THE ROLE OF NEAR REAL TIME ENVISAT ASAR GLOBAL MONITORING MODE DATA IN ARCTIC AND ANTARCTIC OPERATIONAL ICE SERVICES

Nick Walker1, David Simonin1, Kim Partington1,

Towanda Street2, Pablo Clemente-Colon2, Sean Helfrich2, Chris Szorc2, Craig Evanego2, Gary Premo2,

Tommy Skagemo-Andreassen3, Helge Tangen3. 1. Vexcel UK 2. National Ice Center 3. Norwegian Meteorological Institute

ABSTRACT In this paper we describe and assess the role that Near Real Time Envisat ASAR Global Monitoring (GM) mode data has had as a source of information for the derivation of ice products within two leading operational ice services: (i) The U.S. National Ice Center (NIC), who have a global mandate to provide strategic and tactical ice services. (ii) The Norwegian Meteorological Institute, who provide regular ice charts issued 5 days a week covering the area between Greenland and the Kara Sea. ASAR GM mode data can be thought of as filling a “data gap” between low resolution (but high temporal acquisition frequency) sensors such as passive microwave and scatterometers and high resolution sensors (but low temporal acquisition frequency) such as Envisat and RadarSAT’s Wide Swath and Image modes. As such ASAR GM mode data has allowed a range of medium scale key ice features to be viewed with a frequency that was not previously possible. Vexcel UK have been working in collaboration with the NIC and the Norwegian Meteorological Institute to develop new techniques for the provision of sea ice products and have played a key role in integrating ASAR GM mode data as a source of data used by ice analysts. In this paper we describe and evaluate the role of ASAR GM mode within the context of an operational ice service. Specifically we examine: (a) In which Arctic and Antarctic geographic regions is ASAR GM mode most useful, i.e. where does it give the greatest added value in comparison with other available sensors? (b) What ice feature information (e.g. ice edge, ice concentration leads, multi-year versus first year discrimination) can be extracted? (c) How useful is ASAR GM data as a source of information for monitoring ice motion and ice tracking. (d) Data quality and timeliness. (e) And lessons learned for future mission concepts.

1 OVERVIEW AND BACKGROUND Overview: The aim of this paper is to explain how near real time Envisat ASAR Global Monitoring mode data is being used in Arctic and Antarctic operational ice services. To this end we have performed a survey eliciting feedback from the analysts who interpret data at the National Ice Center (USA) and the Norwegian Meteorological Institute. The results from this survey provide useful information concerning the extent and the manner in which ASAR Global monitoring mode data is being used from which recommendations can be made concerning optimum future SAR mission concepts. Background: Over the last three years Vexcel UK has been conducting a programme of research in conjunction with the National Ice Center, based in Washington, in support of their operational requirements. As a part of this work Vexcel UK applied for ESA Category 1 (AO, ID 2376) Global Monitoring mode data. It soon became clear that this data source was going to be valuable as a primary source for analysts to work with as well as for validation and other supplementary applications. Following the enthusiasm shown by the NIC analysts for Global Monitoring mode data, this data source was also integrated and promoted within the context of the ESA GMES PolarView project. As a part of PolarView,

Vexcel UK now provides a series of daily mosaics to both the NIC and the Norwegian Meteorological Institute.

Region Institute Frequency Pixel Size Arctic NIC 2 per day 1 km x 1km Arctic MetNO 1 per day 1 km x 1km Antarctic NIC 2 per day 1 km x 1km Table 1. Vexcel UK mosaics delivered to operational ice services.

2 OPERATIONAL ICE CENTRES

2.1 The National Ice Center The U.S. National Ice Center (NIC) has a mandate to provide US global strategic and tactical ice services. They produce weekly NIC Ice Charts, labelled using the World Meteorological Organization (WMO) international sea ice symbols; "Egg Code" (a typical example of which is shown in Figure 1) and 30 & 90 day ice forecast products.

Figure 1. Typical NIC ice analysis chart, for the Greenland Central region during the period 20-24 June 2005.

