LAND SURFACE TEMPERATURE AND EMISSIVITY
FROM ADVANCED SOUNDERS
Robert O. Knuteson and Henry E. Revercomb
Space Science and Engineering Center, University of Wisconsin-Madison
1225 W. Dayton St., Madison, WI USA 53706
ABSTRACT
Radiance observations from the NASA Atmospheric InfraRed Sounder (AIRS) satellite instrument have been
obtained over a site in the Libyan Desert used previously for AVHRR satellite validation. The spectral
contrast between 9 and 12 µm is large for scenes containing exposed silicate minerals compared with either
vegetated scenes or those containing mostly water. The “Egypt One” site was chosen because it is a large,
fairly uniform sandy desert region suitable for evaluation of the 15-km footprints of the NASA AIRS advanced
sounder. The results suggest a relative emissivity spectral contrast of more than 20% between 12 µm and 9
µm. The spectral contrast at 4 µm relative to 12 µm is also large (>10%) but less than that at 9 µm. These
spectral features are generally consistent with laboratory emission spectra measured for coarse grains of the
mineral quartz. Global, high spectral resolution emissivity maps from AIRS data are being developed to
prepare for the future operational use of high spectral resolution infrared sounding data over land from the
NPOESS CrIS and the METOP IASI sensors.
1. INTRODUCTION
A new generation of infrared sounders for future weather satellites is being developed in Europe and in the
United States for obtaining improved profiles of atmospheric temperature, water vapor, and trace gas
concentrations (Diebel et al., 1997; Aumann et al., 2001; Nelson and Cunningham, 2002). A characteristic of
these advanced sounders is the use of spectrometers with nearly continuous coverage across the thermal
emission spectrum with resolving powers of 1000 or greater. These high spectral resolution sounders have
the advantage of being able to resolve individual absorption lines of water vapor and carbon dioxide and
thereby provide a number of transparent “microwindows” that require a smaller atmospheric correction than
broad-band instruments.
The Atmospheric InfraRed Sounder (AIRS) is a research instrument developed by NASA to provide high
spectral resolution observations of outgoing infrared emission from the Earth surface and atmosphere for
use in the study of weather and climate. The AIRS was launched into a sun-synchronous polar orbit aboard
the NASA EOS Aqua platform in June 2002. The high spectral resolution AIRS observations are a prelude to
similar observations that will be obtained by the CrIS and IASI sensors on the operational NPOESS and
METOP platforms. The AIRS uses a grating spectrometer with over 2000 detector elements to obtain a
spectral resolving power of about 1200 and a field of view diameter of approximately 15-km at nadir. This
paper describes the formulation of the infrared radiative transfer theory used to interpret high spectral
resolution infrared observations over land and presents a case study analysis of AIRS data over the Libyan
Desert and the Red Sea. A preliminary evaluation of the AIRS observations suggest that these data will be
invaluable in creating global, high spectral resolution infrared emissivity maps of the Earth’s land surface.
2. THEORY
This paper will follow the notation of Moriyama and Arai (1994) for the cloud-free radiative transfer equation,
neglecting solar radiation and scattering effects, of a downlooking infrared sensor viewing a homogeneous
surface
0
Z
∂τ ( z, Z )
∂τ ( z, Z )
(1)
dz,
Iν = ∫ Bν [T ( z )] ν
dz + εν ⋅ Bν (TS ) ⋅ τ ν (0, Z ) + (1 − εν ) ⋅ τ ν (0, Z ) ∫ Bν [T ( z )] ν
0
∞
∂z
∂z
where Iν , εν , Bν , TS , τ ν ( z1 , z2 ) , Z , and T (z ) are observed spectral radiance, spectral emissivity,
spectral Planck function, the surface temperature, spectral transmittance at wavenumber ν from altitude z1 to
z2, sensor altitude and air temperature at altitude z, respectively. The first term of Eq. (1) is the emission
from the atmosphere above the surface, the second term is the direct emission from the surface that reaches
the sensor, and the third term is the downwelling atmospheric emission reflected off the ground under the
approximation of a lambertian surface. Eq. (1) applies at monochromatic resolution and has been accurately
implemented in several line-by-line radiative transfer models. An extension of this equation to fields of view
containing mixed scene types can be found in Knuteson et al. (2004). The formal solution for the effective
emissivity as a function of the effective surface temperature is given there as
∧
εν =
↓
[〈 RνOBS 〉 − Νν↑ ] − τ ν Νν
∧
↓
τ ν Bν (TS ) − τ ν Νν
(2),
where the ^ denotes an effective emissivity which is a linear combination of pure scene type emissivities
weighted by the fractional area coverage within the instrument field of view and the up/down arrows denote
the upwelling and downwelling contributions by the atmosphere only. Under the approximation that the
downwelling emission between atmospheric emission lines can be neglected when using high spectral
resolution observations, we can define the relative emissivity as
∧
ε ν ( relative) ≅
〈 RνOBS 〉 − Νν↑
(2)
(3),
τ ν Bν (Tref )
(2)
where Tref is selected to provide a reference temperature. A further approximation can be made if the
atmosphere is sufficiently dry by neglecting the atmospheric emission correction and assuming the
atmospheric transmission is unity. This is often not a very good approximation even in high spectral
resolution microwindows, however it has the advantage that this “raw” relative emissivity can be computed
directly from the observations and thus provides a useful first guess for the land surface emissivity. In
particular, many of the infrared spectral signatures of exposed minerals can be identified from the raw
relative emissivity even without an atmospheric correction. The equation for the raw relative emissivity can
be written as
∧
ε ν ( raw) ≈
〈 RνOBS 〉
(1)
(4)
Bν (Tref )
where Tref(1) is derived from an observed brightness temperature.
