Impacts of form and design policies on urban microclimate: Assessment of zoning and design guideline choices in urban redevelopment projects

Urban form significantly impacts thermal conditions at the microscale. This research aims to understand how existing zoning regulations that prescribe urban form affect the microclimate of densely built redeveloped commercial and mixed-use urban areas. ENVI-met was used to simulate diurnal microclimatic conditions of two former indoor shopping malls in the Denver metropolitan area that were converted to walkable urban outdoor centers in the early 2000s – the 29th street Mall in Boulder, CO and Belmar in Lakewood, CO. Simulations were conducted for a typical summer day (June 29, 2015) and validated through on-site air temperature observations. To compare the impact of urban form on microclimate at both sites, four policy scenarios were developed to apply the hypothetical policy-driven morphology of one site to the other by changing building height, tree density, street direction, and a combination of the three factors. The combined scenario showed the largest differences in air temperature to the base case (up to 2.0 °C), which is in the order of magnitude of the RMSE (1.2 °C–2.4 °C). Site configuration influenced air temperature and wind speed creating hyperlocal cool and warm pockets. Mean radiant temperature (MRT) varied the most between the scenarios with differences of up to 15.0 °C due to tall buildings and increased shading. Urban form policies of Belmar contributed to more effective heat mitigation, particularly with respect to MRT. Findings from this empirical work demonstrate the importance of design guidelines and zoning to achieve desired microclimate outcomes and highlight the potential of planning policies to create climate-sensitive urban forms.