2.2 The Norwegian Meteorological Institute The Norwegian Meteorological Institute (MetNo) provide regular ice charts issued 5 days a week covering the area between Greenland and the Kara Sea. Vexcel UK provides a daily service to deliver GM mode products to the Norwegian Meteorological Institute as part of ESA's GMES ICEMON / PolarView project. Figure 2 shows a typical MetNo ice classification product.

Figure 2. A MetNo ice classification chart from 20/01/2006.

3 GLOBAL MONITORING MODE CHARACTERISTICS Envisat’s ASAR instrument operates according to two main mission scenarios; the global (or background) mission and the regional mission. The global mission mainly comprises the two low bit-rate modes; namely the global monitoring mode (the subject of this paper) and the wave mode. There is also a small component of the background mission which is acquired with full resolution ASAR data. This mainly covers areas of special interest (mostly tectonic, volcanic, subsidence and coastal areas) which are acquired by default if no conflicting user requests have been made. The regional mission comprises all the remaining high data rate modes, such as the: • Image Mode (100 km swath, 30 m resolution on the ground for multi-look detected products, incidence angle selectable from 7 possibilities, HH or VV selectable polarisation) • Alternating Polarisation modes (100 km swath, 30 m resolution on the ground for multi-look detected products, dual band co-polar (HH/VV) or cross-polar (HH/HV or VV/VH) polarisation combinations, slightly reduced radiometric resolution compared to Image mode) • Wide Swath mode (400 km swath, 150 m resolution on the ground for multi-look detected products, HH or VV selectable polarisation) The ASAR instrument can only operate in the high data rate modes for short periods, up to a maximum of 10 minutes per orbit (nominally). The acquired data is transmitted by X- or Ka-band links, either directly or via the Artemis Data Relay Satellite to an ESA receiving station. Whenever ASAR is not acquiring data for the regional mission it systematically records data in either the wave mode over the Earth’s oceans or in GM mode over the Earth’s land or Polar regions. In these low bit rate modes each entire orbit can be recorded and stored on board before being dumped at the Kiruna station in Sweden or the Norwegian Svalbard station. This mission policy ensures that large quantities of ASAR GM data are acquired over both the Arctic and Antarctic regions. Furthermore, the systematic data processing and FTP distribution ensures that ASAR GM products are available to users within approximately 3 hours of acquisition. The key ASAR GM parameters are given in Table 2, together with the ASAR Precision and Wide Swath mode for comparison. It can be seen that ASAR GM mode, with is high temporal coverage and with a medium scale resolution, therefore fills a gap in the spectrum of available data sets covering Arctic regions. In Figure 3 we provide a schematic illustration of this fact in comparison to other cloud penetrating data sources.

Table 2. ASAR Global Monitoring, Precision Image and Wide Swath product specifications. (ENL is the effective number of looks.)

Figure 3. Schematic diagram to illustrate the fact that ASAR GM mode data fills a “data gap” existing between the conflicting “spatial resolution” versus “coverage/temporal sampling” parameters.

Global Monitoring

Precision Image

Wide Swath

Geometric resolution

1 km x 1 km 30m x 30m 150m x 150m

Pixel spacing

500m x 500m 12.5m x 12.5m

75m x 75m

Swath width 400 km 100 km 400 km Continuous coverage

up to 40,000 km (1 orbit)

4,000 km per orbit

4,000 km per orbit

Radiometric resolution (ENL)

~ 8 at near range ~ 6.5 at far range

~3 ~15

Data volume

~ 1.3 Mbytes per 400 km

130 Mbytes per 100 km

59 Mbytes per 400 km

4 Experiences for the Arctic region from NIC Ice Analysts In this section we consider the feedback from the NIC ice analysts for the work that they conduct in the Arctic region. Answers were provided by four analysts.

4.1 Regional analysis We asked the question, “for which regions in the Arctic are the GMM mosaics most useful?” We asked for answers to be given according to the scale shown in Table 3. The regions which score highly, i.e. where ASAR GM mode data is deemed most useful, are highlighted in the bright green and turquoise colours in Figure 4. The regional analysis is shown in full in Table 4. 1 Never useful 2 Occasionally useful 3 Sometimes useful. Helps confirm stuff. 4 Pretty good. Often the most useful source of data. 5 Very useful. Usually the primary source of information. Table 3. Scale of “usefulness”.

