outgoing longwave radiation
taylor, p. c. , 2012: tropical outgoing longwave radiation and longwave cloud forcing diurnal cycles from ceres .
figure 7h - 3: annual (1987) quantity of outgoing longwave radiation absorbed in the atmosphere .
noaa climate data record (cdr) of daily outgoing longwave radiation (olr), version 1. 2
_ _ _ _ _ _ et al. a technique for estimating outgoing longwave radiation from hirs radiance observations .
noaa climate data record (cdr) of daily outgoing longwave radiation (olr), version 1. 2 - urltoken
gruber, a. ; krueger, a. f. the status of the noaa outgoing longwave radiation data set .
schmetz, j. ; liu, q. outgoing longwave radiation and its diurnal variation at regional scales derived from meteosat .
schmetz, j. , and q. liu, 1988: outgoing longwave radiation and its diurnal variation at regional scales derived from meteosat .
hai - tien lee, et al, 2007: development of the hirs outgoing longwave radiation climate dataset. j. atmos. oceanic technol. , 24, 2029–2047
_ _ _ _ _ _; lee, h. - t. ; yanuk, d. validation of a technique for estimating outgoing longwave radiation from hirs radiance observations .
national center for atmospheric research staff (eds). last modified 05 sep 2014 .
the climate data guide: outgoing longwave radiation (olr): avhrr .
retrieved from urltoken
national center for atmospheric research staff (eds). last modified 09 sep 2014 .
the climate data guide: outgoing longwave radiation (olr): hirs .
retrieved from urltoken
ellingson, r. g. et al (1994): validation of a technique for estimating outgoing longwave radiation from hirs radiance observations. j. atmos. oceanic technol. , 11, 357–365
because of the spectral properties of the sun and earth radiation we tend to refer to them as
shortwave
and
longwave
radiation, respectively .
the upward longwave radiation product is a measure of the total outgoing longwave radiation (olr) at the top of the atmosphere, and provides important information regarding the earth’s outgoing energy and overall energy budget at the top of the atmosphere (toa). it is one of three radiation budget parameters that determine the earth radiation budget at the toa. the other two parameters are the incoming solar radiation and the reflected solar radiation. the olr is estimated directly from several abi infrared radiances for each abi pixel, regardless of sky condition. this product, together with the incoming total solar radiation and the reflected solar radiation define the earth’s radiation budget at the toa. understanding of the earth’s radiation budget is important for climate monitoring purposes .
liebmann, b. , and c. a. smith, 1996: description of a complete (interpolated) outgoing longwave radiation dataset. bull. amer. meteor. soc. , 77, 1275–1277
lau, k. m. , and p. h. chan, 1983: short - term climate variability and atmospheric teleconnections from satellite - observed outgoing longwave radiation. part i: simultaneous relationships .
ellingson, r. g. , d. j. yanuk, h. - t. lee, and a. gruber, 1989: a technique for estimating outgoing longwave radiation from hirs radiance observations .
ellingson, r. g. , h. - t. lee, d. yanuk, and a. gruber, 1994: validation of a technique for estimating outgoing longwave radiation from hirs radiance observations .
absorption of longwave radiation by the atmosphere causes additional heat energy to be added to the earth' s atmospheric system. the now warmer atmospheric greenhouse gas molecules begin radiating longwave energy in all directions. over 90% of this emission of longwave energy is directed back to the earth' s surface where it once again is absorbed by the surface. the heating of the ground by the longwave radiation causes the ground surface to once again radiate, repeating the cycle described above, again and again, until no more longwave is available for absorption .
the energy absorbed by the surface is radiated from the earth as terrestrial longwave radiation (l↑). the amount of energy emitted is primarily dependent on the temperature of the surface. the hotter the surface the more radiant energy it will emit. the gases of the atmosphere are relatively good absorbers of longwave radiation and thus absorb the energy emitted by the surface. the absorbed radiation is emitted in all directions with the downward directed portion being longwave atmospheric counter - radiation (l↓) .
otterman, j. , d. starr, t. brakke, r. davies, h. jacobowitz, a. mehta, f. cheruy, and c. prabhakara, 1997: modeling zenithangle dependence of outgoing longwave radiation: implication for flux measurements .
ellingson, r. g. ; ferraro, r. r. an examination of a technique for estimating the longwave radiation budget from satellite radiance observations .
the monthly g s values are defined by on individual 5° by 5° grids, where f ↓ refers to the downwelling longwave radiation at the earth’s surface .
_ _ _ _ _ _ et al. a comparison of erbe and avhrr longwave flux estimates .
in terms of hemispheric averages, the longwave and shortwave cloud forcing tend to balance each other in the winter hemisphere. in the summer hemisphere, the negative shortwave cloud forcing dominates the positive longwave cloud forcing, and the clouds result in a cooling. for deep convection the solar and longwave effects also cancel .
outgoing longwave radiation (olr) data at the top of the atmosphere are observed from the advanced very high resolution radiometer (avhrr) instrument aboard the noaa polar orbiting spacecraft. data are centered across equatorial areas from 160°e to 160°w longitude. the raw data are converted into a standardized anomaly index .
ellingson, r. g. , and r. r. ferrado, 1983: an examination of a technique for estimating the longwave radiation budget from satellite radiance observations .
