Modelling of Ethekwini heat islands under climate change

Abstract

Urbanisation has resulted in the removal of natural surface with artificial surface such as tarred roads and concrete surfaces. This causes more heat storage at the surface, resulting in nighttime temperature difference of average 2°C between urban and rural areas, which impacts climate. This phenomenon is called an urban heat island (UHI’s), and negatively affects human health, energy consumption, and the urban environment. This study investigates the role of urban land surface changes, anthropogenic emissions and urban parameters in the development of UHI over the city of eThekwini, Kwazulu-Natal Province of South Africa (SA). The city is located adjacent to the Indian Ocean, and its proximity to the ocean influences the local climate. The study applies the Conformal-Cubic Atmospheric Model (CCAM), a climate model developed at the Commonwealth Scientific and Industrial Research Organisation (CSIRO) of Australia. The CCAM is run as a regional model with a stretched grid to provides higher resolution output over the area of interest. CCAM is coupled to an urban climate model (UCM) called the Australian Town Energy Budget (ATEB) via a land surface model (LSM) titled the Community Atmosphere–Biosphere Land Exchange (CABLE), which incorporates the 2013 land cover boundary conditions obtained from Moderate Resolution Imaging Spectro-radiometer (MODIS) satellite. The CCAM is driven by two datasets: (i) CCAM-ERA Interim data, from the European Centre for Medium-Range Weather Forecasts (ECMWF) for the period 2005/6 to 2016; and (ii) Climate Model Intercomparison Project Phase 5 (CMIP5) driven simulations for the periods: 2005/6 to 2016, 2039/40 to 2050, and 2089/90 to 2099. The downscaling was performed in various stages, using both ERAI and CMIP5 driven data, with resolution increasing from 50 km (Africa), 8 km (South Africa) and 1 km (eThekwini). At 1 km, CCAM-CABLE incorporates measured urban parameters provided by the city of eThekwini. The CCAM analysis indicates that updating the land cover to the 2013 data version and switching on the urban scheme realistically reproduce the orientation of the city and land cover types. CCAM simulated surface and minimum temperatures, and winds using ERAI and CMIP5 driven data effectively reproduced the UHI seen in surface observations and MODIS data. The CCAM simulations indicate that eThekwini experiences the UHI during DJF season, which is evident in minimum and surface temperatures, longwave radiation and sensible heat flux. The model simulations suggest that the city temperature is at most 2°C warmer than the surrounding rural areas. Projections of minimum and surface temperatures into the near-future indicate that the UHI will persist, will be nearly 1°C higher than in the current climate, and at most 2-4°C higher in the far-future climate. Future projections suggest that the UHI intensity will increase over the city of eThekwini. An analysis of winds shows that both synoptic scale circulations and mesoscale circulations impact the UHI. Wind analysis shows the presence of sea breeze from 06h00 UTC until around 18h00 UTC, and land breeze after 18h00 UTC until 06h00 UTC. These results suggest that human activities such as reducing vegetation, increasing artificial surfaces, and building within the city, have contributed to the development of the UHI. However, atmospheric circulations including land and sea breezes regulate the evolution of the UHI. Different mitigation strategies can be applied to reduce the impact of the UHI in cities, such as increasing water bodies, re-vegetation, and applying high-reflectivity materials. Current findings suggest that CCAM could be an important tool for evaluating and understanding the dynamics of the UHI in cities. Further research is needed to validate the model across different cities under various climatic conditions and to test the capability of the model at sub-kilometer scale. Keywords: city of eThekwini, climate models, updated land cover, surface urban parameters, city climates, urban heat islands, high horizontal resolution.