Lecture 2 Remote Sensing: Radiation Theory and Solar

Radiation

Professor Menglin S. Jin

Department of Meteorology

San Jose State University

• How much energy is emitted by some medium?

• What “kind” of energy (what frequency/wavelength) is emitted by some medium?

• What happens to radiation (energy) as it travels from the “target” (e.g., ground, cloud...) to the satellite’s sensor?

Brief history

• Since the 1960s, most remote sensing has been conducted from satellites

• Prior to that remote sensing is associated mainly with aerial photography, using cameras mounted in aircraft that fly at various altitudes (with scale emcompassed)

• Aircraft remote sensing continues through today but is usually directed towards specific tasks and missions.

"Remote" and "Proximal" Sensing

• “Remote” sensing involves making measurements and collecting data for (and from) objects, classes, and materials that are not in contact with the sensor (sensing device) whereas the “Proximal” sensing includes making direct contact with these targets

if the objective is to measure a person's bodily temperature

• the proximate approach would be

• the remote approach would be

• Both need “Calibrated”

to place a thermometer in or on the body

to hold a radiometer sensitive to thermal energy at some distance from the body

its response as a sensor must be transformable into a good approximation of the actual temperature by determining the response using a target whose temperature range is specifically known.

Passive and Active Remote Sensors

• Remote sensing systems which measure energy that is naturally available are called Passive Sensors. (Sun, surface emission, etc)

• Active sensors, on the other hand, transmit short bursts or 'pulses' of electromagnetic energy in the direction of interest and record the origin and strength of the backscatter received from objects within the system's field of view. Passive systems sense low level microwave radiation given off by all objects in the natural environment.

Example of passive and active remote sensing

In this figure, find out passive and active remote sensing environment

diagram for remote sensing for surafce (1) –solar radiation

diagram for remote sensing for (2) – longwave emission

Electromagnetic Spectrum

• Electromagnetic radiation can be described in terms of a stream of photons, which are massless particles each traveling in a wave-like pattern and moving at the speed of light. Each photon contains a certain amount (or bundle) of energy, and all electromagnetic radiation consists of these photons.

Electromagnetic SpectrumRemote sensing relies on measurements in the

electromagnetic spectrum (except sonar)• Remote sensing of the ground from space

• Need to see through the atmosphere• The ground must have some feature of interest in that

spectral region• Studying reflected light requires a spectral region where solar

energy dominates

• Radar approaches mean we need frequencies that we can generate • Also need to ensure that we are not affected by other radio sources

• Atmosphere should be transparent at the selected frequency

• The wavelengths we are most interested in for climatology and meteorology are between 0.01 and 100 μm

Need to know

• Solar constant

• solar radiaiton at TOA

• TOA radiation budget

• Basic definition

Measuring energy: (Important!)• Radiant energy: Total energy emitted in all

directions (J)• Radiant flux: Total energy radiated in all

directions per unit time (W = J/s)• Irradiance (radiant flux density): Total energy

radiated onto (or from) a unit area in a unit time (W m-2)

• Radiance: Irradiance within a given angle of observation (W m-2 sr-1)

• Spectral radiance: Radiance for range in

Radiance

Toward satellite

Solid angle, measured in steradians(1 sphere = 4 sr = 12.57 sr)

Normalto surface

Radiance is what satellite sensor can measure, but in wavelength

Blackbody radiation

• Examine relationships between temperature, wavelength and energy emitted

• Blackbody: A “perfect” emitter and absorber of radiation... does not exist

Stefan-Boltzmann Law

M BB = T 4

Total irradianceemitted by a blackbody

(sometimes indicated as E*)

Stefan-Boltzmann constant

The amount of radiation emitted by a blackbody is proportional to the fourth power of its temperature

Sun is 16 times hotter than Earth but gives off 160,000 times as much radiation

Planck’s Function

• Blackbody doesn't emit equal amounts of radiation at all wavelengths

• Most of the energy is radiated within a relatively narrow band of wavelengths.

