September 1994 Issue #8

How Shiny Is L.A.?

Cities reflect away some solar radiation and absorb the rest of it, creating higher temperatures and more smog. Thus, an important factor in determining the intensity of urban heat islands is the albedo--or reflectivity--of urban areas. There are, however, few large-scale measurements of albedo. Recent research in Building Energy Measurement and Performance Analysis by Haider Taha measured surface albedo of California's South Coast Air Basin (SoCAB).

Taha performed radiometric measurements from a single-engine plane using two precision spectral pyranometers. The aircraft flew as low as possible (between 200 and 300 m above ground level) to narrow the field of view of a downward-looking pyranometer and to minimize the thickness of the atmospheric layer, thus reducing atmospheric scattering and absorption, between the plane and the ground. Taha scheduled two flights just a few days after the summer solstice and within an hour of solar noon each day to minimize the effects of large air masses (small solar altitude angles) and tall shadows.

The flight plan (see figure) was designed to measure the albedo of a variety of land covers and terrains while avoiding many airspace restrictions in a busy air corridor. The flights covered a total ground track of 200 km and an area of 600 km2. Unfortunately for Taha, the optimum conditions for measurement coincided with the least comfortable flight conditions. Cabin temperatures exceeded 95°F (35°C) while the unsettled midday air caused maximum turbulence.

Taha's flight began in Burbank, continued around the Santa Monica Mountains, over the Pacific Ocean, then northeast through the San Fernando Valley. The path turned south over downtown Los Angeles and the east basin, ending near Pasadena.

The albedo at the location of each measurement was related to the land use of the area. Taha merged the data with a database of land use to produce a gridded map of the SoCAB with 5x5-km cells.

The results show that the SoCAB's urban albedo is higher than the suburban albedo in that region because the suburban land use has more vegetation (vegetation has a lower albedo than light gray buildings). The results also show that the variation in albedo over the SoCAB is not large because of the spatial integration. In general, the measured albedo is lower than 0.20, the conventional value used in non-desert meteorological simulations.

                                    Integrated
Terrain/Land Use                     Albedo

Industrial (grayish/white) no vegetation    0.20
Commercial, high-rise, low vegetation       0.17
Mixed development, moderate vegetation      0.16
Downtown Los Angeles, high-rise             0.15
Park, vegetation, grass                     0.14
Residential with heavy vegetation           0.12
Coastline (highway and hills) and ocean     0.08
Ocean, light wind (no surf), no fog         0.04

-Haider Taha

For more information, please contact Haider Taha, HGTaha@LBL.gov, at (510) 486-7473.

More about the Mitigation of Heat Islands.

Return to the Recent Research index.