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SATELLITE REMOTE SENSING OF TERRESTRIAL CLOUDS
Alexander A. Kokhanovsky
Institute of Remote Sensing, Bremen UniversityP. O. Box 330440 Bremen, Germany
Hot issues in cloud research
Optical properties of ice and mixed clouds
Absorption of solar radiation by clouds
Gas-aerosol-cloud interactions
3-D effects in clouds
Clouds and climate
ENVISAT Start: 28.2.2002
SCIAMACHY Instrument
Characteristics:• UV/Vis/NIR Spectrometer:
240 - 2380 nm• Spectral resolution:
0.2 – 1.5 nm• 8000 spectral points• SCIAMACHY measures:
– Reflected solar Reflected solar light (nadirlight (nadir)
– Scattered solar light (limb)
– Transmitted solar/moon light (occultation)
– Solar Irradiance
SCIAMACHY = SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY
PARAMETERS to be retrieved:
Cloud optical thickness /5-100/
Cloud top height /0.5-10km/
Cloud cover /0-1/
Cloud albedo/0.3-0.8/
Liquid water path /50-200 /
Thermodynamic phase /ice, water or mixed clouds/
Average size of droplets/crystals
2gm
The retrieval techniques1: The look-up-table approach
00 0
0
d , ,cos =- , , +B , ,
d optical thicknesszenith solar angle
zenith observation angle relative azimuth reflected light intensity
B source function
II
I
0
Z
O
2: The semi-analytical approach
0 0 0 0 0
0 0 0
0
0,
, , , = , , -
, , , , , , / cos reflection function
, , reflection function of a semi-infinite cloud spherical albedo of a semi-infinite cloud spherical alb
R R r r K K
R I F
Rrr
0
0 0
edo of a finite cloud( ) escape function
Functions , , , , , ( ) are given by simple analytical equations ( ., , 2003)
Kr r R K
Kokhanovsky et al JGR
Optical characteristics of clouds Trishchenko and Liu, 2001
Optical thickness
ISCCP dataSurface observations
Kokhanovsky, 2003
The physics behind the semi-analytical approach
Satellite
0 0 0
0 0 0
1, , , , , ,
, , , = , , - , ,
r tR R T
R R T
Sun
1
2
1
0 0 0
0 0 0 0 0
0 0
0 0
, , , = , , - , ,
3, , , 1 cos7
1.07 0.75 (1 )0.85 asymmetry parameter (water clouds)
43(1- ) , , , ,mes
R R T
T tK K K
t gg
K Kg R R
The cloud optical thickness determination
The accuracy of the semi-analytical asymptotic theory0.65 m ,2003Kokhanovsky and Rozanov
The accuracy of the semi-analytical asymptotic theory ,2003Kokhanovsky and Rozanov 1.55 , 6
efm a m
The accuracy of the semi-analytical asymptotic theory ,2003Kokhanovsky and Rozanov 1.55 m
Image of Western Europe from Sea-viewing Wide Field of view Sensor 1-km spatial resolution
2001 10 11
Retrieved cloud optical thickness distribution
Retrieved cloud albedo distribution
Frequency distribution: optical thickness
Frequency distribution:spherical albedo
Hurricane Erin, 1-17 September, 2001
Hurricane Erin grazes Bermuda
September 9, 2001 Posted: 11:04 PM EDT (0304 GMT)
A satellite photo of Hurricane Erin
MIAMI, Florida (CNN) -- Hurricane Erin continued to gain strength Sunday but posed increasingly less threat to land as it strayed farther out in the Atlantic. The worst part of the storm, with maximum sustained winds of 195 km/h, passed to the northeast of Bermuda
Hurricane optical thickness distribution
optical thickness25 50 75 125100 150
0 500 1000
0
500
1000
0
Hurricane optical thickness distribution near its eye
Hurricane optical thickness distribution in the eye
Other applications
1.0 1.5 2.0 2.50.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
wavelength, micrometers
measurements approximation
Experiment/sea foam/, Frouin et al., JGR, C, 2001 Experiment/Antarctic snow /, Grenfell et al., JGR, D,1994
Cloud top height determination from a satellite The geometry of the problem
a i b rR R T R T
Nadir observation, solar zenith angle – 20 degreesCloud optical thickness –20.
The accuracy of the forward model
Cloud top height determination from a satellite
The accuracy of the forward model
Cloud top height determination from a satellite
01 , 5.5 , 60l km h km
Cloud top height determination from a satellite The physical principle behind the retrieval
0250 , 5, 60l m
Preliminary results !
Cloud geometrical thickness determination from a satellite
0 5.5 , =20, 60h km
Table 1. Characteristics of selected space instruments, related to measurements of the backscattered light in the oxygen A-band (7550-7750 ).
Instrument Platform Year Spectral interval,
Spectral resolution,
Spatial resolution, km2
GOME ERS-2 1995 5760-7940
3.3 40*320
SCIAMACHY ENVISAT 2002 6040-8050
4.8 30*60
MERIS ENVISAT 2002 7600
25 0.3*0.3or1.1*1.1
GLI ADEOS-II 2002 7630
80
0.25*0.25 and1.0*1.0
POLDER ADEOS-II 2002 76337651
100400
6.0*7.0
A
A
A
Typical SCIAMACHY spectra in the oxygen A-band
Cloud top height determination from SCIAMACHY
PMD1 23-Jan-2003 4700/4701
University of Bremen, IUP/IFE, [email protected]
Cloud top height determination from SCIAMACHY
Cloud top height determination from SCIAMACHY
Cloud top height determination from GOME data using oxygen A-band information as compared to
ATSR-2 IR retrievals (ERS-2 satellite)
23.07.1995, orbits 50723095, 50723114, ERS-2
24.03.1998, 80324174, ERS-2
Conclusions
• Most of important cloud parameters can be retrieved using spectral top of atmosphere reflectance.
• Cases of inhomogeneous clouds can bias retrieval results considerably. This should be clarified in future research.• The information on clouds obtained on a global and regional scale should enhance our studies of climate change, including anthropogenic influences on cloud microphysical and optical properties
and
Clouds are important
and
Hurricane Erin, Sept 9th, 2001
Clouds are beautiful!
0
20
40
60
80
100
10-1 100 101 102 103 104
Latitude: 79o N
Longitude 140o E
b) State 26, Orbit 1778, July 3, 2002a) State 23, Orbit 1778, July 3, 2002
NLC signal
optisch dünnes Regime
optischdichtesRegime
"Knie"
= 260 nm = 270 nm = 280 nm = 290 nm = 300 nm
Limbradianz [a.u.]
Tang
ente
nhöh
e [k
m]
0
20
40
60
80
100
10-1 100 101 102 103 104
Limbradianz [a.u.]
Tang
ente
nhöh
e [k
m]
Latitude: 61o N
Longitude 31o E
No NLC present
"Knie" = 260 nm = 270 nm = 280 nm = 290 nm = 300 nm
SCIAMACHY Limb profileWith NLCs
SCIAMACHY Limb profile without NLCs
SCIAMACHY observes NLC
NLC Season 2002 NLC Season 2002/2003
Global detection of NLC with SCIAMACHY
Noctilucent clouds
Photos: Pekka Parviainen
