Download Lecture #7_radiation - UMD | Atmospheric and Oceanic Science

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.
Solar Constant measured by satellites at TOA
11-yr solar cycle
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2
How does the Earth Respond?
Forces Acting
On the Earth
System
Earth
System
Response
IMPACTS
Feedback
Of the total forcing of the climate system, 40% is due to the
direct effect of greenhouse gases and aerosols, and 60% is
from feedback effects, such as increasing concentrations of
water vapor as temperature rises.
Major Climate System Elements
Carbon Cycle
Atmospheric Chemistry
Water & Energy Cycle
Coupled
Chaotic
Nonlinear
Atmosphere and Ocean
Dynamics
Radiative Forcing from 1750 to 2000
Anthropogenic Forcings
IPCC, 2001
Human Influence on Climate
Carbon Dioxide Trends: 100yr lifetime
Methane Trends
Sulfate Trends
Global Temperature Trends
From M. Prather University of California at Irvine
Global Radiation Budget
Daily 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.
8 Sun
3) At5/2/2017
the SS of NH, daily solar insolation reaches a Max at NP. At the WS of NH, the
does not rise above north of about 66.5o, where solar insolation is zero.
Top-of-Atmosphere Radiation Budget
(Incoming Solar = Outgoing Longwave)
A = Planetary Albedo
S0 = Solar Irradiance
Te = Earth Radiative Temperature
Ts = Equilibrium Surface
Temperature
1% relative error in A
 1 W m-2 flux error  0.5C error in Ts
2xCO2 => +4 W m-2
The Greenhouse Effect
Solar Radiation
Longwave 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 of12
uncertainty in predicting potential future climate change
Cloud Radiative Forcing
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 (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
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fraction,
and varies with season.
235 W m-2
265 W m-2
342 W m-2
57 W m-2
342 W m-2
107 W m-2
285 W m-2
Earth (No Clouds)
235 W m-2
Earth (With Clouds)
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
Data
6 Months
6 Months
6 Months
6 Months
CERES Calibration/
Location
ERBE
Inversion
ERBE
Averaging
ERBE-Like
Products
Cloud Imager
Data
18 Mo.
Cloud Identification;
TOA/Surface Fluxes
Atmospheric
Structure
36 Mo.
Surface and
Atmospheric Fluxes
Geostationary
Data
30 Mo.
24 Mo.
Angular
Distribution
Models
36 Mo.
Diurnal
Models
CERES Surface
Products
42 Mo.
Time/Space
Averaging
42 Mo.
CERES Time Averaged
Cloud/Radiation
TOA, SFC, Atmos
Algorithm Theoretical Basis Documents:
http://asd-www.larc.nasa.gov/ATBD/ATBD.html
Validation Plans:
http://asd-www.larc.nasa.gov/valid/valid.html
CERES Advances over Previous Missions
•
•
•
•
•
•
•
•
•
Calibration
Angle Sampling
Offsets, active cavity calib., spectral char.
Hemispheric scans, merge with imager
matched surface and cloud properties
new class of angular, directional models
Time Sampling CERES calibration + 3-hourly geo samples
new 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 ModelsTake 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 Instrument
TRMM:
Jan-Aug 98
and Mar-Apr 2000
overlap with Terra
Terra:
Mar 00 - present
planned life: 2006
Aqua:
July 02 start
Now in checkout
Planned life to 2008
NPOESS:
TBD: gap or overlap?
2008 to 2011 launch
CERES LW Terra Results - July 2000
CERES Clear-Sky TOA
Longwave Flux (W m-2)
CERES TOA Longwave
Cloud Forcing (W m-2)
CERES SW Terra Results - July 2000
CERES Clear-Sky TOA
Shortwave Flux (W m-2)
CERES TOA Shortwave
Cloud Forcing (W m-2)
CERES Net Cloud Forcing (July, 2000)
Li and Leighton (1993)
Li and Leighton (1993)
Solar Energy Disposition
(in percentage)
100
242
0
28
•
30
46
50
The upper values are from satellite, middle
42 ones from GCMs
and the bottom from limited surface data
Forces Acting on Climate
(inClimate
Watts perForcings
meter2)
3
greenhouse gases
other anthropogenic forcings
natural forcings
2.3
N20
2)
2)
Forcing
F(W/m (W/m
2
CFCs
CH4
1
CO2
0
-1
0.4
well-mixed tropospheric
greenhouse
ozone
gases
stratospheric tropospheric
ozone
aerosols
-0.2
-0.2
-0.4
0.4
volcanic
aerosols
sun
(indirect via 0 3 )
-1
(0.2,-0.5)
Confidence Levels
Time Line
pre-satellite
forced
cloud
changes
vegetation
and other
surface
alterations
Moderate
Very Low
Very Low
Very Low
Very Low
1979-1999
High
Low
Low
Very Low
Very Low
2000-2010
High
Moderate
High
Moderate
predictions
Low
Moderate
Low
Low
High
Moderate
Very Low
Very Low
Low
Low
Low
Moderate
High
High
High
Low
Low
Low
Bargraph: estimated change of radiative forcings between 1850 and 1997. Confidence levels are estimated for the
pre-satellite era, satellite era (1979-1999), upcoming decade of satellite data, and model predictions for the first half
of next century. Models play a role in past and future estimates.
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.
EGRETT ALTITUDE CORRECTED TO TOA AND
GOES-8 PIXEL AND EGRETT FOV INTEGRATED
SW BROAD-BAND ALBEDO
SSP DATA
DATA OVER CF
SW ScaRaB
SW ALBEDO [%]
75
75
70
70
65
65
60
60
GOES-8
55
55
TSBR
50
50
45
40
4000
6000
8000
SW
(1)
=SW (GOES-8) - SW (TSBR) = 5.0 %
SW
(2)
=SW (SSP) - SW (TSBR) = 13.4 %
10000
TIME, SEC
12000
14000
16000
45
40
Relatioship between TOA albedo and atmospheric
transmittance
100
slope
slope
slope
slope
slope
TOA ALBEDO [%]
80
=
=
=
=
=
-0.817
-0.789
-0.774
-0.615
-0.571
ScaRaB 94-95
SSP BASED DATA
GOES-7 APRIL 94
GOES-8 ARESE 95
TSBR
60
40
20
30 MINUTES STANDARD
DEVIATION F <20 W m
0
0
20
-2
40
60
80
100
ATMOSPHERIC TRANSMITTANCE [%]
A summary of the consistency among the data
collected by various instruments
0.06
GOES-8
TSBR
0.14
TDDR
0.08
SSP
BSRN, SIROS, MFRSR, MWR, RADAR
GOES-7
ScaRaB
Evidence from the following Investigations
1.
2.
3.
4.
5.
Validation of satellite SRB estimates to check if the
difference increases with cloud cover
Use of TOA satellite and ground-based BB SRB data to
determine atmospheric absorption
Use of measurements of surface, atmospheric and cloud
variables to compute and compare TOA and surface
solar fluxes
Use of NB satellite spectral data to retrieve cloud optical
properties from which BB fluxes are compared and
compared with satellite BB fluxes
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 approach
Issues with CRF approach
4.
Representative of measurements – surface albedo
Home Work
Due on Apr. 6 (email me)
•
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.