Air Quality forecasts 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. These potential sources of error include model dynamics, model resolution and model physics. 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. Model resolution can also affect the accuracy of predicted tracer distributions. Low resolution simulations (50km grid length) were unable to resolve a change in wind direction associated with the cold front, this led to an error in the transport direction and hence an error in tracer distribution. High resolution simulations (12km grid length) captured the change in wind direction and hence produced a tracer distribution that compared better with the observations. The representation of convective mixing was found to have a large affect on the vertical transport of tracer. Turning off the convective mixing parameterisation in the UM significantly reduced the vertical transport of tracer. Finally, air quality forecasts were found to be sensitive to the timing of synoptic scale features. Errors in the position of the cold front relative to the tracer release location of only 1 hour resulted in changes in the predicted tracer concentrations that were of the same order of magnitude as the absolute tracer concentrations