Coupling of the WRF and ACASA models for urban environments: two case studies

The nature of the exchanges of carbon and energy exchange over urban landscapes has received more attention in recent years. The accurate assessment of urban landscape fluxes of energy, water and carbon and their impact on climate change is important for management of urban centers and the mitigation of climate change.

Surface-atmosphere interactions and the distribution of trace gas have been simulated using the Advanced Canopy Atmosphere Soil Algorithm (ACASA). ACASA is a multi-layer model that has already been applied over natural and agricultural ecosystems. ACASA was recently modified to work properly in urban environments and to account for the anthropogenic contribution to heat and carbon production. ACASA was coupled with a mesoscale weather model to more accurately simulate the mass and energy exchanges across urban regions. These regional simulations in combination with remotely sensed data will be used to provide constraints on the land surface types and the exchange of carbon and energy fluxes from urban centers as part of the European Project “BRIDGE”. Its objective is to develop a sustainable urban planning decision support system accounting for urban metabolism.

This study will present the use of the ACASA model coupled to the Weather Research and Forecasting (WRF-ARW) mesoscale model to simulate the urban fluxes at a horizontal resolution of 200 meters for urban areas of roughly10 by10 km. For this study, ACASA-WRF was applied to simulate energy and mass fluxes over two different urban regions: 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 recent, rapid urbanization that requires a substantial amount of energy for heating, while Firenze is representative of older European cities of lower latitudes, with substantial cultural heritage, a huge tourist flow, and an architectural footprint that remains comparatively constant in time.

In-situ comparisons of point measurements and ACASA presented by Marras et al. are promising and this study shows the application of this methodology at a regional scale with high spatial resolution. In general, simulated fluxes matched the point observations well.