spatial variations of l-band emissivity in antarctica, first results from the smos mission.pdf

IGARSS Summer 2011 Vancouver, Canada

Spatial variations of L-band emissivity in Antarctica, first results from the SMOS mission.

G. Picard(1), Y. Kerr(2)Y. Kerr(2), G. Macelloni(3), N. Champollion(1), M. Fily (1), F. Cabot(2), P. Richaume(2), M. Brogioni(3)

UJF-Grenoble 1 / CNRS, LGGE UMR 5183, Grenoble, F-38041, FranceCESBIO (CNES,CNRS,IRD,UPS) 18 avenue Edouard Belin 31401 Toulouse, FranceIFAC-CNR, via Madonna del Piano 10 – 50019 Sesto Fiorentino, Italy

ContextContext

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)

ContextContext

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, T

B = Tsnow, snow temperature might

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, T

B should be nearly constant over time.

→ The Antarctic plateau could be a good external calibration target.

Objective of this talk:

- Test 1 and 2

Tsnow

OutlineOutline

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

SMOS data processingSMOS data processing

L1C datareprocessed 2010

from Brockmann Consult

Read and XY2HV rotation subroutines

from CESBIO

- Area selection- Flag selection

- Daily average- Projection to the “standard”

stereographic polar projection

at 25 km resolution

The result of these processing steps is a cube of

TB (x,y, t, θ, p)

space time Incidenceangle

polarisation

SMOS data processingSMOS data processing

TB (x,y, t, θ, V)

TB (x,y, t, θ, H)

Physical annual-meansnow temperature

Incidence angle

TB

Angular diagram of TB with all the data in 2010 at Dome C (75oS, 123oE)

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 ?

Dome C

L-band

C-band

L-band

C-band

Temporal variations of TTemporal variations of TBB

TB (x,y, t, θ=55o, p=V-pol)

Daily-mean TB on the Larsen C ice shelf

in the Peninsula

L-band brightness temperature is fairly constant in the dry zone. We can work with averaged T

B.

Brightness temperature at any frequency increases sharply when the snowpack becomes wet.

Daily-mean TB at Dome C (-75oS, 123oE)

Spatial variations of L-band TSpatial variations of L-band TBB

TB (x,y, <t>, θ=55o, p=V-pol)

Two very different zones: the wet zone (low emissivity) and the dry zone (high emissivity)...


Number of days with melt during the austral summer 2009/2010 (derived from SSM/I).T

B (x,y, <t>, θ=55o, p=V-pol)

Why the emissivity is low in the wet zone ?


In the wet zone, during the summer, the liquid water is responsible for the peaks noticed in T

B time-series at every frequency

Tsnow

Wet snow (snow + max 8% of liquid water) causes very strong absorption.

According to Kirchoff law, emissivity is close to 1The physical temperature of wet snow is 273K by definition

TB ~ 273 K

Tsnow


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

TB < 200 K


TB (x,y, <t>, θ=55o, p=V-pol)

Focus on the dry zone:

ERA Interim annual-mean air temperature

The scales are slightly different

TB at V-polarisation and Brewster angle is close to the physical temperature...


It means, the emissivity at V-polarisation and Brewster angle is close to 1, but how close ?

e=1

e=0.95

e=0.97

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.

Each dot corresponds to a pixel in the dry zone, SMOS T

B

versus ERA temperature.


E.g. at Dome C where accurate snow temperature is measured routinely by LGGE:

TB(SMOS) = 213 K

Tair(ERA) = 224 K → e=0.951Tsnow = 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...

Angular diagramAngular diagram

TB (x,y, t, θ, p)

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:

Results:

- TB is over-estimated at both

polarizations and any incidence angle

- The difference between H and V at high incidence angles is underestimated.

- Our interpretation is that the measured density profile is too smooth...

TB

Incidence angle

V-pol

H-pol

Angular diagramAngular 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.

Concluding remarksConcluding 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?

Thank you for your attention

Remains of a wind-crust layer, 5m deep (=50 years old) at Dome C

spatial variations of l-band emissivity in antarctica, first results from the smos mission.pdf

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