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Ground Reflectance

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Ground reflectance (also refers to albedo) is defined as the fraction of incident radiation reflected by the ground. It varies with a number of factors, such as the properties of ground surface material, solar position, sky clearness, ground vegetation, snow coverage, etc..

Contents

Ground reflected solar radiation

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Direct, sky diffuse and ground reflected radiation
Ground reflectance is chiefly used to compute ground reflected solar radiation on the tilt surfaces (like, vertical walls and windows) for building energy simulation, as it will affect cooling and heating load. Solar radiation on tilted surface consists of three major components: solar beam radiation, sky diffuse and ground reflected radiation. In general, ground reflected solar radiation is dominated by diffuse reflectance, thus the fraction of solar reflected radiation onto a building surface depends on the view factor between target surface and the ground, in an open country area, this view factor roughly equals to 0.5 for vertical surfaces. It is noteworthy that in dense urban area, there will be a smaller portion of ground that can be “seen” by the building surfaces; beside, total solar radiation received by the ground is not uniform. In this circumstance, solar reflection from other exterior obstructions cannot be neglected any more (also see adjacent building section for more detail) so that advanced ray tracing calculation is needed.

Material reflectance

As discussed above, surface reflection can be divided into specular reflection and diffuse reflection. For the surfaces like glass or polished metal, specular reflection is predominant such that reflectivity will be close to zero at all other angles except at the reflected angle defined by the law of reflection. Ground is a mixture of many surfaces with distinct properties, diffuse reflection accounts for a major part for most surfaces like grass, concrete, and asphalt. The following table lists the reflectance of some common materials.

Material/Category Reflectance References
Impervious Road
concrete 0.23 Oke 1987, Clarke, J.A. 2001
cement board 0.25 Levinson and Hashem 2001
brick (Clay brick) 0.3 Oke 1987
limestone 0.28 Oke 1987
steel 0.18 Akbari and Desjarlais 2007
shingles, asphalt 0.14 Berdahl and Bretz 1997
granite 0.33 Jackson, 2010
sandstone 0.35 Jackson, 2010
wood, unpainted 0.40 Jackson, 2010
wood, painted 0.38 Jackson, 2010
iron 0.13 Oke 1987
Soil and vegetation
Soil (Dark &wet) 0.05~0.4 Ahrens, C. D. 2006
Soil (Light & dry) 0.15~0.45 Ahrens, C. D. 2006
Grass (Long to short) 0.16~0.26 Ahrens, C. D. 2006
Agricultural crops 0.18~0.25 Ahrens, C. D. 2006
Tundra 0.18~0.25 Ahrens, C. D. 2006
Forest(Delicious) 0.15~0.20 Ahrens, C. D. 2006
Forest(Coniferous) 0.05~0.15 Ahrens, C. D. 2006
Snow (old to fresh) 0.40~0.95 Ahrens, C. D. 2006

Average ground reflectance

As ground consists of a variety of materials, ground reflectance varies both spatially and temporally. Monthly averaged ground reflectance is always derived to approximate solar reflection from different components of the ground. Usually, 0.2 was recommended as a monthly averaged ground reflectance for most applications when ground-albedo measurements are not available, but this value was reported too high for some place (Psiloglou,2009). If ground composition can be estimated, averaged ground reflectance can be computed by the following equation.

Ground reflectance Average.png

Where, Ai is the area of component i.

Ground reflectance in the presence of snow

As ground reflectance increases dramatically in the presence of snow, which can vary from 0.75 to 0.95 for fresh snow cover and 0.4 to 0.7 for old snow cover (T.Muneer 2004). Modeling snow effect can be very difficult as snow type, snow depth, ambient temperature, and human activity all influence ground overall reflectance to a certain extent. Simple model was proposed to estimate monthly average ground reflectance in the present of snow.

Simple Snow Model Ground reflectance.png

Where Nsnow,i is the number of days with snow on the ground, Ni is the total days in a month i. In the formula, it is crucial to have a reasonable estimate of snow reflectance. However, snow reflectance will largely depends on land use. For instance, at the city center the building surface may not “see” much snow as it is surrounded by other buildings, trees and parking lots. Hunn and Calafell (Thevenard and Haddad 2006) recommended different snow reflectance for different land use condition, they are summarized in following Table.

Site exposure Reflectance of snow-covered ground
Typical city center 0.2
Typical urban site 0.4
Typical rural site 0.5
Isolated rural site 0.7

References

[1] Ahrens, C. D. 2006. Meteorology Today. An Introduction to Weather, Climate, and the Environment. Eighth Edition. Thompson, Brooks/Cole. United States. 537 pp. ISBN: 0495011622. http://www.eoearth.org/article/Albedo?topic=54300

[2] Akbari, H. and A. Desjarlais. 2007. Online article: Cooling Down the House - Residential roofing products soon will boast "cool" surfaces. National Roofing Contractors Association. http://www.professionalroofing.net/article.aspx?A_ID=609.

[3] Berdahl, P. and S.E. Bretz. 1997. Preliminary Survey of the Solar Reflectance of Cool Roofing Materials. Energy and Buildings 25: 149-158.

[4] Clarke, J.A. 2001. Energy Simulation in Building Design, 2nd ed. Butterworth Heinemann: Oxford.

[5] Jackson, T. L., Feddema, J. J., Oleson, K. W., Bonan, G. B., and Bauer, J. T. (2010). Parameterization of urban characteristics for global climate modeling. Annals of the Association of American Geographers Special Issue on Climate Change 2010.

[6] Levinson, R. and A. Hashem. 2001. Effects of Composition and Exposure on the Solar Reflectance of Portland Cement Concrete. Lawrence Berkeley National

[7] Muneer. (2004). Solar Radiation and Daylight Models for the Energy Efficient. Second edition. Elsevier Press.

[8] Oke, T.R. 1987. Boudary Layer Climates, 2nd edition. Methuen: New York.

[9] Psiloglou, B. E. and H. D. Kambezidis (2009). "Estimation of the ground albedo for the Athens area, Greece." Journal of Atmospheric and Solar-Terrestrial Physics 71(8-9): 943-954.

[10] Thevenard, D. and K. Haddad (2006). "Ground reflectivity in the context of building energy simulation." Energy and Buildings 38(8): 972-980.

[11] UCSB library. Material Emissivity. http://www.icess.ucsb.edu/modis/EMIS/html/em.html

See also

Microclimates

Adjacent Buildings

External links

Reflectivity

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