Nested Scales of Urban Thermal Anomalies: A Study of Heat Relations to Land Cover and an Urban Growth Boundary, and Consequences for Urban Heat Index (Re-)Definition
Abstract
A key characteristic of urban systems and their potential sensitivity to climate warming is their often already-elevated temperature relative to surrounding rural areas, termed the urban heat island (UHI) effect. Already, urban warming and urban-focused heat waves are a leading cause of premature human mortality. Quantifying this positive urban-rural difference as an urban heat index (also UHI) is a key and useful short-hand for identifying the magnitude of urban warming that city design, layout, policy, and bio-climatic zone can induce. For some cities, the typically positive UHI is reversed, with these cities then termed urban cool islands; however, "urban cool island" is also used to refer to the local cooling effect of parks and green-spaces in otherwise warm urban areas. The multiple scales at which urban warm and cool thermal anomalies are expressed, at which their generation mechanisms should be understood, and across which their scaling should be reconciled has become a challenge in the contemporary literature.
This contribution presents perspective and results from a study of the Portland, Oregon / Vancouver, Washington metropolitan area, which is enclosed with enforced urban growth boundaries (UGB), enabling ease of definition of urban interior and exterior. The nature of urban thermal heterogeneity inside and outside the UGB and in relation to land cover (LC) classes was analyzed using MODIS land surface temperature (LST) data, the national LC database data (from Landsat), and statistical tools. Key findings included that a given LC class presenting contrasting LST inside versus outside the UGB, sometimes even bi-modal; that the single-valued definition of UHI typically used to date might be more meaningfully (and feasibly, given modern distributed data availability) replaced instead with a calculated distribution of UHI; that neither canopy cover nor impervious fraction were good predictors of LST at km pixel scale; and that typically cool LC types frequently exhibited anomalously hot temperatures (and vice versa), depending on landscape context. Based on these findings we conclude a need for, and provide, an updated definition of UHI, a multi-scale framework for considering urban thermal anomalies, and hopefully reconciling disparate terminology in the literature to date.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFMGC21I1378M
- Keywords:
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- 0493 Urban systems;
- BIOGEOSCIENCES;
- 0231 Impacts of climate change: agricultural health;
- GEOHEALTH;
- 1622 Earth system modeling;
- GLOBAL CHANGE;
- 1630 Impacts of global change;
- GLOBAL CHANGE