MET 60: Chapter: 4 (W&H) and 2 (Stull) Radiative Transfer Dr. Craig Clements San José State University.
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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
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 = 0.2897 (μm K) T= temperature of the object (K) Stefan-Boltzmann Law: E = radiation emitted (W m -2 ) σ = Stefan-Boltzmann constant= 5.67 x 10 -8 (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 = 0.000001 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