a ll heated objects must emit electromagnetic radiation, particularly so if they are surrounded by empty space. this radiation is referred to as outgoing. as long as the incoming radiative flux is larger than the outgoing, the radiated object will continue to warm, and its temperature will continue to increase. this in turn will result in an increase in the outgoing radiation (according to the stefan - boltzman law the outgoing radiation increases faster than the temperature). at some point the object will emit as much radiation as the amount incoming and a radiative equilibrium (or balance) will be reached. using what we have learned about radiative heat transfer and some geometric calculation we can calculate the equilibrium temperature of an object if we know the amount of incoming energy. here is how we do that in the case of a planet rotating around the sun :
comparisons were made between estimates of the outgoing longwave radiation at the top of the atmosphere derived from narrowband advanced very high resolution radiometer (avhrr) and broadband earth radiation budget experiment (erbe) scanning instruments. four months of measurements are considered: april, july, and october, 1985 and january 1986. two types of comparisons are considered: ... [ show full abstract ]
comparisons have been made between estimates of the outgoing longwave radiation (olr) at the top of the atmosphere derived from narrowband advanced very high resolution radiometer (avhrr) and broadband earth radiation budget experiment (erbe) scanning instruments. four months of measurements are considered: april, july, and october 1985 and january 1986. instantaneous comparisons (i. e. ,... [ show full abstract ]
olr (outgoing longwave radiation) is a measure of the amount of energy emitted to space by earth' s surface, oceans and atmosphere. as such, it is a critical component of the earth' s radiation budget. in a different context, olr values are often used as a proxy for convection in tropical and subtropical regions since cloud top temperatures (colder is higher) are an indicator of cloud height .
trenberth ke, fasullo jt. global warming due to increasing absorbed solar radiation .
all bodies emit radiation and the wavelength (or frequency) and energy characteristics (or spectrum) of that radiation are determined solely by the body' s temperature .
under other circumstances net longwave can be zero or a positive number. if zero the amount of radiation emitted by the surface would be equal to that of the air. this is true when the air and surface have the same temperature. if the air is warmer than the ground a positive value exists. this could occur with heavy cloud cover (clouds are good absorbers and radiators of longwave radiation) or if a warm air mass travels over a colder surface .
in the ideal case (referred to as
black body
) matter will absorb all the energy impinging on it in the form of electromagnetic waves and as a result will warm up and itself become a radiation source. this
give and take
of energy leads to a state of equilibrium, where the outgoing radiation balances the incoming one .
the earth radiation budget part 2: energy from earth and earth' s temperature .
budyko m. the effect of solar radiation variations on the climate of the earth .
it is interesting to note that this exponent does not exactly correspond to the rayleigh - jeans approximation for longwave blackbody radiation (which would provide an expression proportional to t *). a higher value for l * would be required for that .
ohring, g. , a. gruber, and r. ellingson, 1984: satellite determination of the relationship between total longwave flux and infrared window radiance .
). in turn, increasing global surface temperature enhances emission of longwave (lw) radiation to space (the planck response). a schematic of the global energy budget response to a step change in greenhouse gas (ghg) concentrations is illustrated in
: outgoing lw radiation (olr) initially decreases because of enhanced lw absorption by higher ghg levels; as energy accumulates in the climate system, global temperature rises and olr increases until the toa energy balance is restored—when olr once again balances the net absorbed solar radiation (asr). in this canonical view of global warming, the net energy accumulation (shaded green area in
emma c. turner, simon f. b. tett. (2014) using longwave hirs radiances to test climate models. climate dynamics 43, 1103 - 1127
cho, h. , c. - h. ho, and y. - s. choi, 2012: the observed variation in cloud - induced longwave radiation in response to sea surface temperature over the pacific warm pool from mtsat - 1r imagery .
barkstrom, et al. earth radiation budget (erbe) archival and april 1985 results .
the determination of the earth' s radiation budget is essential to atmospheric modeling and climate studies. radiation budget experiments have used satellites to measure the fundamental radiation parameters - - the amount of solar energy received by the planet, the planetary albedo (the portion of incoming solar radiation that is reflected back to space), the emitted terrestrial radiation (also referred to as the outgoing longwave radiation - - olr), and the net planetary energy balance (the difference between the absorbed solar energy and the olr). the most recent experiment to measure these parameters is the nasa earth radiation budget experiment or erbe. results from erbe are presented below, you can find more information about erbe in the papers cited below. as you look at these maps, the month is printed in the upper left hand corner and the legend is given along the bottom of the figure. albedo values are given in units of percent reflectance, all others are in terms of energy per unit time per unit area (watts per square meter) .
bakstrom, b. r. ceres: the start of the next generation of radiation measurements .
recall that under normal conditions air temperature decreases with an increase in altitude through the troposphere. this occurs because the earth is the immediate source of energy for heating the air above it. knowing that heat is transferred from warmer to cooler bodies, this means the surface is normally hotter than the air above. so for most situations, net longwave radiation is a negative value as more longwave radiation is emitted by the earth than it gains from the air. some of this radiation is emitted out to space is lost from the earth system. that which is not lost to space is absorbed by the atmosphere and drives the
greenhouse
effect .
