Download 6. Earth radiation balance under present day conditions Radiation

Radiation balance of the Earth
Top of Atmosphere (TOA) radiation balance
6. Earth radiation balance under present day
conditions
Atmospheric
radiation balance:
Difference
between
TOA and surface
radiation balance
Surface radiation balance
Radiation and Climate Change FS 2016 Martin Wild
Top of Atmosphere (TOA) Radiation balance
Shortwave TOA radiation balance
Requires knowledge of:
•  Solar constant (determines solar incident energy)
> last lecture
• 
Radiation and Climate Change FS 2016 Martin Wild
TOA radiation balance: Planetary albedo
albus= white (Latin)
Albedo= measure of backscattering from diffuse reflecting, nonradiative
surface
Planetary albedo = reflected divided by incident solar radiation at TOA
Planetary albedo (determines solar reflected energy)
Governs fraction of solar energy absorbed by the Earth
Similarly: surface albedo = reflected divided by incident solar radiation at the surface
Radiation and Climate Change FS 2016 Martin Wild
Radiation and Climate Change FS 2016 Martin Wild
Sensitivity of climate system to albedo changes
The average solar energy incident on the Earth’s sphere per m2 is
S/4 = 340
W/m2
with
S=solar constant (1361
Wm-2)
TOA radiation balance: Historic albedo estimates
•  Early estimates from surface data (cloud cover, balloon data)
• 
Abbot and Fowle (1908) first estimate of planetary albedo: 0.37
• 
Dines (1917) planetary albedo: 0.5, Houghton (1954): 0.34, London (1957): 0.35
(current best estimate 0.30)
•  Measurements from Earthshine (reflected sunlight from Earth
Changing planetary Albedo A by 0.01 changes absorption of solar
radiation in the climate system by 3.4 W/m2
illuminates the dark side of the moon). Detected by Leonardo da Vinci
All anthropogenic greenhouse gases over last 150 years result in 3.2 W/m2
forcing (IPCC AR5)
From climate models:
dT / dA ~ -1.5K / 0.01
•  Measurements from Satellites (since 1960)
Increasing A by more than 0.02 would drive us into a new ice age
Radiation and Climate Change FS 2016 Martin Wild
Early measurements too low (0.28) since only morning overpass (no convection)
Radiation and Climate Change FS 2016 Martin Wild
First Satellite Mission: TIROS
Satellite orbits
TIROS
(Television Infrared Observation
Satellite Program)
• 
NASA's first experimental step to
determine if satellites could be useful in
the study of the Earth.
• 
The TIROS Program's first priority was
the development of a meteorological
satellite information system.
• 
TIROS began continuous coverage of
the Earth's weather in 1962.
• 
lead to the development of more
sophisticated meteorological
observation satellites.
.
The very first television picture
from space, taken by the TIROS-I
Satellite on April 1, 1960.
Radiation and Climate Change FS 2016 Martin Wild
Radiation and Climate Change FS 2016 Martin Wild
Polar orbiting satellites
• 
• 
Geostationary satellites
Elevation of orbit: 35790 km
About 800 km above Earth
surface
Time for one orbit equal to period of rotation of the Earth
Temporal resolution: From each
location 1 measurement per day
• 
Sun synchronous: Every day at
same time over the same site
• 
Models required to integrate
measurement taken only under
specific angel over all angels
(23h 56min 4.09sec) > satellite appears to be stationary
Orbit in Equatorial plane > distorted images from high latitude regions
Radiation and Climate Change FS 2016 Martin Wild
Radiation and Climate Change FS 2016 Martin Wild
Geostationary satellites
Narrowband versus broadband measurements
• 
View from geostationary satellite
(Meteosat)
Visible chanel
Advantage
geostationary satellite:
High temporal resolution
Radiation and Climate Change FS 2016 Martin Wild
Operational weather satellites measure only in specific channels
(radiation wavelenghts bands), e.g. AVHRR (Advanced Very High
Resolution Radiometer) instrument, originally 4 channels (on TIROS
satellite launched in 1978), now 6 channels.
