UrbClim – A fast urban boundary layer climate model

• A simple urban boundary layer climate model (UrbClim) is presented.
• This new model is more than 100 times faster than regular urban mesoscale models.
• It has an accuracy that is comparable to that of those more sophisticated models.

We 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. The UrbClim model is subject to several validation exercises, using observations from Toulouse (France), Ghent and Antwerp (Belgium), and Bilbao (Spain), and considering turbulent energy fluxes, wind speed, and urban–rural temperatures. Despite its simplicity, UrbClim is found to be of the same level of accuracy than more sophisticated models, including for the complex terrain characterizing the Bilbao area. At the same time, it 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.