Verbetering van stedelijke parametrisatie in regionale klimaatsmodellen: het belang van stedelijke aerodynamische, thermische en radiatieve eigenschappen, anthropogene warmte en waterretentie

Outdoor thermal comfort is essential to our health and that of the environment. For the last 200 years, the global population has increased sevenfold resulting in a strong urban expansion. Consequential changes in the landscape 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 in large cities, mortality rates are higher during heat waves, such as that of the European summer of 2003. At the same time, it is expected from global climate change that cities are more often exposed to extreme weather, including the increased number of strong heat waves. More severe droughts, floods and the deterioration of air quality will place increasing pressure on the livability of cities as well. As the proportion of people living in cities increases globally, mayors are faced with the big challenge of mitigating climate change and adapting their cities exposed to it.The general goal of this thesis is to provide an efficient and reliable assessment tool for urban climate change at the regional scales available for a wide research network. Herein, we aim for enabling a full assessment for every mid-latitude synoptic season and weather type, including summer heat waves, storms and cold winters. Therefore, we contribute several important improvements to urban-climate modelling research. Finally, we provide a model-based description of the urban climatic drivers of the surface energy balance and the urban heat island effect, including their diurnal cycle, daily variability and seasonal dependency.The diurnal cycle and spatial extent of the urban heat island of Paris are acquired for a heat-wave of 2006 with a model-based approach first. Besides the changes in the surface-energy balance such as the nocturnal excess in surface sensible heat flux, it is demonstrated with an idealized advection model that a reduced vertical adiabatic cooling over Paris compared to cropland brings an additional urban heat island build-up of 25%. The urban heat island and its vertical extent are strongly affected by the boundary-layer temperature lapse rate, nocturnal low-level jet and radiative cooling as well. Therefore, model developments to better represent these nocturnal boundary-layer features in atmospheric models are needed for more reliable assessment of the urban heat island during heat waves.A new efficient urban land-surface parametrization TERRA-URB is developed for the regional climate model CCLM. It is dealt with important deficiencies in state-of-the-art urban-climate modeling. Firstly, an efficient algorithm is developed for calculating the surface-layer turbulent momentum and energy transfer between urban environments and the atmosphere above. Simulations with TERRA-URB configured for an urban site at Toulouse centre demonstrate that our algorithm considerably improves to model performance in terms of the surface-energy balance compared to CCLM’s standard procedure. Furthermore, TERRA-URB coupled to CCLM configured for Belgium is able to simulate the urban heat island of Antwerp, whereas the standard CCLM model fails to. Secondly, we provide a simple formulation for the infra-red opacity of water vapor for urban land-surface modelling evaluation with observations from sensors installed a few tens of metres above roof level. The formulation eliminates an overestimation - reaching values of over 35 Wm −2 for sunny days - for the upward infra-red radiation. Thirdly, a new water-storage parametrization for urban land-use is developed for which a distribution of water reservoirs on the impervious surface is considered. Parameters for urban water storage are obtained for Toulouse centre with model sensitivity experiments. Herewith, TERRA-URB reproduces the timing, magnitude and persistence of evaporation increase after rainfall better than employing more arbitrary water-storage parametrizations. Finally, modelling the annual urban surface energy balance and the urban heat island with TERRA-URB is improved by applying a dataset of anthropogenic heat release (i.e., waste heat from human activity) with an annual and diurnal cycle.Model runs with TERRA-URB for Toulouse centre show augmented evaporation rates of 60 Wm−2 on average after rainfall which lasts for 12 daytime hours on average. The annual-mean evaporation from the urban impervious surfaces (5.7 Wm−2 ) is an order of magnitude lower than that from the natural surroundings (45 Wm−2 ). These results indicate that the water storage, evaporation and run-off induced by urban expansion can affect atmospheric moisture, precipitation, and the occurrence of pluvial flooding.Sensitivity experiments with CCLM coupled to TERRA-URB over Belgium have led to an overall better understanding of the drivers leading to the urban heat island and their seasonal dependency at the regional scales. It turns out that both urban structure, anthropogenic heat release and 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) during winter is smaller than that from the anthropogenic heat (+1.24 K). Conversely, the contribution of urban structure (+1.97 K) dominates that of the anthropogenic heat (+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).Within the general assessment of climate change projections, land-use change scenarios and urban-climate mitigation, we recommend to account for the impact of both urban structure, anthropogenic heat, their interactions and their seasonal dependency on the regional climate. Hereby, their respective influences on the urban thermal comfort both in a positive or negative way needs to be considered as well. Finally when employing policies needed for mitigation and adaptation of climate change on the global scale through urban modifications, one needs to account for the possible consequences on the regional scale in urban areas as well.nrpages: 140status: publishe