• 
• 
For climate purposes we need broadband radiation observations
=> narrow to broadband conversion methods necessary
Satellite missions with broadband radiometers starting in 1984
Radiation and Climate Change FS 2016 Martin Wild
Satellite missions in the 1980s: ERBE
Example results from ERBE
ERBE (Earth Radiation Budget Experiment):
• 
first satellite mission specifically designed to
observe the Earth radiation budget with
broadband radiometers.
New findings from ERBE:
• 
Goal: produce monthly averages of longwave
and shortwave TOA radiation parameters at
regional to global scales.
Total SW absorption/ LW emission:
235 Wm-2
• 
Three identical sets of instruments launched
on 3 separate spacecrafts, by Space Shuttle
Challenger in 1984 (ERBS spacecraft) and on
two National Oceanic and Atmospheric
Administration (NOAA) weather monitoring
satellites (TIROS); NOAA 9 and NOAA 10, in
1984 and 1986.
• 
Planetary albedo: 0.30
In operation
November 1984 - February 1990
http://science.larc.nasa.gov/erbe/
NOAA-9 and NOAA-10 provided global
coverage (polar orbits, sun synchroneous),
ERBS coverage between 60.00 degrees north
and south latitude (mid inclination orbit).
Radiation and Climate Change FS 2016 Martin Wild
Satellite missions in the 1990s: SCARAB
Radiation and Climate Change FS 2016 Martin Wild
Radiation budget satellite missions 1985 - 2000
ScaRaB (Scanner for Radiation Budget)
• 
French-Soviet space cooperation to
promote the ScaRaB program (Scanner
for Radiation Budget). The objective was
to determine the components of the Earth
Radiation Budget and to provide a
continuity of the NASA ERBE (Earth
Radiation Budget Experiment) mission
(1985-1990).
• 
ScaRaB was launched on 25 January 1994
from Plessetsk and provided one year of
data.
• 
4 broadband channels radiometer (visible,
solar, total and infrared)
Operational:
March 1994 - Feb 1995
Radiation and Climate Change FS 2016 Martin Wild
No continuous monitoring of TOA radiation budget
Radiation and Climate Change FS 2016 Martin Wild
Satellite missions in the 2000s: CERES
Example Results from CERES
CERES (Clouds and the Earth’s Radiant Energy System )
• 
CERES instruments launched aboard TRMM satellite
in November 1997 and on EOS Terra satellite in
December 1999. Two additional instruments launched
on EOS Aqua spacecraft in 2002.
• 
Terra and Aqua polar orbiting, sun synchronous,
scanning satellites, shifted by 6h
• 
Resolution 20 km at Nadir (vertically below satellite)
• 
1 scan in 30 sec, 30 footprints
• 
CERES instruments substantially improved over the
ERBE instruments: lower noise, improved calibration,
and smaller fields of view. Levels of accuracy never
before achieved for radiation budget instruments.
• 
“Golden era” of satellite observations:
CERES satellite observations in the 2000s
Courtesy Norman Loeb, NASA Langley
http://ceres.larc.nasa.gov/
Cloud properties are determined using simultaneous
measurements such as the Moderate Resolution
(narrow band) Imaging Spectroradiometer (MODIS).
Operational :
Since 2000
NASA Releases Terra's First
Global 1-Month Composite Images
Radiation and Climate Change FS 2016 Martin Wild
Radiation and Climate Change FS 2016 Martin Wild
Additional reading material
Geostationary satellite GERB
GERB (Geostationary Earth Radiation Budget)
• 
On august 28th 2002 the Meteosat Second Generation (MSG1)
operational weather satellite has been launched with the
Geostationary Earth Radiation Budget (GERB) sensor on board
• 
GERB is the first instrument dedicated to measure the Earth
radiation budget on a geostationary satellite.