≥ 4 > 3

Figure 4. The Arctic regions for which GM mode data is most useful; highlighted in bright green, and the next most useful; highlighted in turquoise.

Region GM usefulness index Region GM usefulness index Baffin Bay 3.0 High West Arctic 4.0 Baltic Sea 2.3 Hudson Bay 3.0 Barents Sea 3.3 Japan Sea 2.0 Beaufort Sea 3.0 Kara Sea 3.0 Bering Sea 2.6 Labrador Sea 2.6 Canadian Arctic E 2.3 Laptev Sea 3.3 Canadian Arctic West 2.3 Okhotsk Sea 2.6 Chukchi Sea 3.0 Siberian Sea East 3.3 Cook Inlet 1.3 Skagerrak 1.0 Greenland Sea 3.3 White Sea 2.6 High East Arctic 4.3 Yellow Sea 1.6 Table 4. NIC ice analysts’ evaluation of the usefulness of ASAR GM mode data for different Arctic regions.

4.2 ASAR GM mode Arctic Coverage The areas for which ASAR GM mode data is most useful is partly a function of where this data is most readily available in comparison to other sources, notably with respect to the higher resolution SAR modes. In Figure 5 we show the ASAR GM mode data and ASAR high resolution mode acquisitions for one day, the 16th January 2006. It can be seen that these two acquisition patterns for a compliment to one another, i.e. when the high resolution mode data is switched off, the GM mode is switched on. In general, the higher resolution acquisitions are more highly concentrated around the marginal ice zone, which are the most commercially important regions. This leaves the GM mode data to fill-in the remainder of the Arctic region, which usually comprises a very large percentage of the high Arctic.

Figure 5. ASAR GM mode data acquired on 16/01/2006.

ASAR high resolution data acquired on 16/01/2006.

In order to get an impression of how often ASAR GM mode data is useful in the high Artic in comparison to all data sources, i.e. including RadarSAT data; we asked the question “In the High Arctic how often is it the case that GMM data is the only useful data source?” We received the following four responses. 1. 70 – 80 % 2. 60 – 70 % 3. 50 – 60 % 4. 10 – 20 % We display the results individually rather than simply providing an average, because one outlying value (response number 4) would give a misleading impression of the results as a whole. If we exclude result number 4 we are left with an average of around 60 – 70 % for the frequency for which GM data seems to be the most useful data source in the high Arctic.

4.3 Other regional points The following points were also made concerning the use of ASAR GM mode data in different Arctic regions. - For High Arctic above East Siberia and Laptev Sea, GMM is very useful. - In the Alaska seas, we will use GMM to fill “gaps” in RADARSAT. - GMM gives a good overview especially if an area has little useful imagery (i.e. no RADARSAT and if

there is lots of cloud cover). - GMM still provide a good overview of an area with lots of RADARSAT. It is useful in confirming

what is already analysed. - It is not useful in relatively small seas (i.e. Cook Inlet, Baltic Sea), or in instances when the spatial

coverage of the ice is small or there are many small ice polygons (i.e. Yellow Sea, Canadian Arctic). We may see 2 swaths a week in Cook Inlet. The data is never available in the Labrador.

4.4 General comparison with other data sources We wanted to obtain a general impression as to where ASAR GM mode data falls within the hierarchy of data sources that are generally available to NIC ice analysts. We asked the following question: “Assuming all possible data sources are available, put the commonly used data sources in order of usefulness for ice analysis”. The ranking, which is dependent on whether data is cloud contaminated, is shown in Table 5. It should be noted that Envisat Wide Swath is not generally used operationally by the NIC, although it does have similar characteristics to RadarSat data. Ranking Data source

(non cloudy) Ranking Data source

(cloudy) 1 RadarSat 1 RadarSat 2 Modis 2 ENVISAT GM 3 DMSP / OLS 3 AMSR-E 4 ENVISAT GM 4 QUIKSCAT 5 AVHRR 5 Modis*

Table 5. The order of “usefulness” for data sources for Arctic ice analysis. * MODIS – Multi-spectral can assess ice through low clouds

