Lecture 1: Global radiation budget

Global energy budget

○ Accounts for how much energy comes into the Earth's climate system from the Sun, how much energy is lost to space, and how much energy remains on Earth and in the atmosphere.

Shortwave radiation:

○ Is radiant energy with wavelengths in the visible, near ultra-violet, and near-infrared spectra. ○ The sun mainly radiates shortwave radiation - visible light, and small amounts of ultraviolet and infrared radiation.

Natural greenhouse effect:

○ Much of the incoming, short wavelength, solar radiation penetrates the atmosphere and heats Earth's surface. ○ Objects on Earth's surface emit long wavelength radiation skyward. - Greenhouse gases are gas in atmosphere that absorb and emit radiation within the thermal infrared range; water vapour, CO2, methane, ozone etc. ○ Greenhouse gases absorb outgoing, long wavelength radiation and reradiate some of this energy Earthward, thus trapping heat in the lower atmosphere. ○ Note: Longwave is absorbed and reradiated.

Atmospheric windows

○ Portion of the electromagnetic spectrum that can be transmitted through the atmosphere without any distortion or absorption. ○ From 8-10wavelength, radiation from surface of planet can escape to space.

Planetary albedo:

○ Reflectivity of a planet ○ Total = 30%, of which 20% from clouds, 6% from atmosphere and 4% from surface.

Electro-magnetic spectrum:

○ The range of all possible frequencies of electromagnetic radiation, characterised by wavelength. ○ The electro-magnetic spectrum of an object is the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object. ○ Hotter objects have more radiation and shorter wavelengths

Watt

A derived unit of power. ○ Newton = Force to accelerate 1kg at 1 m/s ○ Joule = Force of 1N through 1m ○ Watt = 1J per second. (energy flux) ○ 1 W/m2 (Energy flux density - flow per unit of area).

Diffuse & direct shortwave radiation

Diffuse: Indirect radiation to earth's surface from clouds and sky after having been scattered from the direct solar beam by molecules in atmosphere. Direct: Direct radiation from sun to earth's surface

Longwave radiation:

Is the energy radiating from the Earth as infrared radiation (shortwave, middle or thermal infrared) at low energy to space. Typically cooler objects emit longer wavelengths of electromagnetic radiation.

Albedo:

Is the proportion of the incident light or radiation that is reflected by a surface; Shortwave radiation reflectivity (%) ○ Determines incident shortwave radiation available at the surface. ○ Fresh snow & clouds have the highest. Forests & water have the lowest.

Solar constant

○ 1361 W m-2 - outside atmosphere Is a measure of energy glux per spatial area; the mean amount of incoming solar radiation (shortwave) per unit area measured on the outer surface of Earth's atmosphere in a plane perpendicular to the rays. It is not physically constant, it varies in value as Earth orbits.

What are the key radiation components of the global energy budget and what determines their relative sizes?

YELLOW ARROW: Shortwave radiation Represents the incoming shortwave solar energy from the sun. The Sun emits shortwave radiation; radiant energy with wavelengths in the visible, near ultra-violet, and near infrared spectra. This is because the sun is extremely hot, and there is an inverse relationship between range of wavelengths of the radiation that an object emits, and the temperature of the object. Solar radiation spreads apart as it moves further from the sun. The farther a planet from the Sun, the lower amount of radiation it receives. As this energy moves towards Earth, a portion of it is reflected before it reaches the Earth's surface, due to the high albedo of clouds. Thus reflection will be most prominent on an overcast day, but less significant on a fine day. A small portion of solar radiation may also be reflected from the Earth's surface depending on the weather conditions. A smaller portion of shortwave solar radiation is absorbed by the Earth's atmosphere; molecules such as water vapour and CO2, or the ozone layer intercept and absorb radiation at particular wavelengths. A portion of shortwave radiation will reach the Earth's surface and be absorbed. This depends on the weather conditions - on a fine day, greater, on an overcast, smaller due to reflection of clouds. ORANGE ARROW: Longwave radiation Represents the longwave radiation being emitted by the Earth. This is as cooler objects emit radiation with longer wavelengths. Atmosphere, land and ocean emit energy in the form of longwave radiation. A small amount may leave Earth and go into space due to the narrow atmospheric window. Radiation being absorbed by water vapour. When this vapour condenses into clouds, the energy is released into the atmosphere. Much of the incoming, short wavelength, solar radiation penetrates the atmosphere and heats Earth's surface. Objects on Earth's surface emit long wavelength radiation skyward. Greenhouse gases absorb outgoing, long wavelength radiation and reradiate some of this energy Earthward, thus trapping heat in the lower atmosphere. Size depends on amount of heat being radiated from objects on Earth, and amount of gases in atmosphere available to absorb the longwave radiation. Ultimately, these flow balances -incoming energy absorbed and outgoing radiation emitted are equal. Since the temperature of a surface is determined by the amount of energy it absorbs and emits, the Earth's overall temperature tends to remain constant.