The urban heat island, in which air temperatures in urban environments tend to be higher than in rural areas, is a well-known and widely studied phenomenon. During heat waves, the urban heat island is known to exacerbate the impact on population health. Including urban heat island effects in the formulation of heat warnings, climate change adaptation plans is therefore essential and part of a sustainable urban development in general. Given the extreme climate of Qatar, heat stress is a prime concern, not only from health perspective, but also e.g. from an energy consumption perspective (cooling demand). Few studies have however been performed for arid or tropical cities. Results indicate that in desert city areas, the typical pattern of a hot urban core is often inverted, with downtown areas appearing cooler compared to the suburbs , which obviously adds to the complexity of understanding the urban climate dynamics in such cities. An important difficulty often encountered with typical numerical climate models is the limited resolution and long integration time, making them difficult to use when studying urban and intra-urban variations especially in the context of climate change. In this contribution, we will present a new urban climate model, further referred to as UrbClim , designed to cover agglomeration-scale domains at a spatial resolution of a few hundred metres. This model is composed of a land surface scheme containing simple urban physics, coupled to a 3-D atmospheric boundary layer module. In the land surface scheme, urban terrain is represented as an impermeable slab with appropriate parameter values for albedo, emissivity, and aerodynamic and thermal roughness length, and accounting for anthropogenic heat fluxes. Despite its simplicity, UrbClim is found to be of the same level of accuracy than more sophisticated models. At the same time, the urban boundary layer climate model is faster than high-resolution mesoscale climate models by at least two orders of magnitude. Because of that, the model is well suited for long time integrations, in particular for applications in urban climate projections. Within the EU RAMSES (Reconciling Adaptation, Mitigation and Sustainable Development for citiES, http://www.ramses-cities.eu) and NACLIM (North Atlantic Climate, http://www.naclim.eu ) projects, the UrbClim model has been set up for a large number of cities : Antwerp, London, Bilbao, Berlin, Hyderabad, New York, Rio De Janeiro and Skopje. We will present results and comparisons for these cities as well as detailed validations against air temperature measurements. Furthermore, a coupling was established between UrbClim and CMIP5 ensemble climate projections employed by the IPCC taking full advantage of the fast integration time of the model and allowing assessment of the urban heat island effects under future climate conditions. In addition, we will present an application of the UrbClim model on the city of Doha, Qatar, assessing it's applicability for very arid climatic conditions.


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