• 
Nadir Resolution 50 km, 2 Channels 0.32-4um, 0.32 -30 um
• 
GERB-2 on MSG 2 launched in 2005, GERB-3 in 2012 on MSG-3
• 
One scan through earth disk 15 min
• 
Accuracy: Solar < 0.5%; IR < 1%
• 
Advantage geostationary satellite: radiation measurements
every 15 minutes
• 
Disadvantage: does not cover the whole earth, high latitudes
distorted
Operational:
Available from the website
Radiation and Climate Change FS 2016 Martin Wild
Radiation and Climate Change FS 2016 Martin Wild
since 2002
GERB versus CERES
International Satellite Cloud Climatology Project (ISCCP)
http://isccp.giss.nasa.gov/
Combining information from geostationary satellites (diurnal cycle)
and polar orbiting satellites (global coverage)
Radiation and Climate Change FS 2016 Martin Wild
International Satellite Cloud Climatology Project (ISCCP)
• 
ISCCP was established in 1982 to collect radiance
measurements to infer the global distribution of clouds, their
properties, and their diurnal, seasonal, and interannual
variations.
• 
Data collection began on 1 July 1983. The resulting datasets
and analysis products are being used to improve
understanding and modeling of the role of clouds in climate,
with the primary focus on the effects of clouds on the
radiation balance.
• 
Data are collected from the suite of weather satellites
operated by several nations and processed by several groups
in government agencies. No special instrument development,
used whatever is available from geostationary and polar
orbiting satellites.
• 
Only 2 narrowband channels (visible band at 0.6 micron,
infrared at 11 micron), no broadband information
Operational 1983 - present
Radiation and Climate Change FS 2016 Martin Wild
Planetary albedo
Global cloud cover from satellite 1983-2010
From ISCCP (NASA/ Bill Rossow)
International Satellite Cloud Climatology Project
Radiation and Climate Change FS 2016 Martin Wild
Highest value 0.68
Lowest value 0.16
Global mean 0.30
Radiation and Climate Change FS 2016 Martin Wild
Planetary albedo - seasonal
NH summer (JJA)
Absorbed solar radiation in the climate system
Absorbed solar radiation TOA = (1.0 - planetary albedo) x incident TOA radiation
Variation of ITCZ
Highest value 352 Wm-2
Lowest value 56 Wm-2
Global mean 240 Wm-2
NH winter (DJF)
Radiation and Climate Change FS 2016 Martin Wild
Radiation and Climate Change FS 2016 Martin Wild
Absorbed solar radiation and cloud amount
Cloud albedo
•  Cloud albedo is a measure of the reflectivity of a cloud - higher
values mean that the cloud can reflect more solar radiation.
Absorbed solar
•  Cloud albedo varies from less than 10% to more than 90%
•  Cloud albedo depends on
Cloud amount
– 
thickness of the cloud
– 
liquid water or ice content
– 
drop sizes
– 
sun's zenith angle
•  The smaller the drops and the larger the liquid water content,
the larger the cloud albedo, if all other factors are the same
(e.g. polluted clouds more reflective)
Radiation and Climate Change FS 2016 Martin Wild
Radiation and Climate Change FS 2016 Martin Wild
Cloud albedo
Increase in equivalent radius and decrease in liquid water
path result in decreasing cloud albedo
Radiation and Climate Change FS 2016 Martin Wild
Low clouds
High Clouds
• 
High, thin cirrus clouds are highly transparent to shortwave radiation
• 
Albedo approx. 0.2
Radiation and Climate Change FS 2016 Martin Wild
Cloud radiative forcing
Cloud radiative forcing describes effects of clouds on the
energy content of the climate system
Radiative forcing in general terms:
•  A process which alters the energy balance of the Earthatmosphere system is known as a radiative forcing mechanism
•  Radiative forcing : perturbed state - unperturbed (base ) state
In case of clouds: “with clouds” – “without clouds”
•  If perturbation increases energy content of the climate system >
positive radiative forcing
• 
lower clouds are much thicker than high cirrus clouds and therefore
more reflective.
