Comparison of radiative
transfer models for AIRS
R. Saunders, P. Rayer, A. von Engeln,
N. Bormann, L. Strow and S. Hannon,
S. Heilliette, Xu Liu, F. Miskolczi, Y. Han,
G. Masiello, J-L Moncet, G. Uymin, V. Sherlock,
D.S. Turner
Why compare RT models?
ƒIn data assimilation and retrieval applications it
is important to understand the error
characteristics of the forward model and its
gradient (i.e. Jacobian).
ƒBy comparing an ensemble of RT models with
different methodology and based on different
spectroscopy the spread of the differences can
be an indication of the RT model error.
ƒObvious ‘bugs’ in any of the models can also
be identified during the comparison
© Crown copyright 2005
Page 2
AIRS RT model comparison
Compare RT models by:
„
Compute Br. Temps for all 2378 channels
for 52 diverse profiles
„
For some models compute jacobians for a
selection of 20 channels and 52 profiles
„
For some models compute layer to space
transmittances of 20 channels and 52
profiles
„
Use RFM as reference RT model
© Crown copyright 2005
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AIRS RT model Comparison
Model
RTTOV-7
RTTOV-8
Optran
OSS
LBLRTM
RFM
Gastropod
ARTS
SARTA
PCRTM
4A
FLBL
σ-IASI
Hartcode
Participant
R. Saunders, METO
R. Saunders, METO
Y. Han, NESDIS
J-L. Moncet, AER
J-L. Moncet, AER
N. Bormann, ECMWF
V. Sherlock, NIWA
A. Von Engeln, Bremen
S. Hannon, UMBC
Xu Liu, NASA
S. Heilliette, LMD
D.S. Turner, MSC
G. Masiello, IMAA-CNR
F. Miskolczi, NASA
Direct
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Jacobian
Yes
Yes
Yes
Yes
Yes
Yes
Yes
No
No
Yes
Yes
Yes
Yes
No
+ other models being added
© Crown copyright 2005
Page 4
Diverse ERA-40 52 Profile set
0.1
Pressure hPa
1.0
100
10.0
200
300
100.0
190
210
Pressure hPa
1000.0
170
400
500
600
230
250
270
290
Temperature degK700
0
310
330
800
1
1000
0
All 52 profiles dynamically
consistent with all 3 variables
Pressure hPa
900
0.01
10 0.02
Specific humidity kg/kg
100
1000
© Crown copyright 2005
0.03
0
5e−06
1e−05
1.5e−05
Ozone mass mixing ratio kg/kg
2e−05
Page 5
Comparison of AIRS forward models
284
w.v. continuum
RFM
RTTOV−7
ARTS
FLBL
Gastropod
LBLRTM
OPTRAN
PCRTM
OSS
SARTA
4A
HARTCODE
SIGMA−IASI
RTTOV−8
CFCs
Br. Temp (degK)
283.5
283
282.5
282
500
© Crown copyright 2005
550
600
AIRS channel number
650
700
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Mean bias for all 49 diverse profiles
0.10
RTTOV−7
ARTS
FLBL
Gastropod
LBLRTM
OPTRAN
PCRTM
OSS
SARTA
4A
HARTCODE
SIGMA−IASI
RTTOV−8
Model−RFM (degK)
0.05
0.00
−0.05
−0.10
0
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500
1000
1500
AIRS channel number
2000
Page 7
Bias averaged over channels
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Model bias for different bands
Model-RFM for different spectral regions
0.9
650-770cm-1 chans 2-407 T
0.8
770-980cm-1 chans 408-953 Sfc+Cld
1000-1070cm-1 chans 1001-1133 Ozone
0.7
1250-1350cm-1 chans 1328-1487 Q
2150-2250cm-1 chans 1866-1939 T
2350-2420cm-1 chans 2017-2142 T
0.5
2420-2600cm-1 chans 2143-2318 Sfc
0.4
0.3
0.2
0.1