zhang, k. x. , m. d. goldberg, f. y. sun, l. h. zhou, w. w. wolf, c. y. tan, n. r. nalli, and q. h. liu (2017), estimation of near - real - time outgoing longwave radiation from cross - track infrared sounder (cris) radiance measurements, journal of atmospheric and oceanic technology, 34 (3), 643 - 655. urltoken
observe how the major deserts have their largest olr during their summer. this results from the annual temperature cycle - - as the desert surface heats up it emits more longwave radiation. note also the large emission in the vicinity of the oceanic subtropical highs (30n and 30s) .
in general, the absorbed solar radiation exceeds the outgoing longwave radiation in the tropical and subtropical regions, resulting in a net radiative heating of the planet, while in the middle to polar latitudes there is a net cooling. this equator - to - pole difference, or gradient, in radiative heating is the primary mechanism that drives the atmospheric and oceanic circulations. on an annual and long - term basis in which no energy storage and no change in the global mean temperature occurs, this radiative imbalance between the tropics and polar regions must be balanced by meridional heat transport by the atmosphere and oceans .
calvet, j. c. ; viswanadham, y. amazon forest radiation budget from satellite data .
before proceeding to investigate the effect of solar radiation on earth we should take a moment to review the physical laws governing the transfer of energy through radiation. in particular we should understand the following points :
harries, j. e. , and coauthors, 2005: the geostationary earth radiation budget project .
solar radiation occurs over a wide range of wavelengths. however, the energy of solar radiation is not divided evenly over all wavelengths but, as figure 1 shows, is rather sharply centered on the wavelength band of 0. 2 - 2 micrometers (μm = one millionth of a meter). as can be seen from figure 2, the main range of solar radiation includes ultraviolet radiation (uv, 0. 001 - 0. 4 μm), visible radiation (light, 0. 4 - 0. 7 μm), and infrared radiation (ir, 0. 7 - 100 μm) .
the greenhouse effect is the term applied to the warming of the earth’s surface that results from the fact that atmospheric gases have a greater transparency for incoming solar radiation than for outgoing longwave thermal radiation emitted by the earth’s surface. ultraviolet (uv) radiation contributes little directly to this whole global radiation balance. first, uv radiation is a small fraction of extraterrestrial solar radiation, and second, much of it is absorbed in the stratosphere in comparison with other radiation components. however, ozone (o 3) and chlorofluorocarbons (cfcs) are greenhouse effect gases. ozone is important because it absorbs radiation in the middle of the “atmospheric window, ” within the 8 to 12 micrometer wavelength bandwidth where many of the other standard greenhouse - effect gases (water vapor, co 2, ch 4, n 2 o) have little effect. tropospheric and stratospheric ozone play a role in the greenhouse effect. cfcs are important because 1) they have been recently introduced into the earth’s atmosphere, 2) they have a strong warming potential for each increment added to the atmosphere, 3) they will continue to be released to the atmosphere from emissions from closed cell foams and fugitive refrigerant gases. cfc substitutes may have their own greenhouse effect impacts .
the algorithm (a multivariate regression of goes irradiances on olr reference value) considers goes channels 3 (6. 4µm), 4 (11µm) and 5 (12µm), adding a virtual channel in 8 mm and a longwave tail (l > 15µm). nevertheless, similar statistical errors are obtained by a linear combination of channel 4 irradiance and the longwave tail only .
loeb ng, et al. towards optimal closure of the earth’s top - of - atmosphere radiation budget .
olr (outgoing longwave radiation) is a measure of the amount of energy emitted to space by earth' s surface, oceans and atmosphere. as such, it is a critical component of the earth' s radiation budget. in a different context, olr values are often used as a proxy for convection in tropical and subtropical regions since cloud top temperatures (colder is higher) are an indicator of cloud height. olr observations are made via the advanced very high resolution radiometer (avhrr) instrument aboard the noaa polar orbiting spacecraft. the raw ascending and descending swath data have been spatially and temporarally interpolated onto grids to facilitate use .
the monthly olr and surface downwelling longwave radiation in all - sky and clear - sky conditions between 2003 and 2014 are provided by the nasa ceres ebaf product 48. the monthly caf and tcpw are from the same product. the original gridded 1° × 1° data are all interpolated on a 5° by 5° box .
gube, m. radiation budget parameters at the top of the earth' s atmosphere derived from meteosat data .
ramanathan, v. et al. cloud radiative forcing and climate: results from the earth radiation budget experiment .
figure 7 - 12 simple greenhouse model. radiation fluxes per unit area of earth' s surface are shown .
the outgoing longwave radiation (olr) at the top of the atmosphere (toa) is an important meteorological variable, used in several ways. on one hand, it is used by the meteorological community as an index related to convective activity since lower olr values are associated to the top height of well developed clouds (schmetz; liu, 1988). on the other hand, the olr is the infrared component of the earth' s energy balance. together with the solar net radiation flux, it helps to describe the changes in energy availability induced by clouds through the difference between clear - sky and the observed radiative balances (ramanathan et al. , 1989) .
