Evaluation the effects of urban green space scenarios on near-surface turbulence and dispersion related parameters: A numerical case study in Tehran metropolis

Abstract

Two applicable green space development scenarios (surface with vegetation and green roof) are investigated, using Weather Research and Forecasting model (WRF) coupled with the Single Layer Urban Canopy Model (SLUCM) over Tehran Metropolis in an early summertime period, with special attention to variables related to the atmospheric dispersion and circulation. Results are reported for two High and Low Residential areas of Tehran (HR and LR). Reductions in the daytime Turbulence Kinetic Energy (TKE) budget (down to -0.5 m2/s2 ∼25 %), potential temperature (down to -0.2 K) and wind speed are observed. Surfaces with vegetation show more significant reductions of TKE buoyancy forcing in both areas. In the case with mainly shear generation of turbulence, the surface with vegetation shows increased (decreased) values during the day in HR (LR). Green roof (surface with vegetation) approach shows more reduction in wind speed in HR (LR). Slightly increased observed values for heat and momentum eddy diffusivities up to +11.99 m2/s (∼20 %) and +8.63 m2/s (∼10 %), respectively, limit the air circulation and upward transfer of near-surface polluted air especially in HR area, and these values are more pronounced in the first scenario and especially during the convective daytime conditions. The planetary boundary layer growth decreased (by about 50 m in HR, ∼7%) due to the reduction in the daytime near-surface air temperature and hence convective turbulence. Findings indicate that green scenarios (particularly surface with vegetation) could have negative side effects on atmospheric dispersion, natural air ventilation and air circulation, and consequently have undesirable impacts on Tehran air quality which have to be considered in any green space development programs.