• 
Albedo on the order of 0.6 - 0.7
Radiation and Climate Change FS 2016 Martin Wild
•  If perturbation decreases energy content of the climate system >
negative radiative forcing
Radiation and Climate Change FS 2016 Martin Wild
Cloud radiative forcing
Albedo with clouds
Cloud radiative forcing
Albedo without clouds
Cloud Radiative Forcing (CRF) is the difference between the radiation budget
components for average cloud conditions and cloud-free conditions
“all sky” - “clear sky”, i.e. the difference in fluxes when clouds are preset or absent
The total shortwave absorbed TOA radiation Fsw (as e.g. observed by a satellite) in a specific
region or grid cell under any type of weather conditions can be written as:
Fsw (all sky) = Fsw (clear) (1 - N) + NFsw (cloudy)
where N = cloud fraction, Fsw (clear) is the shortwave radiation absorbed by the cloud free
portion of the grid cell and Fsw (cloudy) is that flux associated with absorption by the cloudy
portion of the grid cell. With rearrangement
Fsw (all sky) = Fsw (clear) + N (Fsw (cloudy)- Fsw (clear) )
Fsw (all sky) - Fsw (clear) = N (Fsw (cloudy)- Fsw (clear) ) = SWCRF
Data from ERBE
Jan. 1986
30 %
15 %
Clouds enhance planetary albedo by 15 %
(cf. 1%=3.4 Wm-2, 15 % = 51 Wm-2)
Radiation and Climate Change FS 2016 Martin Wild
Additional reading material
⇒ Cloud radiative forcing dependent on cloud fraction and the modification of the
absorption in the presence of a cloud.
Similarly for longwave component (longwave outgoing radiation at the TOA FLW (all sky)):
FLW (all sky) - FLW (clear) = N (FLW (cloudy)- FLW (clear) ) = LWCRF
where FLW (clear) and FLW (cloudy) is the emission from the clear and cloudy sky, respectively.
Radiation and Climate Change FS 2016 Martin Wild
Shortwave cloud radiative forcing (SWCRF)
Shortwave cloud radiative forcing at TOA (SWTOA CRF) =
SW absorbed (TOA) all sky - SW absorbed (TOA) clear sky
Available from the website
SWCRF = Fsw (all sky)- Fsw (clear)
= N (Fsw (cloudy)- Fsw (clear) )
Radiation and Climate Change FS 2016 Martin Wild
Highest value 0 Wm-2
Lowest value -106 Wm-2
Global mean -50 Wm-2
Radiation and Climate Change FS 2016 Martin Wild
Longwave TOA fluxes
Outgoing longwave radiation (OLR)
Highest emission -286 Wm-2
Lowest emission -124 Wm-2
Global mean -239 Wm-2
Radiation and Climate Change FS 2016 Martin Wild
Radiation and Climate Change FS 2016 Martin Wild
High and low cloud LW radiative forcings
Clouds and OLR
Cloud top pressure
Cloud temperature - surface temperature
Outgoing longwave
Radiation (OLR)
Radiation and Climate Change FS 2016 Martin Wild
Radiation and Climate Change FS 2016 Martin Wild
Longwave cloud radiative forcing
Comparison LW and SW CRF
Longwave cloud radiative forcing at TOA (LWTOA CRF) =
LW outgoing (TOA) all sky - LW outgoing (TOA) clear sky
LW CRF
SW CRF
LWCRF= FLW (all sky) - FLW (clear)
= N (FLW (cloudy)- FLW (clear) )
Highest forcing +70 Wm-2
Lowest forcing + 2 Wm-2
Global mean + 25 Wm-2
Radiation and Climate Change FS 2016 Martin Wild
Total (net) cloud radiative forcing at TOA
Total cloud radiative forcing at TOA (Rnet TOA CRF) =
Rnet (TOA) all sky - Rnet (TOA) clear sky
Radiation and Climate Change FS 2016 Martin Wild
Global mean TOA cloud radiative forcing
All sky
clear sky
Cloud forcing
_______________________________________________________________
Outgoing Longwave
Absorbed Solar
-239 Wm-2
-264 Wm-2
+25 Wm-2
Wm-2
Wm-2
-50 Wm-2
+240
+290
