Intro to High Spectral Resolution
IR Measurements
Lectures in Madison
27 March 2013
Paul Menzel
UW/CIMSS/AOS
ds line
broadening with pressure helps to explain weighting functions
ABC

d /dp
- k u
where  = e
MODIS
kλ
high z
(low p)
A
kλ
mid z
B
kλ
low z

ABC
C

2
line broadening with pressure helps to explain weighting functions
- k u (z)
 (z   ) = e
A
b
s
o
r
p
t
i
o
n
H
e
i
g
h
t
Wavenumber
Energy Contribution
3
For a given water vapor spectral channel the weighting function depends on the
amount of water vapor in the atmospheric column
Wet Atm.
H
e
i
g
h
t
Wet
Moderate
Moderate
Dry Atm.
Dry
0
Tau
100%
=dTau/dHt
CO2 is about the same everywhere, the weighting function for a given CO2
4
spectral channel is the same everywhere
Vibrational Bands
CO2
O3
H2O
CO2
CO2 Lines
6
H2O Lines
7
8
Rotational Lines
CO2
O3
H2O
CO2
Earth emitted spectrum in CO2 sensitive 705 to 760 cm-1
CO2
Lines
10
Broad Band
window
window
CO2
O3
Sampling of vibrational bands
Integration over rotational bands
H2O
11
… in Brightness
Temperature
12
High Spectral Resolution
Sampling over rotational bands
13
14
From E. Weisz
15
From E. Weisz
16
From E. Weisz
17
From E. Weisz
100
100
GOES
(18)
1000
Advanced Sounder
(3074)
1000
Moisture Weighting Functions
High spectral resolution advanced sounder will have more
and sharper weighting functions compared to current GOES
sounder. Retrievals will have better vertical resolution.
18
UW/CIMSS
19
Brightness Temperature (K)
Resolving absorption features in atmospheric windows
enables detection of temperature inversions
Texas
Spikes down Cooling with height
(No inversion)
Spikes up Heating with height
(low-level inversion)
Ontario
GOES
GOES
Wavenumber (cm-1)
Detection of inversions is critical for severe weather
forecasting. Combined with improved low-level moisture
depiction, key ingredients for night-time severe storm
development can be monitored.
20
IASI detection
of temperature
inversion
(black spectrum)
vs
clear ocean
(red spectrum)
21
Ability to detect inversions
disappears with
broadband observations
(> 3 cm-1)
22
On-line/off-line “signal”
Longwave window region
23
“AIRS or IASI-like”
Longwave window region
24
Longwave window region
25
Longwave window region
26
Longwave window region
27
Longwave window region
28
“Current GOES-like”
Longwave window region
29
Twisted Ribbon formed by CO2 spectrum:
Tropopause inversion causes On-line & off-line patterns to cross
15 m CO2 Spectrum
Blue between-line Tb
warmer for tropospheric channels,
colder for stratospheric channels
Signature not available at low resolution
strat
tropopause
trop
30
31
2500
1000
715 cm-1
32
Inferring surface properties with AIRS high spectral resolution data
Barren region detection if T1086 < T981
T(981 cm-1)-T(1086 cm-1)
Barren vs Water/Vegetated
T(1086 cm-1)
AIRS data from 14 June 2002
33
from Tobin et al.
Thin ice cloud over ocean
R = s Bs (1-c) + c Bc
using e-σ = 1 - σ
So difference of thin ice cloud over ocean
from clear sky over ocean is given by
Dust and Cirrus Signals
ΔR = - s c Bs + c Bc
35_98
67
Imaginary Index of Refraction of Ice and Dust
Ice
Dust
0.8
For Bs > Bc and s ~1
~1-σ~1-a
0.7
a
0.6
•
ab
w
•
at
ΔR = - c Bs + c Bc = c [Bc - Bs ]
nI
0.5
0.4
0.3
0.2
As c increases (decreases) then ΔR becomes more
negative (positive)
0.1
0
800
900
1000
1100
Wavenumber (cm-1)
1200
1300
Vo
of
D
Ap
IASI detection of dust
IASI detection of cirrus
red spectrum is from nearby clear fov
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)
35
36
UW
CIMSS
Mt Etna Ash cloud at 500 hPa
37
Ash cloud and clear sky spectra
38
1
0.98
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
10
11
12
13
14
9
10
11 7 12 8 13 79
18
transmission (total)
wavelength
(m) (m)
Wavelength
0.9
transmission
0.96
0.94
0.92
0.9
0.88
0.86
7
3
transmission
Mt Etna volcanic plume
SO2 (left) from 1284-1345
Ash (right) from 832-900
AVHRR channels
transmission (total)
transmission (scattering)
transmission (absorption)
4
5
Ash
14 10
8
11
9
10
12
11
13
wavelengthWavelength
( m)
(m)
12
14
transmission (scattering)
transmission (absorption)
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
T10.8 - T12.0 > 0 water & ice
T10.8 - T12.0 < 0 volcanic ash
7
8
9
10
11
ET-ODRRGOS, Oxford, UK, 1-5 J
2002
12
13
14
39
AIRS Spectra from around the Globe
20-July-2002 Ascending LW_Window
40
Intro to Microwave and Split
Window Moisture
Lectures in Madison
27 March 2013
Paul Menzel
UW/CIMSS/AOS
Earth emitted spectra overlaid on Planck function envelopes
O3
CO2
H20
CO2
42
MODIS IR Spectral Bands
MODIS
43
First order estimation of SST correcting for low level moisture
Moisture attenuation in atmospheric windows varies linearly with optical depth.
- k u
 = e
= 1 - k u
For same atmosphere, deviation of brightness temperature from surface temperature
is a linear function of absorbing power. Thus moisture corrected SST can inferred
by using split window measurements and extrapolating to zero k
Ts = Tbw1 + [ kw1 / (kw2- kw1) ] [Tbw1 - Tbw2] .
Moisture content of atmosphere inferred from slope of linear relation.
44
45
46
In the IRW - A is off H2O line and B is on H2O line
A
A
B
C
Low
B
B
Sfc
IRW spectrum
A
Weighting Function
47
Radiation is governed by Planck’s Law
c2 /T
B(,T) = c1 /{  5 [e
-1] }
In microwave region c2 /λT << 1 so that
c2 /T
e
= 1 + c2 /λT + second order
And classical Rayleigh Jeans radiation equation emerges
Bλ(T)  [c1 / c2 ] [T / λ4]
Radiance is linear function of brightness temperature.
48
19H Ghz
49
10 to 11 um
50
Microwave Form of RTE
ps
'λ(p)
Isfc = ελ Bλ(Ts) λ(ps) + (1-ελ) λ(ps)  Bλ(T(p))
d ln p
λ
o
 ln p
ps
'λ(p)
Iλ = ελ Bλ(Ts) λ(ps) + (1-ελ) λ(ps)  Bλ(T(p))
d ln p
o
 ln p
o
λ(p)
+  Bλ(T(p))
d ln p
ps
 ln p
atm
ref atm sfc
  
