Observational Evidence of Neighborhood Scale Reductions in Air Temperature Associated with Increases in Roof Albedo

Arash Mohegh,George Ban-Weiss,Haley Gilbert,Jiachen Zhang,Yun Li,Ronnen Levinson,Tianbo Tang,Haider Taha

doi:10.3390/cli6040098

Arash Mohegh, George Ban-Weiss + Show 6 more

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https://doi.org/10.3390/cli6040098

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Abstract

The effects of neighborhood-scale land use and land cover (LULC) properties on observed air temperatures are investigated in two regions within Los Angeles County: Central Los Angeles and the San Fernando Valley (SFV). LULC properties of particular interest in this study are albedo and tree fraction. High spatial density meteorological observations are obtained from 76 personal weather-stations. Observed air temperatures were then related to the spatial mean of each LULC parameter within a 500 m radius “neighborhood” of each weather station, using robust regression for each hour of July 2015. For the neighborhoods under investigation, increases in roof albedo are associated with decreases in air temperature, with the strongest sensitivities occurring in the afternoon. Air temperatures at 14:00–15:00 local daylight time are reduced by 0.31 °C and 0.49 °C per 1 MW increase in daily average solar power reflected from roofs per neighborhood in SFV and Central Los Angeles, respectively. Per 0.10 increase in neighborhood average albedo, daily average air temperatures were reduced by 0.25 °C and 1.84 °C. While roof albedo effects on air temperature seem to exceed tree fraction effects during the day in these two regions, increases in tree fraction are associated with reduced air temperatures at night.

Highlights

The urban heat island (UHI) effect describes a phenomenon whereby temperatures in cities are higher than their rural surroundings [1] and is the result of land transformations associated with urbanization, as well as increases in anthropogenic heat
This suggests that variations in solar power reflected from roofs appear to dominate variations in observed air temperature relative to the effects of tree fractions in Central Los Angeles
In Central Los Angeles, variations in solar power reflected from roofs appear to dominate variations in observed air temperature relative to the effects of tree fractions

Summary

Introduction

The urban heat island (UHI) effect describes a phenomenon whereby temperatures in cities are higher than their rural surroundings [1] and is the result of land transformations associated with urbanization, as well as increases in anthropogenic heat. Compounding UHIs is the fact that cities are facing increased warming due to the local impacts of global climate change; Sun et al [2] found that areas in the greater Los Angeles region could see an increase of 60–90 extremely hot days per year by end of century under high emissions scenarios (RCP8.5: A scenario of comparatively high greenhouse gas emissions), posing threats to public and environmental health in addition to straining energy resources. While warming from climate change requires global action to mitigate, the UHI is a city-specific phenomenon with potential local solutions. There are actions that cities can take to mitigate UHIs, which will decrease the threat of future extreme heat dangers from climate warming. To design and implement appropriate countermeasures, cities need to characterize urban heat and its causes [4,5]

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