CSU AT 605 - LECTURE NOTES (27 pages)

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LECTURE NOTES



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LECTURE NOTES

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Lecture Notes


Pages:
27
School:
Colorado State University- Fort Collins
Course:
At 605 -

Unformatted text preview:

Revised Tuesday January 21 2014 1 2 What makes it go Copyright 2014 David A Randall The Earth s radiation budget An upper boundary condition on the global circulation Radiation is almost the only mechanism by which the Earth can exchange energy with the rest of the Universe The solar energy flux at the mean radius of the Earth s orbit is about 1365 W m 2 One way to get an intuitive grasp of this number is to imagine fourteen 100 Watt light bulbs per square meter Another is to consider that 1365 W m 2 is equivalent to 1 365 GW km 2 which is the energy output of a large power plant for each square kilometer of area normal to the solar beam The globally averaged top of the atmosphere radiation budget is summarized in Table Incident solar radiation 341 W m 2 Absorbed solar radiation 239 W m 2 Planetary albedo 0 30 Outgoing longwave radiation 239 W m 2 Brightness temperature of the Earth 255 K Table 2 1 Summary of the annually averaged top of the atmosphere radiation budget after Trenberth et al 2009 2 1 These numbers are now known to three significant digits The Earth s albedo is near 0 30 independent of season this number has been known to better than 10 accuracy only since the 1970s The energy absorbed by the Earth is a2 Sabs S 1 2 4 a 1 S 1 240 W m 2 4 annualmean 1 An Introduction to the Global Circulation of the Atmosphere Revised Tuesday January 21 2014 2 Here Sabs is the average absorbed solar energy per unit area S is the solar constant a is the radius of the Earth and is the planetary albedo Note that this rate of energy absorption is referred to the total surface area of the Earth i e 4 a 2 rather than to the cross sectional or projected area presented to the solar beam which is four times smaller The most important upper boundary condition on the global circulation of the atmosphere is the incident solar radiation also called the insolation It is determined by the energy output of the sun and the geometry of the Earth s orbit see Fig 2 1 including the



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