Atmospheric Radiation

GCC Summer SchoolMontreal - August 7, 2003

Glen LesinsDepartment of Physics and Atmospheric Science

Dalhousie UniversityHalifax

glen.lesins@dal.ca

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