the retrieval of snow properties from space: theory and applications a. a. kokhanovsky 1, m. tedesco...
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The retrieval of snow properties from The retrieval of snow properties from space: theory and applicationsspace: theory and applications
A. A. Kokhanovsky1 , M. Tedesco2,3, G. Heygster1 , M. Schreier1, E. P. Zege4
1) University of Bremen, Bremen, Germany2) University of Maryland, Baltimore County, USA3) NASA – Goddard Space Flight Center, Maryland, USA4) Institute of Physics, Minsk, Belarus
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IntroductionIntroduction
A new snow retrieval algorithm that makes use of visible and near-infrared measurements in which snow is modeled as a semi-infinite weakly absorbing medium is developed
The shape of grains is accounted for by means of a fractal snow grain model
The technique is applied to study the changes of snow properties before and just after snow fall in Colorado as seen by two MODIS sensors on TERRA and AQUA satellites
The snow grain size and snow albedo have been retrieved from AATSR onboard ENVISAT data over Greenland
Preliminary comparisons with ground measurements have been performed
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1. Snow physical model
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1. Snow physical model
semi-infinite horizontally homogeneousplane-parallel medium
composed of fractal ice grains suspended in air
SunSatelliteclear sky: • gases • aerosols
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2. Snow optical model
g=0.75 in the visible
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0
0 00
1-( ) ( )( , , ) , s= 4
4( ) 3 1-g ice
a b c p dR d
3. Snow radiative transfer model
0.087 0.014( ) 11.1 1.1p e e
- a=1.247, b = 1.186 and c= 5.157
- the function p is the snow grain phase function
Snow spectral reflectivity
R0 = Reflectivity of a semi-infinite snow layer at zero absorption
0 0 0 0( , , ) ( , , ) exp( 4 ( , , ))R R sf
0 0 00
0 0
( ) ( )( , , )
( , , )
K Kf
R
Escape function
Kokhanovsky and Zege, 2004; Appl. Optics
0 0 0
31 2cos
7K
cos cos
0
0
cos , cos
sin , sin
Kokhanovsky, 2006; Optics Letters(methane adsorption;Domine et al., 2006)
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3. Snow radiative transfer model: albedo determination
0 0 0 0( , , ) ( , , ) exp( 4 ( , , ))R R sf
exp( 4 )r s
Reflectivity:
Albedo:
0( , , )0 0 0( , , ) ( , , ) fR R r
0 0
0 0
49 ( , , ) ( , , )exp ln
9 1 cos 1 cos ( , ,clear clear
pol
R Rr
R
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3. Snow radiative transfer model
0 0 0
0 0
36 1 2 1 2( , , ) ( , , ) exp( )
49 ( , , ) iceR R dR
0 1
•grain size d
•spectral snow albedo r
exp 4 icer d
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4. Validation: Hokkaido
0 20 40 60 800.0
0.2
0.4
0.6
0.8
1.0
1.2
2210nm
1240nm
1050nm
545nm
45 degrees
refle
ctio
n fu
nct
ion
observation zenith angle, degrees
0 20 40 60 800.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
1.2
2210nm
1240nm
1050nm
545nm
90 degrees
refle
ctio
n fu
nct
ion
observation zenith angle, degrees
0 20 40 60 800.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
1.2
2210nm
1240nm
1050nm
545nm
180 degrees
refle
ctio
n fu
nct
ion
observation zenith angle, degrees
Solar zenith angle=54deg
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4. Validation: Hokkaido
0 20 40 60 800.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
545nm
theory
180
90 45
0
refle
ctio
n fu
nct
ion
observation angle, degrees
0 20 40 60 800.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
theory
experiment
2210nm
1240nm
1050nm
545nm
0 degrees
refle
ctio
n fu
nct
ion
observation zenith angle, degrees
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4. Validation: Antarctica
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.40.0
0.2
0.4
0.6
0.8
1.0
sph
eri
cal a
lbe
do
wavelength, m
0.19mm
ExperimentHudson et al., 2006JGR
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4. Validation: North Pole
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.60.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
R=0.94exp(-3.5(d)0.5)
refle
ctio
n fu
nct
ion
wavelength, micrometers
theory (d=0.12mm) experiment
DAMOCLES IP 2005-2009: Developing Arctic Modeling and Observing Capabilities for Long-term Environmental Studies
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0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0.4
1
0.4
6
0.5
3
0.6
1
0.6
9
0.7
9
0.9
1
1.0
4
1.1
8
1.3
5
1.5
5
1.7
7
2.0
2
2.3
1Wavelength [micron]
Sp
he
rica
l A
lbe
do
100 micron
200 micron
400 micron
800 micron
Sensitivity of albedo to grain size
4. Satellite retrievals: grain size from MODIS data
2002
1999
Band 5Band 4
Band 6
MODIS Band 5 offers sensitivity to grain size
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Multi-scale, multi-sensor approach to build comprehensive data set needed to meet NASA Earth Science Enterprise science objectives.
