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C h a p t e r 8 Sea Ice Monitoring by Remote Sensing Authors: Stein Sandven and Ola M. Johannessen 8.1 INTRODUCTION 8.1.1 The Role of Sea Ice in the Cl imate and Weather System Sea ice is a part of the cryosphere that interacts continuously with the underlying oceans and the overlaying atmosphere. The growth and decay of sea ice occur on a seasonal cycle at the surface of the ocean at high latitudes. As much as 30 million km2 of the Earth’s surface can be covered by sea ice. In the Northern Hemisphere, sea ice extent fluctuates each year from a minimum in September, when most of the ice is confined to the central Arctic Ocean, Greenland Sea and Canadian Archipelago, to a maximum in March, when the ice covers almost the entire Arctic Ocean and many adjacent seas. In the Southern Hemisphere, the annual fluctuation is even greater, from a minimum in February to a maxi- mum in September when the ice surrounds the Antarctic continent and extends equatorward to 55°–65° S (Gloersen et al. 1992). Figure 8-1 shows an example of maximum and mini- mum ice extent obtained from passive microwave satellite data. Figure 8-1 Maps of maximum and minimum sea ice extent and concentration in the Arctic (top) and Antarctic (bottom) obtained from passive microwave satellite data from the Special Sensor Microwave Imager (SSM/I). The maps are from March 1993 (upper left and lower left) and September 1993 (upper right and lower right). Ice concentrations, expressed as percent coverage and indicated by the color bar, were computed from SSM/I data using the Norwegian NORSEX algorithm. Courtesy: NERSC. 2 Sea Ice Monitoring by Remote Sensing The largest volume of sea ice is found in the Northern Hemisphere in March, 0.05 million km3, which is nearly twice the maximum sea ice volume in the Southern Hemi- sphere. The reason for this is that the mean thickness of Arctic sea ice is about 3 m, whereas the mean thickness of Antarctic sea ice is 1.0–1.5 m. Sea ice research and monitoring is important for many countries at high latitudes, and also those who operate in Antarctica. Sea ice imposes severe restrictions on ship traffic in the Arctic. It is a sensitive climate indicator, and plays an important role in exploration and exploitation of marine resources. Sea ice has many roles in the global climate system. It serves as an effective insulator between the ocean and the atmosphere, restricting exchange of heat, mass, momentum and chemical constituents. During winter when there is a large temperature difference between the cold atmosphere and the relatively warm ocean surface, ocean-to-atmosphere heat transfer is essentially limited to areas of open water and thin ice within the pack. The winter flux of oceanic heat to the atmosphere from open water can be two orders of magnitude larger than the heat flux through an adjacent thick ice cover. As a result, the distribution of open water and thin ice is particularly important to the regional heat balance. Another important role of sea ice in the global climate system is that it affects surface albedo. Ice-free ocean generally has an albedo below 10–15%, whereas snow-covered sea ice albedos average about 80%. A fresh snow cover on ice can increase the surface albedos to values as high as 98%, whereas melt ponds can decrease the ice albedos to as low as 20%. Because the albedo of snow-covered sea ice is high relative to that of open water, the pres- ence of sea ice considerably reduces the amount of solar radiation absorbed at the Earth’s surface. This is most significant in summer when the insolation, or solar heating, is high. Sea ice processes also affect oceanic circulation directly by the rejection of salt to the underlying ocean during ice growth. This increases the density of the water directly under the ice, thereby inducing convection that tends to deepen the mixed layer. This convection contributes to driving the thermohaline circulation of the ocean and, in regions with density structures that were initially weak or unstable, can lead to overturning and deep water for- mation. Much of the world oceans’ deep and bottom water is believed to be formed in polar latitudes by these mechanisms. Conversely, the input of relatively fresh water to the ocean during ice melt periods tends to increase the stability of the upper layer of the ocean, inhib- iting convection. Furthermore, the net equatorward transport of ice in each hemisphere produces a positive freshwater transport and a negative heat transport. On a hemispheric scale, the seasonal variability of ice extent and ice edge location is controlled by atmospheric and oceanic forcing, which include ocean temperature and salin- ity and atmospheric temperature and winds. The location of the ice edge will in turn feed back on several atmospheric and oceanic processes that affect the regional weather, such as generation of polar lows. On a regional scale, surface roughness