Radiation Balance

In atmosphere, radiation can be… transmitted absorbed reflected

1. transmission Radiation passes through atmosphere unimpeded. Shortwave and longwave “windows”

2. absorption Energy is transferred to absorber; absorber emits energy Energy emitted in photons (energy bundles). Each level/orbit represents different amount of energy. Atmospheric gases selectively absorb and emit only at certain wavelengths. Shells or orbits correspond to energy levels; when an atom receives E, electron moves to a different E level; when it goes back to previous state, it gives off energy Photons: discrete bundles of energy

3. reflection Energy re-directed; not absorbed Our eyes detect reflected visible wavelengths. Albedo is the reflective quality of a surface Percent of incoming radiation reflected

Earth’s average albedo, March

Albedo is an important variable in global climate change “A drop of as little as 0.01 in Earth’s albedo would have a major warming influence on climate—roughly equal to the effect of doubling the amount of carbon dioxide in the atmosphere, which would cause Earth to retain an additional 3.4 Wm-2 ”.

Albedos of various surfaces: Earth’s surface 0.31 (31%) Cumulonimbus clouds 0.9 (90%) Stratocumulus clouds 0.6 (60%) Cirrus clouds 0.4 -0.5 (40 – 50%) Fresh snow 0.8 – 0.9 (80 – 90%) Melting snow 0.4 – 0.6 (40 – 60%) Sand 0.3 – 0.35 (30 – 35%) Grain crops 0.18 – 0.25 (18 – 25%) Deciduous forest 0.15 – 0.18 (15 – 18%) Coniferous forest 0.09 – 0.15 (9 – 15%) Tropical rainforest 0.07 – 0.15 (7 – 15%) Water bodies 0.06 – 0.10 (6 – 10%) increases at low sun angles

Scattering / diffuse radiation A form of reflection

Types of scattering: Rayleigh Mie Nonselective

1. Rayleigh Happens when diameter of gas is < 1/10th diameter of wavelength of incoming radiation favors smaller wavelengths Scatters forward and back

Longer path through atmosphere at decreasing angle of sun; other wavelengths have been scattered away; leaving long wavelengths (red)

Optical path at point of tangency is 20 x as long as at SSP.

2. Mie Caused by aerosols: Scatter forward particles in atmosphere microscopic but larger than gas molecules (pollen, dust, smoke, small water droplets ) Scatter forward Do not favor short wavelengths; scatter all visible wavelengths

Pollution: high aerosol content Grey sky : aerosols scatter entire visible range towards surface

3. nonselective No wavelength preference; particles much larger than wavelength Big water droplets; large dust particles E.g., fog and clouds reflect all wavelengths of light, appear white or grey

Radiation Balance Balance maintained by earth and atmosphere between incoming and outgoing radiation Mention that this is averaged over entire globe over entire year.

Imagine shortwave solar radiation entering the top of the atmosphere as total we start with. 70% is absorbed by earth/atmosphere 30% is reflected by earth/atmosphere (albedo = 30%) “100%” incoming

70% absorbed by: Ground (47%) Gases, dust in atmosphere Clouds (23%) Shortwave!

30% reflected by Ground (7%) Clouds (17%) Scattered by atmosphere (6%) Albedo of earth/atmosphere = 30% Shortwave!

23 Shortwave absorption Point out 47 shortwave absorbed by earth; 23 by atmosphere 47

Earth absorbs far more solar radiation than atmosphere. Consider absorption: Earth absorbs far more solar radiation than atmosphere.

Why aren’t we boiling up? Because energy is transferred between atmosphere and earth. Shortwave solar radiation is absorbed and longwave radiation is emitted. Earth’s surface emits 116 % longwave !

116 units (%) of longwave emitted from earth surface to atmosphere 104 absorbed by atmosphere 12 transmitted to space 12 Space 104 Atmosphere 116 Earth’s surface: ABSORBS SHORTWAVE, EMITS LONGWAVE Surface

Longwave emission from earth 12 104 23 116 47

Agents in atmosphere that absorb longwave : (clouds, water vapor, carbon dioxide, ozone, other greenhouse gases) Their energy level is raised; emit longwave 12 Space 104 Atmosphere 116 Earth’s surface: ABSORBS SHORTWAVE, EMITS LONGWAVE Surface

Atmosphere (clouds, water vapor, greenhouse gases) absorb and emit longwave: 98 emitted back to earth 58 emitted to space 58 Space Atmosphere Surface 98 Notice: Amount re-emitted (98 + 58) exceeds amount absorbed (104)

12 Longwave emission from atmosphere 104 23 58 98 116 47

Net radiation Difference between amount emitted and amount absorbed. Shortwave 1. For atmosphere: Net shortwave radiation = amount emitted ? None! amount absorbed ?

