Profile conserving extension of an high-complexity urban surface exchange parametrization

Urban surfaces exhibit unique exchange processes leading to heat and moisture fluxes that are typically not well represented in traditional SVAT schemes used in mesoscale meteorological models. But, pollutant dispersion models rely on accurate forecasts of wind fields and boundary layer structure, especially over urban areas which contribute as major sources of primary pollutants.

In this study, we implement the Martilli et al. (2002) building effects parametrization (BEP) into the Lokal Modell (LM), a fully non-hydrostatic mesoscale model. Simulations are validated using measurements over the city of Basel during the intensive observation period of the BUBBLE measurement campaign. At a horizontal model grid spacing of 2.2 km BEP has a significant impact on low level wind and temperature fields. Simulations with BEP reproduce well the amplitude of the urban heat island (UHI) in Basel.

The impact of BEP is observed to be strongly dependent on the low-level vertical resolution of the mesoscale model. This is shown to be due to the interpolation between the mesoscale model and the parametrization at each timestep. We extend the BEP parametrization to conserve an high-resolution vertical profile within the parametrization. Results show that this modification is successful at removing the strong resolution dependence.

Our work presents a step towards the operational implementation of urban parametrizations in mesoscale models. Other issues such as urban hydrology remain to be addressed in future work.