1. ICE-ARC airborne campaigns 2015-17
Observation of sea ice and ocean dynamic topography for validation of satellite measurements
A. Di Bella on behalf of H. Skourup, S. M. Hvidegaard, R. Forsberg, J. Wilkinson, J. King, A. Rösel, S. Gerland, G. Spreen, V. Helm, C. Polashenski, and G. Liston

2. ICE-ARC spring campaign 2015
Triangle
RV Lance
Collaboration: NPI (N-ICE2015), BAS/DTU Space (EU FP7 ICE-ARC), CRELL, NASA OIB
Overflight of RV Lance:
OIB: March 19
ICE-ARC: April 19
ICE-ARC: April 24
ALS freeboards

3. ASIRAS radar and laser scanner
Laser scanner (Riegl LMS Q240i
Near-infrared, 40 Hz
At 300m flight altitude
Resolution: 1m x 1m
Swathwidth: 300m
ASIRAS Radar:
ku-band (13.5 GHz)
Resolution: 10m x 3m
Credits: Hendricks et al, IGARSS 2010
In 2006 and 2008 DTU Space has been flying with a combination of radar and laser as part of the CryoVEx program
The laser ...
The radar ASIRAS Airborne Synthetic Aperture and Interferometric Radar Altimeter system
GPS for precise positioning
INS for aircraft attitude
GPS kinematic positioning
INS movements of the aircraft

4. Main purpose for ICE-ARC
Airborne measurements with scanning lidar provide a detailed look on the structure of the sea ice, and together with snow depth measurements (from combined radar/laser measurements or models) provide a detailed measurement of sea ice thickness and ridge/lead distribution.
The sea ice data from aircraft provide a detailed validation of satellite sea ice thickness data, as well as data on sea surface topography from e.g. CryoSat-2, Sentinel-3

5. Validation of CryoSat-2
Here is given a short summary of main findings in recently submitted paper to JGR special issue on N-ICE 2015 data:
Comparison of freeboard retrieval and ice thickness calculation from ALS, ASIRAS, and CryoSat-2 in the Norwegian Arctic, to field measurements made during the N-ICE 2015 expedition
By King, J., H. Skourup, S.M. Hvidegaard, A. Rösel, S. Gerland, G. Spreen, V. Helm, C. Polashenski, and G. Liston
The ALS and ASIRAS data is from ICE-ARC flights around RV Lance on April 19 and 24, 2015

6. Radar penetration depths
Laboratory experiments with ku-band radar on sea ice with a layer of cold dry snow,
Beaven et al, 1995
Laser
We investigate main hypothesis of radar penetration depths ..
On the sea ice laboratory experiments have shown that ku-band radar on sea ice with a layer of cold dry snow reflects on the snow ice surface.
However, the penetration is highly dependent on e.g. Snow properties such as density, temperature and snow structure.
Laser reflects on the air-snow surface and can be used as reference for measuring the radar penetration. Thus a combination of laser and radar is ideal for estimating the penetration depth of radar signal. Previous studies suggest that this breaks down for temperatures above -10C

7. N-ICE in situ measurements
Drillings from April 17-24, 2015:
Sea ice freeboard is between and 0.15m, with mean 0m
Sea ice thicknesses between 0.81 and 2.70m, with mean 1.49m
Snow depth between 0.13 and 1.12m with mean 0.48m
GPS snow probe:
The mean and mode snow depth was 0.40 cm.
Snow depth had mean 0.56m and mode 0.5m.

8. ALS, ASIRAS and CryoSat-2 freeboards
PDF
PDF
Freeboard (m)
Freeboard (m)
Section
ALS frb mean (std)
Unit m
ASIRAS frb mean (std)
CryoSat-2 frb* mean (std)
0.36 (0.25)
0.32 (0.28)
0.35 (0.13)
0.41 (0.26)
0.38 (0.29)
0.41 (0.30)
*CryoSat-2 frb taken from ESA CryoSat-2 L2i product

9. ALS, ASIRAS and CryoSat-2 freeboards
There is very little or no (0-4 cm) penetration of the radar signal into the snow layer, and nothing comparable to the sea ice freeboard
PDF
PDF
Freeboard (m)
Freeboard (m)
Section
ALS frb mean (std)
Unit m
ASIRAS frb mean (std)
CryoSat-2 frb* mean (std)
0.36 (0.25)
0.32 (0.28)
0.35 (0.13)
0.41 (0.26)
0.38 (0.29)
0.41 (0.30)
*CryoSat-2 frb taken from ESA CryoSat-2 L2i product

