Prediction of visibility and aerosol within the operational Met Office Unified Model. II: Validation of model performance using observational data

The operational version of the Met Office Unified Model forecasts aerosol and visibility products, as described in Part I. The modelling relies on parametrizations for the aerosol size distribution which account for the coagulation of aerosols into the accumulation mode, and for the hygroscopic uptake of water by aerosols. Aircraft and surface-based measurements are used to assess these parametrizations and to assess the performance of the modelled aerosol mass mixing ratio and visibility. Measurements of aerosol chemical composition with an airborne aerosol mass spectrometer suggest that much of the aerosol is a complex mixture of sulphate, nitrate and organic material, rather than ammonium sulphate alone, as assumed in the model. Some significant improvements can be made to the hygroscopic parametrizations, along with more minor adjustments to the aerosol size distribution, and aerosol extinction efficiency. Comparisons of the measured aerosol mass mixing ratio reveal measurement-model agreement within a factor of two for the four cases investigated here, while comparisons of visibility reveal a systematic bias to low visibilities in the model, particularly in clean atmospheric conditions. Formulations and strategies to improve the model are suggested. (C) Royal Meteorological Society and Crown Copyright, 2008