Recent scientific studies indicate that industrial andagricultural activities are responsible for the rapidchange in environmental variables that directly orindirectly affect the climate. It thus becomes essen-tial to quantify accurately the potential effect of hu-man activity on the Earth’s climate. This in turnimplies that it is necessary to distinguish among thevarious components of the global radiative forcingthat act upon the Earth’s surface: tropospheric andstratospheric trace gases and aerosol distribution, so-lar flux temporal variability, and cloud–radiation in-teractions.
The increase since the preindustrial period in an-
P. Chazette is with the Laboratoire des Sciences du Climat et del’Environnement, Commissariat a l’Energie Atomique, InstitutPierre-Simon-Laplace, C. E. Saclay, Batiment 709, Orme des Meri-siers, F-91191 Gif-sur-Yvette Cedex, France. C. Clerbaux and G.Megie are with the Service d’Aeronomie du Centre National de laRecherche Scientifique, Institut Pierre-Simon-Laplace, UniversitePierre et Marie Curie, B 102, 4 Place Jussieu, F-75252 Paris Cedex05, France.
Received 27 May 1997; revised manuscript received 26 Septem-ber 1997.
thropogenic emissions of tropospheric and strato-spheric greenhouse gases, such as carbon dioxide~CO2!, methane ~CH4!, nitrous oxide ~N2O!, and thehalocarbons, induces additional radiative forcingthat could lead to climatic global warming. The ra-diative forcing is estimated to range from 2.1 to 2.6Wm22.2–4 Predictions from climatic models providea mean temperature increase of the Earth’s surfaceranging from 0.2 to 3.4 K for the next 500 years,associated with a new ecosystem distribution and arise in mean level ranging from sea level 0.2 to 1.3 m.5
Methane is an important contributor to this addi-tional forcing. The present estimated value of theadditional radiative forcing associated with methaneis 0.47 6 0.07 Wm22 ~615%! since the preindustrialperiod, and it represents ;20% of the direct globalgreenhouse effect.1 Four hundred years ago the av-erage methane concentration was ;0.7 parts in 106
per volume ~ppmv! in the lower troposphere, whereasthe actual mixing ratio is estimated to be 1.7 ppmvwith a mean growth rate of 0.6%yyear over the de-cade beginning in 1984.1 The main known sourcesof methane are wetlands, rice paddies, fossil-fuel-related sources, biomass burning, and cattle. Themost important sink is oxidation by the hydroxyl rad-ical OH.
Methane mixing ratios are also influenced by emis-
20 May 1998 y Vol. 37, No. 15 y APPLIED OPTICS 3113
sion of other gases, leading to chemical reactions thatin turn modify the methane content.6 Indeed, meth-ane is one of the major tropospheric ozone ~O3! pre-cursors. Today the budget of methane is not exactlybalanced; hence the methane contribution to the ad-ditional radiative forcing is difficult to anticipate.Nevertheless, if its trend stays at the current rate,the methane concentration will reach 3.6 ppmv in 100years, inducing a total additional radiative forcinggreater than 1 Wm22.1
The chemical lifetime of methane is ;10 years.6Because of the strong spatial variability of both sur-face temperature and methane sources and sinks, theradiative forcing of methane and its associated cli-matic effects are difficult to assess without global-scale measurements. Spaceborne spectrometricobservations might allow us to estimate the radiativeforcing of greenhouse gases on a global scale and tofollow their temporal evolution. This mission is in-cluded in the measurement objectives of the space-borne Interferometric Monitor for Greenhouse Gases~IMG! and Infrared Atmospheric Sounding Inter-ferometer ~IASI!, which will provide spatiotemporaldistribution of tropospheric constituents such asozone, methane, carbon monoxide, nitrous oxide, andchlorofluorocarbons. Other similar instruments,such as the High-Resolution Interferometer Sounder~HIS! and the Airborne Infrared Spectrometer~AIS!,9,10 are currently flying onboard airplanes, andthe Atmospheric Infrared Sounder ~AIRS! and theTropospheric Emission Spectrometer ~TES!,11,12 arescheduled for launch on polar-orbiting satellites.