another part of the energy going to space from the earth is the electromagnetic radiation emitted by the earth. the solar radiation absorbed by the earth causes the planet to heat up until it is emitting as much energy back into space as it absorbs from the sun. because the earth is absorbing only a tiny fraction of the sun' s energy, it remains cooler than the sun, and therefore emits much less radiation. most of this emitted radiation is at longer wavelengths than solar radiation. unlike solar radiation, which is mostly at wavelengths visible to the human eye, the earth' s longwave radiation is mostly at infrared wavelengths, which are invisible to the human eye. when a cloud absorbs longwave radiation emitted by the earth' s surface, the cloud reemits a portion of the energy to outer space and a portion back toward the surface. the intensity of the emission from a cloud varies directly as its temperature and also depends upon several other factors, such as the cloud' s thickness and the makeup of the particles that form the cloud. the top of the cloud is usually colder than the earth' s surface. hence, if a cloud is introduced into a previously clear sky, the cold cloud top will reduce the longwave emission to space, and (disregarding the cloud albedo forcing for the moment) energy will be trapped beneath the cloud top. this trapped energy will increase the temperature of the earth' s surface and atmosphere until the longwave emission to space once again balances the incoming absorbed shortwave radiation. this process is called
cloud greenhouse forcing
and, taken by itself, tends to cause a heating or
positive forcing
of the earth' s climate. usually, the higher a cloud is in the atmosphere, the colder is its upper surface and the greater is its cloud greenhouse forcing .
the measured outgoing longwave radiation and albedo also indicate regional forcing mechanisms. for example, in the tropics east - west variations can be as large as the north - south averages and are associated with east - west circulations. tropical regions, in general, display a net radiative heating, the sahara is often experiencing a net radiatively cooling. this is due to the high surface albedo, the warm surface temperatures and the dry and cloud free atmosphere. the radiative cooling is maintained by subsidence warming, which also has a drying effect and therefore helps maintain the desert .
the product contains the 1 - degree by 1 - degree daily mean outgoing longwave radiation flux at the top of the atmosphere derived from hirs radiance observations onboard noaa tiros - n series and metop satellites. the olr retrieval uses multispectral regression models (ellingson et al. , 1989, lee, 2014). the cdr processing includes hirs radiance calibration, inter - satellite hirs olr calibration, normalization of geostationary - based olr retrieval to hirs, and grid - based 7 - day boxcar temporal integration assisted with imager - based olr derived from gridsat cdr data (lee, 2014) .
abel, p. g. , and a. gruber, 1979: an improved model for the calculation of longwave flux at 11 μm. noaa technical report, ness 106, ntis pb80 - 119431, 24 pp .
in order to maintain a stable climate, the earth must be in energetic equilibrium between the radiation it receives from the sun and the radiation it emits out to space. from this equilibrium we can calculate the effective temperature te of the earth .
figure 7 - 11 efficiency of absorption of radiation by the atmosphere as a function of wavelength. major absorbers are identified .
which is the derivative of the function f (l) representing the total radiation flux in the wavelength range [ 0, l ]. the total radiation flux ft emitted by a unit surface area of the object, integrated over all wavelengths, is
the greenhouse effect is well - established. increased concentrations of greenhouse gases, such as co 2, reduce the amount of outgoing longwave radiation (olr) to space; thus, energy accumulates in the climate system, and the planet warms. however, climate models forced with co 2 reveal that global energy accumulation is, instead, primarily caused by an increase in absorbed solar radiation (asr). this study resolves this apparent paradox. the solution is in the climate feedbacks that increase asr with warming—the moistening of the atmosphere and the reduction of snow and sea ice cover. observations and model simulations suggest that even though global warming is set into motion by greenhouse gases that reduce olr, it is ultimately sustained by the climate feedbacks that enhance asr .
radiation goes through a transformation when it encounters other objects (solid, gas or liquid). that transformation depends on the physical properties of that object and it is through this transformation that radiation can transfer heat from the emitting body to the other objects .
to calculate the radiation transmitted through a slab of length l, we integrate (7. 27) by separation of variables :
harrison, e. f. , d. r. brooks, p. minnis, b. a. wielicki, w. f. staylor, g. g. gibson, d. f. young, f. m. denn, and the erbe science team, 1988: first estimates of the diurnal variation of longwave radiation from the multiple - satellite earth radiation budget experiment (erbe), bull. am. meteorol, soc. , 69, 1144 - 1151 .
starting from this radiative equilibrium situation, we now perturb the equilibrium (step 2) by adding dm of species x, keeping everything else constant including temperature. if x is a greenhouse gas, then adding dm will decrease the outgoing terrestrial flux at the top of the atmosphere by an amount df; df is the radiative forcing caused by increasing the mass of x by dm. more generally, if fin and fout are the incoming and outgoing radiation fluxes in the radiative equilibrium calculation (fin = fout), then the radiative forcing associated with any perturbation to this equilibrium situation, and calculated with the same procedure as above, is defined as df = fin - fout .
because of its spherical shape incoming solar radiation is not equally distributed over the planet. at each instant, only the sun lights only half of the planet' s surface, with maximum radiation coming in at local noon and less in other times of the day .
figure 7 - 7 solar radiation spectra measured from a satellite outside earth' s atmosphere (in bold) and at sea level .
goes - 8 imager radiances in water vapor and infrared channels 3, 4 and 5 were used for assessing outgoing longwave radiation (olr) at the top of the atmosphere. estimation by itpp5 software applied to hirs / noaa 14 passes over brazil was considered as
true reference
. imagery from both satellites is currently acquired and processed at cptec / inpe. goes full - resolution imagery allows assessment of mean irradiance for sets of goes pixels contained within the area of a single hirs pixel. additional goes variables were estimated, such as: an
equivalent channel
at 8 mm and a
longwave tail
for l > 15 mm (this one is not detected neither by hirs nor goes sensors). isotropic outgoing radiance was assumed. multivariate regression of goes irradiances on olr itpp estimates provided a goes estimator with accuracy comparable with others already published and based on avhrr / noaa information. it was found that a regression based only on channel 4 and the longwave tail yielded estimates with the same accuracy: mean deviation of 3 w. m - 2 and standard deviation of 11 w. m - 2. the application for another period in the year, averaged over 1º´1º grid cells, yielded similar mean deviation and standard deviation of 7 w. m - 2. these results suggest that the algorithm applied in this work has physical rather than purely statistical meaning and could be used for olr monitoring in daily and seasonal scales .
