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 Top of Atmosphere (TOA) Radiation balance Shortwave TOA radiation balance Requires knowledge of: Solar constant (determines solar incident energy) > last lecture 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
Sensitivity of climate system to albedo changes The average solar energy incident on the Earth s sphere per m 2 is S/4 = 340 W/m 2 with S=solar constant (1361 Wm -2 ) Changing planetary Albedo A by 0.01 changes absorption of solar radiation in the climate system by 3.4 W/m 2 All anthropogenic greenhouse gases over last 150 years result in 3.2 W/m 2 forcing (IPCC AR5) 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 illuminates the dark side of the moon). Detected by Leonardo da Vinci From climate models: dt / da ~ -1.5K / 0.01 Increasing A by more than 0.02 would drive us into a new ice age Measurements from Satellites (since 1960) Early measurements too low (0.28) since only morning overpass (no convection) 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..
Polar orbiting satellites Geostationary satellites About 800 km above Earth surface 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 Elevation of orbit: 35790 km Time for one orbit equal to period of rotation of the Earth (23h 56min 4.09sec) > satellite appears to be stationary Orbit in Equatorial plane > distorted images from high latitude regions Geostationary satellites Narrowband versus broadband measurements 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. View from geostationary satellite (Meteosat) Visible chanel Advantage geostationary satellite: High temporal resolution For climate purposes we need broadband radiation observations => narrow to broadband conversion methods necessary Satellite missions with broadband radiometers starting in 1984
Satellite missions in the 1980s: ERBE ERBE (Earth Radiation Budget Experiment): first satellite mission specifically designed to observe the Earth radiation budget with broadband radiometers. Goal: produce monthly averages of longwave and shortwave TOA radiation parameters at regional to global scales. New findings from ERBE: Planetary albedo: 0.30 Example results from ERBE 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. 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). Satellite missions in the 1990s: SCARAB 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. Operational: March 1994 - Feb 1995 4 broadband channels radiometer (visible, solar, total and infrared) No continuous monitoring of TOA radiation budget
Satellite missions in the 2000s: 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. Cloud properties are determined using simultaneous measurements such as the Moderate Resolution (narrow band) Imaging Spectroradiometer (MODIS). Operational : http://ceres.larc.nasa.gov/ Since 2000 Example Results from CERES Golden era of satellite observations: CERES satellite observations in the 2000s NASA Releases Terra's First Global 1-Month Composite Images Courtesy Norman Loeb, NASA Langley Additional reading material Geostationary satellite GERB Available from the website 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: 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) 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 Planetary albedo Global cloud cover from satellite 1983-2010 From ISCCP (NASA/ Bill Rossow) International Satellite Cloud Climatology Project Highest value 0.68 Lowest value 0.16 Global mean 0.30
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) Absorbed solar radiation and cloud amount Absorbed solar Cloud albedo Cloud albedo is a measure of the reflectivity of a cloud - higher values mean that the cloud can reflect more solar radiation. 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)
Cloud albedo High Clouds Increase in equivalent radius and decrease in liquid water path result in decreasing cloud albedo High, thin cirrus clouds are highly transparent to shortwave radiation Albedo approx. 0.2 Low clouds Cloud radiative forcing Cloud radiative forcing describes effects of clouds on the energy content of the climate system lower clouds are much thicker than high cirrus clouds and therefore more reflective. Albedo on the order of 0.6-0.7 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 If perturbation decreases energy content of the climate system > negative radiative forcing
Cloud radiative forcing Cloud radiative forcing Albedo with clouds 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 Data from ERBE Jan. 1986 30 % 15 % Clouds enhance planetary albedo by 15 % (cf. 1%=3.4 Wm-2, 15 % = 51 Wm-2) Fsw (all sky) = Fsw (clear) + N (Fsw (cloudy)- Fsw (clear) ) Fsw (all sky) - Fsw (clear) = N (Fsw (cloudy)- Fsw (clear) ) = SWCRF 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)): F LW (all sky) - F LW (clear) = N (F LW (cloudy)- F LW (clear) ) = LWCRF where FLW (clear) and FLW (cloudy) is the emission from the clear and cloudy sky, respectively. Additional reading material Shortwave cloud radiative forcing (SWCRF) Shortwave cloud radiative forcing at TOA (SW TOA 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) ) Highest value 0 Wm -2 Lowest value -106 Wm -2 Global mean -50 Wm -2
Longwave TOA fluxes Outgoing longwave radiation (OLR) Highest emission -286 Wm-2 Lowest emission -124 Wm-2 Global mean -239 Wm-2 High and low cloud LW radiative forcings Clouds and OLR Cloud top pressure Cloud temperature - surface temperature Outgoing longwave Radiation (OLR)
Longwave cloud radiative forcing Longwave cloud radiative forcing at TOA (LW TOA CRF) = LW outgoing (TOA) all sky - LW outgoing (TOA) clear sky Comparison LW and SW CRF 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 Total (net) cloud radiative forcing at TOA Total cloud radiative forcing at TOA (Rnet TOA CRF) = Rnet (TOA) all sky - Rnet (TOA) clear sky Global mean TOA cloud radiative forcing All sky clear sky Cloud forcing Outgoing Longwave -239 Wm -2-264 Wm -2 +25 Wm -2 Absorbed Solar +240 Wm -2 +290 Wm -2-50 Wm -2 Net forcing -25 Wm -2 Albedo 30 % 15 % Highest value +13 Wm -2 Lowest value -80 Wm -2 Global mean -25 Wm -2 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 Ø Therefore total cloud forcing is -25 Wm -2 Ø Clouds cool the planet
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 balance and meridional energy transport Radiation budget at the Earth surface TOA Radiation balance Northward energy transport Surface Radiation balance Rnet R = SW (1 α) + LW LW net
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 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 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]
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 Highest value 286 Wm -2 Lowest value 60 Wm -2 Global mean 185 Wm -2 Surface albedo Absorbed SW radiation at the surface Surface absorbed (Net) SW radiation = (1- albedo surface ) x Downward SW radiation at surface Radiation and Climate Change FS 2010 Martin Wild Highest value 270 Wm -2 Lowest value 22 Wm -2 Global mean 161 Wm -2
Downward longwave radiation Main factors that influence downward longwave radiation: Greenhouse gases (water vapour, CO2 ) Temperature of emitting atmosphere 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 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 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
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 Surface net radiation 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
Atmospheric SW absorption Atmospheric SW absorption (Atmospheric SW divergence) = SW Net (TOA) - SW Net (Surface) Effect of clouds on atmospheric SW absorption Global mean +79 Wm -2 High cloud enhances absorption, but reduces clear sky absorption of solar beam => atmospheric column absorption does not change much Low cloud enhances absorption, withouth reducing clear sky absorption of solar beam => atmospheric column absorption increases 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 Atmospheric LW divergence Atmospheric LW divergence (Atmospheric heat loss) = OLR (TOA) - LW Net (Surface) Global mean -184 Wm -2
Total (SW + LW) atmospheric divergence Global mean -105 Wm -2