Impact of different external parameters on Turin UHI with COSMO at 1km

In an increasingly urbanized world, the numerical weather prediction models need to better represent the urban areas, in order to capture the micro-climate phenomena induced by the cities. The parameterization TERRA_URB (TU) (Wouters et al., 2016), recently implemented in COSMO (Bucchignani et al., 2019), not only represents a novelty in this field but has proved to correctly reproduce the Urban Heat Island effect over different European cities (Garbero et al., 2021). TU provides a heterogeneous description of the urban-atmosphere interactions, through the definition of several urban external parameters, such as the anthropogenic heat flux (AHF), the impervious surface area fraction (ISA), and other urban canopy parameters such as the building area fraction (BF), the mean building height (H) and the height-to-width ratio (H/W). In this study we performed simulations with COSMO model at 1 km resolution with the aim of a better characterization of the UHI over the city of Turin. In particular, we compared the results by using AHF and ISA from the EXTPAR preprocessor and from the Local Climate Zones (LCZ) classification system (Stewart and Oke, 2012). Furthermore, we focused on the influence of the urban parameters BF, H and H/W by comparing two different approaches: as a default, their values are assumed constant for all the urban grid points, while a different 2-D approach consists in deriving their values for each urban grid point based on LCZ classification (Demuzere et al., 2019). A sensitivity analysis was then performed to detect which of the 2-D urban parameters have a greater impact on the results, with an emphasis on the Surface Energy Balance (SEB). With the purpose of unravelling the driving mechanism behind the UHI, we analyzed the individual SEB components and evaluated how much each flux contribute to the urban heat island effect.