The coupling of the weather research and forecasting model with the urban canopy models for climate simulations

Abstract

Understanding the urban heat island (UHI) phenomenon aids in the better predictions and analyses of its mitigation strategies. The mesoscale Weather Research and Forecasting Model (WRF) tool has been increasingly proposed to assess the UHI and to perform microclimate analysis. In this tool, mesoscale models are coupled separately with urban canopy models (UCMs) to predict the heat emission and moisture fluxes from the urban boundaries to the atmosphere. The three commonly considered UCMs to represent the canopy are slab (SB), single layer (SL), and multilayer (ML). These models account for the buildings in terms of roughness elements, two-dimensional structures, and three-dimensional urban surfaces, respectively. In this chapter, the WRF-UCM interaction is explained, and the capabilities and shortcomings of each UCM are described. The WRF-UCMs are used to assess two specific heat wave periods in the case study of the Greater Toronto Area (GTA) in Canada. To evaluate the WRF-UCM, the simulated variables are compared against the measured data obtained, showing the strong value of these new approaches. The UHI intensity (UHII) is further evaluated by the differences in ambient temperature in urban and rural areas during two heat waves, in July 2011 and 2018. The results illustrate that the daily UHI intensity is around 1.2-1.5 °C, while in the daytime it is near 0.7 °C. This chapter, based on the analysis of the selected case study, shows that the SL-UCM is reliable for climate simulations. Finally, to precisely evaluate the UHI intensity and to analyze more sophisticated canopies within cities, the ML-UCM needs to be applied to consider fully both the turbulent kinetic energy and the radiation reflections in the urban canopy.