3. OBSERVATIONS
A case study is provided for two granules (six minutes each) of top of the atmosphere radiance observations
from the NASA AIRS spectrometer on the EOS Aqua platform. The observations are from a night-time
(00:00-00:06 UTC) and a daytime (11:00-11:06 UTC) overpass of the Libyan Desert in North Africa on 16
November 2002. The area over Egypt and the Red Sea is largely cloud-free as can be seen in the coincident
visible imagery from the MODIS sensor shown in Figure 1. The 0.5 degree gridded analysis fields from the
European Center for Medium-range Weather Forecasting (ECMWF) are compared in Figure 2 with the AIRS
brightness temperature from a narrow microwindow at about 12 µm. A square symbol in Figure 2 marks the
location of a region of the Libyan desert used for satellite validation referred to as “Egypt One” (27.12 N,
26.10 E).
Figure 1. MODIS color composite image of the Libyan desert and the Red Sea from an Aqua overpass
at 11:00-11:06 UTC on 16 November 2002. Most of the region of interest is cloud-free.
Figure 2. ECMWF (a) surface pressure map and (c) skin temperature for the 12 UTC analysis
compared to AIRS observations of (b) 12 µm brightness temperature (830-832 cm-1) at nominal 15-km
resolution and (d) gridded to match the ECMWF 0.5 degree spatial resolution for an NASA Aqua
overpass at 11:00-11:06 UTC on 16 November 2002. The square symbol marks the “Egypt One”
satellite validation site (27.12 N, 26.10 E). The AIRS L1B 12 µm brightness temperature is fairly
uniform at the location of the Egypt One site. The ECMWF model suggests that cool, moist air in the
atmospheric boundary layer has pushed down from the Mediterranean Sea in the north, thereby
suppressing the daytime surface skin temperatures over the Libyan Desert on this date.
4. RESULTS
A desert site was chosen to illustrate the infrared spectral signature of silicate minerals (coarse sand) that is
present in the high spectral resolution AIRS observations. Laboratory and ground-based observations of
coarse sand suggest that a large emissivity contrast is expected between the 12 µm and the 9 µm regions
(Salisbury and D’Aria, 1994). The brightness temperature contrast between 12 µm (830-832 cm-1) and 9 µm
(1092-1099 cm-1) is shown in Figure 3 both as spatial maps and as brightness temperature histograms for
the daytime AIRS observations. Note that both the ocean and vegetation regions (Nile river and coastal
zone) have a 9 minus 12 µm brightness temperature difference close to zero while the exposed sand and
rocks in the Libyan desert show brightness temperature differences up to 20°. This is due to the fact that the
infrared emissivity of both vegetation and water have relatively little spectral contrast across the 8 to 12 µm
infrared window compared to the spectral signatures of exposed minerals. In particular, the fractional
coverage of vegetation in the AIRS instrument field of view determines the magnitude of the observed
spectral contrast, with higher vegetation fractions leading to reduced spectral contrast.
Figure 3. AIRS observations of (a) 12 µm brightness temperature at nominal 15-km resolution and (b)
the corresponding brightness temperature histogram for a NASA Aqua overpass at 11:00-11:06 UTC
on 16 November 2002. The difference between the AIRS 9 µm and 12 µm brightness temperature is
shown in panel (c) and (d). Note that the 9 minus 12 µm observed brightness temperatures are close
to zero for the water and vegetation scenes but can reach up to 20° for scenes containing mostly
exposed silicate minerals.