The IMG and the IASI instruments are Fourier-transform spectrometers that measure the radiancecoming through the atmosphere in the infrared spec-tral range from 3.3 to 14 and from 3.62 to 15.5 mm,respectively. The IMG was onboard the JapaneseADEOS platform, which was launched on 17 August1996 on a polar Sun-synchronous orbit and stopped toprovide data at the end of June 1997. The IASI isscheduled to be launched in 2002 for a 5-year-long-duration mission on the European polar platformMETOP ~Meteorological Operational Satellite! andwill be able to scan a swath of ;2000 km. The IMGprovides atmospheric spectra with an apodized spec-tral resolution of 0.1 cm21, whereas the spectral res-olution associated with the IASI will vary from 0.5 to1.5 cm21 after apodization, depending on the spectralrange considered.
We demonstrate here that the instantaneous radi-ative forcing of methane can be determined fromwell-chosen nadir spectral radiances provided by aspaceborne spectrometer ~Section 2!. Relevant spec-tral intervals have been selected ~Section 3! and in-troduced into a radiative transfer model to permit usto calculate the radiative forcing that is due to meth-ane ~Section 4!. Sensitivity studies are performedas a function of the spectrometer’s spectral resolu-tion, noise, and interfering gases by use of the three-dimensional chemical-transport model IMAGES~Intermediate Model of the Annual and Global Evo-lution of Species!.13
3114 APPLIED OPTICS y Vol. 37, No. 15 y 20 May 1998
2. Retrieval of the Radiative Forcing from MeasuredRadiances
A classic two-step procedure can be used to estimatethe radiative forcing associated with methane: in-version of radiances to retrieve concentration andthen computation of radiative forcing associated withthese concentrations. The latter step requires pre-cise knowledge of the vertical profiles for both meth-ane concentration and temperature. A majordifficulty then arises from the fact that concentrationprofiles are only poorly determined by nadir-lookinginstruments.
An original method is proposed for determining themethane’s radiative forcing directly from measuredspectra. This procedure is well adapted to space-borne measurements and provides accurate calcula-tions in a short computing time. The basic idea is tofind statistical relationships between the radiativeforcing and the selected measured radiances.
The radiative forcing FCH4is given by the following
expression:
FCH45 *
2pS *
IR spectrum
@LCH4~s, 12 km!
2 ε~s!BT~s!#dsDcos~u!dV, (1)
where LCH4~s, 12 km! is the spectral radiance at the
wave number s emerging through the top of the at-mospheric layer at tropopause altitude for an atmo-sphere consisting only of methane. BT~s! and ε~s!are, respectively, the blackbody radiance at the sur-face thermodynamic temperature T and the Earth’ssurface emissivity. u is the observation angle fromthe nadir-looking mode, and V is the solid angle cor-responding to the instrument’s field of view. Inte-grations are done over V in the upper hemisphere ~2psr! and over s in the infrared spectrum. Calcula-tions are made with a radiative transfer model de-veloped for internal use at the Laboratoire desSciences du Climat et de l’Environnement, Gif-sur-Yvette, France. This code is restricted to the ther-mal infrared spectral range and applies to clear-skyand aerosol-free conditions. It has been validatedfor these conditions with other widely used radiativetransfer codes.
The differential radiance LDs that comes throughthe atmosphere in the limited spectral range Ds cen-tered on a methane absorption line is given by
LDs 5 *Ds
@L~s, `! 2 ε~s!BT~s!#ds, (2)
where L~s, `! is the radiance measured by the spa-ceborne spectrometer in the nadir-looking mode foreach wave number. Calculating the surface emis-sion function requires knowledge of surface temper-atures and emissivity. They can be retrieved fromthe same instrument by use of the atmospheric win-dow located in the spectral intervals 770–980, 1080–
Table 1. Selected Narrow Spectral Intervals for the Main Isotope of Methane and Relative Contributions of Interfering Gases to CalculatedDifferential Radiance as a Function of Instrumental Resolution
Spectral Interval~cm21!