  
  
  
  
  
__________
sfc
In the microwave region c2 /λT << 1, so the Planck radiance is linearly proportional to the
brightness temperature
Bλ(T)  [c1 / c2 ] [T / λ4]
So
o
λ(p)
Tbλ = ελ Ts(ps) λ(ps) +  T(p) Fλ(p)
d ln p
ps
 ln p
where
λ(ps)
Fλ(p) = { 1 + (1 - ελ) [
]2 } .
λ(p)
51
Transmittance
(a,b) = (b,a)
(a,c) = (a,b) * (b,c)
Thus downwelling in terms of upwelling can be written
’(p,ps) = (ps,p) = (ps,0) / (p,0)
and
d’(p,ps) = - d(p,0) * (ps,0) / [(p,0)]2
52
WAVELENGTH
cm
FREQUENCY
m
WAVENUMBER
Hz
10-5
0.1
Near Ultraviolet (UV)
1,000
3x1015
4x10-5
Visible
0.4
4,000
7.5x1014
7.5x10-5
0.75
Near Infrared (IR)
7,500
4x1014
13,333
2x10-3
Far Infrared (IR)
20
2x105
1.5x1013
500
0.1
Microwave (MW)
103
3x1011
GHz
cm-1
Å
300
10
53
54
55
Microwave spectral bands
23.8 GHz
31.4 GHz
60 GHz
120 GHz
183 GHz
dirty window H2O absorption
window
O2 sounding
O2 sounding
H2O sounding
56
57
23.8, 31.4, 50.3, 52.8, 53.6, 54.4, 54.9, 55.5, 57.3 (6 chs), 89.0 GHz
AMSU
23.8
31.4
50.3
GHz
58
AMSU
23.8
dirty
window
atm Q
warms
BT
31.4
window
50.3
GHz
59
Low mist over ocean (MW)
m , m
rs , s
Tb = s Ts m + m Tm + m rs m Tm
Tb = s Ts (1-m) + m Tm + m (1-s) (1-m) Tm
using e-σ = 1 - σ
So temperature difference of low moist over ocean from clear sky
over ocean is given by
ΔTb = - s m Ts + m Tm + m (1-s) (1-m) Tm
For s ~ 0.5 and Ts ~ Tm this is always positive for 0 < m < 1
60
Low Mist over ocean (IRW)
R = s Bs (1-m) + m Bm
using e-σ = 1 – σ and ~1-σ~1-a
So difference of low mist over ocean
from clear sky over ocean is given by
ΔR = - s m Bs + m Bm
For s
~1
ΔR = - m Bs + m Bm = m [Bm - Bs ]
So if [Bm - Bs ] < 0 then as m increases ΔR becomes more negative
AMSU
52.8
53.6
54.4
GHz
63
AMSU
54.4
54.9
55.5
GHz
64
65
Pressure (hPa)
ATMS Weighting Functions
67
Spectral regions used for remote sensing of the earth atmosphere and surface from
satellites.  indicates emissivity, q denotes water vapour, and T represents temperature.
68