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We have generated consistent sea ice extent and area data records spanning 18.2 years from passive-microwave radiances obtained with the Nimbus 7 scanning multichannel microwave radiometer and with the Defense Meteorological Satellite Program F8, F11, and F13 special sensor microwave/imagers. The goal in the creation of these data was to produce a long-term, consistent set of sea ice extents and areas that provides the means for reliably determining sea ice variability over the 18.2-year period and also serves as a baseline for future measurements. We describe the method used to match the sea ice extents and areas from these four multichannel sensors and summarize the problems encountered when working with radiances from sensors having different frequencies, different footprint sizes, different visit times, and different calibrations. A major obstacle to adjusting for these differences is the lack of a complete year of overlapping data from sequential sensors. Nonetheless, our procedure reduced ice extent differences during periods of sensor overlap to less than 0.05% and ice area differences to 0.6% or less.

The most comprehensive large-scale characterization of the global sea ice cover so far has been provided by satellite passive microwave data. Accurate retrieval of ice concentrations from these data is important because of the sensitivity of surface flux (e.g., heat, salt, and water) calculations to small changes in the amount of open water (leads and polynyas) within the polar ice packs. Two algorithms that have been used for deriving ice concentrations from multichannel data are compared. One is the NASA Team algorithm and the other is the Bootstrap algorithm, both of which were developed at NASA's Goddard Space Flight Center. The two algorithms use different channel combinations, reference brightness temperatures, weather filters, and techniques. Analyses are made to evaluate the sensitivity of algorithm results to variations of emissivity and temperature with space and time. To assess the difference in the performance of the two algorithms, analyses were performed with data from both hemispheres and for all seasons. The results show only small differences in the central Arctic in winter but larger disagreements in the seasonal regions and in summer. In some areas in the Antarctic, the Bootstrap technique shows ice concentrations higher than those of the Team algorithm by as much as 25%; whereas, in other areas, it shows ice concentrations lower by as much as 30%. The differences in the results are caused by temperature effects, emissivity effects, and tie point differences. The Team and the Bootstrap results were compared with available Landsat, advanced very high resolution radiometer (AVHRR) and synthetic aperture radar (SAR) data. AVHRR, Landsat, and SAR data sets all yield higher concentrations than the passive microwave algorithms. Inconsistencies among results suggest the need for further validation studies.

We have recently completed an analysis that examines in detail the spatial and temporal variations in global sea-ice coverage from 26 October 1978, through 20 August 1987. The sea-icemeasurements we analyzed are derived from data collected by a multispectral, dual-polarized, constant incidence-angle microwave imager, the Scanning Multichannel Microwave Radiometer (SMMR) on board the NASA Nimbus 7 satellite. The characteristics of the SMMR have permitted a more accurate calculation of total sea-ice concentrations (fraction of ocean area covered by sea ice) than earlier single-channel instruments and, for the first time, a determination of both multiyear sea-ice concentrations and physical temperatures of the sea-ice pack. An estimate of the SMMR wintertime total ice concentration accuracy of ± 7% in both hemispheres has been obtained. As this is an improvement over the estimated accuracies of previous microwave sensors, we are able to present improved calculations of the sea-ice extents (areas enclosed by the 15% ice concentration boundaries), sea-ice concentrations, and open-water areas within the ice margins. This analysis will be published in a book, Arctic and Antarctic sea ice, 1978–1987: satellite passive microwave observations and analysis, due for publication in1992. Some highlights from the analysis are presented in this paper.

Previous research studies have focused on producing algorithms for extracting geophysical information from passive microwave data regarding ice floe size, sea ice concentration, open water lead locations, and sea ice extent. These studies have resulted in four separate algorithms for extracting these geophysical parameters. Sea ice concentration estimates generated from each of these algorithms (i.e., NASA/Team, NASA/Comiso, AES/York, and Navy) are compared to ice concentration estimates produced from coincident high-resolution synthetic aperture radar (SAR) data. The SAR concentration estimates are produced from data collected in both the Beaufort Sea and the Greenland Sea in March 1988 and March 1989, respectively. The SAR data are coincident to the passive microwave data generated by the Special Sensor Microwave/Imager (SSM/I).

Selected data obtained during the first year of operation of the scanning multichannel microwave radiometer (SMMR) on board the Nimbus 7 satellite (launched in late October 1978) have been used to calculate, on a global basis, various geophysical parameters over open oceans, polar regions, and terrain. Over open oceans these calculations have provided values for sea surface temperatures, near-surface winds, atmospheric water vapor in a column, and rainfall rates. In polar regions, sea ice concentration, multiyear ice fraction, and radiating temperatures have been obtained. Finally, the extent and water equivalence of snow cover over terrain have been calculated. These parameters have been compared with in situ measurements of the same geophysical parameters, where available, and the results of these comparisons are described. The self-consistency of the global displays of all the parameters is discussed along with the plans for archiving them for subsequent research purposes. A description of the SMMR calibration and data processing scheme is also given.

