The document summarizes initial results from the SMOS satellite mission regarding spatial variations of L-band emissivity in Antarctica. Key findings include: 1) L-band brightness temperature is fairly constant in dry snow zones, suggesting potential to retrieve snow temperature from SMOS data in these areas. 2) Wet snow zones exhibit more temporal brightness temperature variations due to liquid water absorption and formation of icy layers during melt-refreeze cycles. 3) Radiative transfer modeling suggests snowpack density profiles are important for accurately modeling brightness temperature, particularly at horizontal polarization. Further data is needed to refine emissivity estimates and potential snow temperature retrievals from SMOS in dry snow zones.

1. Spatial variations of L-band emissivity in Antarctica,
first results from the SMOS mission.
G. Picard(1), Y. Kerr(2), G. Macelloni(3), N. Champollion(1), M. Fily (1),
Kerr(2)
F. Cabot(2), P. Richaume(2), M. Brogioni(3)
UJF-Grenoble 1 / CNRS, LGGE UMR 5183, Grenoble, F-38041, France
CESBIO (CNES,CNRS,IRD,UPS) 18 avenue Edouard Belin 31401 Toulouse,
France
IFAC-CNR, via Madonna del Piano 10 – 50019 Sesto Fiorentino, Italy
IGARSS Summer 2011 Vancouver, Canada

2. Context
General context:
- SMOS L-band (1.4 GHz) microwave radiometer acquires radically new data that may be of
interest for the cryosphere in general and the Antarctic in particular.
Objective of our work:
- Explore the information content of SMOS
data in the continental Antarctic and propose
applications of interest in climate and
glaciology sciences.
SMOS track in Antarctica (12 Jan 2010)

3. Context
SMOS main characteristics:
L-band (1.4GHz), full polarizations, variable incidence angles,
~35km resolution.
What was expected before SMOS launch:
1 - In dry snow, scattering by snow grain is weak at low frequencies
→ the emissivity at L-band should be high, and close to 1 for
incidence angles close to Brewster angle (50-55o) and for V-
polarization. In such a case, TB = Tsnow, snow temperature might
Tsnow
be retrieved everywhere in Antarctica !
2 - Ice absorption at L-band is very weak.
→ Penetration depth in dry snow is expected to be several
hundreds of meters, TB should be nearly constant over time.
→ The Antarctic plateau could be a good external calibration
target.
Objective of this talk:
- Test 1 and 2

4. Outline
1 – Processing of SMOS data in Antarctica
2 – Temporal variations of TB
3 – Spatial variations of TB
4 – Radiative transfer modeling at L-band.
5 – Concluding remarks

5. SMOS data processing
L1C data
reprocessed 2010
from Brockmann Consult The result of these processing steps is a cube of
TB (x,y, t, θ, p)
Read and XY2HV
rotation subroutines
from CESBIO
- Area selection space time Incidence polarisation
- Flag selection angle
- Daily average
- Projection to the “standard”
stereographic
polar projection
at 25 km resolution

6. SMOS data processing
Angular diagram of TB with all the data in 2010 at Dome C (75oS, 123oE)
Physical
annual-mean
snow temperature
TB (x,y, t, θ, V)
TB
TB (x,y, t, θ, H)
Incidence angle Dome C
TB is indeed close to the snow physical temperature near the
Brewster angle (50-55o) at V-polarisation
→ how temporal and spatial variations look like at this viewing
configuration ?

7. Temporal variations of TB
TB (x,y, t, θ=55o, p=V-pol)
Daily-mean T on the Larsen C ice shelf
B
Daily-mean T at Dome C (-75oS, 123oE) in the Peninsula
B
L-band
C-band
C-band
L-band
L-band brightness temperature is fairly Brightness temperature at any frequency
constant in the dry zone. We can work with increases sharply when the snowpack
averaged TB. becomes wet.

