Application of the ACASA model in urban environments: two case studies

In recent years, more attention has been dedicated to the study of urban metabolism due to the increasing growth of urban populations. Urban communities consume material and energy inputs, process them into usable forms, and produce waste as output of the process. Urban flows of energy, water and carbon have an important impact on climate change and their study is crucial in the future design and management of cities. Several techniques are used to estimate energy and mass fluxes exchanged by a city and more accurate models are developed to study the urban metabolism at different spatial and temporal scales. In this research, as a part of the European Project “BRIDGE”, the higher-order closure model ACASA (Advanced Canopy-Atmosphere-Soil Algorithm) was applied to simulate energy and mass fluxes over two different cities: Helsinki (Finland) and Firenze (Italy). These cities were selected because they represent different topographical and environmental characteristics: Helsinki is located at a high latitude and is characterized by a rapid urbanization that requires a substantial amount of energy for heating, while Firenze is a representative European old city with substantial cultural heritage and a huge tourist flow. ACASA is a multi-layer model already applied over natural and agricultural ecosystems. ACASA was recently modified to properly work in urban environment and to account for the anthropogenic contribution to heat and carbon production. Model behavior in the two different case studies was investigated and the model results were compared with in situ Eddy Covariance energy and mass flux measurements. A selected period of the year 2008, for each city, was chosen for flux simulations and analysis. Preliminary results are shown and statistical analysis was performed in order to evaluated the model performance for each city. From this first analysis, it appeared that ACASA model was able to adequately reproduce the energy and mass fluxes over the two cities. In general, simulated fluxes matched the observations well, with only small differences for most of the fluxes in both cities. The use of the ACASA model to simulate the urban fluxes therefore is promising and future applications for studies at both local and meso-scale spatial resolution are planned.