olr (outgoing longwave radiation) is a measure of the amount of energy emitted to space by earth' s surface, oceans and atmosphere. as such, it is a critical component of the earth' s radiation budget. in a different context, olr values are often used as a proxy for convection in tropical and subtropical regions since cloud top temperatures (colder is higher) are an indicator of cloud height. olr observations are made via the advanced very high resolution radiometer (avhrr) instrument aboard the noaa polar orbiting spacecraft. the raw ascending and descending swath data have been spatially and temporarally interpolated onto grids to facilitate use. olr estimates are a key component for the standard madden - julian oscillation (mjo) clivar diagnostics which are used to study the mjo and for climate model evaluation .
a gas molecule absorbs radiation of a given wavelength only if the energy can be used to increase the internal energy level of the molecule. this internal energy level is quantized in a series of electronic, vibrational, and rotational states. an increase in the internal energy is achieved by transition to a higher state. electronic transitions, that is, transitions to a higher electronic state, generally require uv radiation (< 0. 4 mm). vibrational transitions require near - ir radiation (0. 7 - 20 mm), corresponding to the wavelength range of peak terrestrial radiation. rotational transitions require far - ir radiation (> 20 mm). little absorption takes place in the range of visible radiation (0. 4 - 0. 7 mm) which falls in the gap between electronic and vibrational transitions .
we examine in this chapter the role played by atmospheric gases in controlling the temperature of the earth. the main source of heat to the earth is solar energy, which is transmitted from the sun to the earth by radiation and is converted to heat at the earth' s surface. to balance this input of solar radiation, the earth itself emits radiation to space. some of this terrestrial radiation is trapped by greenhouse gases and radiated back to the earth, resulting in the warming of the surface known as the greenhouse effect. as we will see, trapping of terrestrial radiation by naturally occurring greenhouse gases is essential for maintaining the earth' s surface temperature above the freezing point .
loeb ng, et al. observed changes in top - of - the - atmosphere radiation and upper - ocean heating consistent within uncertainty .
diekmann, f. j. , and g. l. smith, 1989: investigation of scene identification algorithms for radiation budget measurements .
the terrestrial radiation flux absorbed by the atmospheric layer is fsto4. the atmospheric layer has both upward - and downward - facing surfaces, each emitting a radiation flux fst14 (kirchhoff' s law). the energy balance of the (earth + atmosphere) system, as viewed by an observer from space, is modified from equation (7. 10) to account for absorption and emission of radiation by the atmospheric layer :
because of the quantized nature of radiation, an object can emit radiation at a certain wavelength only if it absorbs radiation at that same wavelength. in the context of our simple model of figure 7 - 3, a particle can emit at a certain oscillation frequency only if it can be excited at that oscillating frequency. a blackbody is an idealized object absorbing radiation of all wavelengths with 100% efficiency. the german physicist max planck showed in 1900 that the flux distribution function flb for a blackbody is dependent only on wavelength and on the temperature t of the blackbody :
this climate data record (cdr) contains the daily mean outgoing longwave radiation (olr) time series in global 1 degree x 1 degree equal - angle gridded maps spanning from january 1, 1979 to december 31, 2013, and continuing daily with a two - day lag. the olr is estimated directly from the hirs radiance observations for all sky conditions. the observations from imagers onboard international operational geostationary satellites are incorporated to improve the sampling of the olr diurnal variation. the daily olr cdr is at its initial version 1. 2. the data file format is netcdf - 4 with cf metadata, and it is accompanied by algorithm documentation, data flow diagram and source code for the noaa cdr program .
luther, f. m. , and r. g. ellingson. 1985. carbon dioxide and the radiation budget, p. 25–55 .
by contrast, in the strong co2 absorption band at 15 mm, radiation emitted by the earth' s surface is absorbed by atmospheric co2, and the radiation re - emitted by co2 is absorbed again by co2 in the atmospheric column. because the atmosphere is opaque to radiation in this wavelength range, the radiation flux measured from space corresponds to emission from the altitude at which the co2 concentration becomes relatively thin, roughly in the upper troposphere or lower stratosphere. the 15 mm blackbody temperature in figure 7 - 8 is about 215 k, which we recognize as a typical tropopause temperature .
