Context Modeling TB
Modeling emissivity Conclusions
Modeling microwave emission at 19 and 37 GHz
in Antarctica :
influence of the snow grain size
Ludovic Brucker, Ghislain Picard and Michel Fily
Laboratoire de Glaciologie et Géophysique de l’Environnement
Grenoble, France
Workshop on Remote Sensing and Modeling of Surface Properties,
2009
ludovic.brucker@lgge.obs.ujf-grenoble.fr
Modeling microwave emission in Antarctica
Context Modeling TB
Modeling emissivity Conclusions
Passive microwave remote sensing
TB depends on :
. the snow temperature profile
. the snowpack properties
(grain size and density)
Objective : explain by modeling the microwave emission.
ludovic.brucker@lgge.obs.ujf-grenoble.fr
Modeling microwave emission in Antarctica
Context Modeling TB
Modeling emissivity Conclusions
Microwave emission modeling
Dense Media Radiative Transfer theory (Tsang and Kong, 2001)
Multi-Layered model : DMRT-ML
DMRT-ML is driven by vertical profiles of :
- snow temperature
- sphere radius (grain size parameter)
- snow density
ludovic.brucker@lgge.obs.ujf-grenoble.fr
Modeling microwave emission in Antarctica
Context Modeling TB
Modeling emissivity Conclusions
Outline
1. Modeling the time series of brightness temperature at Dome C
2. Modeling the emissivity at large scale in Antarctica
3. Conclusions
ludovic.brucker@lgge.obs.ujf-grenoble.fr
Modeling microwave emission in Antarctica
Context Modeling TB
Modeling emissivity Conclusions
Dome C, Antarctica
Dome C is on the East Antarctic
Plateau (3240 m a.s.l)
ludovic.brucker@lgge.obs.ujf-grenoble.fr
Modeling microwave emission in Antarctica
Context Modeling TB
Modeling emissivity Conclusions
Method to model TB (t) using snow measurements
3 snow property profiles :
temperature
Measured routinely since 2007 down to 21 m deep with 35 probes
density and grain size
Measured in Dec. 2006 in a snowpit down to 3 m deep
ludovic.brucker@lgge.obs.ujf-grenoble.fr
Modeling microwave emission in Antarctica
Context Modeling TB
Modeling emissivity Conclusions
surface
Snow grain size profile
Near Infrared Photography method
near-IR Photography ←− − − − − − − − − −
↓
reflectance (ω)
↓
Specific Surface Area (SSA) profile
using a ω − SSA relationship (Matzl and Schneebeli, 2006)
↓
sphere radius profile
rsphere =
3
SSA · ρice
ludovic.brucker@lgge.obs.ujf-grenoble.fr
Modeling microwave emission in Antarctica
3 m deep photograph
This approach provides microstructure measurements with a high
vertical resolution (Matzl and Schneebeli, 2006)
Context Modeling TB
Modeling emissivity Conclusions
Snow property profiles at Dome C
0
0
a)
b)
c)
d)
0,5
0,5
1
1,5
1,5
2
2
2,5
2,5
3
200 250 300 350 400 450
density (kg/m^3)
0,6
0,7
0,8
0,9
IR reflectance (ω)
ludovic.brucker@lgge.obs.ujf-grenoble.fr
1
10
20
30
SSA (m²/kg)
40
0
0,5
1
1,5
radius (mm)
Modeling microwave emission in Antarctica
2
3
depth (m)
depth (m)
1
Context Modeling TB
Modeling emissivity Conclusions
Snow property profiles at Dome C
0
0
a)
b)
c)
d)
0,5
0,5
1
1,5
1,5
2
2
2,5
2,5
3
200 250 300 350 400 450
density (kg/m^3)
0,6
0,7
0,8
0,9
IR reflectance (ω)
1
10
20
30
SSA (m²/kg)
40
0
0,5
1
1,5
radius (mm)
There is an increase in grain size with depth
ludovic.brucker@lgge.obs.ujf-grenoble.fr
Modeling microwave emission in Antarctica
2
3
depth (m)
depth (m)
1
Context Modeling TB
Modeling emissivity Conclusions
Snow property profiles at Dome C
0
0
a)
b)
c)
d)
37 GHz
0,5
0,5
1
1
1,5
1,5
2
2
2,5
2,5
3
200 250 300 350 400 450
density (kg/m^3)
0,6
0,7
0,8
0,9
IR reflectance (ω)
1
10
20
30
40
SSA (m²/kg)
At Dome C penetration depth at 37 GHz
19 GHz
ludovic.brucker@lgge.obs.ujf-grenoble.fr
0
0,5
1
1,5
radius (mm)
0.8 m
3.7 m
Modeling microwave emission in Antarctica
2
3
depth (m)
depth (m)
19 GHz
Context Modeling TB
Modeling emissivity Conclusions
2 calibrated parameters : α
r z>3m
same α and same r z>3m at 19 and 37 GHz
ludovic.brucker@lgge.obs.ujf-grenoble.fr
Modeling microwave emission in Antarctica
Context Modeling TB
Modeling emissivity Conclusions
α ' 2.8
r z>3m ' 1.14mm
RMSE19 =0.3K
RMSE37 =1.3K
RMSE=0.9K
ludovic.brucker@lgge.obs.ujf-grenoble.fr
Modeling microwave emission in Antarctica
Context Modeling TB
Modeling emissivity Conclusions
Why TB are predicted with a low RMSE ?
