: Potential sources of error

The meteorological processes responsible for transporting tracer during the second ETEX (European Tracer Experiment) release are determined using the UK Met Office Unified Model (UM). The UM predicted distribution of tracer is compared with observations from the ETEX campaign and predictions from the UK Met Office operational air quality forecast model (NAME). The dominant meteorological process is a warm conveyor belt which transports large amounts of tracer away from the surface up to a height of 4km over a 36 hour period. Convection is also an important process, transporting tracer to heights of up to 8km.

Potential sources of error when using an operational numerical weather prediction model to forecast air quality are also investigated. In the UM a semi-Lagrangian advection scheme is used with polynomial interpolation. This can result in unrealistic negative values of tracer which, when combined with a positive definite scheme in which negative values of tracer are set to zero, results in a failure of the model to conserve tracer mass. This problem occurs where sharp gradients of tracer concentration exist, such as are found close to source locations and leads to an overprediction of tracer concentrations.