ITSC
-
12
Cloud processing in IASI
context
Lydie Lavanant
Météo
-
France, Centre de Météorologie Spatiale, BP 147, 22300
Lannion Cedex France
Purpose:
Retrieval of
cloud
information and atmospheric profiles in cloudy
conditions
Steps
:
»
Test case description
»
CO
2
-
slicing method
»
Avhrr cloud description in IASI fov
»
IASI channel selection in cloudy conditions
»
Preliminary results of profile retrieval in cloudy conditions
01/03/02
Test Case
Global IASI orbit simulation. Feb. 1996
13000 situations with:
•
simulated IASI cloudy and noisy spectra 0.25 cm
-
1 (R. Rizzi model)
•
Colocated atmospheric profiles:
NWP analyses in T, Q, O on RTIASI levels
Cloud description (cover, CLWV, CIWV) on 31 NWP levels
Dataset provided by Eumetsat (ISSWG)
CO
2
slicing method
Ref: Menzel and Stewart 1983, Smith and Frey 1990
[(
Rclr
–
Rmeas
)
k
/ (
Rclr
–
Rmeas
)
ref
]
–
[N
e
k
(
Rclr
-
Rcld
)
k
/ N
e
ref
(
Rclr
-
Rcld
)
ref
]
= f
pc
Rmeas: measured radiance
Rclr: clear radiance computed from the colocated forecast
Rcld: black
-
body radiance at the cloud level n
k= channel from 690 cm
-
1
to 810 cm
-
1
Ref= reference channel = 899.75 cm
-
1
For each channel k: cloud pressure
= pressure which minimises equation
P_co2 =
S
(p_co2(k) w
2
(k)) / Sw
2
W =
d
f
pc
/
d
lnp
N
e
= (
Rclr
–
Rmeas
)
ref
/ (
Rclr
-
Rcld
)
ref
Assumption: one thin cloud layer
Rejections: (
Rclr
–
Rmeas
) < sqrt(2)*radiometric noise
N
e
< 0
Preliminary results of CO2 method using CDS cloudy
spectra
100
-
200
23
158.4
102.4
200
-
300
358
47.1
103.3
300
-
400
396
-
9.7
132.7
400
-
500
658
-
31.8
131.7
500
-
600
802
-
51.3
120.2
600
-
700
913
-
97.2
112.3
700
-
800
1057
-
126.6
125.9
800
-
900
793
-
147.6
135.8
900
-
1000
1534
-
129.9
164.0
1000
-
1050
72
-
51.4
71.4
Method:
•
Adapt RTIASI for implementing RTTOV7 cloudy routines
developed by F. Chevallier and al. (2001)
•
Simulate cloudy noisy IASI spectra
Rmeas
for all CDS situations
using:
»
NPW profiles (T,H2O,.., CC, CLWV, CIWV)
»
radiometric noise
•
Compute clear noisy radiances
Rclr for the same fov
using:
»
RTIASI clear
»
noisy NWP profiles (apply forecast errors)
•
Apply CO2
-
slicing method
Examples of IASI
cloudy spectra
Variation with the number of channels
1 cloud layer
N
e
> 0.3
1 cloud layer
RTIASI cloudy + noise
Profile= analysis
24 channels. r
e
solution:5cm
-
1
Variation with emissivity
Variation with
emissivity
Several cloud layers
RTIASI cloudy + noise
Profile= analysis
Variation with
emissivity
1 cloud layer
RTIASI cloudy
Profile=forecast
P_Co2,
e
_Co2
1 cloud layer
RTIASI cloudy + noise
Profile=forecast
s
= 0.2
-
0.25
Cloud top pressure
CDS dataset cloud pressure
CO2 retrieved cloud pressure
AVHRR Cloud mask in IASI fov
Operational routine for HIRS fov
(inside AAPP)
»
Based on a threshold technique
applied
.