InterferingSpecies
Relative Contribution to the Differential Radiance ~%!
High Resolution~0.01 cm21!
IMG~0.1 cm21!
IASI~1–1.5 cm21!
1237.7–1238.3 ~n4 band! H2O 2.9N2O 9.4CO2 1.2
Global 13.5 13.5 14.51240.7–1241.3 ~n4 band! H2O 2.2
N2O 7.1CO2 0.6
Global 9.8 10.1 17.82741–2743.4 ~n3 band! H2O 0.8
N2O 0.03CO2 0.3
Global 1.2 1.2 2.42847.4–2848 ~n3 band! H2O 0.6
N2O ,0.01CO2 ,0.01
Global 0.6 0.8 —
1150, and 2420–2720 cm21, as for the Advanced VeryHigh Resolution Radiometer.14,15
Our aim is to provide a relationship between FCH4
and LDs:
FCH45 h~LDs!, (3)
which could be used for both the IMG and the IASIinstruments.
3. Selection of Radiance Intervals
To meet the mission objectives of the two instrumentsIMG and IASI, i.e., the determination of radiativeforcing with good accuracy, isolated lines or group oflines in the vibrational n3 ~2800–3200-cm21! and n4~1200–1400-cm21! bands of methane were chosen.The selection was made among the intense and non-saturated methane lines that do not overlap otherlines associated with interfering gases. The radi-ances coming through the atmosphere were simu-lated by a radiative transfer model that uses theHITRAN spectroscopic database.16 The main tropo-spheric interfering gases in this spectral range, i.e.,water vapor, nitrous oxide, and carbon dioxide, weretaken into account. Some narrow spectral intervalswere selected in the infrared spectral range accessi-ble to the IMG and the IASI instruments and arelisted in Table 1. Note that in the n3 band the2847.4–2848-cm21 interval is outside the IASI mea-surement spectral range. Three of these four se-lected intervals are characterized by one intenseabsorption line that belongs to the main isotope ofmethane. These intervals are located at 1238.02cm21 @P~10!#, 1241.00 cm21 @P~10!#, and 2847.72cm21 @R~0!#. Their associated transmittances do notexceed 0.45. The 2741–2743.4-cm21 spectral rangeof the n3 band contains a group of lines with a similarintensity of ;2.6 3 10219 cmymolecule.
For each interval the potential contribution of in-
terfering gases to the measured radiances was inves-tigated at a spectral resolution of 0.01 cm21. Thesimulations were carried out with representative cli-matic profiles for nitrous oxide and carbon dioxide.Because of the strong variability of the water-vapormixing ratios as a function of air-mass types, theTIGR ~TOVS Initial Guess Retrieval! database wasused for realistic simulations.17 The influence of wa-ter vapor is larger in the tropical regions where themean integrated water-vapor content is 2.9 6 1.35gycm2, compared with 1.23 6 0.79 and 0.55 6 0.44gycm2 in the mid-latitudes and the polar regions,respectively. Considering the global temperaturevariability, the calculated contributions of thesegases to the differential radiance were averaged overthe globe and are provided in Table 1. In the n4 bandthe main interferences are attributed to nitrous oxide~7.1 and 9.4%! and water vapor ~,3%!. The contri-bution of carbon dioxide to the differential radiance is;1%. In the n3 band, interferences that are due tothese species are even weaker. The water-vapor ra-diative effect is the more significant contribution andis less than 1%.