Passive microwave measurements of the Bering Sea ice cover were made with the NASA CV-990 airborne laboratory and with the Nimbus-7 Scanning Multichannel Microwave Radiometer (SMMR) during February 1983 as part of the Bering Sea marginal zone experiment (MIZEX West). Microwave data obtained with the CV-990 include radiances from both imaging and dual-polarized fixed-beam radiometers spanning a range of frequencies from 10 to 183 GHz. The imagery at 19 and 92 GHz provide a description of the marginal ice zone delineating regions of open water, ice compactness, and ice-edge structure. Comparison of these images with ice concentration maps derived from the SMMR data shows that the maps provide an accurate description of the ice edge and interior pack polynyas. Analysis of the fixed-beam data shows that spectral differences increase with a decrease in ice thickness and the polarization at both 18 and 37 GHz distinguishes among new, young, and first-year sea ice types. Finally, these results are shown to agree with surface-based radiometric measurements.

Microwave images of sea ice obtained by Nimbus-5 and the NASA CV-990 airborne laboratory are used to determine the time variation of the sea-ice concentration and multiyear ice fraction within the pack ice in the Arctic Basin. The images, constructed from data acquired from the electrically scanned microwave radiometer, are analyzed for four seasons during 1973-1975. Observations indicate significant variations in the sea-ice concentration in the spring, late fall, and early winter. Sea-ice concentrations as low as 50% were detected in large areas in the interior of the Arctic polar sea-ice pack. The applicability of passive-microwave remote sensing for monitoring the time dependence of sea-ice concentration is considered.

Microwave and infrared data were obtained from a research aircraft over the Beaufort Sea ice from the shoreline of Harrison Bay northward to a latitude of almost 81 deg N. The data acquired were compared with microwave data obtained on the surface at an approximate position of 75 deg N, 150 deg W. Over this north-south transect of the polar ice canopy it was discovered that the sea ice could be divided into five distinct zones. The shorefast sea ice was found to consist uniformly of first-year sea ice. The second zone was found to be a mixture of first-year sea ice, medium size multiyear floes, and many recently refrozen leads, polynyas, and open water; considerable shearing activity was evident in this zone. The third zone was a mixture of first-year and multiyear sea ice which had a uniform microwave signature. The fourth zone was found to be a mixture of first-year sea ice and medium-to-large size multiyear floes which was similar in composition to the second zone. The fifth zone was almost exclusively multiyear ice extending to the North Pole.

This paper presents an overview of recent remote-sensing techniques as applied to geophysical studies of floating ice. The current increase in scientific interest in floating ice has occurred during a time of rapid evolution of both remote-sensing platforms and sensors. Mesoscale and macroscale studies of floating ice are discussed under three sensor categories: visual, passive microwave, and active microwave. The specific studies that are reviewed primarily investigate ice drift and deformation, and ice type and ice roughness identification and distribution.

A method of determining sea ice parameters using Nimbus 7 polarized multispectral radiance data obtained with the Nimbus 7 Scanning Multichannel Microwave Radiometer (SMMR) is presented. Observed radiances from selected areas in the Arctic region for the period February 3-7, 1979 were used in computing algorithm coefficients. Polar maps of sea ice concentration, multiyear fraction, and ice temperature are illustrated for this period. The variation of the mean and standard deviation of ice concentration and multiyear ice fraction for a region of perennial ice cover over the first 11 months of SMMR operation is also presented. Comparisons are made between the calculated sea ice parameters and information obtained from previous studies using aircraft, submarine and surface observations. The absolute accuracy of the SMMR parameters remains uncertain.

Synoptic views of the entire polar regions of earth were obtained free of the usual persistent cloud cover using a scanning microwave radiometer operating at a wavelength of 1.55 cm on board the Nimbus-5 satellite. Three different views at each pole are presented utilizing data obtained at approximately one-month intervals during the winter of 1972-1973. The major discoveries resulting from an analysis of these data are as follows: (1) Large discrepancies exist between the climatic norm ice cover depicted in various atlases and the actual extent of the canopies. (2) The distribution of multiyear ice in the north polar region is markedly different from that predicted by existing ice dynamics models. (3) Irregularities in the edge of the Antarctic sea ice pack occur that have neither been observed previously nor anticipated. (4) The brightness temperatures of the Greenland and Antarctica glaciers show interesting contours probably related to the ice and snow morphologic structure.