every AVHRR pixel in sounder fov
. to various combinations of channels
»
Combinations of channels depend on:
. geographical location of the pixel
. solar illumination and viewing geometry
»
Thresholds computed in
-
line with:
. constant values from experience
. tabulated functions defined off
-
line
through RTTOV simulations on
climatological data
-
set
. TWVC retrieved from colocated AMSU
-
A
Current products:
»
percentage
clear
AVHRR
in
FOV
»
surface
temperature
from
AVHRR
split
-
window
»
black
body
cloud
coverage
in
FOV
»
cloud
top
temperature
for
the
black
body
layer
»
clear/cloudy
flag
for
each
AVHRR
pixel
Next
version
:
»
Ts,
Tcld,
Cloud
type
for
each
Avhrr
pixel
»
-
>
number
of
clouds
AVHRR Cloud mask in IASI fov
AVHRR Cloud mask in IASI fov
Validation over Europe
correctly detected
Cloudy targets
Cloud free targets
sea ; day
1774 (99.8%)
584 (87.8%)
sea ; glint
269 (98.8%)
72 (89%)
sea ; twilight
59 ( 98.3%)
12 (90%)
land ; day
995 (99.5%)
638 (80.9%)
land ; twilight
27 (100%)
11 (67.4%)
7007 targets of 5x5 AVHRR
pixels
Noaa12
,
14
,
15
for
3 years
38
cloud
types
Mask comparison with visual
analysis of satellite imagery by
CMS nephanalysts
March
2001
Clear land
Clear sea
Cloudy
land Cv>50
Cloudy sea
Cv>50
N
937
103
136
55
Bias (K)
0.02
0.64
0.49
0.15
Std (K)
0.99
0.39
0.98
0.65
Comparison of satellite obs.
and Hirs 8 RTTOV6 Tbs using:
* NWP profile,
* AVHRR clear cover +Ts,
* AVHRR black
-
body cloud
cover +Tn
Channels selection and retrieval in clear conditions on CDS
•
Rodgers DFS selection
•
Guess error matrix = forecast
•
Use a mean profile for mid
-
latitude
conditions
the 300 most informative channels
Clear situations
nbsit= 187 (1/10)
Channels selection above the cloud
Select channels from the 300 most informative channels in clear conditions
Ex: for p_cloud=850 hPa
. uncontaminated channels above the cloud top level: about 65% channels selected
. cloud contaminated
channels
with
(
Tbobs
–
Tbgucld
)
<
0.
3
K
:
about
85%
channels
selected
Profile retrieval in cloudy conditions. CDS dataset
un
-
contaminated channels above the cloud
600
<
p_cloud
<
700
Nbsit= 132 (1/3)
700
<
p_cloud
<
800
Nbsit= 146 (1/4)
800
<
p_cloud
<
900
Nbsit= 138 (1/7)
900
<
p_cloud
<
1000
Nbsit= 166 (1/5)
w
ith
un
-
contaminated
channels above
cloud
:
1DVar in clear
conditions
Profile retrieval in cloudy conditions
with cloud
information as control variable
all P_cld
>
800hPa
1807 situations
(15% of situations)
P_cld
e
_cld
Cloud c
ontrol variables: ln(p_cld),
e_
cld
Cloud g
uess: CO2 p_cld and
e_
cld
selected channels: Tbobs
–
Tbgucld
<
0.
3
K
-
>
more than
8
0% channels selected
before
1d_var
after
1d_var
a
ll selected
channels
: 1DVar
cloudy
forecast
as
background
Summary:
•
Create IASI cloudy spectra using NWP analyses (T,H2o,.., CC, CLWV,
CIWV)
•
Use CO2 method to determine the cloud top pressure and emissivity
•
Retrieve temperature profile in cloudy condition with CO2 cloud
parameters as guess
•
validate on CDS dataset
Future:
•
Consolidate the results on recent NWP data (with cloud profile
information on 60 levels)
-
> package
»
add the water vapor profile
»
Combine IASI, AVHRR and AMSU information
•
Validate on AIRS observations
•
Adapt the method to the IASI stand
-
alone package
•
Test a cloud
-
clearing method (J. Joiner)
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