Proceedings Template - WORD - Bedford Institute of Oceanography

Comparing ice chart parameters against ice observations Simon Prinsenberg and Ingrid Peterson Fisheries and Oceans Canada, Bedford Institute of Oceanography, Dartmouth, N.S., Canada

ABSTRACT Operational ice centres produce daily ice charts for offshore operators to help them operate safely and efficiently in ice-infested waters. The charts describe only the level ice component of the ice thickness, and not the deformed component. Using a helicopter-borne electromagnetic (EM)-laser system and downward-looking video camera, ice thickness distributions and video mosaics were obtained along long flight tracks, and compared to parameters shown on ice charts. The level ice component of the EM-measured ice thicknesses were within the range of the thicknesses shown on the ice charts. However deformed ice increased the mean total ice thickness by 20-80%.

KEYWORDS: Ice thickness; ice charts; Electromagnetic ice sensor INTRODUCTION Ice properties forecasted and displayed on Canadian Ice Service ice charts are used by the Canadian Coast Guard and offshore shipping industry to navigate efficiently and safely through pack ice. Although these charts have also become a valuable historical data source, the accuracy of the ice thickness data is not well understood. Pack ice properties are needed to quantify factors such as ocean-atmosphere heat and mass fluxes, and fresh-water and heat budgets. Our goal is to determine the accuracy of the daily ice charts relative to surface observations. We describe first the construction of the ice chart and its data sources. Lastly, ice thicknesses described by the Ice Egg symbols on the ice charts are compared with observations.

ICE CHART The daily ice chart (Fig. 1) is a description of ice conditions at 18:00 UTC (Coordinated Universal Time) and incorporates all observations and imagery available to the forecaster at this time. The chart is divided into areas (solid lines) expected to have homogeneous ice conditions as indicated in reconnaissance flight data and from satellite imagery. Some areas are further subdivided by dashed lines to show minor expected changes in ice conditions. The dashed-triangle line is the offshore limit of icebergs. As indicated in the chart key, the ice characteristics of the "Ice Egg" are total concentration, and partial

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concentrations by stage of development (age), and by floe size. Ice drift vectors provide a forecast of the ice drift rate of freely moving ice over the next 24 hours in nautical miles per day at several points on the chart, but are not part of this study. The "Based on Reports" box lists the observation dates of data sources used to prepare the analysis which can include: - RADARSAT SAR (Synthetic Aperture Radar). C-band, HH polarization data with 50-100m resolution over 300-500km swaths. 3-day repeat coverage, main data source for chart production. - Airborne SAR or SLAR (Side-looking Airborne Radar). Radar data collected over two 100km swaths along flight tracks up to 3000km long. Data, collected once every three days, provide very good representation of ice type. - Observed chart. A real-time analysis of SLAR data supplemented by visual observations from 1000 to 3000m altitude up to 25km on either side of track. This chart provides very good representation of ice concentration and ice type. - Helicopter reconnaissance. Visual observations within 15km of aircraft along tracks up to 500km long. Data provide very good indication of ice concentration and ice type. - NOAA AVHRR. Advanced Very High Resolution Radiometer data along 2500 km swath with repeat coverage every 12 hours. Cloud-free areas provide good ice concentrations but only fair indication of ice type. - Ship reports. Visual observations within 5km of the ship. Spot reports every 6 hours and track reports every day provide excellent indication of ice concentration and ice type. - Shore reports. Visual observations within 5km of the shore once per day provide a fair indication of ice concentration and ice type (Carrieres et al, 1996). The egg code on the daily ice charts shows the partial concentrations of the dominant ice types or stages of development (ages), and each ice type is associated with a thickness range for level ice. The charts do not include information on deformed ice, i.e. on the degree of rafting, ridging and rubble conditions. For aerial observations, ice type is identified from local ice climatology, ice topography and ice colour. For shipboard observations, ice type can be identified from the