First application to MODIS First application to MODIS data: The CLPX datasetdata: The CLPX dataset
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Elevation and forest cover of Elevation and forest cover of the test areathe test area
Meters
Forest cover fractionGreen very sparseBlue/Black very denseWhite no forest
Elevation [m]
ground measurements
Global Land Cover
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Grain size retrieval: Feb.19, Grain size retrieval: Feb.19, 20032003
TERRA (AM) AQUA (PM)
micrometers10:30am 1:30pm
cloudsnow
forest
morning: snowfall
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Forest effectForest effect
Grain size values retrieved from MODIS-TERRA vs. those retrieved from MODIS-AQUA on February 19, 2003
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Preliminary Preliminary validation(d=(a+b)/2)validation(d=(a+b)/2)
0
100
200
300
400
500
600
700
800
900
0 100 200 300 400 500 600 700 800 900
Measured grain size [micron]
Retri
eved
gra
in s
ize
[mic
ron]
CLPX-1 campaign, North Park, Colorado, USA, 2003
Terra
Aqua
Feb_21
Feb_22
elevation:2.5km
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5. Satellite retrievals: AATSR
-65 -63 -60 -58 -55 -53 -50
76.5
76.0
75.5
75.0
74.5
74.0
73.5
73.0
72.5
longitude, degrees
latitude, degrees
0.1 0.2 0.3 0.4 0.5
reflectance (1600nm)
-65 -63 -60 -58 -55 -53 -50
76.5
76.0
75.5
75.0
74.5
74.0
73.5
73.0
72.5
longitude, degrees
latitude, degrees
0.3 0.5 0.8 1.0
reflectance (550nm)
0.0 0.1 0.2 0.3 0.4 0.5
0
25
50
75
100
125
150
175
200
225
250
275
300
325
reflectance (1600nm)
frequency
0.3 0.5 0.8 1.0
0
250
500
750
1000
1250
1500
1750
2000
reflectance (550nm)
frequency
Reflectances
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5. Satellite retrievals: AATSR
-65 -63 -60 -58 -55 -53 -50
76.5
76.0
75.5
75.0
74.5
74.0
73.5
73.0
72.5
longitude, degrees
latitude, degrees
0.20 0.40 0.60 0.80 1.00
diameter(mm)
-65 -63 -60 -58 -55 -53 -50
76.5
76.0
75.5
75.0
74.5
74.0
73.5
73.0
72.5
longitude, degrees
latitude, degrees
0.0 0.2 0.4 0.6 0.8 1.0
snow albedo(550nm)
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5. Satellite retreivals
0.1 0.2 0.3 0.4
0
13
25
38
50
63
diameter(mm)
frequency
0.80 0.85 0.90 0.95 1.00
0
13
25
38
50
63
75
albedo (550nm)frequency
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6. Account for snow pollution in the visible
abs soot sootK A c 13ext iceK d c
01 abs sootsoot
ext ice
K cM d
K c
0.75g
0
0
1- s=
3 1-gsoot
sootice
cN d
c
0 0 0 0( , , ) ( , , ) exp( 4 ( , , ))R R sf
22
9, Rayleigh soot particles
2
nA
n
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Observations and future work
A new approach is to be developed (A. Lyasputin, UMBC/NASA; von Hoyningen-Huene, University of Bremen) for simultaneous retrieval of AOT and surface BRDF. This will improve MODIS snow BRDF product.
The cloud mask must be improved. A comprehensive validation and calibration campaign is
needed. This will be performed using measurements in Greenland (M. Tedesco, PI of the Proposal submitted to NASA NNH06ZDA001N-IPY).