Intercepted Solar Irradiance

What happens to the sun’s rays that hit the Earth’s terrestrial disk?

Since the sun is at a very large distance away they are practically parallel to one another.

Therefore the projection from the sun’s rays are?

a circular disk of radius equal to the Earth’s radius.

At the summer solstice that “disk” is:

tangent to the tropic of cancer

At the winter solstice that “disk” is:

tangent to the tropic of Capricorn.

Earth’s radiative equilibrium describes

that to maintain temperature constant there is no energy gain or loss due to radiation

What happens to solar radiation once it enter at the TOA of the atmosphere?

transmission, reflection, scattering and/or absorption

Outcome: Transmission

EM wave passes straight through medium

Outcome: scattering

deflection of light in all directions by air molecules, small particles, droplets etc.

Outcome: absorption

EM radiation absorbed by air molecules, aerosol particles or cloud droplets

Outcome: reflection

light sent backward by an interface between two media, such as air and water.

Detail of reflection

specular (mirror effect) or diffuse (reflection in many directions — scattering)

What happens to EM waves once they hit a terrestrial body?

they may be reflected back to space

Albedo:

represents reflectivity of a surface to shortwave radiation… gives a ratio alpha

What does this albedo value depend on?

Depends on type of surface and on angle of solar irradiance

What is the albedo of the planet earth at the top of the atmosphere?

About 30%

What is the solar irradiance budget?

balance between the amount of energy the earth receives from the sun and the amount it emits back into space

Solar irradiance budget distribution:

~30% reflected/scattered back to space, ~19% absorbed by air molecules and clouds, ~ 51% absorbed by surface

Radiative equilibrium requirement considering the terrestrial system’s mean albedo α where:

α x E_TOA is reflected back to space and (1-α) x E_TOA enters the terrestrial system

What does there no energy gain or loss due to radiation do to Earth’s temp?

It ensures that the average temperature of Earth does not change over time, this means that the outgoing and incoming amount of radiant flux must be equal.

Earth’s Radiative Equilibrium Temperature (T_e)

temperature that the black body Earth acquires for equal amounts of incoming and outgoing radiation

Emitted Infrared Radiation (earth)

Intercepted Solar (earth)

What is the total intercepted solar radiant flux by a “disk” of radius R at the TOA?

given by: S₀ x (πR²)

The solar radiant flux (S_o) is absorbed by what?

absorbed per 1m² by the circular disk… which is the projection of the sunlit hemisphere on a plane perpendicular to the sun rays

A black body emits σT_e⁴ W from every:

1m², meaning that a spherical surface of radius R_e will emit as a black body a total radiant flux of σT_e⁴ x (4π)W

Note*, that the emission to space

is from both hemispheres (sunlit and dark)

For radiative equilibrium we need:

mean absorbed radiant flux = mean emitted radiant flux

Does the atmosphere behave like a black body?

No, unlike the Earth’s surface which does. the atmosphere allows reflection, scattering, transmission (certain wavelengths) and absorption (certain wavelengths of EM waves

mirror effect

reflection back at a specific angle