of the ice and the drag coefficient depend upon ridging and rafting, both of which can be produced by wind- or wave-induced ice convergence. Ice observation is therefore important for weather forecasts at high latitudes. Data from polar-orbiting satellites are used extensively in research, as well as for moni- toring sea ice extent and other ice parameters (e.g., Johannessen et al. 1992, 1995, 1999, 2005; Jackson and Apel 2004). Large-scale ice drift can be estimated from passive micro- wave data (Kwok et al. 1998; Liu and Cavalieri 1998; Martin and Augstein 2000) and from scatterometer data (Zhao et al. 2002), while ice deformation and ice growth can be derived from systematic SAR coverage (Kwok et al. 1995; Kwok and Cunningham 2002). Ice thick- ness, another important ice parameter, has been difficult to measure accurately by spaceborne instruments (Wadhams 1994), but new methods using radar altimeters are under develop- ment (Laxon et al. 2003). One of the key objectives in sea ice science is to achieve the Introduction 3 capability of synoptically measuring sea ice thickness in both hemispheres. Data on ice thickness are very sparse, especially in the Antarctic. Present estimates of sea ice volume, which are mainly based on model results due to lack of data, can have errors of + 50%. In the Arctic there are some synoptic surveys of ice thickness obtained by submarine sonar over the last four decades (Rothrock et al. 1999) and point measurements from expeditions and drifting ice stations. The data sets available provide some information about regional and seasonal variability, but are too sparse to provide a coherent picture. General circulation models predict enhanced climatic warming in polar areas, and this is expected to be reflected in a reduced sea ice area and a decreased mean sea ice thickness (Johannessen et al. 2004). Only a satellite-borne method can achieve the required coverage to monitor this change in time and space without prohibitive costs. Satellite observing tech- nique will have to measure elevation and topography of sea ice directly in order to retrieve thickness. The ICESat mission, launched in 2002 (Zwally et al. 2002), provides height measurements using laser altimeter. With systematic observation of ice thickness, com- bined with ice area and ice drift measurements using satellite data, it will be possible to estimate global sea ice volume, fluxes and variabilities more accurately. 8.1.2 Sea Ice as a Barrier for Ship Traffic, Fisheries and Offshore Operations The presence of sea ice represents a major limitation for ships and offshore operations at high latitudes in both hemispheres. The sea ice, which is on average 2–3 m thick, can only be penetrated by ice-strengthened vessels or icebreakers with a sufficient ice class. Most ships and fishing vessels are not ice-strengthened and must therefore avoid all ice areas. In many cases, when the ice concentration is 100% and the ice pressure is high, even the most powerful icebreakers have problems moving forward through the icepack. Off- shore platforms for ice-covered areas must have much stronger construction than is re- quired in ice-free waters. Harbors and loading terminals on the coast require stronger construction in areas of sea ice. In such areas, it is therefore of primary importance to monitor the sea ice daily and produce ice forecasts to assist ship traffic, fisheries and other marine operations. In the last 10 years, high-resolution Synthetic Aperture Radar (SAR) images from RADARSAT-1 have been established as the main data source for ice monitor- ing in several countries (Bertioa et al. 2004). Canada, in particular, uses large amounts of wide-swath SAR images to monitor the Canadian ice regions. Figure 8-2 shows an ex- ample of an ice chart in the western Canadian Arctic produced by the Canadian Ice Ser- vice. In the Northern Sea Route, the longest ice navigation route in the world, Russia has built up an extensive ice service to support sea transportation and ice operations. Use of SAR data is not yet established as a regular service in the Northern Sea Route, but several demonstrations have been performed during dedicated icebreaker expeditions, as shown in Figure 8-3 (Pettersson et al. 1999; Johannessen et al. 2005). Several other countries are developing operational SAR ice-monitoring systems to support sea transportation, ice navi- gation and offshore operations in the ice-covered areas in the Northern Hemisphere. 