Towards standard parametrization of COSMO(-CLM)

Abstract

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 the urban heat island effect, for which cities are exposed to higher air temperatures than those in the natural surroundings, especially for the night-time during heat waves. Urbanization also leads to drastic changes to the air flow and enhanced turbulent transport, and therefore affecting the precipitation and air quality. All these phenomena affect the outdoor comfort, but also causes serious health risks for many people living and working in cities. Therefore, it has been decided to implement a standard urban parametrization working out-of-the-box for both numerical weather prediction with COSMO and regional climate studies with COSMO-CLM. It aims for better weather forecasts for temperature and precipitation in cities with COSMO, and an improved assessment of urban outdoor hazards in the context of global climate change and urban expansion with COSMO-CLM. This work starts from the simple urban parametrization TERRA_URB recently developed for COSMO-CLM, which has been applied for different cities over Europe. In a first step, the EXTPAR tool is updated for representing the cities into the land cover over the entire globe. Hereby, global datasets in the standard EXTPAR tool are used to retrieve the ’Paved’ or ’sealed’surface Fraction (PF) referring to the presence of buildings and streets. Furthermore, new global data sets are incorporated in EXTPAR for describing the Anthropogenic Heat Flux (AHF) due to human activity, and optionally the Surface Area Index (SAI) derived from the Floor Space Index (FSI). In a second step, the urban parametrization is implemented in the standard version of COSMO by means of the a tile approach for the urban tiles. Besides for the reduction in vegetation, it accounts for the buildings and streets as a rough water-impermeable slab, adopting bulk parameters for (thermal) roughness length, albedo, emissivity, heat capacity and conductivity. It is focussed on the urban/rural contrast in terms of turbulent transport within the surface layer by means of sensitivity experiments. Hereby, we investigate for the consistency between empirical functions for thermal roughness length and the underlying theory of the TKE-based surface-layer transfer scheme.status: publishe