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phy392_lecture05_web_2011

# phy392_lecture05_web_2011 - PHY392S Physics of Climate...

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PHY392S Physics of Climate Lecture 5 Web Notes Solar Radiation The Solar Constant Changes in the Solar Input The Solar Cycle PHY392 - Physics of Climate Lecture 5, Page 1

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PHY392 - Physics of Climate Lecture 5, Page 2
surface particles: 0.0065-0.002 Wm -2 (mainly protons) and magnetic fields SUN EARTH photons: 1365 Wm -2 atmosphere magnetosphere sunspot faculae bow shock plasmasphere solar wind heliosphere galactic cosmic rays NRL LASCO coronagraph on SOHO 0.0000007 Wm -2 solar eruptions: flares, coronal mass ejection Slide courtesy of Judith Lean, Naval Research Laboratory

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The Solar Constant Recall: The solar constant is the solar flux density incident at Earth’s orbit (at the top of the Earth’s atmosphere) 1370 W m -2 So, how constant is the solar constant? Let’s consider the solar input and planetary motion. Planets orbit the Sun in elliptical orbits, so the solar radiation incident on the planet must vary with time of year (season). point of nearest approach (maximum solar input) - perihelion point of furthest approach (minimum solar input) - aphelion The orbit of a planet can also be described using its average distance from the Sun and its eccentricity: For Earth, e = 0.017, so the solar radiation varies by about 7%, because the orbital distance varies by 3.5% and the radiation varies with the square of this distance (1.035 2 = 1.07). 2 2 a b 1 axis major axis minor 1 e = = = zero for a circular orbit PHY392 - Physics of Climate Lecture 5, Page 4
In fact, seasonal changes do not correlate with this orbital motion. different hemispheres have different seasonal phases perihelion occurs in the Northern hemisphere winter The seasonal change is more a function of the planetary inclination , which is the angle that the planetary rotation vector makes with the normal to the orbital plane. This inclination causes the average angle which a portion of the planet’s surface makes with the solar beam to vary. If the “surface” of the atmosphere is not normal to the solar beam then the energy input (ignoring orbital changes) is Fcos θ, where θ is the angle between the solar beam and the vertical - the solar zenith angle .

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phy392_lecture05_web_2011 - PHY392S Physics of Climate...

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