4 Sea Ice Monitoring by Remote Sensing Figure 8-2 Example of weekly ice chart for the western Canadian Arctic produced by the Canadian Ice Service based on RADARSAT SAR images. Courtesy: Canadian Ice Service. Figure 8-3 Ice navigation in the Northern Sea Route. (a) Picture of the Russian nuclear- powered icebreaker Siberia leading a convoy of cargo vessels sailing through 2-m thick first-year ice in the Northern Sea Route; (b) The Finnish icegoing tanker Uikku operating in sea ice; (c) SAR mosaic based on RADARSAT ScanSar and ERS-2 data obtained during the ARCDEV expedi- tion in 1998. The line superimposed on the mosaic is the sailing route for the expedition from Murmansk to Ob Gulf. The two western–most images are described in more detail in Figure 8-20. Source: Pettersson et al. 1999. Courtesy: Murmansk Shipping Company, the ARCDEV Project, NERSC, The Canadian Space Agency & The European Space Agency. Brief History of Satellite Sea Ice Monitoring 5 8.2 BRIEF HISTORY OF SATELLITE SEA ICE MONITORING Sea ice observation from coastal stations and ships has a history of more than 100 years. Regular sea ice charting, however, using aircraft and satellites has developed mostly since World War II. Aircraft survey was the main observation method until the 1980s, but use of satellite data has developed gradually over the last three decades and is now the most im- portant observation method. The first satellite sensors providing views of the large-scale structure and motion of sea ice utilized visible and infrared channels, such as those onboard the early Nimbus, Tiros, and Earth Resources and Technology Satellite (ERTS, later renamed Landsat). By the late 1960s, it was apparent that the sequential synoptic observations needed for sea ice and climate studies could not be acquired by visible sensors, which are limited to cloud-free and well-illuminated conditions. Sea ice exists in regions that are dark for several months and are frequently cloudy in the remaining months of the year (Gloersen et al. 1992). Therefore, it has been necessary to develop observation methods using microwaves that are able to penetrate clouds and are not dependent on light conditions. The first passive microwave remote sensing systems for satellites were launched on the Russian Cosmos 243 and Cosmos 384 in 1968 and 1970, respectively. In the US, passive microwave technology was first used in remote sensing of sea ice during the late 1960s and early 1970s, when a prototype of the Electrically Scanning Microwave Radiometer (ESMR) was flown on Nim- bus-5 over the polar regions (Campbell 1973). The first atlas of Antarctic sea ice based on passive microwave data was produced by Zwally et al. (1983). The period since 1970 has been one of great advancement in remote sensing of sea ice. After the ESMR period 1973–1976, a more advanced satellite instrument, the Scanning Multichannel Microwave Radiometer (SMMR) was operated on Nimbus-7 for nine years, from 1978 to 1987. A similar instrument, the Special Sensor Microwave Imager (SSM/I) followed after the SMMR and has provided continuous measurements for more than 25 years. This series of similar spaceborne instruments provided the longest and first regular time series of global sea ice data, allowing studies of variability and trends of the ice area and extent in both hemispheres (Cavalieri et al. 1997; Johannessen et al. 1999). Passive microwave observations have fairly coarse resolution (typically 30 km) and are more suit- able for large-scale or global monitoring than for regional and local observations. New passive microwave systems, such as AMSR-E, provide improved resolution of ice concen- tration charts, typically 6–10 km. Active microwave systems, such as real-aperture Side- Looking Radars (SLR) and Synthetic Aperture Radar (SAR), were developed during the 1970s and 1980s for aircraft surveillance and used in ice monitoring to provide detailed maps of the ice conditions, especially in areas of heavy ship traffic. Satellite SLR systems were used extensively in Russian ice monitoring during the 1980s and 1990s (Johannessen et al. 2000; Alexandrov et al. 2000). Spaceborne SAR combines high spatial resolution with independence of cloud cover and light conditions, making it possible to observe sea ice with much better accuracy than visible and passive microwave methods. In 1978 Seasat was the first satellite that provided high-resolution SAR images of sea ice, but it only operated for about three months. The European Remote Sensing (ERS) program, which started in 1991, represented a major mile- stone in satellite SAR remote sensing of sea ice, because the two satellites ERS-1 and ERS- 2 have operated continuously for more than 10 years and delivered tens of thousands of SAR images of ice-covered regions around the world. Since 1996 the Canadian RADARSAT- 1 has delivered wide-swath SAR images over large parts of the Northern Hemisphere sea ice. Arctic sea ice deformation fields and linear kinematics features have been derived from regular ScanSAR images and used to estimate ice area and volume production (i.e., Kwok 6 Sea Ice Monitoring by Remote Sensing and Cunningham 2002). Since 2003 the European ENVISAT satellite has delivered wide- swath ASAR data, and ice observation is one of the main applications (Flett 2004; Sandven et al. 2004; Johannessen et al. 2005). Other microwave systems such as scatterometer and radar altimeter data have also shown promising results for observation of ice parameters. 