Clouds_F10_Lect8

Clouds_F10_Lect8 - Clouds Fall 2010, Lecture 8 1 Problems...

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Clouds Fall 2010, Lecture 8 1
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Problems with Clouds Clouds are a major problem for climatologists trying to understand how the atmospheric system works, and to make predictions about how GHG will change the climate system The IPCC has indicated clouds as a major problem for their predictions, and as an area nearing much more research 2
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Interaction with Radiation Clouds interact with both visible and infrared radiation o Clouds reflect visible light, should act as a negative feedback, helping to cool the planet o Clouds trap infrared radiation, and should act as a positive feedback, warming the planet 3
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Altitude Clouds can occur at different altitudes within the troposphere The interaction with radiation depends greatly on the cloud altitude o High altitude clouds play a large role in blocking IR radiation, but reflect little visible light, so they heat the planet o Low clouds reflect a great deal of visible radiation, but are ineffective at trapping IR, so they cool the planet 4
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Clouds and Altitude Clouds are good examples of blackbody radiators They radiate to space at whatever temperature the cloud top is at They trap IR radiation emitted from the much warmer ground They radiate back to the ground at the temperature of the bottom of the cloud This means they are net contributors of energy to the atmosphere below the cloud, and high altitude clouds are much more effective than low altitude clouds 5
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Scattering of Visible Light The electric field of incoming visible light excites vibrations in the water molecules They then reradiate the same frequency light, mostly in a direction similar to the direction of the incoming radiation o The shape of the droplet influences scattering direction The process is called scattering 6
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Scattering vs. Absorption In scattering, the reradiated light is visible, at the same frequency as the incoming radiation In absorption, the incoming light is converted to thermal motion of the absorbing molecule, and is reradiated in the IR There is always competition between scattering and absorption o Some light is scattered, some is absorbed 7
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Scattering Fraction The amount of light scattered depends on a number of factors o The most critical factor is the length scale of all of these structural features relative to the wavelength of the light being scattered o Density o Anisotropy (unequal variation of physical properties within a particle) o Concentration fluctuations o And others 8
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The light scattering problem is complex, and our understanding of this and other factors associated with clouds means that no physically realistic models have been developed for clouds, or how clouds might
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Clouds_F10_Lect8 - Clouds Fall 2010, Lecture 8 1 Problems...

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