The seasonal dependency of urban heat islands in Belgium with COSMO-CLM + TERRA-URB

Changes in the landscape by urbanization have lead to drastic climate modifications, which ranks among the most significant human impacts on the environment. Most remarkable is that cities are exposed to higher air temperatures than those in the natural surroundings. This phenomenon - known as the urban heat island - causes serious health risks for many people in the city. Especially inlarge cities, mortality rates are higher during heat waves, such as that of the European summer of 2003. It is expected from global climate change that cities are more often exposed to extreme weather, including the increased number of strong heat waves. Sensitivity experiments with the regional climate model CCLM coupled to a new efficient urban land-surface parametrization TERRA-URB over Belgium have led to an overall better understanding of the climatic drivers of the urban heat island and their seasonal dependency at the regional scales. It turns out that both urban structure and energy waste into the atmosphere by human activity, but also their interaction determine the seasonal variability of the urban heat island intensity. Remarkably, the averaged contribution of urban structure to the nocturnal urban heat island for the cities in and around Belgium (+0.41 K for Brussels) is smallerthan that from the energy waste (+1.24 K) during winter. Conversely, the contribution of urban structure (+1.97 K) dominates that of the energy waste (+0.68 K) during summer. The respective contributions mostly counteract each other during summer (-0.21 K), whereas they enhance each other during winter (+0.25 K). The above-mentioned findings allow to take wiser decisions for keeping the climate in cities comfortable within the context of climate change and urban expansion. Hereby, it is recommended to account for both the impact of urban structure, energy waste, their interactions and their seasonal dependency on urban climate. Hereby, the respective influences on the urban thermal comfort both in the positive as in the negative sense needs to be considered. It is expected that putting in place policies in terms of urban modifications – that are needed to temper climate change on the global scale - will influence the climate in cities on the local scale as well.status: publishe