Earth s Energy Balance and the Atmosphere

Transcription

1 Earth s Energy Balance and the Atmosphere Topics we ll cover: Atmospheric composition greenhouse gases Vertical structure and radiative balance pressure, temperature Global circulation and horizontal energy transport

2 Main questions part 1- composition What are the main constituents of the atmosphere? Which gases are most important for the greenhouse effect? Which gases are short-lived and which are long-lived? At what wavelengths is the atmospheric window found? How do greenhouse gases interact with different kinds of E-M radiation? What is a blackbody? How is a gray body different? How is a non-gray body different?

3 Main questions part 2 vertical structure and radiative balance How does pressure change with altitude? Why is it a simple function? What is the adiabatic lapse rate? What are the two types? Explain what the temperature structure of the Earth looks like, and what occurs in the different layers to cause this. Why is the temperature structure more complex than the pressure structure? What is the Stefan-Boltzman equation? What is a blackbody? What is a gray body? What is Wien s Law? What is Kirchhoff s Law? What is the effective emission temperature of the Sun? How about the Earth? What are the characteristic wavelengths for each? What is the surface T of Earth using a single (perfect) GHG layer? Calculate this value. How do clouds form? How do clouds affect the radiative balance?

4 Main questions part 3 horizontal structure, a.k.a. circulation How is incoming solar radiation distributed? How is outgoing longwave radiation distributed? What does this tell us about horizontal transport of energy? How is this energy transported? In what forms? How does the three-cell model of atmospheric circulation work? How does it relate to energy transport? How is the three-cell model inaccurate? What is a better model? What are the different climatic zones and associated cloud types? How does a global map of shortwave reflectance and longwave emission relate to these features? N.B. Any short-answer questions from this lecture for the final exam will be taken directly from the past three slides no surprises! The answers to these questions are found either in the reading or in this presentation.

5 Atmospheric Composition

6 Which gases are reactive? Which are greenhouse gases?

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8 5800 K 288 K Division between shortwave and longwave around 3 to 4 um

9 Blackbodies emit the maximum amount of radiation at a given wavelength Gray bodies emit less than the maximum radiation but it s a constant fraction across wavelengths Non-gray bodies (here called a selective radiator ) has an emissivity that depends on wavelength. The integral under the black body curve is given by the Stefan- Boltzmann equation (reading). Same is true for gray bodies, where the emissivity must now be factored in.

10 Note that the units are weird the absorption increases downwards. This is the atmospheric window the low absorption region between 8 and 12 um that permits Earth s IR to escape directly to space.

11 Atmospheric Structure and Radiative Balance

12 Pressure decreases exponentially with altitude This occurs mainly because pressure is determined by the integral of all the air above some altitude, so the pressure is insensitive to local changes.

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14 Temperature structure more complex than pressure structure In contrast to pressure, temperature is determined by the local energy balance. Therefore, local changes will be expressed and thereby cause the temperature structure to be more complex.

15 Dry adiabatic lapse rate is a result of trading gravitational potential energy for internal energy. Adiabatic = no exchange of energy between a system and its environment. Dry = no liquid or solid water is present at any time. [Water vapor can be present.] For a dry air parcel, its energy has two components: 1. Internal energy (temperature of air) 2. Gravitational potential energy If the air parcel rises, it gains (2); if it is adiabatic, then it must lose (1), i.e. it must cool.

16 Moist adiabatic lapse rate is smaller than dry version because of phase change If the dew point temperature of the air is reached, then further increases in altititude and temperature lead to condensation. Condensation releases energy, which causes the cooling rate to be lower. Water condenses into small droplets, thus forming clouds.

17 Temperature structure more complex than pressure structure Temperature increase with height due to absorption of UV by O 2. Temperature decrease with height due to adiabatic lapse rate Temperature increase with height due to absorption of UV by O 3. Temperature decrease with height due to adiabatic lapse rate

18 A simple 1-D radiative balance model illustrates the basic features of the greenhouse effect (see reading) Assumes: 1. Earth radiates as a blackbody 2. an isothermal GHG layer that radiates like a gray body Note: this diagram has an error so don t solve it. [Come talk to me after is you want to know what s wrong.]

19 This is a more sophisticated view of the Earth s energy balance. The real world is more complicated than a simple 1-D model!

20 Global Circulation Differences in temperature lead to winds and global atmospheric circulation. However, the circulation affects patterns in temperature. Hence radiative balance and circulation are coupled.

21 To maintain steady state latitudinally, there must be horizontal transport of energy. Somewhere between 30 and 40 N/S latitude is the transition between net energy export and net energy import.

22 The atmosphere transports at least 2/3 of the energy, with remaining by the ocean. How would Earth s temperature change if the ocean circulation simply shut down? Region of net energy import Region of net energy export Region of net energy import [Note that we believe that if this happened, the atmosphere would take on most of the remaining needed energy transport, so the impact would not be as large as implied here.] The imbalance in energy (last slide) causes atmospheric circulation (winds). The winds transport energy and balance the energy budget. Northward heat transport (PW)

23 The Three-Cell model of global circulation that is driven by the pole-to-equator temperature gradient. Winds are then affected by coriolis.

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25 <show movie one year simulation>

26 These images depict average for Mar Data from CERES satellite.

27 Upward longwave fluxes (top) and shortwave fluxes (bottom) on Feb 26, 2000 (CERES satellite). All data ~ late morning local time.