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thickness of upturned ice blocks (Canadian Ice Service, 2002). SAR imagery has become an important data source in ice chart production as it can provide information on concentrations of various ice types and on ridging; however it cannot provide the absolute thickness of the identified ice type. Ice property identification remains difficult, particularly in the near-range of SAR products and in low pack ice concentration (Ramsay et al., 1998). Field experiments are continually done to evaluate ice signatures in SAR imagery for chart production. The ice chart of March 6, 1996 (Fig. 1) was based on SLAR reconnaissance flights and helicopter reports on March 5th, and RADARSAT imagery and ship reports on March 6th. However, the SLAR flight track and the observed reconnaissance chart only covered the southern Gulf of St. Lawrence and Northumberland Strait. The ship and helicopter reports were mainly confined to the near-shore shipping route to Charlottetown, Prince Edward Island (PEI). The area of interest for comparison with ice thickness data collected by helicopter-borne sensors is area C west of the Magdalen Islands. This pack ice is compressed against the Islands by NW winds and according to the ice chart has an ice concentration of greater than 9/10 (9+). 2/10 of the pack ice is 70-120cm thick (type “1.”), 5/10 is 30-70cm thick (type “7”) and the remaining 3/5 is 15-30cm thick (type “5”). Floe sizes

are characterised as being small to medium in size (4, 5 and 4). If the mean thickness of each ice type is assumed to be the centre of the thickness range, the total mean ice thickness of area C is 0.51m (Table 1). HELICOPTER-BORNE SENSOR DATA Ice-plus-snow thickness profiles were collected over many tens of kilometers using a helicopter-borne Electromagnetic Induction (HEM) system. It consists of a 4m-long cylindrical sensor package or “bird” towed 30m below the helicopter and 15-20m above the ice surface. The system estimates the distance to the ice-seawater interface by means of the electromagnetic induction method, which uses frequencies in the 1100 kHz range. The distance between the bird and the surface of the ice (or snow if it is present), is measured with a laser altimeter also contained in the bird. The difference between the two distances gives the ice-plus-snow thickness (Rossiter and Holladay, 1994), with an accuracy of about ±10 cm for level ice. Unlike the ice charts which give the thickness of level ice only, the HEM system gives the thickness of both level and deformed ice. The accuracy for deformed ice is expected to be lower than that for level ice, however the inversion process solves for bulk ice conductivities up to a constraint of 0.5 S/m to allow for high porosity in deformed ice.

Figure 1. Ice chart for March 6, 1996 (18:00 UTC) for the Gulf of St. Lawrence produced by the Canadian Ice Service. The square shows the approximate area covered by the RADARSAT SAR image shown in Fig. 2.


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Table 1. Ice thickness comparison displayed on ice charts and observed by EM helicopter-borne sensor. To calculate the mean ice thickness from the ice charts it is assumed the ice thickness for an ice type is equal to the mean of its thickness range, and for “Thick FY” a value of 1.6m was taken. The mean ice thickness from the ice charts was calculated as

H = ∑ Ai H i

, where Ai is the areal fraction of ice with mean thickness Hi.

i

Mean ice thickness 06 Mar 1996 Offshore EM 06 Mar 1996 Offshore Chart-C 08 Mar 2001 Inshore EM 06 Mar 2001 Inshore Chart-U 07-08 Mar 2001 Inshore Chart-X 08 Mar 2001 Offshore EM 06 Mar 2001 Offshore Chart-N 07 Mar 2001 Offshore Chart-S 08 Mar 2001 Offshore Chart-C

Thick FY Fraction (4.) >120cm 160cm 0.1 0.7 0.4 0.1

Medium FY Fraction (1.) 70-120cm 95cm 0.3 0.2 0.2 0.3 0.4 0.1 0.1 0.3 0.3

Thin FY Fraction (7) 30-70cm 50cm 0.5 0.5 0.1 0.7 0.2 0.4 0.5 0.7 0.6

Grey-white Fraction (5) 15-30cm 22.5cm 0.1 0.3

Deformed thickness (m)

Mean (Min-Max) thickness (m)

0.22

0.72 m 0.51 (0.34-0.68) 1.65 m 0.64 (0.42-0.85) 1.12 (0.88- ) 0.52 m 0.44 (0.28-0.59) 0.64 (0.42-0.85) 0.61 (0.41-0.81)

0.75

0.2 0.4

0.07

0.1

Figure 2 ScanSAR Narrow image of the southern Gulf of St. Lawrence from March 6, 1996 (10:26 UTC, 64 km by 64 km) with EM-measured ice thickness profiles superimposed; the vertical scale denotes 4 m thickness. © CSA/ASC, 1996.


