Atmospheric Radiation

NASA photo gallery

Introduction The major source of earth is the sun. The sun transfer energy through the earth by radiated electromagnetic wave. In vacuum, electromagnetic waves travel at speed of light (300,000 km/s). The continuous electromagnetic spectrum distinguishes different types of waves based on wavelength and frequency.

Heat Transport in Atmosphere Transport of heat is accomplished by conduction, convection and radiation radiation: energy transfer which can take place in a vacuum, with no intervening physical medium (unlike conduction and convection) conduction: transfer of kinetic energy of molecules by collisions of molecules convection: transport of heat within a fluid by bulk motion of fluid itself heat transported by conduction and convection is "sensible heating"

Solar Insolation the solar insolation is the amount of solar radiation received at the earth's surface earth is closest to sun (147 million km) on January 3 (perihelion), and farthest from sun (152 million km) on July 4 (aphelion)

As the earth revolves about the sun, it is tilted on its axis by an angle of 23.5?. The earth's axis always points to the same area in space (as viewed from a distant star). Thus, in June, when the Northern Hemisphere is tipped toward the sun, more direct sunlight and long hours of daylight cause warmer weather than in December, when the Northern Hemisphere is tipped away from the sun. (Diagram, or course, is not to scale.)

Annual Heat Imbalances

General Circulation Model

Basic Concepts The subject of atmospheric radiation is concern with the transfer energy within the atmosphere by photons, or equivalently by electromagnetic wave. The relevant photon fall into two classes Solar Photon (Shortwave) 0.1-4.0 m? Ultraviolet? Visible? infrared Emitted by the sun Thermal Photon (Longwave) 4.0 100 m? Infrared Emitted by Earth s surface (Terrestrial) or Atmosphere

1. Planck s Law A black body (a theoretical construct) is define as body that complete absorbs all radiation falling on its. Sun and atmosphere-earth systems are not perfect black bodies, but we can still apply black body radiation laws with useful results

2. Stefan-Boltzmann law Rate at which object radiates heat is proportional to fourth power of its temperature intensity of radiation = (constant) x (temperature) 4 Thus intensity of radiation emitted from solar photosphere (temperature 5800 K) is many times that from Earth's surface (temperature 15 o C or 288 K) [increase is a factor of 160,000 = (5800/288) 4 per unit area]

3. Wien s Law As temperature of an object increases, wavelength of most intense radiation emitted decreases wavelength of maximum intensity (microns) = 3000/T (wavelength in microns, a millionth of a meter; temperature in degrees K) For example, a heated piece of iron glows dull red when it gets hot, and changes to orange and eventually to white as it gets hotter, i.e., visible radiation of decreasing wavelength

Ultraviolet Visible Infrared radiance E b (W/m 2 sr m) Earth Average of earth surface temperature is 288 K. The Sun ( m) Logarithm of the black-body spectral radiance (the amount of light that passes through or is emitted from a particular area, and falls within a given solid angle in a specified direction) E (T) and for each temperature

Irradiance (W/m 2 ) 6000 K 288 K The black body irradiance plots against the logarithm of wavelength for T = 6000 K and 288 K

Solar radiation solar radiation is emitted from solar photosphere (surface of sun), at a temperature of 5800 K it is most intense at a wavelength of about 0.5 microns (micrometer), which lies in the visible part of the electromagnetic spectrum also known as shortwave radiation

Terrestrial radiation terrestrial radiation is emitted from surface of Earth, at a temperature of 15 o C or 288 K it is most intense at wavelength of about 10 microns, which lies in the invisible, infrared part of the spectrum also known as longwave radiation

Atmospheric Influence on Radiation

Atmospheric Influence on Radiation Upon entering the atmosphere the incoming solar radiation encounter atmospheric gases and aerosol. Gasses Atoms Molecules Particles (Dust, Large molecule, etc) Clouds These constitutes of the atmosphere influence how much and where solar radiaiton ultimately makes it to the earth s surface

Atmospheric Influence on Radiation Some Radiation is Scattering Some Radiation is Transmitted Some Radiation is Absorbed

Atmospheric Scattering Scattering is the process where an atom, molecule, or particle redirects energy. Incident Radiation Scattered Radiation

Types of Scattering Rayleigh Scattering Mie Scattering Non-selective Scattering Atmospheric atom and small molecule Aerosol Cloud

Rayleigh Scattering Atmospheric gas or small molecule scatter radiation by a process known as Rayleigh Scattering Not all wavelengths of radiation are scattered equally; In visible part of the spectrum, shorter wavelength is scattered preferentially than longer wavelength

Rayleigh Scattering Raman scattering is the scattering in which the wavelength (or photon energy) of the scattered light is different from the wavelength of the incident light. In the process energy from the incident photons is transferred to the molecule, leaving the molecule in a higher vibrational state and the photon at lower energy. Scattering of a photon off a molecule in a higher vibrational state will result in scattered photons with higher energy and a molecule in a lower vibrational state h h

Rayleigh Scattering Rayleigh scattering redirects the radiation about equal in all directions. Rayleigh scattering affects about 10% of all incoming solar radiation. Rayleigh scattering is responsible for Blue skies and Red Sunsets

Rayleigh Scattering

Mie Scattering Mie scattering is scattering due to particles that are larger than the wavelength of the incident light. Mie scattering is not (or hardly) wavelength dependent. Scattered light will look white or light bluish, depending on the size of the scattering particles.