Combined use of active and passive microwave imagery is the optimum way to observe the morphology and dynamics of near-shore ice. Such data are compared with the electrically scanning microwave radiometer imagery of the Nimbus-5 satellite. Data from the synthetic aperture radar, scanning multichannel microwave radiometer, Seasat-1, and Nimbus-7 can provide an increased understanding of Beaufort Sea nearshore ice. Background The current need for sea ice information has occurred at a time of rapid evolution of remote-sensing platforms and sensors. These timely technical advances are beginning to eliminate the observational barriers that have limited our knowledge of a natural phenomenon existing in areas that are dark and/or cloud-covered much of the year. Furthermore, as sea ice undergoes large spatial variations on short time scales, the new sensing techniques are acquiring for the first time the sequential synoptic observations needed for cause-and-effect studies. Considerable emphasis in recent years has been placed on the microwave remote sensing of sea ice because it offers the possibility of an all-weather day-or-night capability. Both passive and active microwave remote sensing techniques have been explored. An overview of these activities has been given in Ref. 1. Early work in this area used passive microwave techniques because such techniques were the first to be incorporated aboard earth-viewing satellites. The most important series of aircraft flights that demonstrated the feasibility and usefulness of ice observations by means of passive microwave sensors were those that occurred during the NASA Arctic Ice Dynamics Joint Experiment (AIDJEX). A series of three AIDJEX pilot field experiments were performed during the spring of 1970, 1971, and 1972 in the southern Beaufort Sea. During each of these experiments, the NASA CV-900 Galileo I performed a variety of flights ranging in altitude from 150 m to 11 km. A wide variety of visual and infrared sensors were operated in addition to an imaging radiometer operated at a wavelength of 1.55 cm and fixed-beam radiometers operated at wavelengths of 0.81, 2.8, 6.0, and 21 cm. The 1970 data showed that it was possible to distinguish sea ice from liquid water both through the clouds and in the dark. This finding was useful because it pointed the way to an "all-time" ability to observe leads and polynyas. These data also showed that strong microwave emissivity differences occur on the ice surface itself. However, the lack of sufficient ground-truth data prevented a determination of the reason for these differences. The ground-truth measurements and mesoscale microwave mosaic maps (10 000 km) acquired during the 1971 AIDJEX experiments allowed Gloersen et al. 3 to show that the observed emissivity differences of sea ice at a wavelength of 1.55 cm are associated with the age of the ice, with multiyear ice having low emissivities (cold brightness temperatures, 210 K) and first-year ice having higher emissivities (235 K). This was important because it suggested that passive microwave imagery could provide an all-time capability of distinguishing between old (thick) and new (thin) ice and of tracking ice motion as well as lead and polynya dynamics.

A microwave remote sensing program of sea ice in the Beaufort Sea was conducted during the Arctic Ice Dynamics Joint Experiment (AIDJEX). Several types of both passive and active sensors were used to perform surface and aircraft measurements during all seasons of the year. In situ observations were made of physical properties (salinity, temperature, density, surface roughness), dielectric properties, and passive microwave measurements were made of first-year, multiyear, and first-year/multiyear mixtures. Airborne passive microwave measurements were performed with the electronically scanning microwave radiometer while airborne active microwave measurements were performed by synthetic aperture radar, X- and L-band radar, and a scatterometer.

A combination of remote sensing from an aircraft and simultaneous surface measurements have confirmed the feasibility of identifying old and new sea ice according to its emission of thermal radiation at wavelengths between 0.3 and 3 cm. Emissivity of first-year thick ice with a surface temperature of about 260 K is 0.95 or greater for wavelengths between 0.81 and 11 cm; the emissivity of multiyear ice is 0.8 at 0.81 cm and 0.95 at 11 cm, increasing monotonically in this wavelength interval. The ease with which multiyear ice can be distinguished from first-year ice using a passive microwave radiometer is demonstrated by comparing mosaics prepared both from photographs and images of 1.55-cm radiation.

An algorithm has been developed which utilizes the multi- spectral data available from the Nimbus-7 Scanning Multichannel Microwave Radiometer (Nimbus-7 SMMR) to infer sea ice concentration (C), multiyear ice fraction (F), and ice surface temperature. The method is expected to provide an improved capability for inferring C as contrasted with the Nimbus-5 ESMR algorithm which utilized only one channel of radiometric data to predict C to about 10–15%. With Nimbus-7 SMMR, the predicted capability is 3–5% for C. A sensitivity analysis has been completed for all of the derived sea ice parameters using computed noise amplification factors and the measured noise figures from the Nimbus-7 SMMR channels.

The current increase of scientific interest in all forms of floating ice – sea ice, lake ice, river ice, ice shelves and icebergs – has occurred during a time of rapid evolution of both remote-sensing platforms and sensors. The application of these new research tools to ice studies has generally been both piecemeal and sporadic, partly because the community of ice scientists has not kept up with the rapid advances in remote sensing technology and partly because they have not made their needs known to the space community. This paper seeks to help remedy the latter shortcoming. The remote sensing requirements for floating ice studies are given, and the capabilities of various existing and future sensors and sensor combinations in meeting these requirements are discussed. The desirable future sensors are also discussed from both the research and operational points of view.

