Earth’s Radiation Balance and Cloud Radiative Forcing
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Transcript of Earth’s Radiation Balance and Cloud Radiative Forcing
The Earth’s surface is kept warm
through one source: the Sun. It is the
primary source for Earth’s energy.
Some of the incoming sunlight and
heat energy is reflected back into
space by the Earth’s surface, gases in
the atmosphere, and clouds; some of
it is absorbed and stored as heat.
When the surface and atmosphere
warm, they emit heat, or thermal
energy, into space. The “radiation
budget” is an accounting of these
energy flows. If the radiation budget
is in balance, then Earth should be
neither warming nor cooling, on
average.
Clouds, atmospheric water vapor and
aerosol particles play important roles
in determining global climate through
their absorption, reflection, and
emission of solar and thermal energy.
Earth’s Radiation Balance and Cloud Radiative ForcingEarth’s Radiation Balance and Cloud Radiative Forcing
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Solar Constant measured by satellites at TOA
11-yr solar cycle
EarthEarthSystemSystemResponseResponse
How does the Earth Respond?
IMPACTS
Feedback
Of the total forcing of the climate system, 40% is due to the Of the total forcing of the climate system, 40% is due to the direct effect of greenhouse gases and aerosols, and 60% is direct effect of greenhouse gases and aerosols, and 60% is from feedback effects, such as increasing concentrations of from feedback effects, such as increasing concentrations of
water vapor as temperature rises.water vapor as temperature rises.
Forces ActingForces ActingOn the EarthOn the EarthSystemSystem
Major Climate System ElementsWater & Energy CycleCarbon Cycle
Atmospheric Chemistry
CoupledCoupledChaoticChaoticNonlinearNonlinear
Atmosphere and OceanAtmosphere and OceanDynamicsDynamics
Radiative Forcing from 1750 to 2000Radiative Forcing from 1750 to 2000
IPCC, 2001IPCC, 2001
Anthropogenic Forcings
From M. Prather University of California at Irvine
Human Influence on ClimateHuman Influence on Climate
Carbon Dioxide Trends: 100yr lifetime
Methane Trends
Sulfate Trends
Global Temperature Trends
Global Radiation Budget
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Daily mean solar flux at TOADaily mean solar flux at TOA
1) The Sun is closest to the Earth in Jan. So more solar energy received in SH than in NH.2) At the equinoxes, the solar insolation is at a Max at the equator and is zero at the poles.3) At the SS of NH, daily solar insolation reaches a Max at NP. At the WS of NH, the Sun does not rise above north of about 66.5o, where solar insolation is zero.
1% relative error in A
1 W m-2 flux error 0.5C error in Ts
2xCO2 => +4 W m-2
Top-of-Atmosphere Radiation Budget(Incoming Solar = Outgoing Longwave)
A = Planetary Albedo
S0 = Solar Irradiance
Te = Earth Radiative Temperature
Ts = Equilibrium Surface Temperature
The Greenhouse Effect
Longwave Radiation
Solar Radiation
Clouds have been classified as the highest priority in climate change by the U.S. climate change research initiative because they are one of the largest sources of uncertainty in predicting potential future climate change
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The effect of clouds on the Earth's radiation balance is measured as the difference between clear-sky and all-sky radiation results
FX(cloud) = FX(clear) – FX(all-sky)
FNet(cloud) = FSW(cloud) + FLW(cloud)
where X= SW or LW
Negative FNet(cloud) => Clouds have a cooling effect on Climate
Positive FNet(cloud) => Clouds have a warming effect on Climate
Cloud Radiative Forcing
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Cloud Radiative Forcing (CRF)
Since cloud-base temperature is typically greater than the clear-sky effective atmospheric radiating temperature, CRFLW is generally positive.
The magnititude of CRFLW is strongly dependent on cloud-base height (i.e., cloud-base temperature) and emissivity.
Conversely, clouds reflect more insolation than clear sky, therefore, CRFSW is always negative over long time averages or large spatial domains. The magnititude of CRFSW cooling strongly depends on the cloud optical properties and fraction, and varies with season.
Earth (No Clouds) Earth (With Clouds)
57 W m-2342 W m-2107 W m-2
342 W m-2
235 W m-2285 W m-2
235 W m-2265 W m-2
FSW (cloud) =-50 W m-2
FLW (cloud)= 30 W m-2
=> Net Effect of Clouds = -20 W m-2
A brief history of ERB missions
CERES Data Processing Flow
CERES Calibration/Location
Cloud Identification;TOA/Surface Fluxes
Surface andAtmospheric Fluxes
Time/SpaceAveraging
ERBEInversion
ERBEAveraging
AngularDistribution
Models
DiurnalModels
CERES Surface Products
CERES Time AveragedCloud/Radiation
TOA, SFC, Atmos
ERBE-Like Products
Algorithm Theoretical Basis Documents: http://asd-www.larc.nasa.gov/ATBD/ATBD.htmlValidation Plans: http://asd-www.larc.nasa.gov/valid/valid.html
