Earth's Energy Balance- 2/27

albedo

-A portion of the incoming solar radiation is absorbed by the surface and a portion is also reflected away. The proportion of light reflected from a surface is the albedo. -The average albedo of Earth is approximately 0.31 (31%)

specific radiation balance

-About 30% of the available solar radiation at the top of the atmosphere is reflected or scattered back to space by particulates and clouds before it reaches the ground. -The gases of the atmosphere are relatively poor absorbers of solar radiation, absorbing only about 20% of what is available at the outer edge of the atmosphere. The remaining solar radiation makes its way to surface as direct and diffuse solar radiation.

net shortwave radiation

-the difference between incoming and outgoing shortwave radiation expressed as: K*= (S+D) - (S+D)a -During the day, K* is a positive value as incoming always exceeds outgoing shortwave radiation. At night, K* is equal to zero as the Sun is below the horizon. -incoming solar radiation: K↓ = (S+D) -reflected solar radiation: K↑ = (S+D)a

dominant components of the atmosphere

4 billion years ago: 80% CO2, 10% nitrogen, 10% water today: 78% nitrogen and 21% oxygen

non-radiative heat transfer

Available net radiation is used to do work in the Earth system. The principal use of this energy is in the phase change of water (latent heat, LE), changing the temperature of the air (sensible heat, H), and subsurface (ground heat, G) or, Q* = H + LE + G

electromagnetic radiation

Electromagnetic radiation travels through space in the form of waves. Unlike heat transfer by convection or conduction, heat transfer by electromagnetic radiation can travel through empty space, requiring no intervening medium to transmit it.

net radiation balance

Gathering all the radiation terms together we have net radiation: Q*= [(S+D) - (S+D)a] + L↓ - L↑ Q*: Net Radiation S: direct solar radiation D: diffuse radiation a: albedo L↓: atmospheric radiation (longwave) L↑: surface radiation (longwave) -Net radiation can be positive, negative, or even zero. Net radiation is a positive value when there is more incoming radiation than outgoing radiation. This typically occurs during the day time when the sun is out and the air temperature is the warmest. At night, net radiation is usually a negative value as there is no incoming solar radiation and net longwave is dominated by the outgoing terrestrial longwave flux. Net radiation is zero when the incoming and outgoing components are in perfect balance, which doesn't occur too often.

shortwave

Shortwave radiation from the Sun penetrates through space to the outer edge of the atmosphere unimpeded by the vacuum of outer space. If one places a surface oriented perpendicular to an incoming beam of light, 1360 W m2 of solar radiation will be received. This value is known as the solar constant but actually varies by a small amount as the Earth-Sun distance changes through the year. When the solar constant is averaged over the entire Earth surface we get a value of 342 W m2. Once solar radiation begins to penetrate through the atmosphere this amount begins to decrease due to absorption and reflection.

greenhouse effect

The amount of energy emitted is primarily dependent on the temperature of the surface. The hotter the surface the more radiant energy it will emit. The gases of the atmosphere are relatively good absorbers of longwave radiation and thus absorb the energy emitted by the Earth's surface. The absorbed radiation is emitted downward toward the surface as longwave atmospheric counter-radiation (L↓) keeping near surface temperatures warmer than they would be without this blanket of gases. This is known as the "greenhouse effect".

net longwave radiation

The difference between incoming and outgoing longwave radiation is net longwave radiation expressed as: L* = L↓ - L↑

longwave radiation

The energy absorbed at the surface is radiated by the Earth as terrestrial longwave radiation (L↑).

maximum wavelength- function of temperature

The maximum wavelength at which a body emits radiation depends on its temperature. Wein's (pronounced "weens") Law states that the peak wavelength of radiation emission is inversely related to the temperature of the emitting body. -That is, the hotter the body, the shorter the wavelength of peak emission. The figure below shows the wavelengths over which the sun and earth emit most of their radiation. The Sun being a much hotter body emits most of its radiation in the shortwave end and the Earth in the longwave end of the spectrum.

energy carried in a wave

The quantity of energy carried in a wave is associated with the height or amplitude of the wave. Everything else being equal, the amount of energy carried in a wave is directly proportional to the amplitude of the wave. The type or "quality" of radiation depends on the wavelength, the distance between successive crests. The greater the distance between wave crests, the longer the wavelength.

radiation balance

The radiation balance of the Earth system is an accounting of the incoming and outgoing components of radiation. These components are balanced (incoming = outgoing) over long time periods and over the Earth as whole. If they weren't the Earth would be continually cooling or warming. However, over a short period of time, radiant energy is unequally distributed over the Earth, resulting in periods of localized heating and cooling.

subsurface heat (ground)

The third major use of radiant energy is to warm the subsurface of the Earth. Heat is transferred from the surface downwards via conduction. Like in the case of sensible heat transfer, a temperature gradient must exist between the surface and the subsurface for heat transfer to occur. Heat is transferred downwards when the surface is warmer than the subsurface (positive ground heat flux). If the subsurface is warmer than the surface then heat is transferred upwards (negative ground heat flux).

total incoming solar radiation or insolation

Together, direct and diffuse shortwave radiation accounts for the total incoming solar radiation or insolation (K↓) K↓=S+D

greenhouse gases

Water vapor and carbon dioxide do not significantly absorb solar (short wave) radiation, however, they do readily absorb long wave (thermal) radiation. In this way, H2O and CO2 in the atmosphere acts much like the glass on a greenhouse. These gases allow the sunlight (solar radiation) to enter and warm the surface. However, they act to trap the outgoing heat, resulting in rising air temperatures. These gases are termed "greenhouse gases" and the absorption of outgoing longwave radiation by these gases is referred to as the Greenhouse Effect.

conduction/convection- transfer from surface (land and water) to atmosphere

We feel the transfer of sensible heat as a rise or fall in the temperature of the air. Heat is initially transferred into the air by conduction as air molecules collide with those of the surface. As the air warms it circulates upwards via convection. Thus the transfer of sensible heat is accomplished in a two-step process. Because air is such a poor conductor of heat, it is convection that is the most efficient way of transferring sensible heat into the air.

thermals (convective processes)

a column of rising air in the lower altitudes of the Earth's atmosphere. Thermals are created by the uneven heating of the Earth's surface from solar radiation, and an example of convection. The Sun warms the ground, which in turn warms the air directly above it.

atmosphere

a layer of gases surrounding the planet that is retained by Earth's gravity

sensible heat

heat energy transferred between the surface and air when there is a difference in temperature between them.

temperature/heat

is proportional to the average speed of the atoms or molecules in an object (matter) -for a constant volume, P1/T1 = P2/T2

direct solar radiation

shortwave radiation able to penetrate through the atmosphere without having been affected by constituents of the atmosphere in any way.

diffuse radiation

shortwave radiation that has been scattered by gases in the atmosphere. Scattering is a process whereby a beam of radiation is broken down into many weaker rays redirected in other directions

specific heat (relative values for water and air)

the amount of energy needed to raise the temperature of 1 gram of material by 1oC. -Specific heat is different for every substance, a function of the molecular structure.

latent heat (changes in energy state during evaporation/condensation)

the energy released or absorbed when a substance changes from one state to another (evaporation, condensation and freezing of water).