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The HEM system uses frequencies of 30 and 90 kHz, and a horizontal coplanar coil configuration which has a footprint of about 3.7 times the height of the bird (Kovacs et al., 1993), or 55-74 m for a flying height of 15-20 m; the footprint of the laser is a few centimeters. Since the EM footprint exceeds 50m, the HEM system does not resolve ridge shapes, keel depths, or the sharp boundaries between level ice and deformed ice. A GPS orientation sensor (Trimble TANS Vector) in the bird is used to correct for pitch and roll with an accuracy of 0.1° (Holladay et al., 1997). The laser altimeter is an IBEO PS-100E and the sampling rate is 28 Hz. Ice surface elevation profiles were extracted from the laser data by removing the bird altitude variation using the three-step procedure described by Hibler (1972). Because of ice drift, the HEM data collected in 1996 were repositioned to the acquisition times of the imagery by tracking features near the end-points of the EM flight lines in the RADARSAT imagery acquired near the time of the EM flights. Direct measurements of snow and ice thickness were obtained through ice holes, mostly at ice beacon deployment sites, and along lines used for calibration and validation of the HEM system.

RESULTS A ScanSAR Narrow sub-image (64 by 64 km) acquired on March 6, 1996 (10:26 UTC) shows pack ice in the Gulf of St. Lawrence, west of the

Magdalen Islands (Fig. 2). Many large dark-toned floes can be seen, surrounded by lighter-toned areas and a few dark leads. In the field, the large floes were observed to be very smooth with little snow cover, and were surrounded by smaller ridged floes and leads containing young brash ice. EM-measured ice thickness profiles collected on the same day (~17:00 UTC) are superimposed on the image. The profiles measured over the dark-toned floes (segments B and C) are clearly less variable and contain fewer high ice thicknesses than the profiles measured over the brighter areas in the SAR image (segments A and D); the high-pass laser altimeter data are also less variable over the dark-toned floes. Although the higher backscatter in the areas surrounding the large level ice floes is in part due to ridging, it is mostly due to the young brash ice; ridging usually occurs in regions of thinner ice. Ice thickness distributions for segments A, B, C and D are shown in Figure 3. Note that because the EM footprint exceeds 50m, these histograms would be expected to differ from histograms derived from point measurements (e.g. from drilling). For segment B, the dark uniform floe, the mean thickness is 51cm; a peak is also seen at this thickness for segment C containing some smaller dark floes. For segments A and D however, the ice thickness distributions appear to be bimodal, with peaks at both 30-60cm and 70-90cm, suggesting that they represent a mixture of deformed and level ice.

Figure 3 Ice thickness distributions for line segments A, B, C and D shown in Figure 2.


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Ice-plus-snow thicknesses were measured directly through a total of 91 auger holes at 14 measurement sites, mostly where ice beacons were deployed. The sites chosen were level ice, first-year floes which were very safe to land on and were large (0.5-4 km) to ensure that the

beacons lasted many days. The mean ice thickness from the auger holes is 49cm (Figure 4a), and is similar to the mean ice thickness from segment B, 51cm, obtained over a dark-toned floe. It is also similar to the ice chart mean thickness for area C, 51cm.

Figure 4 Ice thickness distribution from (a) auger hole measurements (b) EM-measurements of Flight 53, lines 10050-10080.

Figure 5 Ice chart for March 8, 2001 (18:00 UTC) for the Gulf of St. Lawrence produced by the Canadian Ice Service. The square shows the approximate area covered by the RADARSAT SAR image shown in Fig.6.


