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

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

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

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

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

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

N-ICE in situ measurements Drillings from April 17-24, 2015: Sea ice freeboard is between -0.17 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: 19-04-2015 The mean and mode snow depth was 0.40 cm. 24-04-2015 Snow depth had mean 0.56m and mode 0.5m.

ALS, ASIRAS and CryoSat-2 freeboards 19-04-2015 24-04-2015 PDF PDF Freeboard (m) Freeboard (m) Section ALS frb mean (std) Unit m ASIRAS frb mean (std) CryoSat-2 frb* mean (std) 19-04-2015 0.36 (0.25) 0.32 (0.28) 0.35 (0.13) 24-04-2015 0.41 (0.26) 0.38 (0.29) 0.41 (0.30) *CryoSat-2 frb taken from ESA CryoSat-2 L2i product

ALS, ASIRAS and CryoSat-2 freeboards 19-04-2015 24-04-2015 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) 19-04-2015 0.36 (0.25) 0.32 (0.28) 0.35 (0.13) 24-04-2015 0.41 (0.26) 0.38 (0.29) 0.41 (0.30) *CryoSat-2 frb taken from ESA CryoSat-2 L2i product

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. 19-04-2015 Blue 24-04-2015 Red

Freeboard to thickness  

Freeboard to thickness  

Freeboards from N-ICE Drillings from April 17-24, 2015: Sea ice freeboard is between -0.17 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)

Freeboard to thickness   From Kern et al. 2016, Remote Sensing, 8 (538), doi:10.3390/rs8070538

HEM thicknesses 19-04-2015 24-04-2015 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 7-120 cm.

HEM thicknesses 19-04-2015 24-04-2015

HEM thicknesses 19-04-2015 24-04-2015 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.

HEM thickness vs CryoSat-2 freeboards

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.

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

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.

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 201822 March 201822 March 201822 March 2018

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.

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