this absorption of energy by the earth must be balanced by emission of terrestrial radiation out to space. the earth is not a blackbody at visible wavelengths since the absorption efficiency of solar radiation by the earth is only e = 1 - a = 0. 72. however, the earth radiates almost exclusively in the ir where the absorption efficiency is in fact near unity. for example, clouds and snow reflect visible radiation but absorb ir radiation. we approximate here the emission flux from the earth as that of a blackbody of temperature te, so that the energy balance equation for the earth is
the albedo, olr and net radiation are closely related to surface type and the weather regime. for example, look at the sahara desert and the amazon basin in the summer and winter. the incoming solar radiation is a function of latitude and time of year. the desert is approximate 20 degrees north latitude while the amazon basin is approximately 20 degrees south of the equator. so the incoming solar radiation in the amazon in january (southern hemisphere summer) is nearly the same as the incoming solar radiation over the sahara in july (northern hemisphere summer). the two regions also have very high albedos during their respective summers - - but for two different reasons. the high albedos of the amazon are the result of highly reflecting deep - convective cloud systems. over the desert, there are few clouds, but the surface, which is mostly dry soil, is highly reflective. the olr is very different for these two regions. the amount of terrestrial radiation is a function of temperature, the tops of the convective clouds are very cold, and so the outgoing energy is small. in contrast the desert surface is very warm, and so the olr is large. what is the difference in the net radiation for these two regions ?
and 2 2 y 3 (t v) = represents the monochromatic transmittance for (initially) isotropic diffuse radiation emitted at the optical depth t v .
the monthly g a values are calculated by on each 5° by 5° grid 42, where σ is the stenfan–boltzmann constant (5. 67 × 10 −8 w m −2 k −4). g a represents the difference between the heat emitted by the earth’s surface and the longwave radiation escaping from the toa. the former energy is estimated based on the assumption of blackbody radiation; thus, the emissivity (ε) is close to one for every underlying surface. this assumption is confirmed by the global surface emissivity map (fig. s4) derived using the nasa ceres ebaf products 48 .
the study of clouds, where they occur, and their characteristics, play a key role in the understanding of climate change. low, thick clouds primarily reflect solar radiation and cool the surface of the earth. high, thin clouds primarily transmit incoming solar radiation; at the same time, they trap some of the outgoing infrared radiation emitted by the earth and radiate it back downward, thereby warming the surface of the earth. whether a given cloud will heat or cool the surface depends on several factors, including the cloud' s altitude, its size, and the make - up of the particles that form the cloud. the balance between the cooling and warming actions of clouds is very close although, overall, averaging the effects of all the clouds around the globe, cooling predominates .
the solar and terrestrial properties of clouds have offsetting effects in terms of the energy balance of the planet. in the longwave, clouds generally reduce the radiation emission to space and thus result in a heating of the planet. while in the solar (or shortwave), clouds reduce the absorbed solar radiation, due to a generally higher albedo than the underlying surface, and thus result in a cooling of the planet. view the maps of cloud forcing given above. does the presence of low level clouds over oceans heat or cool the planet? what about the convective clouds over the oceans ?
cite dataset when used as a source: hai - tien lee and noaa cdr program (2011): noaa climate data record (cdr) of daily outgoing longwave radiation (olr), version 1. 2. [ indicate subset used ]. noaa national climatic data center. doi: 10. 7289 / v5sj1hh2 [ access date ]. , ncei cannot assume liability for any damages caused by any errors or omissions in the data, nor as a result of the failure of the data to function on a particular system. ncei makes no warranty, expressed or implied, nor does the fact of distribution constitute such a warranty. ncei can only certify that the data it distributes are an authentic copy of the records that were accepted for inclusion in the ncei archives .
the planck blackbody formulation for the emission of radiation is generalizable to all objects using kirchhoff' s law. this law states that if an object absorbs radiation of wavelength l with an efficiency el, then it emits radiation of that wavelength at a fraction el of the corresponding blackbody emission at the same temperature. using kirchhoff' s law and equation (7. 3), one can derive the emission spectrum of any object simply by knowing its absorption spectrum and its temperature :
sinha, a. , and j. e. harries, 1997: the earth’s clear - sky radiation budget and water vapor absorption in the far infrared .
the model calculates the distribution of atmospheric temperatures necessary to achieve a global radiative equilibrium for the earth system, that is, an exact balance between the incoming solar radiation flux at the top of the atmosphere (fs / 4), the outgoing solar radiation flux reflected by the earth system (fsa / 4), and the terrestrial radiation flux emitted by the earth system (fs (1 - a) / 4). this equilibrium is necessary for a stable climate; as we will see below, even a small deviation would cause a large temperature perturbation. the model used for the calculation may be as simple as a 1 - dimensional (vertical) formulation of radiative equilibrium, or as complicated as a gcm; the choice of model is not too important as long as the calculated temperature profiles are reasonably realistic .
the absorption or scattering of radiation by an optically active medium such as the atmosphere is measured by the optical depth d of the medium. we have seen above how gas molecules absorb radiation; they also scatter radiation (that is, change its direction of propagation without absorption) but this scattering is inefficient at visible and ir wavelengths because of the small size of the gas molecules relative to the wavelength. scattering is important for aerosols, which we will discuss in the next chapter. consider in the general case a thin slab [ x, x + dx ] of an optically active medium absorbing or scattering radiation (figure 7 - 17) :
thus the radiation decays exponentially with propagation distance through the slab. we define d = n (sabs + sscat) l as the optical depth of the slab :
a typical object emits radiation over a continuous spectrum of frequencies. using a spectrometer we can measure the radiation flux df (w m - 2) emitted by a unit surface area of the object in a wavelength bin [ l, l + dl ]. this radiation flux represents the photon energy flowing perpendicularly to the surface. by covering the entire spectrum of wavelengths we obtain the emission spectrum of the object. since df depends on the width dl of the bins and this width is defined by the resolution of the spectrometer, it makes sense to plot the radiation spectrum as df / dl vs. l, normalizing for dl (figure 7 - 4) .
