Simulated impact of urban expansion on the future temperature heatwaves in Sydney.

The combined effect of climate change and urbanisation on 2-m temperature are investigated over Greater Sydney using the Weather Research and Forecasting (WRF) model. The climate of the region is simulated at 2km horizontal spatial resolution for the present (1990-2009) and a possible future (2040-2059) climate scenario (following the A2 emission scenario). The model default land use is replaced with a more accurate dataset that covers the Sydney area, which also includes the expected urban expansion in the future simulation according to local government urbanisation plans. The areas with projected land use changes are identified and compared with the rest of the region to evaluate how urban expansion and global warming will act together on the persistence of warm conditions.
Larger heat capacity of urban structures and inhibited evaporation by impervious surfaces are responsible for changes in the surface energy flux partition, which in turn affect near-surface temperature. In addition to changes driven by climate change alone, future urbanisation will greatly affect minimum temperature, and thus nighttime heatwaves. The urbanisation footprint on minimum temperatures is noticeable throughout the year but it is particularly strong during winter and spring, when the differences with the surroundings are especially marked. Daytime heatwaves are also projected to increase over Greater Sydney, with larger changes over the interior, but urban expansion has no perceptible impact on the occurrence of maximum temperature heatwaves, which suggests that the changes are caused by climate change alone.
Seasonal differences for each of the temperature daily extremes, as well as between maximum and minimum temperature, are partly explained by the prevalent direction of winds that compensate or intensify the effect urbanisation on local temperatures.

Figure 3: a Present climate seasonal frequencies of minimum temperature heat waves from AWAP (1990- 2009). b Projected changes in seasonal frequencies of minimum temperature heat waves from WRF (2040- 2059 vs. 1990-2009). A heat wave is defined as a period of at least 3 consecutive days with maximum temperature above the 90th percentile of the corresponding calendar month.