Mie Scattering Predominantly scatters light (radiation) in the forward direction. For example, the solar radiation is typically scattered towards the surface and not back to space.

Non-selective Scattering Light passing through clouds is an excellent example of nonselective scattering Water droplet are curved and behave like lenses that bend (reflect) light. The size of the water droplets in the atmosphere is important because the curvature of the water droplet changes with size so the scattering changes.

Transmission Transmission is the unimpeded, direct passage of light (radiation) through the atmosphere. On sunny day, in the absence of pollution and water vapor, as much as 80% of the solar radiation may transmitted to the surface. The transmission of solar radiation decreases with increasing clouds, water vapor, and aerosol. On cloudy day, the transmission of solar radiation may approach 0%.

Absorption Any material which absorbs solar energy and reduces the amount of solar radiation that reaches the surface. The molecular collision is always occurred in the atmosphere. The collision are likely to occur before reemission take place, leading to transfer of the excitation energy to other form of energy.

Absorption The atmosphere does not absorb radiation the same for all wavelengths. Solar radiation (shortwave radiaiton) pass through quite easily Most of the high-energy UV radiation is almost completely absorbed by Stratospheric Ozone Water vapor and Carbon Dioxide absorb near infrared radiation

500 nm. or 0.5 m; green color Solar Radiation Terrestrial Radiation

Atmospheric absorptivity Major of atmospheric composition Atmospheric window for infrared radiation Atmospheric window for solar radiation. Therefore, earth atmosphere is transparent for solar radiation Solar Radiation Terrestrial Radiation wavelength

Albedo

Albedo The albedo of an object is the extent to which it reflects light, defined as the ratio of reflected to incident electromagnetic radiation. Albedo = Amount of Reflected Amount of Incoming Average of Earth s albedo = 30%

Percentage of reflected sun light in relation to various surface conditions of the earth

Albedo Cloud albedo 20% Atmospheric albedo 4% Albedo is reflectivity of an object for incident sunlight Earth surface albedo is around 30% Earth surface albedo 6%

A simple radiative model

A simple radiative model The incident flux of solar energy at the Earth s mean distance from the sun is F s =1370 Wm -2 Earth cross section area = a 2 Average albedo (A) = 0.3 Incoming solar energy = (1-A) F s a 2

A simple radiative model According to Stefan-Boltzmann law, power emit per unit area = T 4 Power emit in all directions from total surface area = 4 a 2 Total power emit = 4 a 2 T 4

A simple radiative model Assume earth is black body then Energy incoming = Energy outgoing (1-A) F s a 2 = 4 a 2 T 4 In this condition, T 255 K It is lower than the observed mean surface temperature; 288 K What happen?

A simple model of the greenhouse effect

A simple model of the greenhouse effect In natural condition, layer of atmosphere exist. Temperature of atmosphere is assume to be uniform T a The atmosphere transmit fraction of any incident solar radiation s and fraction of thermal radiation t What happen for the non-transmit radiation? The top of atmosphere irradiance (F o ) is F o =? (1 - A)F s F o 240 W/m 2 Solar radiation reach earth surface is s F o

A simple model of the greenhouse effect The ground emit an upward flux F g = T g 4 Upward flux reach top of atmosphere is Atmosphere is not a block body, but emit radiation both upward and downward Atmospheric irradiance is F a = (1 - t ) T a 4

A simple model of the greenhouse effect Balance energy Therefore, at top of atmosphere, Incident solar radiation at top of atmosphere = atmospheric radiation + thermal radiation reach top of atmosphere F o = F a + t F g At earth surface, thermal radiation = solar radiation reach surface + atmospheric radiation F g = F a + s F o

A simple model of the greenhouse effect F g = T g4 = F o 1 + s 1 + t For rough estimate, s 0.9 and t 0.2 So F g = 1.6Fo, leading to surface temperature of T g 286 K, which is close to the mean observed temperature 288 K

A simple model of the greenhouse effect Atmospheric radiation can find from F a = (1 - t ) T a4 = F o 1 + s 1 + t Temperature at the shallow atmosphere in this model, T a 245 K

2 5Wm 237 102 342 3 Ramanathan, Barkstrom and Harrison, Phys Today, 1989

Summary of Atmospheric Radiation

Global Warming Is it really man-made?

Vostok

Keeling Curve Dr. Charles Keeling (left) The rise of carbon dioxide as measured by Charles D. Keeling and collaborators on the top of Mauna Loa. The unit?ppm? stands for?parts per million by volume.? The rise results from burning of fossil fuels, with some contribution from deforestation and production of cement.

Aerosol and Atmospheric Radiation

Aerosol Radiative forcing (1) Direct forcing: absorption or scattering of solar radiation Semi-direct effect: Black carbon absorbs solar radiation warming the atmosphere and cooling the surface This stabilizes the atmosphere Suppressing clouds and convection The variation in cloud cover and convection then affects the radiation budget.

Aerosol Radiative forcing (2) Same volume, more surface area Cloud drop Indirect forcing Effect of aerosols on the optical properties and lifetime of clouds Aerosols act as CCN More aerosols more cloud droplets With fixed water vapor smaller cloud droplets More smaller droplets brighter clouds and longer-lived clouds