The contributions associated with all the interfer-ing gases were added to provide a global mean per-turbation. Total perturbations that are due tointerferences with other gases were found to be ap-proximately 13.5%, 9.8%, 1.2%, and 0.6% for the se-lected intervals ~see Table 1!. We performedsimulations to study the effect of the instrumentalresolution on these values. Results are provided inTable 1 and were obtained at 0.1 cm21 ~IMG!, 1 cm21
~IASI n4 band!, and 1.5 cm21 ~IASI n3 band!. For thepurpose of illustration, Fig. 1 shows the radiancespectrum simulated for both the IMG and the IASIspectrometers for the 2741–2743.4-cm21 selectedspectral interval. It can be seen that the effect of theinstrumental function is important. The IMG spec-
20 May 1998 y Vol. 37, No. 15 y APPLIED OPTICS 3115
tral resolution of 0.1 cm21 allows us to resolve themain methane lines in the spectral range considered.But at the IASI spectral resolution the lines aremerged into only one spectral signature.
For lower spectral resolutions the wings of the in-strumental function can introduce interference fromnon-methane-absorbing lines located outside the se-lected spectral ranges. At the IMG spectral resolu-tion the contribution of interfering species is smalland the values are not or are only slightly modified.On the contrary, at the IASI spectral resolution theperturbation increases differently for each spectralinterval. In the worst case it varies from 10.1% to
Fig. 1. Spectral differential radiance as a function of wave num-ber for both the IMG and the IASI spectral resolutions in theselected 2741–2743.4-cm21 spectral range.
3116 APPLIED OPTICS y Vol. 37, No. 15 y 20 May 1998
17.8% ~see Table 1!. This variation is due mainly tothe water-vapor lines that interfere with the selectedmethane line. At this spectral resolution water va-por becomes the main interfering species for both then4 and the n3 bands. As a consequence, the accuracyof the proposed method in retrieving radiances de-pends on the spectral resolution.
4. Methane Radiative Forcing Estimated from IMG andIASI Spectra
We first determine the h function in Eq. ~3!, not tak-ing into account potential interfering species. In asecond step we consider the radiative effects associ-ated with these interfering gases ~water vapor, ni-trous oxide, and carbon dioxide! to evaluate theuncertainty on the retrieved methane radiative forc-ing. A third step is the study of the contribution ofthe instrumental noise to assessment of the h func-tion.
The concentration fields calculated with the IM-AGES model were used for this study.13 The 2520vertical mixing ratio profiles of methane simulatedfor summertime were considered. The ECMWF~European Center for Medium-Range Weather Fore-casts! temperature vertical profiles used in the IM-AGES model were introduced, along with theassociated methane profiles, into the radiative code toenable us to calculate the corresponding radiativeforcing. The calculated values are plotted in Fig. 2.It can be seen that the spatial distribution of thecalculated radiative forcing values is far from beinghomogeneous over the globe. The calculated meanvalue for the methane contribution is 2.9 Wm22, with
Fig. 2. Methane radiative forcing computed by a line-by-line radiative transfer model for summertime with the IMAGES model outputs.The largest values are observed above the warm surfaces and close to the methane sources.
a standard deviation of 1.13 Wm22. Radiative forc-ing values are strongly correlated to the surface tem-peratures, and the radiative effect is more importantabove the warm surfaces. To evaluate the radiativecontribution that is due to the spatial distribution ofmethane alone, we considered a constant mean tem-perature profile ~midlatitude condition!. A changein the methane concentration from a low to a highconcentration level ~source region! provides two radi-ative forcing values that differ by ;12%. We con-sider this value a minimum for the precision to beachieved by the inversion method to quantify effi-ciently the spatial variability of the methane radia-tive forcing.
A. Determination of the Radiative Forcing Law
Figure 3 shows the variation of the radiative forcingassociated with methane ~FCH4
! as a function of thenormalized differential radiance ~LDs! for a spectralresolution of 0.1 cm21 and for both 1240.7–1241.3-and 2741–2743.4-cm21 spectral intervals. The airmass types ~polar, midlatitude, and tropical! are alsorepresented. As expected, the forcing is smaller inthe case of polar air masses and larger for tropical airmasses. The data distribution in Fig. 3~a! ~n4 band!behaves as a linear function, and a mean-squares fitprovides the following h function:
Fig. 3. Methane radiative forcing as a function of the differentialradiance for the nadir-looking mode at 0.1-cm21 spectral resolutionfor ~a! the 1240.7–1241.3-cm21 and ~b! the 2741–2743.4-cm21 in-tervals. The air-mass types are represented in various shades ofa gray scale: black for polar, dark gray for mid-latitude, andlighter gray for tropical air masses.
