Impacts of future urban expansion on urban heat island effects during heatwave events in the city of Melbourne in southeast Australia

Hosen M Imran,Anne W M Ng,Shobha Muthukumaran,Jatin Kala

doi:10.1002/qj.3580

Abstract

AbstractThe city of Melbourne in southeast Australia is planning to expand urban areas substantially by the year 2050 and this expansion has the potential to alter the Urban Heat Island (UHI), that is, higher temperatures in urban areas as compared to surrounding rural areas. Moreover, Melbourne has been experiencing more frequent heatwaves for last two decades, and the intensity and duration of heatwaves is expected to increase in the future, which could exacerbate the UHI. This study evaluates the potential impacts of future urban expansion on the urban meteorology in Melbourne city during four of the most severe heatwave events during the period 2000–2009. Urban expansion is implemented as high‐density urban with a high urban fraction of 0.9 to investigate the maximum possible impact. Simulations are carried out using the Weather Research and Forecasting model coupled with the Single‐Layer Urban Canopy Model with current land‐use and future urban expansion scenarios. Urban expansion increases the near‐surface (2‐m) UHI (UHI2) by 0.75 to 2.80 °C and the skin‐surface UHI (UHIsk) by 1.9 to 5.4 °C over the expanded urban areas during the night, with no changes in existing urban areas. No substantial changes in UHI2 and UHIsk occur during the day over both existing and expanded urban areas. This is largely driven by changes in the storage heat flux, with an increase in storage heat at night and a decrease during the day; that is, excess storage heat accumulated during the day is released at night, which causes a slower decrease of near‐surface temperature and increase in the UHI. Urban expansion did not affect human thermal comfort (HTC) in existing urban areas and there were no marked differences in HTC between existing and expanded urban areas.

Highlights

Changes in land cover due to increased urbanization can affect the urban environment and climate substantially (Seto and Shepherd, 2009)
The diurnal cycle of the surface energy balance is shown in Figure 3 for the control and experiments with urban expansion, averaged over urban grid sign of storage heat indicates the flow of heat from the surface to the atmosphere and vice versa for negative storage heat
The spatial changes in daily mean sensible and latent heat fluxes are shown in Figure 4

Summary

Introduction

Changes in land cover due to increased urbanization can affect the urban environment and climate substantially (Seto and Shepherd, 2009). The UHI is driven by the higher thermal heat capacity and heat storage of urban infrastructure and reduced evapotranspiration due to the loss of vegetation and increase in impermeable surfaces. The UHI results in higher air temperatures at screen level in urban areas that can contribute to heat-related illnesses including heart disease, which can lead to mortality, during summer, and these effects can be exacerbated during heatwaves in Australia (Department of Infrastructure and Regional Development, 2013). Higher temperatures in urban areas affect urban ecosystems, as well as human thermal comfort, and increase the rate of energy consumption (Block et al, 2012)

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