Abstract
Urban heat advection (UHA) can extend the climatic impact of a city into the surrounding countryside. This may lead to an intensification of already well‐documented urban heat island (UHI) impacts on health and infrastructure, and challenge the representativeness of long‐term reference temperature records taken near urban areas. However, previous UHA studies have been unable to accurately quantify surface‐level UHA due to challenges arising from complex urban land‐use patterns. To address this, the numerical Weather Research and Forecasting (WRF) mesoscale model coupled with the Building Energy Parameterization urban canopy scheme is used to simulate meteorological fields for idealized land‐use cases. Hypothetical square cities (up to 32 km in size) are simulated for a year's period. A time‐mean 2‐m temperature field (representing the canopy UHI) shows that the mean UHI intensity (up to 4.3 °C [SD 1.7 °C]), wind speed <3.9 m/s) is linearly related to the logarithm of city size. This finding, entirely derived from numerical modelling, is consistent with the log‐linear relationships previously found in the observational data of many cities in the world. A UHA methodology was then applied to the temperature fields to separate UHA from the UHI, with up to 2.9 °C (SD 1.7 °C) of UHA found downwind of the largest city size. For this hypothetical city size, an UHA intensity of 0.5 °C is found up to 24‐km downwind from the urban boundary. In addition, the UHA‐distance profiles along the central horizontal transect for various urban sizes are found to follow a scaling rule as a good approximation. As a result, the findings of this paper can be used as a starting point for climate impact assessments for areas surrounding urban areas without the need for complex, computation‐intensive simulations.
Highlights
The urban heat island (UHI) is a zone of warmer air and surface temperatures caused by differential heating and cooling rates between urban and rural land-use types
Surface-level urban heat advection (UHA) has been hypothesized for a while (Lowry, 1977) and acknowledged in several UHI studies (Brandsma et al, 2003; Unger et al, 2010; Brandsma and Wolters, 2012), UHA is rarely considered in UHI studies. This is in part due to a lack of spatial information caused by an observational paucity within the urban environment; the challenges associated with siting and maintaining urban meteorological networks are discussed in Muller et al (2013) and Chapman et al (2014). Another obstacle is the lack of an effective approach to enable UHA to be separated from UHI, because the air temperature at a given location is influenced by a combination of locally generated heat and the heat transported from upwind sources
The modelled time-mean UHI intensity (UHII) fields (ΔT, as illustrated by [C1] in the top diagram of Figure 2) for the five urban size cases are shown in Figure 3a, where it is clear that an increase in UHI area and intensity are related to an increase in urban size (LU)
Summary
The urban heat island (UHI) is a zone (the “island”) of warmer air and surface temperatures caused by differential heating and cooling rates between urban and rural land-use types. This is in part due to a lack of spatial information caused by an observational paucity within the urban environment; the challenges associated with siting and maintaining urban meteorological networks are discussed in Muller et al (2013) and Chapman et al (2014) Another obstacle is the lack of an effective approach to enable UHA to be separated from UHI, because the air temperature at a given location is influenced by a combination of locally generated heat (i.e., due to underlying land use, topography and aspect) and the heat transported from upwind sources. The results will be used to develop a simple statistical model that can be used to estimate UHAI without the need for computationally expensive simulations
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