Robert  Ricker

1  Alfred  Wegener  Ins-tute  for  Polar  and  Marine  Research,  Bremerhaven,  Germany  

2  DTU-­‐Space,  Technical  University  of  Denmark,  Copenhagen,  Denmark  

Robert  Ricker¹,  Stefan  Hendricks¹,  Veit  Helm¹,  Henrie5e  Skourup²,  Rüdiger  Gerdes¹  

Introduc>on  

 

CryoSat-­‐2  measures  the  distance  between   satellite  and  surface.  

 

It  is  crucial  to  measure  the  reflec>ng  horizon   very  accurately.      

ESA  

ESA  

Measuring  sea-­‐ice  freeboard  

h_i     Sea-­‐ice  thickness   h_s   Snow  depth  

h_fs   Snow  freeboard  

h_fi   Ice  freeboard  

h_ssh   Sea-­‐surface  height  

h_GPS   CryoSat-­‐2/aircraO  al>tude  

H   Measured  distance  

h

_fs

 =  h

_GPS

 -­‐  H  -­‐  h

_ssh  

Introduc>on  

Objec've  of  this  study:  

Tes>ng  the  hypothesis  with  a  comparison  between   airborne  laser,  radar  and  electromagne>c  sounding   and  CryoSat-­‐2  measurements  over  mul>  year  ice  in   the  Lincoln  Sea.  

Hypothesis:    

It  is  assumed  that  the  CryoSat-­‐2  radar  is   penetra>ng  a  cold  and  dry  snow  layer.  

 

CryoSat-­‐2  measures  the  distance  between   satellite  and  surface  

 

It  is  crucial  to  measure  the  reflec>ng  horizon   very  accurate      

ESA  

ESA  

CryoSat  Valida>on  Experiment  (CryoVEx)  

Airborne   Radar   Al>meter:  

ASIRAS  

Airborne  Laser  Scanner   Airborne  Laser  Scanner  

EM  Induc>on   Sounding:  

EM  BIRD  

CryoSat sea ice freeboard modified with SSH from laser scanner

70˚W 60˚W

50

˚W

83˚N 84˚N 85˚N 86˚N

CryoSat sea ice freeboard modified with SSH from laser scanner

70˚W 60˚W

50

˚W

83˚N 84˚N 85˚N 86˚N

0 100

km N

1.22 1.42 1.62 1.82 2.00 2.19

0.17 0.02 0.12 0.03 0.01 0.07

0.35 0.54 0.74 0.95 1.15 1.35

0.02 0.01 0.03 0.08 0.12 0.07

t in h:

d in km:

CS Orbit 5399 CS Orbit 5428

Lincoln Sea

Alert

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Sea Ice Freeboard in m

Data  resolu>on  -­‐  overview  

Laser  scanner  

~  0.3  m  

~  1  m   CryoSat-­‐2  

~  300  m  

~  1000  m  

ASIRAS  

~  2.3  

~  12  m   m  

EM  Bird  

~  5  m  

2.  Laser  scanner:  A  weighted  

average  is  formed  of  across-­‐track   data  points.  

3.  Laser,  ASIRAS,  EM:  Assigning  to   the  respec>ve  CryoSat  footprint   and  averaging  the  enclosed  

points.    

Averaged  laser  and   ASIRAS  eleva>on   CryoSat-­‐2  footprint  

Laser  scanner   measuring  points  

ASIRAS   footprint  

Data  and  methods  

0 50 100 150

Track distance in km 17

18 19 20 21 22 23

Elevation in m

SSH (Laser)

1.  A  manually  picked  sea-­‐surface  height   from  laser  scanner  data  is  used  to   calculate  the  CryoSat-­‐2  and  ASIRAS   freeboard.  

Results:  Sea-­‐ice  freeboard  along  track  

Lead  

Lead  

0 50 100 150

Track distance in km 0.0

0.5 1.0 1.5

Freeboard in m

Laser CryoSat ASIRAS

ORBIT 005399

0 50 100 150

Track distance in km 0.0

0.5 1.0 1.5

Freeboard in m

Laser CryoSat ASIRAS

ORBIT 005428

Sea-­‐ice  freeboard  distribu>on  

Radar  modal  freeboard  is  shiOed  to  lower  values   compared  to  the  Laser  freeboard  which  indicates  a   penetra>on  of  the  K_u  band  signal.        

