The Role of Near Real Time Envisat ASAR Global Monitoring Mode Data in Arctic and Antarctic Operational Ice Services (2006)

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 Xor 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”.