h 5 5.92x 1 9.38 3 1022, (4)
whereas @Fig. 3~b!# for the n3 band the h function isbetter fitted with a power law function:
h 5 5.46~x!0.49, (5)
where x is the normalized differential radiance forthe larger value LD
max observed in the tropics ~x 5LDsyLDs
max!.When the spectral resolution is changed from 0.1 to
1 cm21 the general behavior of the relationship re-mains the same but the coefficients of the h functionschange slightly because of the decrease of the differ-ential radiance with the degradation of spectral res-olution. As an example, the h~0! value for the n4band varies from 0.094 to 0.112.
The shapes of the h functions were found to besimilar in each spectral interval that belongs to thesame vibrational band ~n3 or n4!. The radiative forc-ing is strongly correlated ~95%! to the surface tem-perature. One can thus explain the generalbehavior of this relationship by using the blackbodyPlanck function for each spectral interval ~n4 and n3!.The value of the Planck function in the selected spec-tral range of the n4 band is sensitive to the temper-ature in the same way as the sensitivity of theintegrated Planck function is in the absorption rangeof methane. As a consequence, the h function fol-lows a linear law for the selected intervals that be-long to the n4 band. To demonstrate that the hfunction behaves as a power function in the n3 bandone can use the fact that the Planck function forintervals of the n3 band is itself a power function ofthe Planck function for intervals of the n4 band.
For surface temperature ~TS! conditions the black-body Planck function for a given wave number in then4 band ~s4! can be expressed as a function of theblackbody Planck function for a wave number in then3 band ~s3! by the following logarithmic relation:
ln@BTs~sn4
!# 5ln@BTs
~sn3!#
Ç
~a!1
3 lnSsn4
sn3
DÇ
~b!1
hckTs
~sn32 sn4
!Ç
~c!,
(6)
where h and k are the Planck and the Boltzmannconstants, respectively, and c is the velocity of light invacuum. It can easily be calculated that, for atmo-spheric temperatures, term ~c! is small comparedwith term ~a! 1 term ~b! and that term ~c! induces adeviation from the linear dependence in this relationthat does not exceed 1.5% as an average value.
The relative uncertainties on the adjusted h func-tions were 3.5% and 6% for the n4 and n3 selectedspectral intervals, respectively. We now study howthese uncertainties vary when interfering gases aretaken into account.
B. Sensitivity to Both Interfering Gases and SpectralResolution
The influence of the interfering gases on the h func-tion was investigated by a procedure similar to thatdescribed in Section 3. Simulations were carried out
20 May 1998 y Vol. 37, No. 15 y APPLIED OPTICS 3117
with mean concentrations profiles for nitrous oxideand carbon dioxide, which have homogeneous distri-bution over the globe, and with the TIGR data basefor water-vapor profiles.17
As we discussed above, the selected spectral inter-vals in the n3 band are less sensitive to interferinggases than those in the n4 bands. As a consequence,for the former vibrational band the h function pro-vided by Eq. ~5! remains valid, the dispersion beingapproximately the same as in Fig. 3~b!. Interfer-ences are more important in the n4 band; hence thedispersion of the data around an adjusted functionincreases with the degradation of the spectral reso-lution. The calculated radiative forcing as a func-tion of the differential radiances is plotted in Fig. 4for the selected 1240.7–1241.3-cm21 interval at theIASI spectral resolution. It can be seen that thequality of the fit has decreased, mainly because of thedispersion of the data associated with the tropical airmasses. The standard deviations ~1s! for the least-squares fit are 6.5% @Fig. 3~a!# and 19.4% ~Fig. 4!.