4.5 GM Mode for Ice Concentration We asked the NIC analysts to comment on the suitability of ASAR GM mode data for assessing ice concentration. The following responses summarise the findings: - It is very useful. The main use. - It usually possible to identify the ice concentration as defined by the Egg Codes directly using only the

GMM data. - Pack Ice is pretty easy, but fast ice and openings can sometimes be confusing as fast ice may show up

like an opening. Also along the edge it can sometimes be misleading on the concentrations. - Due to off nadir signal noise the concentrations can be severely underestimated, even within the pack

ice. Also belts and strips can be washed out making the concentration of ice appear greater along the edge that higher resolution or AVHRR imagery is able to discern.

4.6 GM Mode for Ice Type Classification We asked the NIC analysts to comment on the suitability of ASAR GM mode data for assessing ice type classification. The following responses summarise the findings: - GMM data may be used to perform a full analysis on an area for which there are no other useful data

sources, or it can be used to “fill in gaps” in data. - Time of year: Sometimes can be used to identify ice types, but this depends on the time of year.

During the freeze-up (Oct-May in north) this is possible. During melt out Jun-Sep GMM data is less accurate.

- Multi-year ice: Especially good in the High Arctic, but there is debate on its ability to discriminate between Old Ice or Second Year Ice.

- First-year ice: Not always the case, found it can pick out New Ice good, and at times Young Ice, although, not so much on the different types of First Year Ice.

4.7 GM Mode for Lead Detection It was found that ASAR GM mode data is quite often good for lead detection. In general even very small leads (<3 km) are discernable using ASAR GM mode data In order to facilitate lead detection Vexcel UK have developed a “Lead Detection” algorithm, which highlights regions which are believed to be cracks or leads in the ice. Figure 6 shows an example of this algorithm applied to a region north of Canada during the 20th November 2004. This is a product that will be distributed as a part of the ESA GMES programme.

Figure 6. Vexcel UK lead detection for 20/11/2004.

4.8 GM Mode Data for Ice Motion In general the NIC ice analysts reported that they are not routinely making use of ASAR GM mode data for tracking ice fields in the Arctic. However, Vexcel UK have developed a simple tracking algorithm that demonstrates that this data source can be useful for this application. Figure 7 shows the ice motion that occurred between the dates; 7th & 9th Jan 2006.

Figure 7. The ice motion vectors derived from the Vexcel UK algorithm, between the period 07/01/2006 and 09/01/2006. The turquoise vectors indicating the ice motion direction, have a magnitude equal to five times that of the actual ice motion displacement. This scaling ratio is used simply to make the vectors clearer in the cases of low ice movement.

5 The Norwegian Meteorological Institute Experiences in the Arctic The Norwegian Meteorological Institute (MetNo) provide regular ice charts issued 5 days a week covering the area between Greenland and the Kara Sea. It was reported that the primary use of Global Monitoring mode was the identification of the ice edge position. To a lesser extent ASAR GM is also being used in limited classification tasks, such as being able to identify more features like wide leads and dark areas of new ice.

5.1 General comparison with other data sources As in section 4.4 we wanted to obtain a general impression as to where ASAR GM mode data falls within the hierarchy of data sources that are generally available to MetNo ice analysts. We asked the following question: “Assuming all possible data sources are available put the commonly used data sources in order of usefulness for ice analysis”. The ranking, along with a number of supplementary notes, is shown in Error! Reference source not found. Ranking Data source

1 NOAA AVHRR (when not cloudy, which is seldom) 2 AQUA/TERRA MODIS (only in summer and when it is not cloudy) 3 Radarsat ScanSAR or Envisat WideSwath when available (which is most of the 5 days a week

we draw ice charts, and covers maybe 10-30% of our area) 4 Envisat Global Mode Mosaic (when coverage) 5 QuikScat (Enhanced resolution data retrieved from Univ B.Young and made into ice

concentration product at met.no) 6 SSM/I (OSI SAF product)

Table 6. The order of “usefulness” for data sources for MetNo Arctic ice analysis.