- Snow properties are measured with a high vertical resolution;
- State-of-the-art model.
ludovic.brucker@lgge.obs.ujf-grenoble.fr
Modeling microwave emission in Antarctica
Context Modeling TB
Modeling emissivity Conclusions
Outline
1. Modeling the time series of brightness temperature at Dome C
2. Modeling the emissivity at large scale in Antarctica
3. Conclusions
ludovic.brucker@lgge.obs.ujf-grenoble.fr
Modeling microwave emission in Antarctica
Context Modeling TB
Modeling emissivity Conclusions
Emissivities in Antarctica derived from observations
Mean annual SSM/I emissivities in dry-snow regions
19.3 GHz
37 GHz
0˚
0˚
30
˚
0˚
˚
30
270˚
90˚
90˚
30
˚
˚
270˚
60
60
0˚
33
0˚
30
0˚
33
0˚
12
12
1.00
0˚
0˚
0.95
24
24
0˚
1.00
0.90
0.85
0.80
21
0˚
0
15
180˚
0.80
21
˚
0.75
0.70
˚
0
15
0˚
180˚
0.65
0.75
0.70
0.65
(Picard et al., 2009)
ludovic.brucker@lgge.obs.ujf-grenoble.fr
0.95
0.90
0.85
Modeling microwave emission in Antarctica
Context Modeling TB
Modeling emissivity Conclusions
Observed emissivities in a 19-37 space
The emissivities have close values
at 19 GHz and 37 GHz
ludovic.brucker@lgge.obs.ujf-grenoble.fr
Modeling microwave emission in Antarctica
Context Modeling TB
Modeling emissivity Conclusions
Observed emissivities in a 19-37 space
The emissivities have close values
at 19 GHz and 37 GHz
Question : which snow property can explain such a distribution
(spectra) of emissivity at 19 and 37 GHz ?
ludovic.brucker@lgge.obs.ujf-grenoble.fr
Modeling microwave emission in Antarctica
Context Modeling TB
Modeling emissivity Conclusions
Homogeneous snowpack
0,1
1
0,1
0,2
0,3
19.3 GHz
37 GHz
0,4
0,2
0,5
0,3
0,6
0,4
0,7
0,9
0,8
0,5
Emissivity
0,9
0,6
0,8
0,7
0,7
0,8
0,9
0,6
0,2
0,4
0,6
Sphere radius (mm)
0,8
1
ludovic.brucker@lgge.obs.ujf-grenoble.fr
Modeling microwave emission in Antarctica
Context Modeling TB
Modeling emissivity Conclusions
Homogeneous snowpack
0,1
1
0,1
0,2
0,3
0,5
0,3
0,1
0,2
1
19.3 GHz
37 GHz
0,4
0,2
0,3
0,6
0,4
0,4
0,7
0,9
0,9
0,5
Emissivity at 37 GHz
0,8
0,5
Emissivity
0,9
0,6
0,8
0,7
0,6
0,8
0,7
0,8
0,7
0,7
0,8
0,9
0,9
0,6
0,2
0,4
0,6
Sphere radius (mm)
0,8
1
ludovic.brucker@lgge.obs.ujf-grenoble.fr
0,6
0,6
0,7
0,8
Emissivity at 19.3 GHz
0,9
Modeling microwave emission in Antarctica
1
Context Modeling TB
Modeling emissivity Conclusions
Homogeneous snowpack
0,1
1
0,1
0,2
0,3
0,5
0,3
0,1
0,2
1
19.3 GHz
37 GHz
0,4
0,2
0,3
ra
0,6
0,4
ec
t
0,7
sp
0,4
0,9
w
0,9
0,5
no
ec
t
ra
ss
sp
al
ou
0,7
sn
ow
sp
Fl
a
ec
ts
tra
no
w
m
0,6
m
al
0,7
0,8
no
or
0,7
0,7
0,8
N
Emissivity
0,9
0,6
0,8
A
Emissivity at 37 GHz
0,8
0,5
0,8
0,9
0,9
0,6
0,2
0,4
0,6
Sphere radius (mm)
0,8
1
ludovic.brucker@lgge.obs.ujf-grenoble.fr
0,6
0,6
0,7
0,8
Emissivity at 19.3 GHz
0,9
Modeling microwave emission in Antarctica
1
Context Modeling TB
Modeling emissivity Conclusions
Homogeneous snowpack
0,1
1
0,1
0,2
0,3
19.3 GHz
37 GHz
0,4
0,2
0,5
0,3
0,6
0,4
0,7
0,9
0,8
0,5
Emissivity
0,9
0,6
0,8
0,7
0,7
0,8
0,9
0,6
0,2
0,4
0,6
Sphere radius (mm)
0,8
1
