Ariane Middel, Shai Kaplan & Anthony Brazel, Arizona State University, Tempe, AZ, USA

Kathrin Hab, University of Kaiserslautern, Kaiserslautern, Germany

Presentation Title: Summer daytime oasis effect at the Las Vegas desert-urban fringe



Previous research suggests that desert cities exhibit a daytime oasis effect, mainly driven by moisture availability and shading, which is the converse of the urban heat island effect at night. This study aimed to quantify the daytime oasis effect, at the micro-scale level, of a residential neighborhood at the Las Vegas desert-urban fringe. A typical development pattern along the northern extent of the city, adjacent to the Mojave Desert was chosen as the study site. The study highlights the importance of urban form, solar access and air flow for the creation of daytime oases that addresses the hypothesis that the oasis effect does not decay uniformly, but has a complicated pattern dependent on local climate zones and landscape types.

Key Lessons Learned

  • Desert-adapted shade trees have a strong potential to mitigate the urban heat island in xeric Phoenix residential neighborhoods; the cooling benefit is greater at the microscale (2.5 °C) than at the local scale (1.1 °C).
  • ENVI-met simulations suggest that adding trees to residential neighborhoods in Phoenix can reduce two meter air temperatures by 0.14 °C per one percent increase in tree canopy cover on a mid-afternoon summer day; An increase in tree canopy cover from currently 10% to 25% in a Phoenix residential neighborhood resulted in an average daytime 2 m air temperature reduction of up to 2.0 °C at the local scale.
  • A distinct park cool island of up to 3.6 °C was documented over a large park in Tempe, Arizona, consisting of irrigated turf and bare soil.
  • The park cool island effect can extend to non-vegetated surfaces.
  • Daytime heat in desert urban areas can be mitigated through vegetation and albedo modification of surfaces, but also by manipulating the form and spatial arrangement of urban features.
  • In mid-afternoon, dense urban forms can create local cool islands.
  • Intra-urban spatial differences in cooling are strongly related to solar radiation and local shading patterns.

Policy/Practice Implications of Research

  • Findings provide comprehensive design guidelines for practitioners to conduct environmental impact assessments of new developments.
  • Environmental assessment tools will become increasingly important, as more and more cities are developing climate action plans and working on mainstreaming climate considerations into housing developments.
  • A challenge will be to make the transition from sustainability-science infused municipal operations to private sector operations, which still dominate urban expansion in the US Southwest.

Knowledge Gaps and Needs

  • With regard to the ENVI-met simulations, further enhancement of the model is required to:
    • Better represent night-time situations, including heat storage or materials;
    • include variable wind speeds in a diurnal simulation;
    • include precipitation and irrigation parameters; and,
    • Comprehensively represent anthropogenic heat (e.g., waste heat release from air-conditioning systems).
  • With regard to this study:
    • Support findings by in-situ measurements (transects from Las Vegas city fringe into desert);
    • comparison of results to other desert cities;
    • assessment of building energy performance; and,
    • Economic assessment analysis of heat mitigation measures.