We can overcome this effect by calculating a differ-ent regression law for each of the air-mass types. Inthis new calculation an averaged water-vapor con-tent is associated with each air-mass type ~polar,mid-latitude, and tropical! to improve the quality ofthe fit. Figure 5 illustrates these linear fits and pro-vides the corresponding adjusted h functions.
The relative uncertainty associated with the ad-justment of these functions is summarized in Table 2for each air mass and for each selected spectral in-terval. It can be seen that the quality of the fitdepends strongly on the air-mass type and variesfrom 1.5% to 12%. The largest uncertainty is asso-ciated with polar air masses, which have the lowestradiative contribution. As explained above, this de-composition in air-mass type is not required for the n3band, for which an average uncertainty of 6% is
Fig. 4. Methane radiative forcing as a function of the differentialradiance for the nadir-looking mode at the IASI resolution ~1 cm21!for the 1240.7–1241.3-cm21 spectral range. Interferences thatare due to water vapor were included in the calculation with theTIGR database. The air-mass types are represented in variousgray-scale shades as for Fig. 3.
3118 APPLIED OPTICS y Vol. 37, No. 15 y 20 May 1998
found. Furthermore, the additional uncertainty as-sociated with the use of a mean climatic profile in-stead of the true water-vapor vertical profile was alsoassessed and is provided in Table 2 for each case.The obtained values range from 0.2% for polar airmasses to 3.6% for the tropical air masses.
To evaluate the total uncertainty on the retrievedmethane radiative forcing one needs to consider theinfluence of instrumental noise, which varies as afunction of the wave number.
C. Sensitivity to Instrumental Noise
The two instruments have approximately the samenoise level, which reaches 0.23 3 1023 and 0.16 31024 W m22 cm sr21 for the spectral bands n4 and n3,respectively.8 It includes all noise contributions ~de-tectors, amplifiers, analog-to-digital converters, andprocessing! and all error sources ~errors that weredue to field-of-view motion, fluctuation of wavelengthcalibration, and a priori knowledge of the spectralresponse function!.
The relative uncertainty on the calculated radia-tive forcing that is due to the instrumental noise foreach air-mass type as a function of both the selectedspectral intervals and the spectral resolution of thespectrometers is also summarized in Table 2. Theadditional uncertainty that is due to instrumentalnoise on the methane radiative forcing retrieval isless than 1.5% in the selected spectral domains of then3 band where the average signal-to-noise ratio isbetter than 15. It is, however, larger in the selected
Fig. 5. Methane radiative forcing as a function of the differentialradiance for the nadir-looking mode at the IASI resolution ~1 cm21!for the 1240.7–1241.3-cm21 spectral range. Representative aver-age value profiles were used for interferences that are due to watervapor. A linear fit has been adjusted on each type of air mass, andthe corresponding h functions are provided.
Table 2. Error Budget ~rms! Calculated for Each Type of Air Mass ~Polar, Midlatitude, Tropical, and Global! for Each Selected Spectral Intervala
aIncludes the uncertainties associated with the adjustments of the h function, the water-vapor variability, and the instrumental noise.For each case, the lower uncertainty value is highlighted in boldface type.
spectral domains of the n4 band, mainly because ofthe increased sensitivity of the polar air masses forwhich the signal-to-noise ratio is less than 8, at theIASI spectral resolution. The relative uncertaintyon the radiative forcing of methane for polar airmasses ranges from 2.7% to 4.8% for the IMG and theIASI spectral resolutions, respectively. For theother types of air mass it stays below 0.7%.
D. Assessment of Accuracy of the Direct RetrievalMethod
All uncertainties ~e.g., standard deviation on the ad-justed h functions, use of mean value profiles forinterfering gases, and contribution of the instrumen-tal noise! were added to provide the total uncertaintyfor each spectral interval. This calculation was car-ried out for each air-mass type separately, and aroot-mean-square total uncertainty was then calcu-lated over all air-mass types ~see Table 2!. It can beseen that the optimum spectral interval to be used forthe direct retrieval of radiative forcing depends onthe air-mass type. For polar air masses, spectralintervals that belong to the n3 band are recom-mended, whereas for mid-latitude and tropical airmasses the use of spectral intervals that belong to then4 band is more appropriate ~see the uncertainty val-ues highlighted in boldface type in Table 2!. Thiscalculation leads to a maximum uncertainty that iscalculated to be better than 7% in all cases. Weassume that both the IMAGES methane profiles andthe temperature profiles used are statistically repre-sentative of the dispersion of the atmospheric pro-files.