Net forcing
-25 Wm-2
Albedo
30 %
15 %
_______________________________________________________________
Ø  Clouds increase planetary albedo by 15 %
Ø  This reduces absorption of solar radiation by 50 Wm-2
Ø  This reduction is partly offset by reduced of outgoing longwave by 25 Wm-2
Highest value +13 Wm-2
Lowest value -80 Wm-2
Global mean -25 Wm-2
Radiation and Climate Change FS 2016 Martin Wild
Ø  Therefore total cloud forcing is -25 Wm-2
Ø  Clouds cool the planet
Radiation and Climate Change FS 2016 Martin Wild
Net radiation TOA
Zonally averaged TOA balances
Net radiation TOA (“planetary radiation budget”) = sum of incoming and
reflected shortwave radiation, and outgoing longwave radiation
SW absorbed and OLR
Wm-2
Highest deficit -126
Highest gain + 88 Wm-2
Global mean + 1 Wm-2
TOA Radiation Balance
(Net Radiation TOA)
Radiation and Climate Change FS 2016 Martin Wild
Radiation balance and meridional energy transport
Radiation and Climate Change FS 2016 Martin Wild
Radiation budget at the Earth surface
TOA
Radiation balance
Northward energy
transport
Surface Radiation balance Rnet
R = SW ↓ (1− α) + LW ↓−LW ↑
net
Radiation and Climate Change FS 2016 Martin Wild
Radiation and Climate Change FS 2016 Martin Wild
Significance of surface radiation balance (I)
Significance of surface radiation balance (II)
Rnet: Major driver of surface temperature
Surface energy balance in long term mean (equilibrium)
Surface energy balance:
∂T
ρCΔz
= R − H − LE − G − M
∂t
ρCΔz
∂T
= R − H − LE − G − M
∂t
0
approx.0
s
net
with
€
Rnet:
surface radiation balance
H:
turbulent sensible heat flux
LE:
turbulenter latent heat flux
G:
Ground heat flux (into soil)
M:
Melt energy (if snow is present)
Surface radiation components largest terms in surface
energy balance equation
Radiation and Climate Change FS 2016 Martin Wild
Surface net radiation as driver of global water cycle
s
net
R = H +LE
net
€
Globally, 80 % of Rnet goes into LE, 20 % into H
LE = Energy equivalent of evaporative flux which balances
€
precipitation in long term mean
⇒  global mean Rnet in first order proportional to global mean
evaporation/precipitation
⇒  Rnet principal driver of global water cycle
Radiation and Climate Change FS 2016 Martin Wild
Solar radiation at the Earth surface
R = H +LE
net
LE
€
Rnet
LE combination of latent heat of vaporisation L [J kg-1] and Evaporation flux [kg m-2s-1]
Radiation and Climate Change FS 2016 Martin Wild
Radiation and Climate Change FS 2016 Martin Wild
Determination of surface solar radiation
Downward SW radiation at the surface
From surface observations
Advantage: accurate measurements with well-calibrated instruments at single points, long
time series
Disadvantage: insufficient global coverage, spatial representativness
Observation stations from
Global Energy Balance Archive (GEBA)
From Satellite data
Advantage: global coverage
Disadvantage: only top of atmosphere measurements => need physical or empirical
radiative transfer models to derive surface fluxes, satellite calibration problems, short and
discontinuous time series
Radiation and Climate Change FS 2016 Martin Wild
Surface albedo
Highest value 286 Wm-2
Lowest value 60 Wm-2
Global mean 185 Wm-2
Radiation and Climate Change FS 2016 Martin Wild
Absorbed SW radiation at the surface
Surface absorbed (Net) SW radiation =
(1- albedosurface) x Downward SW radiation at surface
Highest value 270 Wm-2
Lowest value 22 Wm-2
Global mean 161 Wm-2