The AIRS infrared spectrum closest to the the Egypt One site, found in Figure 4, clearly shows the main
emissivity feature of silicate minerals (coarse quartz) at 8 to 9 µm and a smaller feature at 13 µm. The gaps
in the spectrum are spectral regions not measured by the AIRS grating spectrometer. In contrast, the IASI
interferometer is expected to make nearly continuous measurements across the infrared spectrum from
about 4 µm to about 15 µm. Figure 5 shows a comparison of observations from the Egypt One desert site
and from an ocean field of view in the Red Sea and the raw relative emissivity computed using Eqn. 4 with
Tref(1) equal to the mean observed brightness temperature in the 12 µm microwindow between 830 cm-1 and
832 cm-1. Note that an atmospheric correction has not yet been applied to these raw relative emissivity
results. Figure 6 shows the raw relative emissivity derived from the AIRS L1B observations as a color-filled
contour plot for both day-time and night-time granules. The 4 µm relative emissivity can only be determined
at night-time due to the contribution of reflected solar radiation during the day-time.
MW
SW
LW
∆T ≅ 17K
12 µm
9 µm
Figure 4. AIRS observed brightness temperature spectrum over the “Egypt One” validation site for a
NASA Aqua daytime overpass at 11:03 UTC on 16 November 2002.
Egypt One
Red Sea
12 µm
9 µm
Figure 5. The upper panel shows a region of the AIRS observed brightness temperature spectrum for
the Egypt One site (sand dunes) and for a field of view in the Red Sea (ocean water) for the same
daytime overpass at 11:03 UTC on 16 November 2002. The lower panel shows the “raw” relative
emissivity computed for the two scenes (without atmospheric correction). The black dots indicate
the AIRS “microwindows” for which the raw relative emissivity approximation is valid.
(a)
(b)
(c)
Figure 6. Raw relative emissivity shown as a contour map derived from the 15-km L1B AIRS
radiances at (a) 9 µm during a daytime overpass and at (b) 9 µm and (c) 4 µm during a nightime
overpass of the Aqua satellite on 16 November 2002. The observed brightness temperature at 12 µm
is used as the reference temperature. At 9 µm the vegetated Nile river basin and the ocean coastal
zones show up as high emissivity (>0.95) while the exposed desert sands have low emissivity (<0.8).
The night-time measurements at 4 µm show less spectral contrast than 9 µm for most of the desert
regions with the notable exception being a region in Greece where the relative emissivity at 4 µm is
much lower than at 9 µm, perhaps indicating a smaller mineral grain size. In these maps, the cloud
covered regions show up as black with emissivities greater than 1. The square symbol marks the
location of the validation site in the Libyan Desert. Note that an atmospheric correction has not yet
been applied to these results.
5. CONCLUSIONS
The Atmospheric InfraRed Sounder (AIRS) is a research instrument on the NASA Aqua platform that is
being used to develop data assimilation and product retrieval techniques in preparation for the upcoming
operational sounders; IASI on METOP and CrIS on NPP/NPOESS. One of the unique uses of the AIRS
observations is to produce global infrared emissivity maps with the high spectral resolution sampling needed
for input to radiative transfer model calculations. An initial approach for the production of these high spectral
resolution emissivity maps is presented in this paper. Illustrations have been provided of the first step in the
production of these IR surface emmissivity maps, i.e. the computation of the raw relative emissivity using
only AIRS observations themselves. Contour maps of the Libyan desert created from AIRS observations
from 16 November 2002 are presented for both day and night-time overpasses. The unique aspect of the
AIRS measurements is the combination of high spectral resolution sampling of the infrared spectrum which
allows the use of numerous micro-windows between water vapour absorption lines and the global coverage
provided by cross-track scanning in a sun-synchronous polar orbit.
An estimate of the spectral contrast in surface emissivity has been obtained with high spectral resolution
over a satellite validation site in the Libyan Desert. The “Egypt One” site was chosen because it is a large,
fairly uniform, sandy desert region suitable for evaluation of the 15-km footprints of the NASA AIRS
advanced sounder. The results show a spectral contrast of more than 20% between 12 µm and 9 µm. The
spectral contrast in the raw relative emissivity at 4 µm is also large (>10%) but less than that at 9 µm. These
spectral features are generally consistent with laboratory emission spectra measured for coarse grains of the
mineral quartz.
The next step in this process is to extend the validity of the relative emissivity by using the ECMWF model
atmospheres and a forward radiative transfer model to make a correction in the high spectral resolution
micro-windows for the atmospheric emission and transmission. Future work includes the combined use of
AIRS with coincident Meteosat Second Generation observations to characterize the land surface emissivity
down to the smaller time and spatial scales of MSG.
6. ACKNOWLEDGEMENTS
We gratefully acknowledge the support of the NASA EOS project contract NAS5-31375, the Integrated
Project Office (IPO) contract 50-SPNA-1-00039, and NOAA NESDIS for support of this and related activities.
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