Summary
of Spectral Signatures
Labs in Bertinoro
23 Aug – 2 Sep 2004
Paul Menzel
NOAA/NESDIS/ORA
Solar (visible) and Earth emitted (infrared) energy
Incoming solar radiation (mostly visible) drives the earth-atmosphere (which emits
infrared).
Over the annual cycle, the incoming solar energy that makes it to the earth surface
(about 50 %) is balanced by the outgoing thermal infrared energy emitted through
the atmosphere.
The atmosphere transmits, absorbs (by H2O, O2, O3, dust) reflects (by clouds), and
scatters (by aerosols) incoming visible; the earth surface absorbs and reflects the
transmitted visible. Atmospheric H2O, CO2, and O3 selectively transmit or absorb
the outgoing infrared radiation. The outgoing microwave is primarily affected by
H2O and O2.
Solar Spectrum
VIIRS, MODIS, FY-1C, AVHRR
CO2
O2
H2O
O2
H2O
H2O
H2O
O2
H2O
H2O
CO2
MODIS IR Spectral Bands
MODIS
GOES Sounder Spectral Bands: 14.7 to 3.7 um and vis
II II I
|I I
ATMS Spectral Regions
MODIS
wavelength
High ice cld
Midlevel cld
Midlevel cld
Low water cld
13.9
13.4
13
12.5
12.1
11.7
11.4
11
10.7
10.4
10.1
9.86
9.6
9.36
9.13
8.91
8.7
Ice reflectance
8.5
Reflection [%]
5
4.5
4
3.5
3
2.5
2
1.5
1
0.5
0
0.8
Variation with Particle Size (reff)
(IWP= 10 g m
-2
e
Reflection Function (2.142 µm)
0.6
60
4.5 micron
50
40
30
20
4 µm
0.5
8 µm
0.3
16 µm
0.2
22.5 micron
10
4
0
700
r = 2 µm
0
70
Cloud Forcing (K)
 = 26°,  = 40°,  = 42°
; 10.8-10 km)
6
12
8

2
800
900
1000
1100
-1
W avenumber (cm )
1200
1300
0.0
0.0
0.2
24 32
16
0.4
48
32 µm
80
c
0.6
0.8
Reflection Function (0.664 µm)
1.0
Kaolinite
montmorillonite
Kaolinite
montmorillonite
Optical properties of cloud particles: imaginary part of refraction index
Imaginary part of refraction index
0.6
Ice
0.5
0.4
Water
0.3
0.2
0.1
0
1
3
5
7
9
11
13
wavelength [microns]
SW & LW channel differences are used for cloud identification
{4 m - 11m}, {4.13 m - 12.6m}, and {4.53 m - 13.4m}
15
Source: Dr. M. Watson, Michigan Technical University
AVHRR channels
transmission (total)
transmission (scattering)
transmission (absorption)
transmission
transmission
AVHRR channels 1
1
1
0.9
0.9
0.9
0.8
0.8
0.8
0.7
0.7
0.7
0.6
0.6
0.6
0.5
0.5
0.5
0.4
0.4
0.4
0.3
0.3
0.3
transmission
(total)
0.2
0.2
0.2
transmission
(scattering)
0.1
0.1
0.1
4
5
Ice
transmission (absorption)
0
0
0
8
979
119
12
13
10
11 7 1412 8 13 79
1108
transmission (total)
wavelength
(m) (m)
Wavelength
0.9
0.8
0.7
0.6
0.5
0.4
0.3
4
5
Ash
14 10
8
11
9
10
12
11
13
wavelengthWavelength
( m)
(m)
transmission (scattering)
transmission (absorption)
T10.8BT11-BT12
- T12.0 >>00 water
for ice & ice
BT11-BT12 < 0 for volcanic ash
T10.8 -Frank
T12.0Honey
< 0 volcanic
ash
1980s
12
14
SO2 calculations from F. Prata
Fog Detection over Snow Surfaces
"Non-detection" of fog over snow surfaces with VIS channels:
thick clouds and snow have the same reflectance
• Emissivity more variable
near 3.9 m
• Sandy areas appear 5-10 K
cooler at IR3.9 than at
IR10.8 (at night, dry
atmosphere)
IR3.9
Dry sand: 0.8
IR10.8
0.95
Emissivity as a function of wavelength
and surface type
• Different appearance of land
surfaces during daytime,
depending on surface type
Dust and Cirrus Signals
Imaginary Index of Refraction of Ice and Dust
• Both ice and silicate
Ice
Dust
0.8
absorption small in 1200 cm-1
window
• In the 800-1000 cm-1
atmospheric window:
0.7
0.6
Silicate index increases
0.5
nI
Ice index decreases
0.4
with wavenumber
0.3
0.2
0.1
0
800
900
1000
1100
Wavenumber
(cm-1)
wavenumber
1200
1300
Volz, F.E. : Infrared optical constant of
ammonium sulphate, Sahara Dust,
volcanic pumice and flash, Appl Opt 12
564-658 (1973)
Dust IR spectra – green clr sky vs dust particles of different size
Dust IR spectra – green clr sky vs dust layers at different heights
Fog and Low Stratus
Comparison of snow reflectance in VIS and NIR 1.6 channels
Energy
spectrum
Source:
EUMETSAT
Ch08
• Ch08 is in the centre of the O3 absorption band around 9 
Weighting
functions
Source:
EUMETSAT
Signals from lower parts of troposphere;
But:
Secondary maximum from higher than 100 hPa
Figure 3c
AIRS radiance changes (in deg K) to atm & sfc changes
43
Inferring surface properties with S-HIS high spectral
44
resolution data - Note the large change, especially for bare soil, in
surface emissivity between 960 and 1060 cm-1. The HES minimum
mission would not cover both regions.
Pure Vegetation
1.0
Aircraft
S-HIS
LSE
0.85
S-HIS OBS
Bare Soil
12 m
9 m
Wavenumber (cm-1)