Abstract
High-resolution numerical simulations of the urban heat island (UHI) effect with the widely-used Weather Research and Forecasting (WRF) model are assessed. Both the sensitivity of the results to the simulation setup, and the quality of the simulated fields as representations of the real world, are investigated. Results indicate that the WRF-simulated surface temperatures are more sensitive to the planetary boundary layer (PBL) scheme choice during nighttime, and more sensitive to the surface thermal roughness length parameterization during daytime. The urban surface temperatures simulated by WRF are also highly sensitive to the urban canopy model (UCM) used. The implementation in this study of an improved UCM (the Princeton UCM or PUCM) that allows the simulation of heterogeneous urban facets and of key hydrological processes, together with the so-called CZ09 parameterization for the thermal roughness length, significantly reduce the bias (<1.5 °C) in the surface temperature fields as compared to satellite observations during daytime. The boundary layer potential temperature profiles are captured by WRF reasonable well at both urban and rural sites; the biases in these profiles relative to aircraft-mounted senor measurements are on the order of 1.5 °C. Changing UCMs and PBL schemes does not alter the performance of WRF in reproducing bulk boundary layer temperature profiles significantly. The results illustrate the wide range of urban environmental conditions that various configurations of WRF can produce, and the significant biases that should be assessed before inferences are made based on WRF outputs. The optimal set-up of WRF-PUCM developed in this paper also paves the way for a confident exploration of the city-scale impacts of UHI mitigation strategies in the companion paper (Li et al ).
Highlights
The ‘urban heat island’ (UHI) is probably the most wellknown environmental impact of urbanization (Oke 1982), and the most well-documented example of anthropogenic climate modification through land use change (Arnfield 2003)
It is clear that the simulated UHI depends significantly on the planetary boundary layer (PBL) schemes and the thermal roughness length parameterizations, which are the only two parameters that vary in figures 2(c)–(f)
When the modified Zilitinkevich relationship is used with the two PBL schemes (figures 2(e), (f)), the simulated Land Surface Temperature (LST) are very similar, implying that the LST at this particular time is controlled by the parameterizations for thermal roughness length, rather than by the PBL scheme
Summary
The ‘urban heat island’ (UHI) is probably the most wellknown environmental impact of urbanization (Oke 1982), and the most well-documented example of anthropogenic climate modification through land use change (Arnfield 2003). The combined effects of UHIs, global climate change, and soaring urban populations pose significant challenges to energy and water sustainability and to human health in urban environments.
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