Impacts of in-canyon vegetation and canyon aspect ratio on the thermal environment of street canyons: numerical investigation using a coupled WRF-VUCM model

The thermal environment of an urban street canyon in summer becomes a great concern for human health under rapid urbanization. For accurate prediction of the in-canyon thermal environment, a realistic representation is required of microscale physical processes within the canyon as well as multi-scale atmospheric interaction between the canopy air and the overlying urban boundary layer. To accomplish this, the Vegetated Urban Canopy Model (VUCM), which interactively parametrizes in-canyon radiative/dynamic/thermodynamic/hydrological processes based on a combined framework of the two-dimensional single canyon and the single tree canopy, is implemented into the Weather Research and Forecasting (WRF) model. Using the coupled WRF-VUCM model, a series of simulations is performed for a hot summer day with a finest grid resolution of 0.333 km to investigate the impacts of in-canyon vegetation (permeable grass/soil surfaces and trees, with the vegatation fraction f_v ranging from 0 to 0.4 which corresponds to 0 to 15% in urban patch area) and canyon aspect ratio (h/w; ranging from 0.5 to 2) on the thermal environment of urban street canyons over the Seoul metropolitan area. The model simulation compares well with the measured 2 m temperatures (above zero-plane displacement height) and canopy air temperatures at 13 urban sites in Seoul, with root mean square errors of 1.0 and 0.96 °C, respectively. The increase of the in-canyon vegetation from 0 to 15% (at h/w = 1) leads to a reduction of the canopy air temperature throughout the diurnal cycle, exhibiting relatively larger cooling effect during daytime (∼1.1 °C on average) than at night (∼0.8 °C on average) under a limited condition for evapotranspiration by the in-canyon vegetation. Provided that the soil moisture is enough for the hydrological effect, the cooling effects significantly increase by a factor of ∼2.5 in both daytime and night-time. The increase of h/w from 0.5 to 2 (at f_v = 0.2) reduces the daytime canopy air temperature (∼1.3 °C on average) but increases the nocturnal canopy air temperature (∼0.3 °C on average). It is also found that the existence of in-canyon vegetation at h/w > 1 has a synergic cooling benefit to the thermal environment of street canyons compared to the effects of no vegetation. These results demonstrate the importance of interactive parametrization of the physical processes and the interplay of in-canyon vegetation and building density (via canyon aspect ratio) effects on accurate prediction of the thermal environment of urban street canyons.