smos over antarctica - a short story of massive...
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SMOS over Antarctica - a short story of massive icebergEwa Slominska, (email:[email protected])W. Marczewski, J. Slominski
Space Research Center PAS
Introduction & Motivation
For nearly a year, the drifting iceberg was tracked on the SMOS data.
An isolated, floating along Antarctica, set of8-10 SMOS DGG pixels is a focal point for thisanalysis. It was characterised by an excess inbrightness temperature of approximately 30K,when compared to surrounding open water.
As part of a natural cycle, ice shelves periodicallycalve icebergs. In March 2000, Antarctica’s RossIce Shelf released a colossal berg, assigned asB-15. The B-15 was the world‘s largest recordediceberg with the area of over 11,000 km2. B-15started to break into smaller pieces in 2002 and2003. One of the pieces from the “B-15 family”(more precisely B-15J) was spotted by theSMOS satellite.
Figure 1: SAR image from ENVISAT taken on
the 25th of August, 2011 at 10:25:04, was used
for the iceberg identification. With a little help
from the CESBIO SMOS Blog followers the
iceberg was identified as the B15J (more de-
tails [1]).
Observation on a massive “natural buoy” could be additional source of information forthe studies on sea - sea-ice - ice interactions.
Goals
Examine the motion of spotted object: trajectory, averaged speed, distance...
Examine the iceberg evolution in terms of brightness temperature measured by SMOS.
Examine polarimetric characteristics for the DGG pixels corresponding to tracked iceberg.
Approach
SMOS Level 1c browse Land and Sea Data were merged in order to produceweekly averaged global maps for the first Stokes parameter. - This provides relevantinsight into dynamics of processes observed by SMOS. Temporal evolution of the firstStokes parameter revealed set of several DGG pixels drifting from the Ross Sea eastwards.Comparison with ENVISAT images confirmed that spotted object is an iceberg (Figure 1).
Figure 2 shows four snapshots with marked iceberg path (from January to December,2011). In order to enhance floating object, the colour scale was modified on purpose. Itis further applied to temporal analysis of I .
For the whole examined dataset, SMOS DGG pixels corresponding to the tracked iceberg,were extracted, in order to examine temporal evolution of brightness temperature. (seeFigure 7)
Figure 2: Set of SMOS L1c browse sea data for ascending passes, illustrating iceberg trajectory. The colour scale has been modified onpurpose, to better distinguish the drifting object. With such an approach areas belonging to the same TB regime are directly shown.
Polarimetric characteristics, iceberg position, drift velocity - Analysis & Results
Polarimetric characteristics for selection of DGG pixels belonging to the iceberg weregrouped into separate sets according to location and time criteria.
Figure 3: Selected set of polarimetric characteristics for the central pixels of the tracked iceberg
Figure 4: Comparison of the first Stokes parameter for the central pixels of the iceberg, and one randomly
picked DGG pixels in the open water, and another one in the closest area of the DOME-C station (reference
and calibration site for SMOS)
Icebergcharacteristics, werecompared with thoseprepared for thefollowing DGG pixels,one located in thevicinity of DOME-C(DGG: 7181359 -123.383 E, 75.272 S)and another one inthe open water (DGG:6221186 - 150.102 W,60.109 S) - Figure 3.
In the same manner,comparison for the 1st
Stokes was done -Figure 4.
I parametercorresponding toiceberg pixels revealsnoticeable change inits shape during theexamined period.
In order to roughly estimate iceberg velocity, the distance betweentwo consecutive locations has been computed with basic haversineformula. Estimated velocities are plotted along the trajectory (seeFigure 5). This result should be compared with the same plot but forthe first Stokes (Figure 6). Results from figures 5 and 6 are combinedin the Figure 7, showing temporal evolution of Vdrift and (I ) . Thegreen line in the Figure 7 marks the averaged velocityV̂drift = 6.22km/day computed with assumption about the great-circledistance (white line in the Figure 5) .
Figure 5: Vdrift along the trajectory, white line marks the great-circle distance used
for estimation of averaged V̂drift.
Figure 6: Values of the I parameter for the central pixel of the iceberg.
Figure 7: Time evolution of Vdrift and (I ).
Discussion & Conclusions
The derived iceberg motion indicated significant change of direction
in the middle of September 2011, when the berg started to move
equatorwards. Straying from Antarctica, was accompanied with
sequential decrease of the brightness temperature. At the end of
December, the signatures of observed iceberg were barely apparent,
making further tracking not feasible.
It seems highly probable that SMOS documented the final stage of
evolution of B-15J.
Estimated velocities depict several cases where dramatic increase of
Vdrift is noted. It is mainly at final days of observations. The
explanation for such an results is highly related to problems of
accurate detection and positioning of the tracked object. Thus all
velocities greater than 40 km/day should be rejected and not
considered as the physical values.
Distance: integrated along the trajectory: Dtraj = 4994.71km,
obtained from great-circle assumption Dg−c−d = 2172.1km
Presented analysis is the initial step for better understanding
various cryospheric processes and is the first approach to the
application of such models as MEMLS (Microwave Emission Model
for Layered Snowpack).
References
[1] CESBIO SMOS blog - http:
//www.cesbio.ups-tlse.fr/SMOS_blog/?tag=iceberg
[2] Full evolution of the drifting B15J -
http://www.cbk.waw.pl/~ewa/SMOS/iceberg
[3] Kerr, Y. H., P. Waldteufel, P. Richaume, P. Ferrazzoli, and
J.-P. Wigneron: SMOS level 2 processor soil moisture
Algorithm Theoretical Basis Document (ATBD), Toulouse,
France: CESBIO, vol. SO-TN-ESL-SM-GS-0001, V3.a, 2008.
Acknowledgements: This work has received fundings in the frame of the PECS program, named SWEX-R, realised between ESA and partners from Poland. SMOS data were provided by the ESA EOPI Cat-1 project SMOS Cal/ Val 3275
http://www.cbk.waw.pl/~ewa/