ludovic.brucker@lgge.obs.ujf-grenoble.fr
Modeling microwave emission in Antarctica
Context Modeling TB
Modeling emissivity Conclusions
Homogeneous snowpack
0,1
1
0,1
0,2
0,3
19.3 GHz
37 GHz
0,4
0,2
0,5
0,3
0,6
0,4
0,7
0,9
0,8
0,5
Emissivity
0,9
0,6
0,8
0,7
0,7
0,8
0,9
0,6
0,2
0,4
0,6
Sphere radius (mm)
0,8
1
The homogeneous snowpack cannot explain simultaneously the
emissivities at 19 and 37 GHz in Antarctica.
ludovic.brucker@lgge.obs.ujf-grenoble.fr
Modeling microwave emission in Antarctica
Context Modeling TB
Modeling emissivity Conclusions
Heterogeneous snowpack
with a linear increase in snow grain size with depth
To increase the snow grain size with depth :
r (z) = rnear
0
0 0,1 0,2 0,3 0,4 0,5 0,6
surf
+Q ·z
0,1
0,2
1
0,3
0,4
-1
0,9
Emissivity at 37 GHz
0,5
Depth (m)
-2
-3
0,6
0,8
0,7
0,8
0,7
-4
0,9
-5
0 0,1 0,2 0,3 0,4 0,5 0,6
Grain size (mm)
0,6
0,6
ludovic.brucker@lgge.obs.ujf-grenoble.fr
0,7
0,8
Emissivity at 19.3 GHz
0,9
1
Modeling microwave emission in Antarctica
Context Modeling TB
Modeling emissivity Conclusions
Heterogeneous snowpack
with a linear increase in snow grain size with depth
To increase the snow grain size with depth :
r (z) = rnear
0
surf
+Q ·z
0 0,1 0,2 0,3 0,4 0,5 0,6
0
1
0,1
10
20
-1
30
0,9
Emissivity at 37 GHz
40
Depth (m)
-2
-3
0,8
0,7
-4
-5
0 0,1 0,2 0,3 0,4 0,5 0,6
Grain size (mm)
0,6
0,6
ludovic.brucker@lgge.obs.ujf-grenoble.fr
0,7
0,8
Emissivity at 19.3 GHz
0,9
1
Modeling microwave emission in Antarctica
Context Modeling TB
Modeling emissivity Conclusions
Heterogeneous snowpack
with a linear increase in snow grain size with depth
To increase the snow grain size with depth :
r (z) = rnear
0
surf
+Q ·z
0 0,1 0,2 0,3 0,4 0,5 0,6
0
1
0,1
10
0,3
20
-1
30
0,9
Emissivity at 37 GHz
40
Depth (m)
-2
-3
0,8
0,7
-4
-5
0 0,1 0,2 0,3 0,4 0,5 0,6
Grain size (mm)
0,6
0,6
ludovic.brucker@lgge.obs.ujf-grenoble.fr
0,7
0,8
Emissivity at 19.3 GHz
0,9
1
Modeling microwave emission in Antarctica
Context Modeling TB
Modeling emissivity Conclusions
Heterogeneous snowpack
with a linear increase in snow grain size with depth
To increase the snow grain size with depth :
r (z) = rnear
surf
+Q ·z
0 0,1
1
10
increasing snow
grain size gradient
30
0,9
0,5
40
Emissivity at 37 GHz
0,3
20
0,8
0,7
0,7
0,6
0,6
ludovic.brucker@lgge.obs.ujf-grenoble.fr
increasing near surface
snow grain size
0,7
0,8
Emissivity at 19.3 GHz
0,9
1
Modeling microwave emission in Antarctica
Context Modeling TB
Modeling emissivity Conclusions
Heterogeneous snowpack
with a linear increase in snow grain size with depth
To increase the snow grain size with depth :
n
r n (z) = rnear
0 0,1
1
10
0,9
1
0,5
Emissivity at 37 GHz
Emissivity at 37 GHz
Emissivity at 37 GHz
0,9
40
50
60
0,8
70
80
90
100
0,7
0,7
0,6
0,6
10
0,5
30
increasing near surface
snow grain size
0,8
Emissivity at 19.3 GHz
0,3
20
0,9
0,5
0,7
0,7
0 0,1
1
0,3
10
0,8
0,6
0,6
1
20
40
0,7
n=3
0 0,1
0,3
30
0,9
+ Qn · z
n=2
n=1
increasing snow
grain size gradient
surf
20
30
40
0,8
50
60
70
80
90
100
0,7
0,7
0,7
0,8
