MET 60: Chapter: 4 (W&H) and 2 (Stull) Radiative Transfer Dr. Craig Clements San José State University

There are three modes of energy transmission in the atmosphere.  Conduction: the transfer of energy in a substance by means of molecular excitation without any net external motion.  Convection: the transfer of energy by mass motions within a fluid or gas, resulting in actual transport of energy.  Radiation: the propagation of electromagnetic waves through space. Energy transmission

Conduction

Convection

Electromagnetic radiation  Radiation is the transfer of energy by rapid oscillations of electromagnetic fields.  The most important general characteristic is its wavelength ( ), ______________________________.  Radiation travels through space at the speed of light (3 x 10 8 m s -1 ) or 670,616,630 MPH. Defined as the crest-to-crest distance

The Spectrum of Radiation Electromatic radiation may be viewed as an ensemble of waves propagating at the speed of light (c*=2.998 x 10 8 m/s through vacuum). As for any wave with a known speed of propagation, frequency, wavelength λ, and wave number, ν (i.e., the number of waves per unit length in the direction of propagation) are interdependent. Wave number is the reciprocal of wavelength

The electromagnetic radiation in a specific direction passing through a unit area (normal to direction considered) is called: monochromatic radiance (or spectral intensity or monochromatic radiance) λ The integral of the monochromatic intensity over some finite range of electromagnetic spectrum is called the intensity or radiance, I [W m -2 ]

The “Spectrum” Wavelength, λ IλIλ λ2λ2 λ1λ1 Spectrum of Monochromatic Intensity (theoretical)

Radiation  What emits radiation? –All objects with a temperature greater than 0°K emit some type of radiation (energy)  Examples:  Radiation laws: –Warmer objects emit more intensely than cold objects. (Stefan-Boltzmann Law) –Warmer objects emit a higher proportion of their energy at short wavelengths than cold objects. (Wien’s Law)

Wien’s Law: E = σT 4 λ = w / T λ = maximum wavelength (μm) w = constant = (μm K) T= temperature of the object (K) Stefan-Boltzmann Law: E = radiation emitted (W m -2 ) σ = Stefan-Boltzmann constant= 5.67 x (W m -2 K -4 ) T= temperature of the object (K)

Review questions  Considering the previous discussion –Which object would emit more (intensity) radiation: Earth or Sun? –If you were examining the radiation emitted by both the Sun and Earth, which would have a longer wavelength? –What wavelength radiation are you emitting right now? Sun Earth infrared

Solar Radiation (Sunlight)  Sunlight is primarily made up of the following: –Visible Light (44%) –Infrared Radiation (48%) –Ultraviolet Radiation (7%) Unit: 1  m = m

Terrestrial or Longwave Radiation  Planets mainly emit infrared radiation  Radiation emitted by planets occurs mainly at wavelengths _____ than those contained in solar radiation Solar Radiation (“Shortwave”) Terrestrial Radiation (“Longwave”) longer

Solar vs. Terrestrial Radiation  The sun is much hotter than planets; therefore, sunlight consists of shorter wavelengths than planetary radiation ;  Thus …

Energy from the Sun  Obviously, the Sun provides the Earth with it’s energy. The question is, how much of the Sun’s energy does the Earth get?  Sun’s energy is either –Scattered (reflected away) or –Absorbed  Scattering happens by bouncing off –Particles in the atmosphere –Earth’s surface  Absorption happens when certain gases absorb the energy –The reality is the only certain gases absorb certain wavelengths.

Absorption of radiation  Absorption of shortwave radiation by atmospheric gas molecules is fairly weak; –most absorption of shortwave radiation occurs at the Earth’s surface.  Most gases do not interact strongly with longwave radiation, however –Greenhouse gas molecules absorb certain wavelengths of longwave radiation.

Absorption of Radiation in the Earth’s Atmosphere

Incoming solar radiation  Each ‘beam’ of incoming sunlight can be either: –Reflected back to space:  Clouds  Atmosphere  Surface –Or absorbed; either by atmosphere (e.g. clouds or ozone) or Earth’s surface. Albedo

Recap  ______________ radiation comes from the sun and is composed of both ultraviolet and visible radiation  __________________ radiation comes from the Earth and is composed of infrared radiation  Recall that everything (above a temperature of 0K) emits some type of radiation (energy) with a particular wavelength. Shortwave or solar Longwave, terrestrial or infrared

Review - sensors that measure radiation  A _________________ measures solar radiation.  A__________________ measures infrared radiation (terrestrial) that comes from the Earth. Pyranometer Pyrgeometer