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Atmos 10209/25/2017
Atmos 1020 – Lecture 2The Radiation Balance of the Atmosphere
Temp.
ERL~5km
255 K 288 K
239 w/m2
IR out
239 w/m2
Solar in
• Increasing the greenhouse gas concentration of the earth causes the ERL to rise to a higher height. • This also requires the 255 K level to rise to maintain energy balance. • Due to the lapse rate, the entire profile migrates to the right and the surface temperature warms!
ERL> 5km
The electromagnetic spectrum:
While the gray model atmosphere provides conceptual understanding of the greenhouse effect in broad terms, to understand the nuances of climate change due to increasing CO2, we must consider the wavelength dependence of the IR transmission spectrum.
Just like white light from the sun is composed of all visible colors, infrared light emitted from the Earth’s surface is composed of a range of infrared colors
transmission=1All the light gets through
transmission=0.5half the light gets through
transmission=0None of the light gets through
Infrared ColorsBluer Redder
Atmos 1020 – Lecture 2The Radiation Balance of the Atmosphere
7 microns wavelengthClouds near ocean are invisible
11 microns wavelengthSee clouds near ocean surface
Atmos 1020 – Lecture 2The Radiation Balance of the Atmosphere
Many of the trace gasses in the earth’s atmosphere are absorbing
Atmospheric Window
Interesting Fact: The Earth’s sfc temperature is ~288 K. This corresponds to most of the energy being emitted to space in the 8 to 12 micron wavelength ranges. This is no accident.
Atmos 1020 – Lecture 2The Radiation Balance of the Atmosphere
Many of the trace gasses in the earth’s atmosphere are absorbing in the IR.We call these greenhouse gasses – water vapor is the most important!
Atmospheric Window
An increase in CO2 causes the partially transmissive part of the atmosphereic window to become less transmissive – more opaque –effectively trapping more IR energy in the atmosphere and reducing the IR emission to space. i.e. Closing the Atmospheric Window!
Atmos 1020 – Lecture 2The Radiation Balance of the Atmosphere
• IncreasedCO2causestheatmosphericwindowtocloseasthepartiallytransmissive bandsnear13micronsbecomemoreopaque.
• Thesurfacethenwarmsinamannersimilartoourpriordiscussions• Thewarmersurfacecausesmorewatertoevaporateintotheaircausingthepartially
transmissive bandsnear7micronstobecomemoreopaque. • Thesurfacethenwarmsmore….
Window closes
FromKiehlandTrenberth(1997)
Atmos 1020 – Lecture 2The Radiation Balance of the Atmosphere
Ein = Eout
Surface Energy Balance
Input Output
Sunlight – 168 Sensible – 24Evap. Water – 78Net IR - 66
Atmosphere Energy Balance Sensible – 24Precip – 78Sunlight - 66
Net IR - 168
Atmos 1020 – Lecture 2The Radiation Balance of the Atmosphere
Ein = Eout
Absorbed Solar Radiation = Emitted IR Radiation
What can throw this equation out of Balance?
1. Changes to… 1. Changes to…
Atmos 1020 – Lecture 2The Radiation Balance of the Atmosphere
Ein = Eout
Absorbed Solar Radiation = Emitted IR Radiation
What can throw this equation out of Balance?
1. Changes to the sun2. Changes to aerosols3. Changes to clouds4. Changes to surface properties
1. Changes to greenhouse gasses (water vapor, carbon dioxide, others)
2. Changes to clouds
Atmos 1020 – Lecture 2The Radiation Balance of the Atmosphere
What of clouds?
Clouds have 2 competing influences on energy balance:
Ein = Eout
Absorbed Solar Radiation = Emitted IR Radiation
Cloud trap IR radiation which warms the system.
Clouds reflect solar radiation which coolsthe system.
Atmos 1020 – Lecture 2The Radiation Balance of the Atmosphere
How do clouds trap IR radiation?
Recall that temperature decreases with height….
Cirrus – Cold Tops
Stratus – warm TopsStratus – warm Tops
Stratus (low level) clouds emit IR radiation at about the same rate as the surface but they are very reflective to solar radiation
Cirrus (high level) clouds absorb the warm IR radiation from below and emit IR at MUCH colder temperatures (~factor of 4 less IR).
IR
Clouds and the Energy Balance
Clouds appear almost uniformly white in terms of reflected sunlight
The picture of clouds in the IR is much more variable with light and dark areas corresponding to cloud top temperatures.
Clouds and the Energy Balance
Low Level Stratus Clouds tend to cool the climate system.
Albedo is HighInfrared is about the same as the surface
Clouds and the Energy Balance
Thick Clouds in the Upper Troposphere
Reflect Solar – a Cooling Trap IR – A warming
The net effect depends on the details of the clouds…