Abstract
Mitigation of the urban heat island (UHI) effect at the city-scale is investigated using the Weather Research and Forecasting (WRF) model in conjunction with the Princeton Urban Canopy Model (PUCM). Specifically, the cooling impacts of green roof and cool (white/high-albedo) roof strategies over the Baltimore-Washington metropolitan area during a heat wave period (7 June–10 June 2008) are assessed using the optimal set-up of WRF-PUCM described in the companion paper by Li and Bou-Zeid (). Results indicate that the surface UHI effect (defined based on the urban–rural surface temperature difference) is reduced significantly more than the near-surface UHI effect (defined based on urban–rural 2 m air temperature difference) when these mitigation strategies are adopted. In addition, as the green and cool roof fractions increase, the surface and near-surface UHIs are reduced almost linearly. Green roofs with relatively abundant soil moisture have comparable effect in reducing the surface and near-surface UHIs to cool roofs with an albedo value of 0.7. Significant indirect effects are also observed for both green and cool roof strategies; mainly, the low-level advection of atmospheric moisture from rural areas into urban terrain is enhanced when the fraction of these roofs increases, thus increasing the humidity in urban areas. The additional benefits or penalties associated with modifications of the main physical determinants of green or cool roof performance are also investigated. For green roofs, when the soil moisture is increased by irrigation, additional cooling effect is obtained, especially when the ‘unmanaged’ soil moisture is low. The effects of changing the albedo of cool roofs are also substantial. These results also underline the capabilities of the WRF-PUCM framework to support detailed analysis and diagnosis of the UHI phenomenon, and of its different mitigation strategies.
Highlights
The diurnal cycles of surface temperatures of different roof and ground subfacets, and those of the complete urban surface temperature and the 2 m air temperature, are illustrated in figure 2 for a simulation with 0% green roofs and 0% cool roofs
The right panel of figure 2 shows the diurnal cycles of the complete urban surface temperature calculated from equation (3) and the 2 m air temperature calculated from equation (6), which provide the basis for estimating the impacts of green and cool roof mitigation strategies in the analyses to follow
This study builds on the companion paper by Li and Bou-Zeid (2014) where the high-resolution simulation of urban heat island (UHI) with Weather Research and Forecasting (WRF) was validated
Summary
UHIs have been studied for decades (see Oke 1982, Arnfield 2003 for reviews) and have been shown to be caused by many factors including the extensive use of man-made materials such as asphalt and concrete in urban areas, which results in the reduction of evapotranspiration and in greater heat storage capacity (Grimmond 2007, Oke 1982). Global climate change is expected to exacerbate the heat conditions in urban environments. A recent study has shown that heat waves, which are projected to become more frequent and last longer under a warming climate, interact nonlinearly with UHIs to produce extremely high heat stresses for urban residents (Li and Bou-Zeid 2013)
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