© Crown copyright 2005
T
eD
N
TT
O
V8
I
R
IA
S
Si
gm
a-
O
D
E
AR
TC
4A
H
SA
R
TA
SS
O
R
TM
PC
AN
PT
R
O
TM
LB
LR
as
tro
G
FL
BL
TS
AR
TT
O
V7
0
R
St. dev (degK)
0.6
Page 9
Comparison with observations
OSS
1.0
−1.0
−1.0
−3.0
−3.0
500
1000
1500
−5.0
2000
0
500
3.0
1.0
1.0
−1.0
−1.0
−3.0
−3.0
500
1000
1500
−5.0
2000
0
500
RTTOV−8
3.0
1.0
−1.0
−3.0
0
500
1000
1500
−3.0
0
500
1000
2000
AIRS channel number
© Crown copyright 2005
−5.0
0
500
1500
2000
1.0
1.0
−1.0
−1.0
−5.0
0
500
1000
1500
2000
0
500
Sigma_IASI
1.0
1.0
−1.0
−1.0
−3.0
−3.0
−5.0
1500
2000
1500
2000
−3.0
−3.0
−5.0
1000
Gastropod
3.0
3.0
1000
−3.0
3.0
3.0
500
2000
−1.0
5.0
5.0
0
1500
1.0
LBLRTM
5.0
−5.0
1000
3.0
5.0
ARTS
5.0
−5.0
−1.0
−5.0
2000
Model − AIRS obs (degK)
3.0
Model − AIRS obs (degK)
Model − AIRS obs (degK)
5.0
0
1500
1.0
4A
SARTA
5.0
−5.0
1000
3.0
1000
1500
2000
RFM
5.0
Model − AIRS obs (degK)
0
5.0
Model − AIRS obs (degK)
Model − AIRS obs (degK)
3.0
1.0
HARTCODE
5.0
5.0
3.0
−5.0
PCRTM
OPTRAN
5.0
0
500
1000
1500
2000
3.0
1.0
−1.0
−3.0
−5.0
0
500
1000
1500
2000
AIRS channel number
Page 10
Summary of model –AIRS observations
© Crown copyright 2005
Page 11
AIRS channels selected for jacobians
300
290
Br. Temp (degK)
280
270
260
250
240
230
220
210
200
0
200
400
600
800 1000 1200 1400 1600 1800 2000 2200
AIRS channel number
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Comparison of Jacobians
Temperature jacobian
Profile 1 AIRS channel 77
0.1
Pressure (hPa)
1.0
10.0
RFM
OSS
Gastropod
PCRTM
OPTRAN
LBLRTM
4A
FLBL
RTTOV−8
RTTOV−7
sigma−iasi
100.0
1000.0
−0.01
0.01
0.03
0.05
0.07
0.09
K/K
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Page 13
Measure of fit
For the jacobians the results from each model were differenced
with RFM one of the line-by-line models in order to be able to
conveniently examine the inter-model differences. For the
jacobians the “measure of fit” adopted by Garand et. al., [2001]
was used defined as:
M = 100 ×
∑( X − X )
∑( X )
i
2
ref
2
ref
where Xi is the profile variable at level i and Xref is the
reference profile variable which was taken to be the RFM
model profile for this study.
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Page 14
Comparison of temperature jacobians
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Page 15
Comparison of water vapour jacobians
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Issues for jacobians
Temperature jacobian
Profile 22 AIRS channel 787
0.10
1.00
Pressure (hPa)
This is a weak
temperature jacobian
but some of the
models (e.g. 4A, PCRTM)
have very unphysical
structures. Does this
matter?
RFM
OSS
Gastropod
PCRTM
OPTRAN
LBLRTM
4A
FLBL
RTTOV−8
RTTOV−7
sigma−iasi
10.00
The measure of fit is
not ideal for assessing
these features.
100.00
1000.00
−0.001
0
0.001
0.002
K/K
© Crown copyright 2005
Page 17
Thanks any questions?
All results and some plots are
on the ITWG web page at:
http://cimss.ssec.wisc.edu/itwg/groups/rtwg/rtairs.html
Paper to appear in JGR soon
© Crown copyright 2005
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