Radiation Balance

In atmosphere, radiation can be… transmitted absorbed reflected

transmission No change

absorption Energy is transferred to absorber; absorber emits energy Energy emitted in photons (energy bundles)

Hydrogen atom as absorber

Absorption: atom receives energy; electron moves to higher energy level

Emission: electron moves to lower energy level; gives off energy as a photon

Remember... Energy levels emit photons of different wavelength Atmospheric gases selectively absorb and emit only at certain wavelengths. Atmosphere does not absorb ALL of the incoming solar radiation. Remember...

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 (51%) Gases, dust in atmosphere (17%) Clouds (2%) 70% Shortwave!

30% reflected by Ground (5%) Clouds (20%) Scattered by atmosphere (5%) Albedo of earth/atmosphere = 30% shortwave

Shortwave absorption Point out 51 shortwave absorbed by earth; 19 by atmosphere

Because energy is transferred between atmosphere and earth. 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.

117 units (%) of longwave emitted from earth surface to atmosphere 111 absorbed by atmosphere 6 transmitted to space 111 6 117 Earth’s surface: ABSORBS SHORTWAVE, EMITS LONGWAVE

Point out 51 shortwave absorbed by earth; 19 by atmosphere

Agents in atmosphere that absorb longwave : (clouds, water vapor, carbon dioxide, ozone, other greenhouse gases) Their energy level is raised; emit longwave 111 117 6

amount re-emitted (160) exceeds amount absorbed (111) Notice! amount re-emitted (160) exceeds amount absorbed (111) 40 24 H2O,CO2, O3 clouds 111 6 96 117 40 + 24 + 96 = 160 atmosphere surface

Difference between amount emitted and amount absorbed Net radiation Difference between amount emitted and amount absorbed For atmosphere: net longwave radiation = 111 – 160 = - 49 For earth’s surface What is net longwave radiation ?

Difference between amount emitted and amount absorbed Net radiation Difference between amount emitted and amount absorbed For atmosphere: net longwave radiation = 111 – 160 = - 49 For earth’s surface: net longwave radiation = 96 – 117 = - 21

Net all wave radiation includes long and shortwave Atmosphere: Shortwave: Absorbs : how much?

Point out 51 shortwave absorbed by earth; 19 by atmosphere

Net all wave radiation includes long and shortwave Atmosphere: Shortwave: Absorbs : 17 + 2 = 19

Net all wave radiation includes long and shortwave Atmosphere: Shortwave: Absorbs : 17 + 2 = 19 Longwave Loses : net of - 49 Net all-wave deficit for atmosphere is 19 – 49 = - 30

Surface: Net all-wave Shortwave Absorbs (gains): 51 Longwave Emits (losses) : - 21 Net all-wave surplus for surface: 51 – 21 = 30

Point out 51 shortwave absorbed by earth; 19 by atmosphere

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

Looking more closely at 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)

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.

Surplus of 30 units of net all wave at surface 7 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 23 are transferred to the atmosphere as latent heat (evaporation) Evaporation of water makes energy available to atmosphere that otherwise would have warmed surface

Point out 51 shortwave absorbed by earth; 19 by atmosphere

“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

Groundhog Day yesterday!

Quiz 4 1. This represents Incoming shortwave Incoming longwave albedo

Quiz 4 2. This represents Outgoing shortwave from surface Albedo c. Outgoing longwave

Quiz 4 2. These represent Shortwave reflection by the atmosphere and clouds. b. Radiation of longwave by clouds and green- house gases to earth and space.

Quiz 4 3. This represents transfer of surplus heat from surface to atmosphere by the combined processes of : and

Quiz 4 4. This represents transfer of surplus heat from surface to atmosphere by:

Review: How did we get a net all wave surplus of 30 for surface and deficit of 30 for atmosphere? Look at other balances

Enhanced greenhouse effect enhancement of normal greenhouse heating caused by increased concentration of greenhouse gases (carbon dioxide, methane, nitrous oxides, etc)

Differences in insolation* / radiation caused by: 1.Latitude 2.Season 3.Atmospheric obstruction *Insolation = intercepted solar radiation

1.Latitude: Low latitudes: net surplus of radiation High latitudes: net deficit Balanced by circulation of ocean and atmosphere

2.Season: energy per unit area diminishes with sun angle

3.Atmospheric obstruction: clouds and dust optical path length

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