Characteristics of spectral emissivity for various types of the surface derived from IMG spectrum data

Y. Ota and R. ImasuThe University of Tokyo, Center for Climate System Research, Tokyo, Japan

1. IntroductionEmissivity of the surface in the thermal infrared region, particularly in the atmospheric window region, is an important parameter when we consider the energy budget in the surface-atmosphere system. Recently global mapping of broad band infrared emissivity have been obtained from MODIS and ASTER data analyses. Spectral emissivity data, however, are still necessary for characterizing the surfaces more precisely. This study presents a procedure to derive spectral emissivity in 8-12 micron region from IMG data.

It is not possible to completely separate the surface temperature and the emissivity at each wavelength, because the number of independent parameter is N+1 ( one thermodynamicaltemperature of surface and emissivities at N wavelengths ) though the measurement is radiance at N channels. This means that we need one additional information or assumption to solve the equations.

Therefore we have set an assumption depending on the surface type which is derived from visible imagery simultaneously observed with high resolution radiance spectra. This method have been applied to the analysis of IMG data, and some interesting characteristics of spectral emissivity have been obtained for various types of the surface such as desert sand and sea ice.

2. MethodThe upwelling radiance at wavenumber in the infrared region which is measured at the top of the atmosphere under no cloud condition can be written as

(1)

where is the radiance from the surface, is the radiance form the atmosphere, is the Planck function at each atmospheric temperature profile , is the atmospheric transmittance profile, and is the surface emissivity.

Equation (1) can be formally written in the form

, (2)

and then if the surface temperature and the vertical profile of atmospheric temperature and absorption gases are known accurately, the spectral emissivity can be derived from the equation (2). Especially the estimation of the surface temperature, whose value almost determines the absolute value of emissivity, is the most significant factor. In this study, we have estimated the surface temperature by using one wavenumber channel at which the observed brightness temperature is the maximum value in the atmospheric window region and the emissivity has been assumed to be unit . Because of this assumption, derived emissivity is relative to the channel which has been assumed the unit emissivity. And the vertical profiles of temperature and water vapor have been retrieved by using the another wavenumber channels at which the total transmittances are regarded as almost zero.

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3. DATAThe IMG ( the Interferometric Monitor for Greenhouse gases ) is the FTS launched on the ADEOS satellite in August 1996 and measured upwelling thermal infrared radiation by nadir-looking until June 1997. In this study the level 1 data of band 3 which have spectral coverage about 5.0 - 15.0μm and are apodized by using Norton-Beer function (medium) with spectral resolution of 0.1 [cm-1] have been used.

To derive the surface type in the IFOV (8km×8km) of IMG, the visible imageries of OCTS ( Ocean Color Temperature and Scanner ) which was also launched aboard the ADEOS has been used.

The LBLRTM ver.7.04 with HITRAN 2000 database were used for line-by-line radiative transfer calculation.

To compare with analyzed results, the emissivity data of “MODIS UCSB Emissivity Library” has been used.

4. RESULTSFigure 1 shows the results of relative emissivity analysis for the various types of surface. The wavenumber dependencies are shown for each surface, except for the range of ozone band (9.6μm) in which the error is dominant due to no ozone information in the procedure of emissivity analysis. In comparison with the measurements of MODIS UCSB Emissivity Library (Figure 2), it appears that the wavenumber dependencies are well derived ( especially ``Sand’’ ). Figure 3 shows the results of analysis along the satellite orbit across the boundary of the Sahara Desert. The difference in wavenumber dependency due to the existence of vegetation is shown.

On the other hand, the absolute values of emissivity have a tendency to be underestimated. ( In particular, snow should be regarded as almost black body. ) This means that the surface temperature is overestimated, and there are some reasons to be considered. One of them may be the case that the surface is colder than the atmosphere of near surface, for example snow and ice field. In this case, the procedure in this study selects the wavenumber channel at the wing of water vapor absorption line to estimate surface temperature.

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Figure 3 : The result of surface emissivity analysis along the satellite orbit near the Sahara Desert ( 26th. Jan. 1997 )

Figure 2 : Comparison of surface emissivity in thermal infrared region. ( MODIS UCSB Emissivity Library, http://www.icess.ucsb.edu/modis/EMIS/html/em.html )

5. CONCLUSIONOne procedure to derive the relative spectral emissivity of surface from high resolution infrared spectral data observed by satellite remote sensing has been demonstrated. This method has been applied to the IMG data which include the various types of surface in its IFOV, and the surface types have been determined by using the OCTS visible imageries.

As a result, the wavenumber dependency of surface emissivity has been derived in terms of comparison with laboratory measurement, and the difference in wavenumber dependencies of various types of surface has also been shown from the IMG data observed above the Sahara Desert. This means that it is not impossible to observe the land surface property by measuring thermal infrared radiation in future. For accurate derivation of the absolute value of surface emissivity, however, the estimation of true surface temperature is the critical issue.

No vegetation ground surface No vegetation mountain

Snowy field Sea ice

Figure 1 : Relative emissivity of the surface derived from IMG spectrum and OCTS imageries. The red arrows and the squares represent the IFOV of IMG band 3.

( 73.7W, 153.7W, 27th. Dec. 1996 )

( 30.0N, 4.0W, 25th. Dec. 1996 ) ( 38.3N, 87.7E, 27th. Dec. 1996 )

( 79.1N, 41.9E, 4th. Apr. 1997 )

Characteristics of spectral emissivity for various types of the surface derived from IMG spectrum dataY. Ota and R. ImasuTh

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