A Verification that Large Albedo Increases in Chicago Resulted from New Reflective Roofs and an Identification of Building Characteristics Producing Large Albedo Increases

Abstract

Early in 2011, Mackey et al. (2011) remotely-sensed significant albedo increases on the magnitude of 0.016 happening over the City of Chicago between 1995 and 2009. These increases were well correlated to drops in remotely-sensed temperature, especially in comparison to effects from vegetation increases, and had regression slopes as high as -21.1, indicating a drop of 2.11 ⁰C for every albedo increase of 0.1. While the group hypothesized that the reflectivity increases were driven primarily by new reflective roofs mandated by new building energy efficiency codes established in 2003, the team could not verify this beyond "qualitative visual observations." This study attempts to quantitatively verify this hypothesis by combining the data from Mackey et al.'s experiment with a vector shape file of all building footprints in Chicago. Additionally, the criteria of building height and number of untis are evaluated in relation to albedo increases to determine the building characteristics producing the most significant albedo increases. Results verified that the albedo increases previously noted were primarily the result of new reflective roofs by showing a stronger correlation and steeper regression of -27.1 against temperature when albedo increases in building footprints were isolated from city-wide increases. Also, the city's albedo increases above 0.2 were found to be composed almost entirely of pixels in building footprints (around 90%-100%), indicating that reflective roofs were driving the largest albedo increases associated with very low temperatures. Analysis of the two building characteristics revealed that the larger increases in albedo tended to happen in low-rise buildings and (in the case of residential buildings) in structures with a greater number of units. While the overall correlation of these variables to albedo increase is not that significant and the conclusion that larger, flatter buildings can host greater albedo increases is not that surprising, these factors might be useful for identifying potential buildings to be targeted for a future reflective roof campaign.

Interactive Map of Analyzed Data:
(Toggle On/Off One Map at a Time Using the Buttons at the Right)

Temperature Change Maps(1995-2009)


Temp. Change Scale (⁰C): Urban Temperature Change Scale

Albedo Change Maps(1995-2009)


Albedo Change Scale: Urban Albedo Change Scale

Building Height Map


Building Height Scale (Stories): Building Height Scale

Building Units Map


Building Units Scale (HousingUnits): Building Units Scale


Temperature and albedo data obtained from LANDSAT 5 via USGS's Glovis
Building footprints and data obtained from the City of Chicago's GIS database

Contents:

Introduction
Methods
Results Part I: Verification that Albedo Increases Resulted from New Reflective Roofs
Results Part II: Identification of Building Characteristics Producing Large Albedo Increases
Discussion

References