Emissivity at 19.3 GHz
0,9
1
0,6
0,6
0,7
0,8
Emissivity at 19.3 GHz
n=3 cannot explain anomalous snow spectra
ludovic.brucker@lgge.obs.ujf-grenoble.fr
Modeling microwave emission in Antarctica
0,9
1
Context Modeling TB
Modeling emissivity Conclusions
Heterogeneous snowpack
with a linear increase in snow grain size with depth
To increase the snow grain size with depth :
n
r n (z) = rnear
0 0,1
1
10
0,9
1
0,5
Emissivity at 37 GHz
Emissivity at 37 GHz
Emissivity at 37 GHz
0,9
40
50
60
0,8
70
80
90
100
0,7
0,7
0,6
0,6
10
0,5
30
increasing near surface
snow grain size
0,8
Emissivity at 19.3 GHz
0,3
20
0,9
0,5
0,7
0,7
0 0,1
1
0,3
10
0,8
0,6
0,6
1
20
40
0,7
n=3
0 0,1
0,3
30
0,9
+ Qn · z
n=2
n=1
increasing snow
grain size gradient
surf
20
30
40
0,8
50
60
70
80
90
100
0,7
0,7
0,7
0,8
Emissivity at 19.3 GHz
0,9
{e19 , e37 } =⇒ {rnear
ludovic.brucker@lgge.obs.ujf-grenoble.fr
1
surf ,
0,6
0,6
0,7
0,8
Emissivity at 19.3 GHz
Qn }
Modeling microwave emission in Antarctica
0,9
1
Context Modeling TB
Modeling emissivity Conclusions
Retrieved snow grain profile parameters
{e19 , e37 } =⇒ {rnear
rnear
surf
ludovic.brucker@lgge.obs.ujf-grenoble.fr
surf ,
Qn }
Qn for n=2
Modeling microwave emission in Antarctica
Context Modeling TB
Modeling emissivity Conclusions
Validations
• in situ measurements acquired along traverses
• IR photographs at Dome C
• climate models
• grain size retrieved by visible and infrared satellite sensors
(POLDER, ATSR-2, Landsat and MODIS)
ludovic.brucker@lgge.obs.ujf-grenoble.fr
Modeling microwave emission in Antarctica
Context Modeling TB
Modeling emissivity Conclusions
Validation at Dome C
0
-1
depth (m)
-2
Grain size derived from :
- IR photography
- inversion of the emissivities
-3
-4
-5
0
0,5
1
1,5
radius (mm)
2
ludovic.brucker@lgge.obs.ujf-grenoble.fr
Modeling microwave emission in Antarctica
Context Modeling TB
Modeling emissivity Conclusions
Outline
1. Modeling the time series of brightness temperature at Dome C
2. Modeling the emissivity at large scale in Antarctica
3. Conclusions
ludovic.brucker@lgge.obs.ujf-grenoble.fr
Modeling microwave emission in Antarctica
Context Modeling TB
Modeling emissivity Conclusions
CONCLUSIONS
- An increase in snow grain size with depth was measured by IR photography;
- With a calibrated α and r z>3m , TB (t) are accurately explained, RMSE<1K;
- Emissivities modeled with a homogeneous snowpack cannot predict
the flat spectra of observed emissivities
=⇒ the snow grain size must increase with depth;
- Considering a simple grain growth relationship and
{e19 , e37 }, it is possible to retrieve {rnear surf , Qn };
- Our retrievals were validated.
FUTURE WORKS
- Explain the horizontal polarization;
- New measure of snow properties.
Acknowledgements
These works are supported by the Programme National de Télédétection
Spatiale, the project VANISH of the Agence Nationale de la Recherche
and the program NIEVE (LEFE, INSU-CNRS).
ludovic.brucker@lgge.obs.ujf-grenoble.fr
Modeling microwave emission in Antarctica