5.2 Improvement in the ice edge accuracy from Global Monitoring mode Given that the key MetNO application for ASAR GM mode data is improving the accuracy of the identification of the ice edge, we asked is this could be quantified. In general it was found that the ice edge accuracy improves by 8-20 km in comparison to the OSI SAF (SSM/I passive microwave) 10 km product. In general this is a similar improvement that can be achieved using MODIS, AVHRR or ASAR high resolution data. ASAR GM mode data is generally most useful where the ice edge moves a lot, which typically is where most activities take place. These regions are at the north west of Svalbard (fishing) and directly east of Svalbard (fishing and transport).

6 National Ice Center Antarctic Operations For the Antarctic region the National Ice Center perform iceberg tracking as well as general ice analysis.

6.1 Regional analysis In a similar manner to Section 4.1 we asked the question, “for which regions in the Antarctic are the GMM mosaics most useful?” We asked for answers to be given according to the scale in Table 7. The resultant scores are displayed in Figure 8. It can be seen that for all regions the ASAR GM mode data scores very highly in terms of its usefulness. 1 Never useful 2 Occasionally useful 3 Sometimes useful. Helps confirm stuff. 4 Pretty good. Often the most useful source of data. 5 Very useful. Usually the primary source of information. Table 7. Scale of “usefulness”.

Figure 8. Regional analysis of the usefulness of ASAR GM mode data in the Antarctic. The scores are illustrated by the red numbers, according to scale shown in the table above.

6.2 Antarctic ASAR GM coverage The fact that ASAR GM mode data is very useful for application in the Antarctic, as evidenced by the results in Section 6.1, is very much a function of the fact that the coverage is very extensive in this region in comparison to other sources. In Figure 9 we show the ASAR GM mode data and ASAR high resolution mode acquisitions for one day, the 16th January 2006. In Figure 10 we show the RadarSAT coverage for one whole week, for the period leading up to the 16th January 2006.

Figure 9. ASAR GM mode data acquired on 16/01/2006.

ASAR high resolution data acquired on 16/01/2006.

Figure 10. RadarSAT SAR data acquired during one week between 10/01/2006 and 16/01/2006.

6.3 GM Mode data used for tracking Antarctic Icebergs The feedback from the NIC analysts was particularly enthusiastic for the application of tracking Antarctic icebergs. The NIC track an iceberg if it measures at least 10 nm along the long axis. Icebergs are tracked as long as they remain south of 60S. The following quote from an NIC analyst provides a good summary of the NIC experience for this application: “From personal experience, Global Monitoring mode data has been a godsend for Antarctic Icebergs, they appear clearly and a week’s worth almost always covers the entire Antarctic. I am able to track and record bergs easily, and do not have to worry about areas with sparse imagery. It also clearly shows any bergs that calve off an ice-shelf or from itself, which is a major tool in early identification. It also plays a major role in identifying shelf ice from fast ice whenever the confusion arises.” As a further indication of how useful the ASAR GM data source is for this application, the NIC iceberg website log was examined on the 21st January 2006. At that time the website logged 42 icebergs in the Antarctic region. Of these their last updated position were given by the following sources, making it very clear that Envisat ASAR GM mode data is by far the most commonly used data source for iceberg tracking. Envisat GM: 38 DMSP: 3 Modis: 1 Total: 42 A further point that was noted is that icebergs appear susceptible to tone reversal in the Southern Hemisphere. Tone reversal may mask the presents of iceberg during the austral summer or at low latitudes. This is possibly caused by surface melt.

7 Conclusion In this paper we have shown that ASAR GM mode’s excellent Polar coverage and near real time delivery is an ideal compliment to existing data sources for operational ice analysis. ASAR GM mode provides a very comprehensive account of ice conditions at the scale most required for ice concentration and ice typing. The key benefits due to ASAR GM mode are felt regionally: - In the high Arctic (it is often the best resolution imagery available) - Over the entire Antarctic (it is often the best resolution imagery available) The key applications for which ASAR GM mode data gives the most benefit are: - Ice concentration mapping - Ice edge mapping (as used by MetNo) - Antarctic iceberg tracking (where ASAR GM mode data has become the primary source of data)