Radiation and Climate Change FS 2010 Martin Wild
Radiation and Climate Change FS 2016 Martin Wild
Downward longwave radiation
Main factors that influence downward longwave radiation:
•  Greenhouse gases (water vapour, CO2…)
•  Temperature of emitting atmosphere
Radiation and Climate Change FS 2016 Martin Wild
Emission sources of downward longwave radiation
Downward longwave radiation at surface
Highest value 431 Wm-2
Lowest value 101 Wm-2
Global mean 342 Wm-2
Radiation and Climate Change FS 2016 Martin Wild
Upward longwave radiation at surface
Contributions from different atmospheric layers to downward longwave
radiation at the surface:
Layer: Earth surface - 10 m:
38%
Layer: Earth surface - 30 m:
50%
Layer: Earth surface - 100 m:
62%
Layer: Earth surface - 300 m:
80%
Layer: Earth surface - 1000 m:
90%
LW emission of Earth surface = εσTs4 (Stefan Boltzmann Law)
with ε: Emissivity of the Earth surface, Ts: surface temperature
Calculations with LOWTRAN 7 radiation code
Radiation and Climate Change FS 2016 Martin Wild
Often for simplicity ε = 1
More precise ε < 1 => Total LW upward flux =
LW surface emission + reflected part of downward LW radiation
= εσTs4 + (1-ε) downward LW radiation
Radiation and Climate Change FS 2016 Martin Wild
Emission coefficient ε of various surface
Water
0.92- 0.96
Snow
0.82 -0.99
Ice
0.96
Dry sand
0.90
Wet Sand
0.95
wet soil (no vegetation)
0.95 -0.98
Dry soil
0.90
Desert
0.90
Grasland
0.90
Forest
0.90
Aluminium
0. 01 -0.05
Iron
0.13 -0.28
Silver
0.02
Human Skin
0.95
Net longwave radiation at surface
Surface Net longwave radiation =
LW downward (surface) - LW upward (surface)
“Surface thermal cooling”
Highest value -10 Wm-2
Lowest value -134 Wm-2
Global mean -56 Wm-2
Radiation and Climate Change FS 2016 Martin Wild
Surface net radiation
Radiation and Climate Change FS 2016 Martin Wild
TOA, atmospheric and surface radiation budget
Surface Net (total) radiation =
Net SW (surface) + Net LW (surface)
Near equilibrium
~ 0 Wm-2
at TOA:
Radiative energy deficit in
atmosphere: -105 Wm-2
Radiative energy surplus
at surface: +105 Wm-2
Highest value +210 Wm-2
Lowest value -16 Wm-2
Global mean +105 Wm-2
Radiation and Climate Change FS 2016 Martin Wild
Radiation and Climate Change FS 2016 Martin Wild
Atmospheric SW absorption
Effect of clouds on atmospheric SW absorption
Atmospheric SW absorption (Atmospheric SW divergence) =
SWNet (TOA) - SWNet (Surface)
High cloud enhances absorption, but
reduces clear sky absorption of solar beam
Global mean +79 Wm-2
Radiation and Climate Change FS 2016 Martin Wild
SW cloud radiative forcing ratio
SW CFR Ratio= SW CRF(surface) / SW CRF (TOA)
Measure for modification of atmospheric absorption due to clouds
SW CRF Ratio = 1 => clouds do not alter SW absorption in atmospheric column
SW CRF Ratio > 1 => clouds enhance SW absorption in atmospheric column
Radiation and Climate Change FS 2016 Martin Wild
=> atmospheric column absorption does not
change much
Low cloud enhances absorption, withouth
reducing clear sky absorption of solar beam
=> atmospheric column absorption
increases
Radiation and Climate Change FS 2016 Martin Wild
Atmospheric LW divergence
Atmospheric LW divergence (Atmospheric heat loss) =
OLR (TOA) - LWNet (Surface)
Global mean -184 Wm-2
Radiation and Climate Change FS 2016 Martin Wild
Total (SW + LW) atmospheric divergence
Global mean -105 Wm-2
Radiation and Climate Change FS 2016 Martin Wild