Earth System Cooling

Development of Earth System Cooling with Reflective Roofs: Experimental Verification and Model-based Evaluation for Selected Cities.

Project Year: 2011-12

Funding Agency: U.S. Department of Energy- Office of Energy Efficiency and Renewable Energy (EERE) and Office of Science)

The project aims to experimentally evaluate the changes in radiative forcing introduced by increasing surface albedo using radiometric satellite observations.  

Roof surfaces were modified (during spring 2011) and monitored at two locations in northern India during June 2011–March 2012. The first site was located at the GB Pant University (Pantnagar; 29.03°N,79.49°E,235 m asl), an agricultural and industrial area. The second site was located at the Aryabhatta Research Institute of Observational Sciences (Nainital; 29.36°N,79.46°E, ~ 1940 m asl), in the central Himalayan region about 30 km north of Pantnagar. The high altitude Himalayan Mountains lie in the Northern and Eastern sectors of the second observation site and 50–130 km away. The Pantnagar university campus has a perimeter of about 30 km and encompasses a large area of about 16 000 acres (about 6.5 km2) and is used primarily for agricultural research. Local sources of aerosol and gaseous emissions near the observation location include residential cooking, diesel vehicles for the university campus and surrounding agriculture. Industrial activity is present in nearby towns. Two roofs at Pantnagar (PW1 and PW2) were painted with white elastomeric coating in June 2011. Two other roofs were left untreated (PD1 and PD2). The roof labeled PD2 could be considered to have an intermediate albedo because it was more reflective than PD1 but less reflective than PW2 and PW1. A similar strategy was followed for painted (NW1 and NW2) and unpainted (ND1 and ND2) roofs at Nainital. The painted and unpainted surface areas were similar at each site.

R&D organisation and academic institutions

IIIT Hyderabad, MNIT Jaipur

Outcomes –

Measured downwelling shortwave fluxes from ground-based radiometers, SW↓SFC, are shown together with AOD at 0.55 μm from MODIS, and predictions of SW↓SFC from RRTMG_SW for the time of IKONOS observations in the following table.

Measured and computed surface and atmospheric properties, and downwelling SW radiation for dates and times of IKONOS observations.

Imaging date RM AOD 0.55 μm (MODIS) SW ↓SFC(measured) (W m−2) SW↓SFC (computed w/o AOD) RRTMG (W m−2) Transmissiont (computed) Reflection r(computed) AOD 0.55 μm (computed)
13 Oct. 2011 (N) 0.104 0.331 802.53 ± 22.19 817.17 ± 14.65 0.864 ± 0.040 0.088 ± 0.004 0.145
21 Oct. 2011 (P) 0.165 0.626 637.35 ± 22.73 759.88 ± 14.30 0.718 ± 0.042 0.154 ± 0.009 0.626
21 Dec. 2011 (N) 0.140 0.092 665.90 ± 35.90 675.71 ± 10.29 0.874 ± 0.061 0.088 ± 0.006 0.092
26 Dec. 2011 (P) 0.126 0.080 472.43 ± 31.34 614.94 ± 13.90 0.641 ± 0.059 0.203 ± 0.019 0.578
20 Mar. 2012 (N) 0.096 0.269 826.25 ± 14.22 916.95 ± 15.03 0.800 ± 0.029 0.113 ± 0.004 0.445
20 Mar. 2012 (P) 0.149 0.762 639.35 ± 16.79 906.57 ± 14.93 0.586 ± 0.028 0.190 ± 0.009 1.484


The radiative transfer model was able to reproduce the surface fluxes (the absolute mean bias was 10.49 W m−2 once the AOD values were modified), but the differences between measured and default (without aerosols) RRTMG_SW results for SW↓SFC can vary by up to ~250 W m−2, demonstrating the importance of the aerosol loads. Except for two days, the AOD values considered in the runs (column 8) were generally higher than the values retrieved from MODIS. One likely source of the discrepancies is that local aerosol sources near the Pantnagar site were not resolved by the 10 km MODIS imaging. However, we cannot uniquely identify this as the cause. Equation used to estimate the atmospheric transmission and reflection (t and r), which differed between Nainital and Pantnagar. The transmission and reflection differences between the two sites are likely due to differences in the vertical profiles of aerosol concentration, and the season dependence of the path ray lengths.