PALM-Traffic 2.0: Implementation of the traffic-induced heat and velocity in the PALM model system

Abstract. The new Traffic model has been developed and integrated into the PALM model system 26.10. Using data typically available for traffic emission modelling, it adds the parameterised impacts of car-induced heat and air flow on modelled tendencies of potential temperature and advection. Model evaluation in a realistic urban environment showed a significant impact of the traffic model at 1 m resolution on street canyon turbulent flow: it changes the wind component aligned with the street canyon by about 2.0 m s⁻¹ and turbulent kinetic energy by about 0.6 m² s⁻². The increase of potential temperature was the strongest during stable stratification at night and early morning by up to 1.5 K; after the start of convection there was a modest increase of ≈ 0.5 K. The 4 m resolution domain showed a lower impact. Comparison with measurements obtained in a dedicated observation campaign in two parallel streets with heavy traffic showed that applying the model typically brings simulated results closer to observations, with good agreement between modelled and observed values. Comparison of the horizontal turbulent spectra showed that applying the Traffic model brings modelled spectra very close to those calculated from observations, with the impact at all frequencies including high frequencies close to the observation and model time resolution 1 Hz. The impact on air quality was tested in a simulation replicating a previously published study of a stable, low-wind winter situation. The results showed that the Traffic model significantly affected PM₁₀ concentrations, changing both their magnitude and spatial distribution and substantially reducing the instabilities found in simulations without traffic effects. It demonstrated that inclusion of the transportation-produced momentum and heat forcing in the model is necessary for reliable realistic simulations of the stable low wind meteorological conditions.