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Atmospheric Radiation
GCC Summer SchoolMontreal - August 7, 2003
Glen LesinsDepartment of Physics and Atmospheric Science
Dalhousie UniversityHalifax
Outline
Introductory concepts Radiation and Climate Radiative Transfer Theory Remote Sensing
Credits
K.N. Liou, An Introduction to Atmospheric Radiation, 2nd Ed., 2002
Web Lecture Notes by Prof. Irina Sokolik, http://irina.colorado.edu/teaching.htm
Kiehl and Trenberth (1997); IPCC (2001)
Global Annual Energy Balance
What is the Solar Constant?
• 1366 W m-2
• How constant?– Earth’s orbit and tilt (annual)– Sunspot cycle (11 years)– Longer time variations
Solar Irradiance Variation from ACRIM
http://science.nasa.gov/headlines/images/sunbathing/sunspectrum.htm
Solar vs. Terrestrial Radiation
Absorption of Radiation by Gases
1. Ionization/Dissociation - UV
2. Electronic Transition - UV
3. Vibrational/Rotational Transition -Visible/IR
4. Pure Rotational - IR
Transmission through the Atmosphere
Solar Terrestrial
IR Window
Radiative Interactions - Dipole Transitions
Vibrational Modes
Electrostatic potential
map shows both end
oxygens are equivalent
with respect to negative
charge. Middle atom
is positive.
Ozone (O3)
OO OO
••••OO••••
••••••••••••••••––++
OO OOOO••••
••••••••••••••••
–– ++
••••
www.facstaff.oglethorpe.edu/mwolf/PowerPoint/ CareyOrgPP/sections1st/Chapter%201bx.ppt
Absorption by Gases
SolarIrradiance
Scattering of Radiation
Particle Size
WavelengthSize Parameter,
r
http://hyperphysics.phy-astr.gsu.edu/hbase/atmos/blusky.html#c2
Rayleigh Scattering
Mie Theory for mr=1.5
NormalizedPhaseFunctionsFromMie Theory
Kiehl and Trenberth (1997); IPCC (2001)
Global Annual Energy Balance
Zonal Average Irradiance
Solar
Terrestrial
Net
MeridionalTransport
Cloud Radiative Forcing from ERBE
Radiative Equilibrium & Role of Convection
Solar Heating Rates from Model
Zonal Annual Average from Satellite
Results from SOCRATES (2-D Radiative-Chemical)
http://acd.ucar.edu/models/SOCRATES/socrates/socrates1.html
http://acd.ucar.edu/models/SOCRATES/socrates/socrates1.html
Annual Mean Net Radiation Flux from SurfaceBased Measurements
Terrestrial IR Spectra
Modelled IR Fluxes
High
Level of Scientific Understanding
1
2
3
0
-1
-2
Medium Medium Low Very
Low
Very
Low
Very
Low
Very
Low
Very
Low
Very
Low
Very
Low
Very
Low
Halocarbons
N2
O
CH4
CO2
Aerosols
Aviation-inducedTropospheric
ozone
Stratospheric
ozone
Black
carbon from
fossil fuel
burning
Organic
carbon
from
fossil
fuel
burning
Aerosol
indirect
effect
Biomass
burning
Land-use
(albedo)
only
Mineral
dust
Sulphate
Contrails Cirrus
Solar
Global mean radiative forcing of the climate
system for the year 2000, relative to 1750R
adia
tive
forc
ing
(W
m-2)
Wa
rmin
gC
oo
lin
g
Kiehl and Trenberth (1997); IPCC (2001)
Global Annual Energy Balance
Radiative Transfer Equation
Source FunctionOptical Depth
Cosine of solar zenith angle
Radiance
Azimuthal Angle
Beer’s Law
Plane Parallel Radiances
Solution to the Radiative Transfer Equation
UpwardRadiance
DownwardRadiance
SUN
Source Function Multiple Scattering Term
Single Scattering Term
Single & Multiple Scattering Source
Surface Reflectance
Bi-directional ReflectanceDistribution Function (BRDF)
Surface Albedo
Remote Sensing of Clouds
Effect of Clouds from Radiative-ConvectiveModel
Solar Albedo of Clouds - Theory
