Air quality modelling using the Met Office Unified Model: model description and initial evaluation

The on-line air quality model AQUM (Air Quality in the Unified Model) is a limited-area forecast configuration of the Met Office Unified Model which uses the UKCA (UK Chemistry and Aerosols) sub-model. AQUM has been developed with two aims: as an operational system to deliver regional air quality forecasts and as a modelling system to enable air quality studies to be conducted to inform policy decisions relating to emissions controls. This paper presents a description of the model and the methods used to evaluate the performance of the forecast system. Results are presented of evaluation studies conducted for a year-long period of operational forecast trials and several past cases of poor air quality episodes. To place the model performance in context we compare AQUM ozone forecasts with those of another forecasting system, the MACC ensemble, for a 5-month period. The results demonstrate that AQUM has a large dynamic range of modelled ozone levels and has a good level of responsiveness to elevated ozone episode conditions – a characteristic which is essential for forecasting poor air quality episodes. An analysis of the variation of model skill with forecast lead-time is presented and the insights this provides to the relative sources of error in air quality modelling are discussed.