8.3 PHYSICAL PROPERTIES OF SEA ICE 8.3.1 Large- scale Ice Parameters Defined by WMO Sea ice terminology has been standardized by the Sea Ice Working Group of the World Meteorological Organization (WMO 2004), which established a nomenclature and the egg code for use in ice map production (Table 8-1 and Figure 8-4). The WMO nomenclature was defined for large-scale properties of sea ice and does not include the characteristics of ice structure on scales from cm to mm. Properties on this scale are important for the inter- action of remote sensing signals with the ice-snow surface. Other limitations of the conven- tional ice codes from an ice navigation point of view are, according to Lensu et al (1996): · The classification of ice types is unnecessarily detailed for younger and thinner ice types that exist only for shorter periods of time in the freeze-up and early-winter season. On the other hand, thicker, deformed and multiyear ice types are not char- acterized in the detail needed. The classification should be proportional to the de- crease of ship speed and increase of damage probability. · There is no quantitative reference to deformed ice types like rafted ice, in spite of the fact that deformed ice can be predominant and several times thicker than level ice. · There is no quantitative reference to ice ridges, i.e., their size and frequency of occurrence. · There is no reference to lead size, frequency or orientation. · The relation of regional ice characteristics to what is experienced by an ice-going vessel is uncertain. · The codes cannot optimally use the information that is available from SAR images. · The terminology has no clear connection to geophysical ice models used in fore- casting. The main reason for these shortcomings is that the WMO nomenclature was defined in 1970, when no operative ice models existed, no high-resolution satellite data were avail- able, and very little data on ice thickness, floe size and ridge distribution existed. The code clearly aims to display visual and mainly qualitative observations, for example those made onboard a vessel. It is not feasible that a single ice code would satisfy all possible require- ments. With systematic use of SAR data and other new ice observation techniques, it is foreseen that new ice codes will be developed. Figure 8-4 The World Meteorological Organization’s Egg code for standard sea ice nomenclature defined in Table 8-1. Courtesy: The World Meteorological Organization Physical Properties of Sea Ice 7 Table 8-1 Definition of the WMO Egg Code. Courtesy: The World Meteorological Organization 8.3.2 Small-scale Ice Structure and Growth of Ice A complete description of ice freezing and melting, the main physical processes respon- sible for the large seasonal variability in sea ice extent and volume, should start with a discussion of the structure of the H2O molecule as it changes in the phase transition be- tween solid and liquid. The fact that solid water has lower density than liquid water has the important implication that sea ice floats on top of the ocean. In this review, we will only 8 Sea Ice Monitoring by Remote Sensing describe the most important physical properties very briefly, focusing on those which are important for remote sensing. The basic physical parameters of sea ice are temperature, salinity, crystal structure that incorporates brine and air, surface roughness, snow cover, and the presence of liquid water on top of the ice, which frequently occurs in summer when ice and snow melts at the ice surface. Since sea ice is formed by the freezing of salt water, there are important effects of the salt during the freezing process; some salt is released from the ice as it is formed and is trapped in brine pockets of varying size. Ice formed during a winter season (first-year ice) contains typically from 6 to 10‰ salt if the ice is formed from normal sea water with salinity of 35‰. The brine gradually drains from the upper part of the ice, causing multiyear ice to have a very low salinity of less than 1‰ in the surface layer. In the first stage of freezing, frazil ice is formed consisting of small ice crystals at the surface. As freezing continues and more ice crystals are formed, the crystals coagulate to form grease ice. Frazil ice and grease ice dampen the short gravity waves at the sea surface, which has significant impact on radar remote sensing of open ocean water. An example of this dampening is shown in the photograph in Figure 8-5a. If the freezing is allowed to continue without disturbance from surface waves, a weakly consolidated layer of elastic ice is formed. When this layer is less than 10-cm thick, it is defined as nilas. As it grows thicker and becomes less elastic, it forms gray ice (10–15 cm) and gray-white ice (15–30 cm). The process of brine drainage gradually replaces the brine pockets with voids of air that change the visual appearance from almost black