Indirect Aerosol Effect - ShiptracksIndirect Aerosol Effect - ShiptracksL1B true color RGB composite (25 April 2001)L1B true color RGB composite (25 April 2001)
60
0
30
15
45
re (µ
m)
Effective radius retrieval(using 2.1 µm band, all phases)
Shiptracks from MODISIndirect Aerosol Effect
July 1, 2003
Kiehl and Trenberth (1997); IPCC (2001)
Global Annual Energy Balance
IR Brightness Temperature from ER-2 (Clear)
BrightnessTemperaturesFrom ER-2(Various Clouds)
Polarization of Sunlight Reflected by Venus
Points=Obs
Lines=Theory
Hansen and Hovenier, 1974
POLDER – Polarization for Ice Habits
Ice Crystal Phase Functions
http://isccp.giss.nasa.gov
Cloud Fraction from Satellites
TERRA - Launched Dec. 18, 1999(MODIS, ASTER, MISR, CERES, MOPITT)
• MODIS– 1-2 day global coverage in 36 wavelengths from 250 m to
1 km resolution• MISR
– Stereo images at 9 look angles• ASTER
– Hi-resolution, multi-spectral images from 15 m to 90 m resolution, plus stereo
• MOPITT
– Global measures of CH4 & CO
• CERES– Measures Earth’s shortwave, longwave, – net radiant energy budget
http://modis-atmos.gsfc.nasa.gov/reference.html
MODIS Atmospheric Products
• Pixel-level (level-2) products– Cloud mask for distinguishing clear sky from clouds– Cloud radiative and microphysical properties
• Cloud top pressure, temperature, and effective emissivity• Cloud optical thickness, thermodynamic phase, and effective
radius• Thin cirrus reflectance in the visible
– Aerosol optical properties• Optical thickness over the land and ocean• Size distribution (parameters) over the ocean
– Atmospheric moisture and temperature gradients– Column water vapor amount
• Gridded time-averaged (level-3) atmosphere product– Daily, 8-day, and monthly products– 1° x 1° equal angle grid– Mean, standard deviation, marginal probability density function,
joint probability density functions
• modis-atmos.gsfc.nasa.gov
MODIS - TERRATrue colour image
Dust over theMediterranianMarch 12, 2003
CO2 Slicing Method
• CO2 slicing method
– ratio of cloud forcing at two near-by wavelengths
– assumes the emissivity at each wavelength is same, and cancels out in ratio of two bands
• The more absorbing the band, the more sensitive it is to high clouds– technique the most accurate for
high and middle clouds
• MODIS is the first sensor to have CO2 slicing bands at high spatial resolution (1 km)– technique has been applied to HIRS
data for ~20 years– retrieved for every 5 x 5 box of 1
km FOVs, when at least 5 FOVs are cloudy, day & night
1000
100
10
0.0 0.2 0.4 0.6 0.8 1.0
Pres
sure
(mb)
Weighting Function dt(,p)/d ln p
Channel 32 33 34 35 36
Central Wavelength (µm)
12.020 13.335 13.635 13.935 14.235
36
1.2
35
34
33
32
Brightness Temperature in 15 m CO2 band
Arrows atWavelengthsMeasured byVTPR
Retrieval of Cloud Optical Depth and Effective Radius
• The reflection function of a nonabsorbing band (e.g., 0.86 µm) is primarily a function of optical thickness
• The reflection function of a near-infrared absorbing band (e.g., 2.14 µm) is primarily a function of effective radius– clouds with small drops (or
ice crystals) reflect more than those with large particles
• For optically thick clouds, there is a near orthogonality in the retrieval of c and re using a visible and near-infrared band
Cloud Optical DepthApril 2001
20
0
10
Cloud Effective Particle RadiusApril 2001
4 m
22
40
Remote Sensing of Aerosols
Kiehl and Trenberth (1997); IPCC (2001)
Global Annual Energy Balance