10. In situ measurements
Temperature profiles from snow pit measurements
This has already been addressed in papers, however, these are only speculating that there is no penetration for air temperatures above -10°C.
On April 19 the snow-air temperature -16 C and snow-ice interface -6.7C. On April 24 these temparetures were -15C and -6.2 C. In between these dates the air temperature was between -13 and -25°C.
However, a dense wind crust was observed near the air-snow surface in snow pits dug on the respective dates, which could possible prevent the radar signal to penetrate further into the snow layer.
Blue
Red

11. Freeboard to thickness

12. Freeboard to thickness

13. Freeboards from N-ICE Drillings from April 17-24, 2015:
Sea ice freeboard is between and 0.15m with mean 0 cm
Sea ice freeboard = (Snow freeboard)ALS – (Snow depth)GPS snow probe
At least half of the grid the sea ice freeboards are negative
Similar conditions are found in Antarctica due to heavy snow load, and one has take this into account in the freeboard to thickness conversion
(a)
Snow depth from GPS snow probe
(b)
Snow freeboard from ALS
(c)
Sea ice freeboard = (b) – (a)

14. Freeboard to thickness
From Kern et al. 2016, Remote Sensing, 8 (538), doi: /rs

15. HEM thicknesses
With respect to slide 15, the correlation between ALS freeboards and HEM thicknesses are high (0.74 on April 19 and 0.70 on April 24). This is also the case for ASIRAS freeboards and HEM (0.81 on April 19 and 0.65 on April 24). The ALS/ASIRAS thicknesses correlation to HEM thicknesses are also high.
In case anyone asks: Conversion of freeboard to thicknesses slide 15; Here is used typical MYI density with snow depth and snow density obtained from N-ICE in situ measurements. Limits for snow depth (gray areas) are cm.

16. HEM thicknesses

17. HEM thicknesses
Slide 17: If we use the radar=sea ice freeboard, the sea ice thickness obtained are 2-3 times too thick, when compared to HEM thicknesses.

18. HEM thickness vs CryoSat-2 freeboards

19. Conclusion/summary
The radar freeboard from CryoSat-2 and ASIRAS is almost equal to the snow freeboard obtained from ALS.
This is peculiar in this case, as the air temperature is below -13°C for the entire period between the 2 overflights.
The cause could be due to a dense wind crust layer close to the air-snow surface.
There are large correlation between ALS, ASIRAS freeboards and HEM thicknesses, but no correlation between HEM thicknesses and CryoSat-2 freeboards.
Converting ALS and ASIRAS freeboards into thicknesses correlates well with observed HEM thicknesses.
If ASIRAS freeboards are converted to thicknesses using the general assumption of radar freeboard = ice freeboard, the thicknesses obtained are 2-3 times too thick, when compared thicknesses obtained by HEM.

20. ICE-ARC spring campaign 2016
A small ICE-ARC 2016 airborne campaign was initiated and carried out to re-fly some flight lines (Triangle+ULS) from 2015, where data collected in 2015 were missing due to malfunctions of the BAS logging system.
The campaign took place in beginning of April 2016 with shared aircraft mobilization costs with the ESA CryoSat-2 Validation Experiment (CryoVEx 2016).
Triangle
ULS

21. ALS long-term sea ice monitoring
The data set is unique and covers 13 years of spring observations, and includes ICE-ARC measurements from 2016.
The dotted lines mark datasets not covering the full flight line.
Data from various campaigns show an overall thinning of the sea ice with large inter-annual changes overlaid.

22. ICE-ARC spring 2017 ?
Flights in the Beaufort Sea to measure sea ice thickness and sea surface height to compare to buoy data (IMB and GPS) and validation of satellite measurements, sea ice thickness and sea surface height primarily from CryoSat-2, but also the freshwater component estimated by GRACE
Sachs harbour
Logistics:
BAS Twin Otter
Base: Sachs Harbour, Banks Island, gateway to the Beaufort Gyre
Instruments Lidar/ASIRAS radar
22 March March March March 2018

23. Development of UAV lidar system
- In support of ICE-ARC field campaign
Velodyne Puck
lidar: 600g
Penguin B is capable of up to 26.5 hour endurance with the 4 kg payload
The Lidar has a range of 100 m, which is a perfect for relative flat surface topography, such as sea ice.
The total weight of the instruments is only 2.5 kg including lidar, INS/GNSS, onboard controller unit, GPS antenna and cables.

24. Time schedule
Purchased instruments; light weight Lidar + integrated INS/GNNS, June 2016
Setup and test of instruments in house, July-September 2016
Installation of instruments in UAV, ongoing
Test flights of UAV with instruments; Beginning of November 2016
Flights to test instruments and validate data in Denmark,
Test flights Greenland Station Nord, first 2 weeks of April 2017
Field campaign from VRS Station Nord with coincident UAV/lidar for sea ice surface topography and under ice measurements of bottom of the ice, 2 last weeks of April 2017
Villum Research Station