8. Spatial variations of L-band TB
TB (x,y, <t>, θ=55o, p=V-pol)
Two very different zones: the wet zone (low emissivity) and the dry zone (high emissivity)...

9. Spatial variations of L-band TB
Number of days with melt during the austral
TB (x,y, <t>, θ=55o, p=V-pol) summer 2009/2010 (derived from SSM/I).
Why the emissivity is low in the wet zone ?

10. Spatial variations of L-band TB
In the wet zone, during the summer, the liquid water is responsible for the peaks noticed
in TB time-series at every frequency
Wet snow (snow + max 8% of liquid water) causes very
strong absorption.
According to Kirchoff law, emissivity is close to 1
The physical temperature of wet snow is 273K by definition
Tsnow TB ~ 273 K

11. Spatial variations of L-band TB
Melt-refreeze cycles during the summer period form coarse grain- or icy- layers.
During the winter, the brightness temperature is low because:
Icy or coarse grain layer causes very strong scattering.
Emanating microwaves are reflected backward
(=downward).
→ Emissivity is very low
Tsnow
TB < 200 K

12. Spatial variations of L-band TB
Focus on the dry zone:
TB (x,y, <t>, θ=55o, p=V-pol) ERA Interim annual-mean air temperature
The scales are slightly different
TB at V-polarisation and Brewster angle is close to the physical temperature...

13. Spatial variations of L-band TB
It means, the emissivity at V-polarisation and Brewster angle is close to 1, but how close ?
e=0.97
e=1
e=0.95
Each dot corresponds to a pixel
in the dry zone, SMOS TB
versus ERA temperature.
If ERAInterim is accurate, the emissivity is in the range [0.95, 0.97] . But ERAInterim is not perfect...
and known to be warm-biased in Antarctica by a few Kelvin. Emissivity may be slightly higher.

14. Spatial variations of L-band TB
E.g. at Dome C where accurate snow temperature is measured routinely by LGGE:
TB(SMOS) = 213 K
Tair(ERA) = 224 K → e=0.951
Tsnow = 218 K → e=0.977
To exploit SMOS brightness temperature at Brewster angle and V-polarization, we need to
refine our understanding of the emissivity at L-band.
One solution is to use radiative transfer modeling...

15. Angular diagram
Ingredients:
DMRT-ML is the snow passive microwave radiative transfer model developed at LGGE
+ Density, grain size and temperature profiles measured at Dome C down to 10m
and extrapolated down to 100m (snow/ice transition).
→ Preliminary results of predicted brightness temperature at L-band:
TB (x,y, t, θ, p)
Results:
V-pol
- TB is over-estimated at both
polarizations and any incidence
angle
TB - The difference between H and V
at high incidence angles is
H-pol underestimated.
- Our interpretation is that the
measured density profile is too
smooth...
Incidence angle

16. Angular diagram
High contrast of density (= refractive index) between layers causes increased difference
between H and V polarisations at high incidence angles.
To test this assumption, a new simulation with noise added to the density profile:
V-pol
H-pol
This result should be considered as preliminary. New
density profiles should be collected to confirm this
assumption.

17. Concluding remarks
First year of SMOS data shows:
- The brightness temperature is fairly stable relative to the noise in the dry zone. The
Antarctic plateau can be used as a calibration target at 50-55o incidence angle and V-
polarisation only.
- In other configurations, changes of the surface state affect the signal like at the higher
frequencies (work in progress...)
- In the wet zone, the signal is dominated by the emissivity variations caused by ice layers.
No expected application in this zone.
- In the dry zone, the signal is close to the snow temperature. Retrieval of climatological
temperature from SMOS data should be achievable if the departure of the emissivity from
unity is corrected.
- Our modeling results at Dome C suggest the density profile is a very important
characteristic to understand H-polarized brightness temperature. Applications?

18. Thank you for your attention
Remains of a wind-crust layer, 5m deep (=50 years old) at Dome C