Global Aerosol Emissions
(Tg / yr)
Annual Global Volcanic Aerosol Loading
Aerosol Optical Weighting Functions
K(a)=a2Qen(a)~Qe/reff
http://www.giss.nasa/gov/data
Model Aerosol Type Optical Thickness
MODIS Aerosol Optical Properties
• Seven MODIS bands are utilized to derive aerosol properties– 0.47, 0.55, 0.65, 0.86, 1.24, 1.64, and 2.13 µm– Ocean
• reflectance contrast between cloud-free atmosphere and ocean reflectance (dark)
• aerosol optical thickness (0.55-2.13 µm)• size distribution characteristics (fraction of aerosol optical
thickness in the fine particle mode; effective radius)– Land
• dense dark vegetation and semi-arid regions determined where aerosol is most transparent (2.13 µm)
• contrast between Earth-atmosphere reflectance and that for dense dark vegetation surface (0.47 and 0.66 µm)
• enhanced reflectance and reduced contrast over bright surfaces (post-launch)
• aerosol optical thickness (0.47 and 0.66 µm)
Gobi Desert Dust Storm - March 20, 2001 MODIS
a (0.55 µm)
0
2.0
1.0
Aerosol Optical Thickness - MODISFine Particle Mode
a (0.55 µm)
0
0.8
0.4
TOMS - Aerosol Index - Feb 26, 2000
http://toms.gsfc.nasa.gov/index.html
LITE - Lidar In space Technology ExperimentSeptember 1994 - Space Shuttle
http://www-lite.larc.nasa.gov/
Deep Convection
Saharan Dust
Cloud-Aerosol Lidar and Infrared Pathfinder Sate llite O bs ervations
EARLINET Sy mposium 11 February2003
· Orbit: 705 km, 98° inclination,in formation with Aqua,CloudSat and Parasol
· Launch end of 2004
· Mission duration: 3 years
· Three co-alignedinstruments:
• 3-channel lidar– 532 nm ||– 532 nm ^– 1064 nm
• Imaging IR radiometer
• Wide-fie ld camera
Mission Concept
Complementary Instruments
• CloudSat radar (cloud profiles)• Aqua CERES (top-of-the-atmosphere radiation)• Aqua AIRS / AMSU-A / HSB (atmospheric state)• Aqua MODIS (aerosol / cloud properties)• PARASOL (aerosol / cloud properties)• Aura OMI (aerosol absorption)
Aqua CALIPSOCloudSat
PARASOL
Aura
Vertical distribution ofaerosols and clouds
Aerosol / cloud properties
Remote Sensing of Gases
Radiative Forcing Between 1850 to 2000
Kiehl and Trenberth (1997); IPCC (2001)
Global Annual Energy Balance
Atmospheric Transmittances in the Microwave
Microwave Emissivity of Ocean Surface
Microwave Brightness Temperature
Precipitable Water
http://www.arm.gov/docs/instruments/static/rl.html
Source/Aerosol 355nmN2 387nmWater Vapour 408nm
Raman Lidar to Measure Water Vapour Profile
GPS Signals to Measure Water Vapour
http://atmos.af.op.dlr.de/projects/scops/
Normalised weighting functions for the High Resolution Infrared Sounder (HIRS) on NOAA satellites. Each function indicates the relative contribution of the atmosphere from a given level to the radiance observed at the satellite through the numbered channel.
Satellite Limb Scanning
Limb ScanningWeightingFunctions
Kiehl and Trenberth (1997); IPCC (2001)
Global Annual Energy Balance
Final Comments
Ultimately radiation drives all processes in the atmosphere
Remote sensing will continue to grow as a source of atmospheric measurements
New suite of satellites will require more atmospheric scientists in this area
Solar Ultra-violet Spectrum
Optical Properties for Typical Stratus and Cumulus
Bidirectional Reflectance and Absorbanceof Cirrus Clouds
LIDARS
Brightness Temperature in 15 m CO2 band
Arrows atWavelengthsMeasured byVTPR
IR Brightness Temperature from ER-2