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The mean ice thickness from all EM measurements is 68 cm (Figure 4b). The ice concentration is 0.94 assuming thicknesses less than 10 cm are open water. If the open water area (6%) is ignored, the mean EMmeasured ice thickness increases to 72cm (Table 1). Thus assuming the auger measurements are representative of level ice, and the EMmeasurements represent both deformed and level ice, the overall mean ice thickness is 40% higher than that of level ice. A second example is for March 8, 2001. The ice chart (Fig. 5) is based on RADARSAT and NOAA imagery and reconnaissance flights, all from March 8th. For most of the area between PEI and the Magdalen

Islands (region C), 3/10 of the pack ice is 70-120cm thick, 6/10 is 3070cm thick and 1/10 is 15-30cm thick, for a mean ice thickness of 4181cm. However within 10km of the north shore of PEI (region X), 4/10 of the ice is >120cm thick, 4/10 is 70-120cm thick and 2/10 is 50-70cm thick, for an average thickness of over 0.88cm, since strong northeasterly winds on March 6 compressed the ice against the shore. Before this wind event, the March 6 ice chart showed that for most of the pack ice along the shore (region N), 1/10 was 70-120cm thick, 5/10 was 3070cm thick and the remaining 4/10 was 15-30cm thick, for a mean ice thickness of only 28-59cm (Table 1).

Figure 6 ScanSAR Wide image of the southern Gulf of St. Lawrence from March 8, 2001 (10:13 UTC) with EM-measured ice thicknesses superimposed; the vertical scale denotes 4 m thickness. © CSA/ASC, 2001.


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The SAR image for March 8 (Fig 6) was acquired at 10:13 UTC, and the superimposed EM thicknesses were collected between 13:40 UTC and 19:40 UTC. The image shows that region X of the ice chart corresponds to a ten kilometer wide belt of high backscatter along the north coast of PEI, along with some relatively low backscatter along the shore at 63o. The mean EM ice thickness for this area (Segments A) is 165 cm (Fig 7). To the north between 46.60 and 47.15°N corresponding to region C of the ice chart, the mean thickness is 52cm (Table 1). However, the EM ice thicknesses are very inhomogeneous: segments B, C and D have mean thicknesses of 41, 130 and 25cm respectively (Fig. 7). Segment D corresponds to an area of low backscatter in the SAR image, and to rafted nilas in the video mosaics (Peterson et al., 2001). Estimation of the deformed portion of the total ice thickness is difficult from the EM-measured ice thickness data alone, because of the large EM footprint. The deformed portion in the inshore and offshore regions was estimated by computing the mean surface elevation from the laser altimeter data, subtracting a background elevation computed for a level ice area, and multiplying by an assumed ratio of total deformed ice thickness to elevation (8.9). Thus this method gives the thickness of ice due to the ice in the ridges above the level ice plus the ice in the keels below the level ice, i.e. no assumptions are made about the water level,

and rafted ice is not included in the deformed ice portion. Subtracting a background level is intended to minimise error due to snowdrifts and the laser itself. Figure 8 shows an example of laser-measured surface elevation and the corresponding EM-measured ice thickness from Segments A in Figure 6. The estimated deformed thickness for March 8 was 75cm for the inshore region and 7cm for the offshore region (Table 1). Thus the level ice thickness portions for the inshore and offshore regions are 90cm and 45cm, and are near the low end of the ice thickness range shown on the March 8 ice charts, >88cm and 41-81 cm. Thus although there was no thick FY and 10-30% medium FY ice along the north coast of PEI in the March 6 ice chart, the ice charts on March 7 and 8 contain 40% thick FY and 40% medium FY. These represent the highest thicknesses ever shown here around this date in all years since 1983, when the ice egg code was first introduced. The only other year containing any thick FY ice along the north coast on this date was 1993, with 40% thick FY and 20% medium FY ice. The mean ice thickness on the March 8 ice chart was also high in the offshore region (61cm) relative to the March 6 ice chart (44cm).

Figure 7 Ice thickness distributions for line segments A, B, C and D shown in Figure 6.


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Figure 8 Example of EM-measured ice thickness data (red upper trace) and laser-measured surface elevation data (blue lower trace) from line segments A shown in Figure 6. Beginning in 2001, ice histograms collected by the towed HEM system and a fix-mounted HEM system were sent to the Canadian Ice Service in real time for use in ice chart production. They may have contributed to the high thicknesses on the March 8 ice chart if some of the deformed ice in the histograms was included. The towed EM data from March 8 were probably too late to be incorporated into the ice chart (18:00 UTC). However, EM data collected in the inshore region on March 7 with a fix-mounted EM system were available to the ice forecaster and were likely incorporated into the March 7 and 8 ice charts. This newer fix-mounted EM system has a footprint of 15m versus the 50m footprint of the towed EM Probe (Prinsenberg et al., 2002).

