Evaluating a building-averaged urban surface scheme in an operational mesoscale model for flow and dispersion

- A building-averaged urban canyon scheme in a mesoscale model is evaluated.
- This scheme simulates the observed near-neutral to weakly unstable conditions at night.
- In contrast, the original slab scheme predicts weakly stable conditions at night.
- A better representation of the observed dispersion by the building-averaged scheme.
- Computational efficiency of the canyon scheme is on par with the slab scheme.

A recently developed building-averaged urban surface scheme as coupled to an operational mesoscale model, TAPM, is evaluated for both flow and tracer dispersion using data from the 2002 Basel UrBan Boundary Layer Experiment (BUBBLE) conducted in the city of Basel, Switzerland. This scheme is based on the so-called town energy balance (TEB) approach and simulates turbulent fluxes using a generic canyon geometry to resolve energy balances for walls, roads and roofs. Air conditioning to close the building energy budget, in-canyon vegetation, and the effects of recirculation and venting of air within the canyon on turbulent fluxes are included. Comparison is also made with the original urban surface scheme of TAPM based on a simple slab approach with separate urban and vegetation-soil tiles and a specified anthropogenic heat flux. The results show that the new scheme leads to an overall improvement in the prediction of surface fluxes, and is able to reproduce the observed near-neutral to weakly unstable conditions at night, which is a feature of urban meteorology. In contrast, the slab scheme predicts stable conditions at night. The observed concentration fields from the tracer experiments are better simulated using the new scheme, but because there were no nighttime tracer releases, the capability of the new scheme under full diurnal conditions could not be demonstrated. For the applications considered here, the computational efficiency of the new scheme in TAPM is on par with the slab scheme.