Chapter 4.4 Final results of the model inter-comparison of very high-resolution simulations with numerical weather prediction models for eight urban air pollution episodes in four European cities

Meteorological conditions during eight air pollution episodes were simulated in the target cities Helsinki, Oslo, Bologna and Valencia with varying participation of three HIRLAM versions, Lokalmodell/LAMI, MM5 and RAMS. The models were used with their operational parameterisations but tested for increased resolution of up to 1 km by multiple nesting. The meteorologically extreme winter episodes in Helsinki, Oslo and Bologna are dominated by ground-based inversions in very stable stratification which are poorly simulated, and the causes investigated. For the less meteorologically extreme summer photochemical episodes in Valencia and Bologna, the models perform better and succeed in simulating the dominating mesoscale circulations. In Bologna, the variable border between an Apennine mountain-valley breeze and a larger Po valley circulation reduces predictability.