barkstrom, b. r. , the earth radiation budget experiment (erbe), 1984: bull am. meteorol. soc. , 65, 1170 - 1185 .
houghton, j. t. , the physics of atmospheres, 2nd ed. , cambridge university press, new york, 1986. blackbody radiation, gray atmosphere model .
this solar radiation flux fs is intercepted by the earth over a disk of cross - sectional area pre2 representing the shadow area of the earth (figure 7 - 9). a fraction a of the intercepted radiation is reflected back to space by clouds, snow, ice... ; a is called the planetary albedo. satellite observations indicate a = 0. 28 for the earth. thus the solar radiation absorbed by the earth per unit time is given by fspre2 (1 - a). the mean solar radiation flux absorbed per unit area of the earth' s surface is fspre2 (1 - a) / 4pre2 = fs (1 - a) / 4 .
the total daily radiation decreases from equator to pole. thus the earth' s surface should inherently be warmer at the equator than it is at the poles. however, …
absorption or scattering is more efficient if the radiation beam falls on the slab with a slant angle q relative to the perpendicular, because the radiation then travels over a longer path inside the slab (figure 7 - 18). the physical path of the beam through the slab is l / cosq, and the optical path is d / cosq :
allen l. h. (1994) uv radiation as related to the greenhouse effect. in: biggs r. h. , joyner m. e. b. (eds) stratospheric ozone depletion / uv - b radiation in the biosphere. nato asi series (series i: global environmental change), vol 18. springer, berlin, heidelberg
as energy from the sun passes through the atmosphere a number of things take place (see figure 7h - 1). a portion of the energy (26% globally) is reflected or scattered back to space by clouds and other atmospheric particles. about 19% of the energy available is absorbed by clouds, gases (like ozone), and particles in the atmosphere. of the remaining 55% of the solar energy passing through the earth' s atmosphere, 4% is reflected from the surface back to space. on average, about 51% of the sun' s radiation reaches the surface. this energy is then used in a number of processes, including the heating of the ground surface; the melting of ice and snow and the evaporation of water; and plant photosynthesis. the heating of the ground by sunlight causes the earth' s surface to become a radiator of energy in the longwave band (sometimes called infrared radiation). this emission of energy is generally directed to space (see figure 7h - 2). however, only a small portion of this energy actually makes it back to space. the majority of the outgoing infrared radiation is absorbed by the greenhouse gases (see figure 7h - 3 below) .
when coefficients obtained for june 2001 are used for olr assessment in august 2001, averages over a 1º×1º grid show errors with standard deviation of ±7 w. m - 2. absolute errors lower than 14 w. m - 2 lie over the most part of noaa overpass. this result suggests that the algorithm describes physical rather than statistical or seasonal properties of outgoing longwave flux. given the better resolution and higher frequency of goes imagery, a detailed monitoring of daily and seasonal cycles of olr over south american region could be allowed. further analysis and improvements of these aspects are recommended, especially comparisons with other olr estimation models available for noaa 14 and 16 and
absolute
measurements like those of ceres sensor .
figure 7 - 13 radiation fluxes emitted to space at three different wavelengths and for the temperature profile in the left panel. opaque regions of the atmosphere are shown in gray shading .
the greenhouse effect is a naturally occurring process that aids in heating the earth' s surface and atmosphere. it results from the fact that certain atmospheric gases, such as carbon dioxide, water vapor, and methane, are able to change the energy balance of the planet by absorbing longwave radiation emitted from the earth' s surface. without the greenhouse effect life on this planet would probably not exist as the average temperature of the earth would be a chilly - 18° celsius, rather than the present 15° celsius .
a terrestrial radiation spectrum measured from a satellite over north africa under clear - sky conditions is shown in figure 7 - 8. as we will see in section 7. 3. 3, the terrestrial radiation spectrum is a combination of blackbody spectra for different temperatures, ranging from 220 to 320 k for the conditions in figure 7 - 8. the wavelength range of maximum emission is 5 - 20 mm. the earth is not sufficiently hot to emit significant amounts of radiation in the visible range (otherwise nights wouldn' t be dark !) .
because the atmosphere is almost transparent to sunlight, all that is absorbed at the surface results in warming and the emission of ir radiation; this radiation cannot freely escape into space because of absorption in the atmosphere by trace gases such as water vapor and carbon dioxide (co 2). these absorbing gases and their surrounding air warm up, emitting radiation downward, towards the earth' s surface, as well as upward, towards space. this effectively traps part of the ir radiation between ground and the lower 10 km of the atmosphere. this reduction in the efficiency of the earth to lose heat causes the surface temperature to rise above the effective temperature calculated above (t e) until finally, enough heat is able to escape to space to balance the incoming solar radiation. the effect is analogous to that of a blanket that traps the body heat preventing it from escaping into the room and thus keeps us warm on cold nights .
if the earth had no atmosphere, a surface temperature far below freezing would produce enough emitted radiation to balance the absorbed solar energy. but the atmosphere warms the planet and makes earth more livable. clear air is largely transparent to incoming shortwave solar radiation and, hence, transmits it to the earth' s surface. however, a significant fraction of the longwave radiation emitted by the surface is absorbed by trace gases in the air. this heats the air and causes it to radiate energy both out to space and back toward the earth' s surface. the energy emitted back to the surface causes it to heat up more, which then results in greater emission from the surface. this heating effect of air on the surface, called the atmospheric greenhouse effect, is due mainly to water vapor in the air, but also is enhanced by carbon dioxide, methane, and other infrared - absorbing trace gases .