23 Shortwave absorption Point out 47 shortwave absorbed by earth; 23 by atmosphere 47

Net radiation Difference between amount emitted and amount absorbed. Shortwave 1. For atmosphere: Net shortwave radiation = amount emitted ? None! amount absorbed = 23 Net shortwave radiation for atmosphere = + 23

Net radiation Shortwave 2. For surface: Net shortwave radiation = amount emitted ? None! amount absorbed ?

23 Shortwave absorption Point out 47 shortwave absorbed by earth; 23 by atmosphere 47

Net radiation Shortwave 2. For surface: Net shortwave radiation = amount emitted ? None! amount absorbed = 47 Net shortwave radiation for surface = + 47

Net radiation B. Longwave 1. For atmosphere: Net longwave radiation = amount emitted ?

12 Longwave emission from atmosphere 104 23 58 98 116 47

Net radiation Difference between amount emitted and amount absorbed. B. Longwave 1. For atmosphere: Net longwave radiation = amount emitted : -58 + -98 = -156 amount absorbed ?

Longwave emission from earth 12 104 23 116 47

Net radiation Difference between amount emitted and amount absorbed. B. Longwave 1. For atmosphere: Net longwave radiation = amount emitted : -58 + -98 = -156 amount absorbed : +104 Net longwave radiation for atmosphere = 104 – 156 = -52

Net radiation B. Longwave 2. For surface: Net longwave radiation = amount emitted ?

Longwave emission from earth 12 104 23 116 47

Net radiation B. Longwave 2. For surface: Net longwave radiation = amount emitted : - 116 amount absorbed ?

12 Longwave emission from atmosphere 104 23 58 98 116 47

Net radiation B. Longwave 2. For surface: Net longwave radiation = amount emitted : -116 amount absorbed : +98 Net longwave radiation for surface = 98 – 116 = -18

Net all wave radiation includes long and shortwave. Atmosphere: + 23 - 52 = - 29 Surface : + 47 - 18 = + 29

The balancing act….. The net deficit of the atmosphere equals the net surplus of the earth’s surface.

But, there’s more… If radiation were only means of transferring energy, our feet would scorch and our heads would freeze.

Energy transfer mechanisms (other than radiation): Conduction: transfer of heat from one molecule to another by collision Only a few cm of air above surface are heated by conduction Convection : transfer of heat from one area to another by physical mixing Warm air near surface transfers heat upward by mixing

conduction and convection… Temperature gradient in upper few centimeters of soil Energy conducted downward during day; upward at night Temperature gradient in laminar boundary layer of air (few mm. thick) A third process transferring heat from surface to atmosphere is EVAPORATION, a latent heat transfer mechanism.

Sensible Heat: can be sensed with thermometer. Types of heat energy: Sensible Heat: can be sensed with thermometer. Latent Heat : heat released or absorbed during phase changes (solid-liquid-gas) Energy used to change phase is not lost energy evaporating water is “held” in water vapor to be released in reverse process Some of energy received at surface is used to evaporate water rather than to raise surface temp. even more.

24 5

Surplus of 29 units of net all wave at surface 5 are transferred to atmosphere by sensible heat transfer of conduction and convection Sensible heat travels by conduction through laminar boundary layer and is dispersed upward by convection 24 are transferred to the atmosphere as latent heat (evaporation) Evaporation of water makes energy available to atmosphere that otherwise would have warmed surface

“Natural” Greenhouse effect Maintains earth’s mean surface temperature at 59°F (15°C) Otherwise -4°F (-20°C) Caused by counterradiation: greenhouse gases in atmosphere absorb longwave emitted from earth’s surface, some of which is radiated back to earth.

Greenhouse analogy Glass of greenhouse allows shortwave radiation IN, but does not allow escape of longwave BUT…UNLIKE the atmosphere, a greenhouse prevents loss of heat by convection