Observational Evidence of Neighborhood Scale Reductions in Air Temperature Associated with Increases in Roof Albedo
2.1. Areas of Analysis
2.2. Defining Aggregation Areas
2.3. Meteorological Data
2.4. Description and Data Sources for Land Use Land Cover Properties
2.5. Deriving Sensitivities of Measured Air Temperature to LULC Properties
- We first detect and remove outlier weather stations for each hour. This is carried out by first performing a standard least squares linear regression. The influence of each point in determining the regression slope is then computed using leverage and residuals. Based on the distribution of influences for each hour and region, data points that have influence beyond 1.5 times the inner quartile range of the distribution are removed. This tends to eliminate points that have too much influence in determining the final regression statistics. After these points are removed, another regression is carried out. This time we use a robust linear regression with a Huber-T objective function . Regression using the Huber objective function gives higher weights to points with lower residuals, whereas standard regression using least-squares gives equal weights to each observation. The combination of outlier removal and robust regression minimizes the role of observations with high leverage, high residuals, or both. The sensitivity of temperature to the LULC parameter is then computed as the slope of the robust regression (i.e., ).
- Next, we determine whether the computed spatial sensitivity is statistically distinguishable from zero. We do so by computing the probability (“p”) value of the aforementioned robust regression. We deem the hourly sensitivity significant if the p-value is less than 0.1. The choice of this value was rather subjective.
- Lastly, for each hour of the day, we compute the number of days in July with statistically distinguishable sensitivities for each land cover property. Those sensitivities with >10 significant days are deemed as having significant relationships for that hour of the day. Those with ≤10 are deemed insignificant. This threshold was chosen subjectively, but roughly corresponds to half the number of sunny days for this month.
3.1. The Sensitivity of Temperature to Solar Power Reflected from Roofs
3.2. The Sensitivity of Temperature to Tree Fraction
3.3. Roof versus Non-Roof Surfaces as Contributors to Variability in Solar Power Reflected from Neighborhoods
4.1. Comparison with Literature
4.2. Dependence of Reported Temperature-Landcover Sensitivities to Neighborhood Characteristics
4.3. Policy-Relevant Take-Away Points
- Observed air temperature reductions are associated with increases in reflected solar power from roofs. Temperature reductions are larger during the day than at night, and peak in the afternoon. The peak effect is 0.31 °C and 0.49 °C reduction in afternoon air temperature per MW increase in solar power reflected from the neighborhoods in SFV and Central Los Angeles, respectively. To put this in more tangible terms, we can report temperature reductions per roof or overall albedo increase (Table 2). The average daily temperature reductions are 0.25 °C and 1.84 °C per 0.1 increase in roof albedo, which translates to 0.96 °C and 6.56 °C reduction per overall albedo increase of 0.1 for SFV and Central Los Angeles, respectively.
- In Central Los Angeles, variations in solar power reflected from roofs (and thus roof albedo) appear to dominate variations in observed air temperature relative to the effects of tree fractions. Note that this is based on current neighborhood-to-neighborhood variability in tree fraction in this region and should not be interpreted as how future additional tree cover would affect temperatures. For SFV, observations suggest an overnight (00:00–07:00 LDT) temperature reduction of up to about 1.5 °C per 0.1 increase in tree fraction.
- As with any observational studies, we are correlating temperature and land use/land cover parameters. Thus, we cannot make definite conclusions about causation. However, we have hypothesized and provided evidence for appreciable temperature reductions at the neighborhood-scale due to increasing reflected solar power through roof albedo increases. To our knowledge, this is the first study to provide observational evidence of roof albedo increases being associated with temperature reductions.
Conflicts of Interest
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|Daily Minimum Temperature (°C)||Daily Maximum Temperature (°C)||Daily Mean Temperature (°C)||Diurnal Temperature Range (°C)||Mean (Standard Deviation) Building Height (m)||Mean (Standard Deviation) Tree Fraction||Mean (Standard Deviation) Roof Fraction|
|SFV||19.4||30.5||24.3||11.1||5.33 (0.49)||0.12 (0.037)||0.26 (0.06)|
|Central Los Angeles||18.9||27.7||22.5||8.8||5.20 (2.72)||0.12 (0.075)||0.25 (0.09)|
|Area||Daily Average Temperature Reduction per 0.1 Increase in Roof Albedo (°C)||Daily Average Temperature Reduction per 0.1 Increase in Neighborhood Albedo (°C)||Afternoon a Temperature Reduction per 0.1 Increase in Roof Albedo (°C)||Afternoon Temperature Reduction per 0.1 Increase in Neighborhood Albedo (°C)||Notes|
|Current study||SFV||0.25 b||0.96 b||0.05||0.19|
|Current study||Central Los Angeles||1.84||6.56||5.52||19.7||Values from a network of weather stations. Afternoon = 14:00–15:00 LDT|
|Santamouris ||Various||0.2 c||0.3 d||0.4 c||0.9 d||Values come from a meta-analysis of previous climate modeling studies. Time of “peak” afternoon temperature reductions vary by study|
|Vahmani et al. ||Southern California||0.35||1.84||Values from climate modeling using the default near-surface air temperature model output. Afternoon = 15:00 LDT|
|Zhang et al. ||Southern California||0.06||0.34||0.09||1.03||Values from climate modeling. Temperatures represent “canyon air temperature” rather than the default near-surface temperature model output. Afternoon = 15:00 LDT|
|Taha et al. ||Downtown Los Angeles||1.0–9.2||Values from mobile measurements taken at various times of day, with the highest sensitivity derived from 11:00–14:00 LDT. Note that sensitivity values for measurements taken at night fall within the reported range.|
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Mohegh, A.; Levinson, R.; Taha, H.; Gilbert, H.; Zhang, J.; Li, Y.; Tang, T.; Ban-Weiss, G.A. Observational Evidence of Neighborhood Scale Reductions in Air Temperature Associated with Increases in Roof Albedo. Climate 2018, 6, 98. https://doi.org/10.3390/cli6040098
Mohegh A, Levinson R, Taha H, Gilbert H, Zhang J, Li Y, Tang T, Ban-Weiss GA. Observational Evidence of Neighborhood Scale Reductions in Air Temperature Associated with Increases in Roof Albedo. Climate. 2018; 6(4):98. https://doi.org/10.3390/cli6040098Chicago/Turabian Style
Mohegh, Arash, Ronnen Levinson, Haider Taha, Haley Gilbert, Jiachen Zhang, Yun Li, Tianbo Tang, and George A. Ban-Weiss. 2018. "Observational Evidence of Neighborhood Scale Reductions in Air Temperature Associated with Increases in Roof Albedo" Climate 6, no. 4: 98. https://doi.org/10.3390/cli6040098