we still need to relate the radiative forcing to change in the earth' s surface temperature, which is what we ultimately care about. such a relationship can be derived using our simple 1 - layer model for the atmosphere in section 7. 3. 2. in this model, the outgoing terrestrial flux for the initial atmosphere in radiative equilibrium (step 1) is (1 - f / 2) sto4, where f is the absorption efficiency of the atmospheric layer and to is the surface temperature (equation (7. 15) ). increasing the abundance of a greenhouse gas by dm corresponds to an increase df of the absorption efficiency. thus the outgoing terrestrial flux for the perturbed atmosphere (step 2) is (1 - (f + df) / 2) sto4. by definition of the radiative forcing df ,
the sensitivity of outgoing longwave radiation (olr) estimation techniques, based on either advanced very high resolution radiometer (avhrr) or high resolution infrared sounder (hirs) data, has been simulated using radiation transfer model calculations for different atmospheric conditions. the simulations have focused on the effects of near - surface temperature discontinuity over desert surfaces and on variations in the strength of temperature and moisture inversion over subtropical high pressure regions. sensitivity tests show that the avhrr technique overestimates the reference model olr when skin temperature exceeds the surface air temperature, with the overestimate increasing with skin–air temperature differences. the magnitude of the overestimates appears similar to what was found in comparisons of avhrr derived and earth radiation budget experiment (erbe), olr fluxes over desert surfaces. unlike avhrr, the hirs technique shows no tendency to over - or - under estimate model calculations for such conditions. in subtropical high - pressure regions, the avhrr olr fluxes tend to be smaller than those observed by erbe. as temperature inversion becomes stronger, the olr difference between model calculations and avhrr estimations increases, but the strength of the inversion does not have much effect on the hirs technique. as the h2o content in the inversion layer increases, the model and hirs derived fluxes decrease while the avhrr derived fluxes do not follow the variations of the radiation balance between the surface and near surface levels. results indicate that large flux differences between erbe and avhrr over desert and subtropical oceanic regions are due to shortcomings in the avhrr technique while the hirs technique is successfully sensitive to the variations in air temperature and water vapor.
outgoing longwave radiation ( olr ) is the energy radiating from the earth as infrared radiation at low energy to space . olr is electromagnetic radiation emitted from earth and its atmosphere out to space in the form of thermal radiation . the flux of energy transported by outgoing longwave radiation is measured in w/m ² . over 99 % of outgoing longwave radiation has wavelengths between 4 µm and 100 µm , in the thermal infrared part of the electromagnetic spectrum . contributions with wavelengths larger than 40 µm are small , therefore often only wavelengths up to 50 µm are considered . in the wavelength range between 4 µm and 10 µm the spectrum of outgoing longwave radiation overlaps that of solar radiation , and for various applications different cut-off wavelengths between the two may be chosen . radiative cooling by outgoing longwave radiation is the primary way the earth system loses energy . the balance between this loss and the energy gained by radiative heating from incoming solar shortwave radiation determines global heating or cooling of the earth system ( energy budget of earth ’s climate ) . local differences between radiative heating and cooling provide the energy that drives atmospheric dynamics . 
outgoing longwave radiation (olr) is the energy radiating from the earth as infrared radiation at low energy to space. olr is electromagnetic radiation emitted from earth and its atmosphere out to space in the form of thermal radiation. the flux of energy transported by outgoing longwave radiation is measured in w/m ². over 99% of outgoing longwave radiation has wavelengths between 4 µm and 100 µm, in the thermal infrared part of the electromagnetic spectrum. contributions with wavelengths larger than 40 µm are small, therefore often only wavelengths up to 50 µm are considered. in the wavelength range between 4 µm and 10 µm the spectrum of outgoing longwave radiation overlaps that of solar radiation, and for various applications different cut-off wavelengths between the two may be chosen. radiative cooling by outgoing longwave radiation is the primary way the earth system loses energy. the balance between this loss and the energy gained by radiative heating from incoming solar shortwave radiation determines global heating or cooling of the earth system (energy budget of earth ’s climate). local differences between radiative heating and cooling provide the energy that drives atmospheric dynamics.
outgoing longwave radiation (olr) is the energy radiating from the earth as infrared radiation at low energy to space. olr is electromagnetic radiation emitted from earth and its atmosphere out to space in the form of thermal radiation. the flux of energy transported by outgoing longwave radiation is measured in w/m ². over 99% of outgoing longwave radiation has wavelengths between 4 µm and 100 µm, in the thermal infrared part of the electromagnetic spectrum. contributions with wavelengths larger than 40 µm are small, therefore often only wavelengths up to 50 µm are considered. in the wavelength range between 4 µm and 10 µm the spectrum of outgoing longwave radiation overlaps that of solar radiation, and for various applications different cut-off wavelengths between the two may be chosen. radiative cooling by outgoing longwave radiation is the primary way the earth system loses energy. the balance between this loss and the energy gained by radiative heating from incoming solar shortwave radiation determines global heating or cooling of the earth system (energy budget of earth ’s climate). local differences between radiative heating and cooling provide the energy that drives atmospheric dynamics.