4c5_L.Lavanant

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3 Νοε 2013 (πριν από 3 χρόνια και 7 μήνες)

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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)