CERESData
Cloud ImagerData
AtmosphericStructure
GeostationaryData
6 Months 6 Months 6 Months 6 Months
18 Mo.
24 Mo.
30 Mo.
36 Mo.36 Mo.
42 Mo.
42 Mo.
CERES Advances over Previous Missions• Calibration Offsets, active cavity calib., spectral char.• Angle Sampling Hemispheric scans, merge with imager
matched surface and cloud propertiesnew class of angular, directional models
• Time Sampling CERES calibration + 3-hourly geo samplesnew 3-hourly and daily mean fluxes
• Clear-sky Fluxes Imager cloud mask, 10-20km FOV• Surface/Atm Fluxes Constrain to CERES TOA, ECMWF
imager cloud, aerosol, surface properties• Cloud Properties Same 5-channel algorithm on VIRS,MODIS
night-time thin cirrus, check cal vs CERES• Tests of Models Take beyond monthly mean TOA fluxes
to a range of scales, variables, pdfs• ISCCP/SRB/ERBE overlap to improve tie to 80s/90s data.• CALIPSO/Cloudsat Merge in 2006 with vertical aerosol/cloud
Move toward unscrambling climate system energy components
CERES InstrumentTRMM:TRMM:Jan-Aug 98Jan-Aug 98and Mar-Apr 2000and Mar-Apr 2000overlap with Terraoverlap with Terra
Terra: Terra: Mar 00 - presentMar 00 - presentplanned life: 2006planned life: 2006
Aqua: Aqua: July 02 startJuly 02 startNow in checkoutNow in checkoutPlanned life to 2008Planned life to 2008
NPOESS: NPOESS: TBD: gap or overlap? TBD: gap or overlap? 2008 to 2011 launch2008 to 2011 launch
CERES Clear-Sky TOA Longwave Flux (W m-
2)
CERES TOA Longwave Cloud Forcing (W m-2)
CERES LW Terra Results - July 2000
CERES Clear-Sky TOA Shortwave Flux (W m-2)
CERES TOA Shortwave Cloud Forcing (W m-2)
CERES SW Terra Results - July 2000
CERES Net Cloud Forcing (July, 2000)
Li and Leighton (1993)
Li and Leighton (1993)
Solar Energy Disposition(in percentage)
• The upper values are from satellite, middle ones from GCMs and the bottom from limited surface data
3030100100
464650504242
242242002828
Forces Acting on Climate(in Watts per meter2)
Forc
ing
(W
/m2)
Assessment of Cloud Absorption and Earth’s
Radiation Budget
• What is going on with recent debate on cloud absorption problem following ARESE ?
• What is the most sound value for global surface solar radiation budget at present?
Li et al. (Nature, 1995)
Validation of satellite SRB estimates to check if the difference increases with cloud cover
Hypothesis to be tested
If CAA exists, satellite retrieval of SRB would not agree with ground-based observations, and the difference would increase with cloud amount
Li (J. Climate, 1998)
Summary of ARESE Studies
• Cloud absorption anomaly is not supported by ground-based, nor space-borne measurements.
• The central piece of information supports cloud absorption anomaly comes from TSBR aboard Egrett, which are inconsistent with other measurements.
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GOES-8
TSBR
SSP DATA DATA OVER CF SW ScaRaB
SW
(1)=
SW(GOES-8) -
SW(TSBR) = 5.0 %
SW
(2)=
SW(SSP) -
SW(TSBR) = 13.4 %
EGRETT ALTITUDE CORRECTED TO TOA ANDGOES-8 PIXEL AND EGRETT FOV INTEGRATED
SW BROAD-BAND ALBEDO
SW
ALB
ED
O [%
]
TIME, SEC
Relatioship between TOA albedo and atmospheric transmittance
0 20 40 60 80 1000
20
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60
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TO
A
AL
BE
DO
[%
]
ATMOSPHERIC TRANSMITTANCE [%]
slope = -0.817 ScaRaB 94-95 slope = -0.789 SSP BASED DATA slope = -0.774 GOES-7 APRIL 94 slope = -0.615 GOES-8 ARESE 95 slope = -0.571 TSBR
30 MINUTES STANDARD
DEVIATION F <20 W m-2
A summary of the consistency among the data collected by various instruments
TSBR
BSRN, SIROS, MFRSR, MWR, RADAR
GOES-7
ScaRaB
0.06
0.14
0.08
GOES-8 TDDR
SSP
Evidence from the following Investigations
1. Validation of satellite SRB estimates to check if the difference increases with cloud cover
2. Use of TOA satellite and ground-based BB SRB data to determine atmospheric absorption
3. Use of measurements of surface, atmospheric and cloud variables to compute and compare TOA and surface solar fluxes
4. Use of NB satellite spectral data to retrieve cloud optical properties from which BB fluxes are compared and compared with satellite BB fluxes
5. Use of ground-based radiation to retreive cloud optical depth from which TOA fluxes are estimated and compared.
Potential Causes for Apparent CAA
1. NB to BB conversion due to the use of non-calibrated NB operational weather satellite data
2. Calibration in satellite and/or aircraft measurements
3. Inadequate analysis method prone to mis-interpretation: Issues with the slope approachIssues with CRF approach
4. Representative of measurements – surface albedo
• When the earth was formed some 5 billion years ago, the sun was about 30% of today’s brightness. When the sun ceases illuminating, its brightness is estimated to be 3 times brighter. Estimate changes in planet temperature relative to the current.
• Based on the global energy balance diagram, summarize the sinks and sources of energy at the top, bottom and inside of the atmosphere.
• When the earth was formed some 5 billion years ago, the sun was about 30% of today’s brightness. When the sun ceases illuminating, its brightness is estimated to be 3 times brighter. Estimate changes in planet temperature relative to the current.
• Based on the global energy balance diagram, summarize the sinks and sources of energy at the top, bottom and inside of the atmosphere.
Home Work
Due on Apr. 6 (email me)
Home Work
Due on Apr. 6 (email me)