Numerical Modeling Study on the Effects of Urban Greening on Microclimate

Abstract

Urban greening is widely promoted as a nature-based remedy for urban overheating, yet its microclimate response in cold, semi-arid, mountain-bounded capital cities such as Ulaanbaatar differs fundamentally from the linear cooling reported in humid climates. The Weather Research and Forecasting model coupled to the Single-Layer Urban Canopy Model was configured in four nested domains down to 1 kilometer over Ulaanbaatar, driven by ERA5 reanalysis. Four scenarios were simulated for 22 June 2025 by modifying the urban impervious-area fraction (FRC_URB) at 1.0, 0.9, 0.5, and 0.1, corresponding to greening levels of 0, 10, 50, and 90 percent. The 0.9 case was retained as the control. The control reproduced the tropospheric temperature profile with a root-mean-square error of 1.2°C and a nocturnal urban heat island of 2.5°C, matching the long-term observed summer mean. The daytime response was moisture-dominated: a shift from 0 to 90 percent greening lowered the 2 m air temperature by only 0.3°C but raised the surface dew point by 1.8°C. The nocturnal urban heat island declined monotonically with greening fraction and reversed sign at the strongly greened limit, falling from 3.5°C through 0.6°C at moderate greening to −2.9°C at 90 percent greening, driven by reduced heat-storage release and weakened surface roughness that allowed katabatic drainage to ventilate the basin. The −2.9°C value is an upper bound sensitive to the model soil-moisture treatment. Intermediate rather than maximum greening minimizes the nocturnal urban heat island in a basin-confined city, and the full cooling potential is realized only when greening is combined with irrigation, permeable surfaces, and rainwater harvesting.