High-resolution global climate simulations: Representation of cities

Ensemble runs of high-resolution (similar to 10 km; N1280) global climate simulations (2005-2010) with the Met Office HadGEM3 model are analysed over large urban areas in the south-east UK (London) and south-east China (Shanghai, Hangzhou, Nanjing region). With a focus on urban areas, we compare meteorological observations to study the response of modelled surface heat fluxes and screen-level temperatures to urbanization. HadGEM3 has a simple urban slab scheme with prescribed, globally fixed bulk parameters. Misrepresenting the magnitude or the extent of urban land cover can result in land-surface model bias. As urban land-cover fractions are severely under-estimated in China, this impacts surface heat-flux partitioning and quintessential features, such as the urban heat island. Combined with the neglect of anthropogenic heat emissions, this can result in misrepresentation of heat-wave intensities (or cold spells) in cities. The model performance in urban areas could be improved if bulk parameters are modelled instead of prescribed, but this necessitates the availability of local morphology data on a global level. Improving land-cover information and providing more flexible ways to account for differences between cities (e.g., anthropogenic emission; morphology) is essential for realistic future projections of city climates, especially if model output is intended for urban climate services.