Simulations were undertaken to compare the re-trieving efficiency associated with the proposed directmethod with the efficiency obtained from a classic
inversion procedure. As both methods require dif-ferent input parameters, this is only a tentative as-sessment of the precision associated with eachretrieving procedure. On the one hand, the directmethod uses mean concentration profiles for interfer-ing species and requires determination of the surfacetemperature. On the other hand, in an inversionprocedure, vertical profiles for methane, interferingspecies, and temperature are needed. To estimatethe uncertainties associated with the inverse proce-dure we assumed in the simulation that integratedcolumns of methane are retrieved from the nadirspectral radiances provided by the instrument withan accuracy of 10%. This value comes from the in-strumental specifications associated with the defini-tion of both the IMG and the IASI instruments andmay be underestimated. We then assumed thatroot-mean-square error of less than 10% could beassociated with each tropospheric layer in the radia-tive code. This simulation provides radiative forc-ings that were found to be accurate to 66%. As wellas in the direct method, interferences that are due tothe variability of water vapor ~1%–4.1%! must also betaken into account. This calculation leads to a min-imum estimated uncertainty that is in the samerange as the one calculated with the budget errorassociated with the direct retrieval method. It canthus be concluded by use of simple assumptions thatthe direct method is at least as efficient as the classicinversion procedure and will provide results in ashorter computing time.
5. Conclusion
This preliminary study was undertaken in the frame-work of the scientific assessment of the spaceborneinfrared Fourier-transform spectrometers IMG and
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IASI. The possibility of using the data to be pro-vided by these instruments without going throughthe usual inversion procedure to estimate methaneradiative forcing was discussed. We developed anoriginal method adapted to cloud-free conditions. Ituses a selected spectral window to retrieve the radi-ative forcing associated with the presence of methanein the troposphere with a mean overall precision ofbetter than 7%, assuming that surface temperatureand emissivity are well determined. This accuracymay still be further improved by use of appropriatespectral intervals, depending on the latitude of thedata acquisition ~polar, mid-latitude, or tropical airmass!.
The direct method provides accurate radiative forc-ing in a short computing time. It requires only theuse of mean concentration profiles for water vapor,nitrous oxide, and carbon dioxide, which absorb inthe same spectral range. Only surface tempera-tures are needed, instead of vertical temperature pro-files as in the inverse method. A rough estimate ofthe precision associated with the latter shows thatthe direct method is at least as efficient, in particularwhen the data are discriminated in terms of airmasses.
As the spatial variability of the radiative forcingassociated with methane is estimated to be ;40%, wehave shown that the spectral radiances provided by anadir-looking instrument will allow us to quantifyhow methane contributes to the increase of the radi-ative forcing of the atmosphere. Such spatiotempo-rally distributed data will be helpful for thevalidation of radiative transfer codes such as thoseused in general circulation models. Moreover, thesedata can be used for climatic studies of greenhouseradiative forcing. Indeed, instruments such as theIASI constitute the next generation of space sensorsdedicated to the operational meteorology that willensure the stability of infrared measured radiancesover more than a decade. Finally, this procedurecould be adapted for use with other greenhouse gases~such as water vapor, ozone, and nitrous oxide! thatwill also be measured by nadir-looking spectrome-ters.
This study has been undertaken in the frameworkof the IASI Sounding Science Working Group activi-ties under the auspices of the European Organisationfor the Exploitation of Meteorological Satellites andthe French Centre National d’Etude Spatiales. Weare grateful to J. F. Muller for providing the outputsof the IMAGES model.
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