• The exact amount of energy emitted at a particular wavelength lambda is given by the Planck function:

Planck’s function

B (T) = c1-5

exp (c2 / T ) -1

Irridance:Blackbody radiative fluxfor a single wavelength at temperature T (W m-2 m-1)

Second radiation constantAbsolute temperature

First radiation constant Wavelength of radiation

Total amount of radiation emitted by a blackbody is a function of its temperaturec1 = 1.19x10-16 W m-2 sr-1

c2 = 1.44x10-2 m K

Planck curve

Wein’s Displacement Law

mT = 2897.9 m K

Gives the wavelength of the maximum emission of a blackbody, which is inversely proportional to its temperature

Earth @ 300K: ~10 mSun @ 6000K: ~0.5 m

Rayleigh-Jeans Approximation

B (T) = (c1 / c2) -4 T

When is this valid: 1. For temperatures encountered on Earth 2. For millimeter and centimeter wavelengthsAt microwave wavelengths, the amount of radiation emitted is directly proportional to T... not T4

(c1 / c2) -4

Brightness temperature (TB) is often used for microwave and infrared satellite data, where it is called equivalent blackbody temperature. The brightness temperature is equal to the actual temperature times the emissivity.

B (T)TB =

Emissivity and Kirchoff’s Law

Actual irradiance bya non-blackbodyat wavelength

Emittance: Often referred to as emissivity

Emissivity is a function of the wavelength of radiation and the viewing angle and) is the ratio of energy radiated by the material to energy radiated by a black body at the same temperature

absorbed/ incident

Absorptivity (r , reflectivity; t , transmissivity)

Solar spectrum composition

• The spectrum of the Sun's solar radiation is close to that of a black body with a temperature of about 5,800 K.

• The Sun does, however, emit X-rays, ultraviolet, visible light, infrared, and even Radio waves

• UV – 0.1-0.4 μm

visible – 0.4-0.7 μm (namely so called light)

infrared – 0.7 μm – 1mm

Intensity and Wavelength of Emitted Radiation : Earth and Sun

Atmosphere Window

window

Solar constant

• The solar constant is defined as the quantity of solar energy (W/m²) at normal incidence outside the atmosphere (extraterrestrial) at the mean sun-earth distance. Its mean value is 1367.7 W/m². The spectral distribution is given in the figure.

The solar constant includes all wavelengths of solar electromagnetic radiation, not just the visible light

Question: How to calculate solar radiation?Assuming sun’s surface temperature is 5780K, Average distance between Sun-Earth is 1.5x108 km mean Sun radius is 7x105 km.

a. Energy from the Sun (E) Using the Stefan-Boltzmann law, calculate t the average irradiance of the sun.

b. Reverse law

The inverse square law is used to calculate this constant:

So = E(sun) x (R(sun)/r)2

How to calculate solar radiaiton at TOA?

The Earth receives a total amount of radiation determined by its cross section (π·RE²), but as it rotates this energy is distributed across the entire surface area (4·π·RE²).

Hence the average incoming solar radiation is one-fourth the solar constant (approximately 342 W/m²)

Is solar radiation at TOA a real constant?

Does this value vary with latitude and season, and local hour?

Answer: At any given moment, the amount of solar radiation received at a location on the Earth's surface depends on the state of the atmosphere and the location's latitude.

Solar Zenith Angle (important)

The angle between the local zenith and the line of sight to the sun.

the extraterrestrial solar illuminance (Eext), corrected for the elliptical orbit by using the day number of the year (dn), is given by

See handout

correction for the elliptical orbit - declination

http://www.srrb.noaa.gov/highlights/sunrise/glossary.html

Declination is analogous to latitude on Earth's surface, and measures an angular displacement north or south from the projection of Earth's equator on the celestial sphere to the location of a celestial body. See Celestial Sphere Figure.

Sun Spot numbers

solar energy incident to earth

• the "solar constant" images above, from a variety of calibrated satellite instruments aboard SOHO.

• SOHO was launched in December 1995 by

an Atlas Centaur rocket and became operational in March 1996. SOHO weighs about two tons and with its solar panels extended stands about 25 feet across.  SOHO will continue operating well past the next solar maximum in 2001. (Image credit: Alex Lutkus)

HW1 – solar radiaiton

• IDL tutorial for HW1