CONCLUSION The Ice Egg symbol shown on ice charts refers to the estimated concentration, thickness and floe size of the dominant stages of development, or level ice thickness categories. It lacks information on the contribution of deformed ice to the total mean ice thickness, i.e. on the degree of rafting, ridging and rubble conditions that are frequently encountered in the marginal ice zone. Electromagnetic-laser sensors however are giving ice observers the tools to obtain spot samples of level ice as well as ice thickness distributions of both level and deformed ice along long flight paths. These sensors have been used in the southern Gulf of St. Lawrence since 1996 to collect data to validate identification algorithms of imagery, but only lately have the data been incorporated into the ice charts. Level ice thicknesses measured in 1996 were in agreement with those shown on the ice chart, but the mean total ice thickness comprising both level and deformed ice was 40% higher than the level ice thickness alone. In the 2001 example, both ice charts and EM data showed higher ice thicknesses within 10km of the north coast of PEI than farther offshore. The level ice thicknesses inferred from the 2001 data were at the low end of the range shown on the ice charts. The total ice thickness was 80% and 20% higher than the level ice thickness in the inshore and offshore areas respectively. EM-measured ice thicknesses within a single ice chart region were very inhomogeneous on scales of about 10km, and showed good agreement with RADARSAT SAR imagery. Beginning in 2001, EM data has been provided to the Can. Ice Service in real-time data as another data source for ice chart production, and may have contributed to the high ice


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thickness values shown on the ice chart for March 8, 2001.

ACKNOWLEDGEMENTS This work was funded in part by the Federal Panel on Energy Research and Development. We wish to thank the many participants of this project from the Canadian Coast Guard, Canada Centre for Remote Sensing, Canadian Ice Service and Fisheries and Oceans Canada.

REFERENCES Carrieres, T, Greenan, B Prinsenberg, S and Peterson, IK (1996). Comparison of Canadian Daily Ice Charts with Surface Observations off Newfoundland, Winter 1992. Atmosphere-Ocean 34(1): 207-226. Canadian Ice Service (2002). MANICE, Environment Canada, Ottawa, Canada. Hibler, WD (1972). “Removal of Aircraft Altitude Variation from Laser Profiles of the Arctic Pack,” J of Geophys. Res. 77: 71907195. Holladay, JS, Lo, B and Prinsenberg, SJ (1997). “Bird orientation effects in quantitative airborne electromagnetic interpretation,” Oceans ’97 Conf. Proceedings, Halifax, Nova Scotia, Canada, pp 1114-1116. Kovacs, A, Holladay, JS and Bergeron, CJ (1993). “Footprint Size of a Helicopter-Borne Electromagnetic Induction Sounding System versus Antenna Altitude,” CRREL Rep 93-12, 13p. Peterson, IK, Prinsenberg, SJ, Holladay, JS, Lalumiere, LA (2002). Validation of Sea Ice Signatures in Radarsat ScanSAR Imagery for the Gulf of St. Lawrence, Proceedings of IGARSS 2002, June 24-28, 2002, Toronto, Canada. Prinsenberg, SJ, Holladay, JS and Lee, J (2002). “Measuring ice helicopter thickness with EISFlowTM a fixed-mounted electromagnetic-laser system”, Proceedings of the 12th Int Offshore and Polar Eng. Conf., Kitakyushu, Japan, May 26–31, 2002, pp 737740. Ramsay, B, Manore, M, Weir, L, Wilson, K and Bradley, D (1998). “Use of RADARSAT Data in the Canadian Ice Service”, Can. J of Remote Sensing 24: 36-42. Rossiter, JR and Holladay, JS (1994). “Ice-Thickness Measurement. In: Remote Sensing of Sea Ice and Icebergs”, John Wiley and Sons, New York, pp 141-176.

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