High-Resolution WRF Simulations with Data Assimilation to Improve the Evaluation of Meteorological Fields over Complex Terrain in the Bolzano Basin (IT)

Air quality scenarios provided by coupling meteorological and dispersion models are key tools in support of policies for monitoring pollutant dispersion and reducing health risks. However, air quality assessments in complex terrain still pose many challenges, due to the inherent difficulties in accurately modeling both atmospheric and dispersion processes. In July 2013 a new incinerator became operative 2 km Southwest of the city of Bolzano, the most populated town of South Tyrol, in the Central Italian Alps. The plant has a maximum garbage treatment capacity of 130000 t y-1, producing a smoke release at 60 m a.g.l. and a rate of 85000 Nm3 h-1 at 413 K. This new plant required policy makers to improve the forecast of dispersion processes within the Bolzano basin. This task is strictly related with the understanding of local meteorological features, especially those connected with critical air quality episodes, such as wintertime ground-based thermal inversions in calm conditions. In order to move to this direction, high-resolution numerical simulations were run with the WRF model over the Bolzano area, focusing on specific wintertime conditions. It is found that the flow field in the Bolzano basin is very complex, due to the diurnal cycle of up- and down-valley winds flowing in each of the tributary valleys merging into the basin, and to the presence of a low-level nocturnal jet at the exit of one of these valleys displaying a narrow canyon-like shape. The simulation of the interaction between these different local circulations, including their correct evolution in space and time, represent a demanding task for the model. Moreover, difficulties were also encountered in the proper reproduction of low levels temperature, as a deep ground-based thermal inversion was present in the area in the simulation period. In order to improve model performance, data assimilation technique is then applied, feeding the model with both conventional and non-conventional measured data: surface temperature and wind speed and direction coming from eight different weather stations distributed within the domain; a vertical wind profile, up to 400m, from a SODAR, located at the centre of the domain, over the incinerator roof; and a vertical temperature profile, up to 1000m, from a thermal profiler also located in the centre of the area. Thanks to the applied data assimilation, meteorological features observed during the simulation period are better captured by the WRF model, thus providing an optimized input for dispersion modeling studies.