PhD research candidate University of New South Wales
Principal Supervisor: Prof. Alan Peters
Co-supervisor: Dr. Paul Osmond >>
The Effect of Architectural-Scale Vertical Surfaces on Urban Microclimate.
Buildings modify the radiative, thermal, moisture and aerodynamic processes that determine the microclimates of cities, contributing to the urban heat island (UHI) effect (Oke 1987). Horizontal and vertical urban surfaces control the conversion and partitioning of incident radiant energy within the urban atmosphere (Voogt 1995), hence each microclimate is “dominated by the characteristics of its immediate surroundings” (Oke 1987, 274). The receipt of direct solar radiation (and consequent surface heating), the decrease in solar radiation due to shading and the “local increase in solar receipt by reflection from sunlit walls” (Oke 1987, 264) are the most significant radiative effects of microclimate modification due to buildings (Oke 1987).
Importantly, the thermophysical properties of materials – both surface (e.g., emissivity and albedo) and molecular (e.g. specific heat, thermal conductivity, admittance, diffusivity, heat capacity and density) – and “the ability of a surface to hold or evaporate moisture” (Quattrochi & Ridd 1994, 2020) – control both the magnitude and temporal dynamics of radiant energy emissions from discrete urban surfaces (Quattrochi & Ridd 1994). The idealized urban canyon-air volume has three “active surfaces” – the two opposing building walls and the street (Oke 1987). The influence of vertical surfaces (i.e., walls) on the surface energy balance, and hence microclimate, increases with increased canyon aspect ratio (H/W), or taller, more densely spaced buildings (Voogt & Oke 1998). Increased vertical surface area and multiple reflections increase absorption of short-wave radiation (Oke 1987). The urban atmosphere within the urban canopy layer is therefore “conditioned by the temperatures of both horizontal and vertical surfaces” (Voogt & Oke 1998, 1999).
However, the spatial patterns and temporal dynamics of vertical surface temperatures are not directly observed by aerial or satellite nadir-pointing remote sensing technologies and when sampled by off-nadir (angled) remote sensors, result in directional bias (Voogt & Oke 1997, 1998). Therefore, “much of the uncertainty associated with surface (radiant) emissions” (Soux et al 2004, 403) is due to the underrepresentation of active vertical surfaces in UHI observations and the variability of vertical surface temperatures at the microscale (Soux et al 2004; Hartz et al 2006). Furthermore, whilst advances in hyperspectral imagery have enabled “automated” micro-scale (i.e., ~100 m) surface material and thermal mapping for precinct and canyon scale climatology (e.g., Heiden et al 2012), thermo-spatial mapping and the development of tools for investigating energy transfer relations at the architectural-scale (i.e., 1m -20m), where individual buildings are considered to be the “fundamental units to create the urban climate” (Cleugh & Grimmond 2012,52)’ remains underdeveloped, particularly in relation to the energy partitioning processes of solar radiation receipt, long-wave radiation emissions and storage heat fluxes from complex vertical surfaces (Sham et al 2013; Grimmond et al 2010; Oke 2006; Voogt & Oke 2003).
The transferability of prior studies (e.g., Sham et al 2013; Samuels et al 2010; Hartz et al 2006; Hoyano et a11999; Voogt & Oke 1998, 1997) to measure, map, model and predict the dynamic thermal behavior of complex vertical urban surfaces (i.e., facades) is limited by uncertainty stemming from:
- Methodological challenges, sensor limitations and inconsistent metadata protocols.
- Paucity of field-based data collection and ground-validation.
- The omission or underestimation of the role of urban vertical vegetation.
- Inaccuracies regarding heat transfer coefficients and emissivity values.
- Unrepresentative sampling and inconsistent spatial/temporal classification, and;
- The difficulties of observing, classifying and modeling the “non-trivial complexity” of actual three-dimensional urban surfaces that exhibit greater temporal and spatialthermal variability than either controlled experiments or idealized models (Grimmond et al 2010; Soux et al 2004; Voogt & Oke 2003, 1997; Roth et aI1989).
In order for architects to adopt microclimatic design principles, they need reliable diagnostic and predictive information about the microclimatic effects of buildings at spatial scales relevant to their decision-making. This research aims to investigate and establish quantitative, modifiable relations between the thermal properties of typical facade configurations (comprised of synthetic and natural vertical surfaces and materials) and observed outdoor surface, air and radiant temperatures, using ground-based infrared thermography, GIS and mobile micrometeorological instruments. The research addresses, and broadens, the fundamental question “whether there are systematic patterns of surface temperature related to street configuration” (Voogt & Oke 1998, 200) by developing an innovative methodology, the “Vertical Surfaces Thermal Typology” (VeSn). Once developed, the VeSn will enable automated vertical surface thermal classification corresponding to representative building facades, including the effects of orientation and materials, and thereby contributing to the predictive capacity of emergent microclimate design tools at scales suitable for architectural decision-making.
Jonathan obtained a Bachelor of Science (BSc) in Engineering from the University of Cape Town and a Bachelor of Architecture (BArch) from the University of the Witwatersrand in Johannesburg, South Africa. He worked in the disciplines of appropriate technology, product and productivity design, and architecture in Johannesburg, and briefly in Hong Kong, prior to establishing his own architectural practice in Sydney. He obtained his Master of Sustainability (MSust) at Sydney University and is now a Ph.D. candidate and sessional lecturer/tutor at the University of New South Wales (UNSW).
Fox, J.: ‘The Effect of Facades on Outdoor Microclimate: A Review’, Conference paper for the 3rd International Conference on Countermeasures to Urban Heat Islands, (IC2UHI) in Venice, Italy, 13-15 October 2014. Download Abstract >>
Cleugh H.A. and Grimmond C.S.B., 2012, “Urban Climates and Global Climate Change”, in Henderson-Sellers A. & Mc Guffie K. (eds.), The Future of the World’s Climate, Elsevier, Chapter 3: 47-76. Oke T. R., 1987, Boundary Layer Climates, 2nd edition, Methuen, London.
Roth M, Oke T.R. and Emery W.J., 1989, “Satellite-derived urban heat islands from three coastal cities and the utilization of such data”, Urban Climatology, International Journal Of Remote Sensing, 10(11): 1699-1720.
Sham J.F.C., Memon SA and Lo T.Y., 2013, “Application of continuous surface temperature monitoring technique for investigation of nocturnal sensible heat release characteristics by building fabrics in Hong Kong”, Energy and Buildings 58:1-10.
Voogt J. A. and Oke T. R., 1997, “Complete Urban Surface Temperatures”, Journal of Applied Meteorology, 36: 1117-1132.
Voogt J. A. and Oke T. R., 1998, “Radiometric Temperatures of Urban Canyon Walls Obtained from Vehicle Traverses”, Theoretical and Applied Climatology, 60:199-217.
Voogt J. A. and Oke T. R., 2003,”Thermal remote sensing of urban climates”, Remote Sensing of Environment 86(3): 370-384.