-0.5 0.0 0.5 1.0 1.5 2.0

Freeboard in m

Relative Units

-0.5 0.0 0.5 1.0 1.5 2.0

-0.5 0.0 0.5 1.0 1.5 2.0

Laser, Mean= 0.58m CryoSat, Mean= 0.35m ASIRAS, Mean= 0.39m ORBIT 005399

-0.5 0.0 0.5 1.0 1.5 2.0

Freeboard in m

Relative Units

-0.5 0.0 0.5 1.0 1.5 2.0

-0.5 0.0 0.5 1.0 1.5 2.0

Laser, Mean= 0.63m CryoSat, Mean= 0.41m ASIRAS, Mean= 0.46m ORBIT 005428

Apparent  Penetra>on:  

App.  10-­‐20  cm  

Sea-­‐ice  thickness  T  (with  penetra>on  term):  

T = F · ^W

W I

+Z · ^{S W}

W I

+P · ^W

W I

Calcula>ng  sea-­‐ice  thickness  

0 50 100 150

Track distance in km -0.2-0.1

0.0 0.1 0.2 0.3 0.4 0.5

Depth in m

Snow depth Penetration

Assumed  snow  depth  func>on:    

Freeboard   Snow  depth   Penetra>on  

Water  density:    1020  kg/m³   Ice  density:    910  kg/m³   Snow  density:    300  kg/m³  

⇢_W

⇢_I

⇢_S F Z P

Results:  Sea-­‐ice  thickness  along  track  

0 50 100 150

Track distance in km 0

2 4 6 8 10 12

Sea ice thickness in m

EM Laser CryoSat

ORBIT 005399

0 50 100 150

Track distance in km 0

2 4 6 8 10 12

Sea ice thickness in m

EM Laser CryoSat

ORBIT 005428

Direct  thickness  measurements  from  

electromagne>c  induc>on  sounding  (EM  Bird)  

Sea-­‐ice  thickness  distribu>on  

Using  the  assumed  density  values,  snow  depth  and  penetra>on   provide  a  good  agreement  with  the  direct  thickness  measurements   from  electromagne>c  induc>on  sounding  

-1 0 1 2 3 4 5 6 7 8 9 10

Sea Ice Thickness in m

Relative Units

-1 0 1 2 3 4 5 6 7 8 9 10

-1 0 1 2 3 4 5 6 7 8 9 10

EM, Mean= 3.23m CryoSat, Mean= 2.83m Laser, Mean= 3.54m ORBIT 005399

-1 0 1 2 3 4 5 6 7 8 9 10

Sea Ice Thickness in m

Relative Units

-1 0 1 2 3 4 5 6 7 8 9 10

-1 0 1 2 3 4 5 6 7 8 9 10

EM, Mean= 3.07m CryoSat, Mean= 3.12m Laser, Mean= 3.95m ORBIT 005428

Conclusion  and  outlook  

 

CryoSat-­‐2  freeboard  generally  coincides  with  the  ASIRAS  freeboard.  

 

Radar  does  not  penetrate  the  snow  cover  completely.  

Outlook:    We  will  inves>gate  the  influence  of  a  snow  layer  and  surface    roughness  with  a  forward  model  for  CryoSat-­‐2  waveforms.  

 

With  an  assumed  snow  depth  of  0.3  m  and  a  penetra>on  of  0.15  m   a  good  agreement  between  Laser,  radar  and  EM  sounding  thickness   retrieval  is  achieved.  

Acknowledgements  

The  measurements  in  the  framework  of  CryoVEx  and  PAM-­‐ARCMIP   2011  campaigns  were  carried  out  by  the  DTU  Space  and  the  Alfred   Wegener  Ins>tute  for  Polar  and  Marine  Research.  The  CryoVEx  

campaigns  are  part  of  ESA’s  Living  Planet  Programme.  The  CryoSat-­‐2   data  are  provided  by  the  ESA.  All  this  is  gratefully  acknowledged.  

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