The prelaunch version of the Nimbus-7 Scanning Multichannel Microwave Radiometer (Nimbus-7/SMMR) sea ice algorithm was utilized to prepare global maps of the sea ice coverage for a test period coincident with the NASA CV-990 airborne laboratory underflights in October-November 1978. Selected areas from the north polar map projections of the sea ice parameters were compared with data obtained concurrently with the Nimbus-7/SMMR simulator and other instruments on board the NASA CV-990, and were found to be in disagreement. This is attributed to offsets in the radiance values used as inputs, as has also been observed in retrieving ocean/atmosphere geophysical parameters from Nimbus-7/SMMR data. As of this writing, in-orbit calibration adjustments are in progress. Some displays of the adjusted radiances, using interim adjustment values, are discussed. Of particular interest is the observation of larger variation in the polarization differences for a given ice type than was anticipated. This will have to be taken into account in the ongoing fine-tuning of the sea ice algorithms.

Observations of the ocean ice, ice sheets, ice edges, and ocean waves in the Antarctic, performed by the Seasat and Nimbus-7 satellites are described. Specific features of the Seasat altimeter, the SAR, the Seasat-A Scatterometer System, and the Scanning Multichannel Microwave Radiometer (SMMR) are discussed, together with the advantages and limitations of each instrument as applied to the study of ice dynamics. The Nimbus-7 SMMR algorithm for calculating sea-ice concentrations is presented. In addition, the Seasat altimeter-derived data on wind speed and wave heights in the Southern Ocean, and on the ice sheets of the Antarctica and Greenland were evaluated.

An algorithm has been developed for estimating total and multiyear sea ice concentration from passive microwave and surface air temperature measurements. The algorithm was made for use with Nimbus 7 scanning multichannel microwave radiometer (SMMR) data. It is based on radiation physics and may thus easily be modified to suit other passive microwave instruments. A comparison between Nimbus 7 SMMR and aircraft microwave measurements indicates that estimates of total ice concentration are accurate to ±3% and those of multiyear ice concentration to ±10%. These accuracies are not valid during the melt season when the snow on the ice is wet. For the first time such a comparison of independent estimates has been performed to validate the capability of measuring sea ice coverage from space. The ability of the SMMR to follow moving patches of multiyear ice and of rain/wet snow areas has been demonstrated. From the concentration estimates the sharpness of the ice edge can be estimated. The need for accurate concentration estimates for reliable heat budget estimates in the Arctic is also discussed.

This paper compares satellite data on the marginal ice zone obtained during the Marginal Ice Zone Experiment in 1984 by Nimbus 7 with simultaneous mesoscale aircraft (in particular, the NASA CV-990 airborne laboratory) and surface observations. Total and multiyear sea ice concentrations calculated from the airborne multichannel microwave radiometer were found to agree well with similar calculations using the Nimbus SMMR data. The temperature dependence of the determination of multiyear sea-ice concentration near the melting point was found to be the same for both airborne and satellite data. It was found that low total ice concentrations and open-water storm effects near the ice edge could be reliably distinguished by means of spectral gradient ratio, using data from the 0.33-cm and the 1.55-cm radiometers.

The extent and area of sea ice and the amount of open water within the ice boundaries have been computed for the Arctic, Antarctic, and global ice covers every 2 days from November 1978 to August 1987 utilizing the Nimbus 7 scanning multichannel microwave radiometer (SMMR) observations. During this 9-year period, no significant trends were found in the ice areas of the Arctic and Antarctic packs or in their sum, the global ice area. Also, no significant trends were found in the maxima and minima of the annual ice extents for either polar pack. However, during these 9 years, significant negative trends were found in the annual global ice extent maxima (5%) and in the annual maxima of the global open water area within the sea ice boundaries (7–14%). Significant negative trends were also found in the annual open water maxima in the Antarctic (5–10%) and the annual open water minima in the Arctic (13–23%). The open water within the ice boundaries averaged approximately 14% in the Arctic and 18% in the Antarctic, at the time of their maximum extents. This negative trend in the global ice extent maxima during 1978–1987 also occurred during 1973–1976, found by an analysis of the monochromatic microwave observations obtained by the Nimbus 5 satellite. The combined 15-year negative trend is 6%. A significant change between the 1973–1976 and the 1978–1987 periods in the Antarctic ice pack is that the very large polynya that formed and persisted in the Weddell Sea during three out of four of the austral winters in the earlier period did not form in any of the 9 years of the latter period. This study, in conjunction with recent studies of global temperature change, lends support to the thesis that climatic changes in the global average temperature might be detectable by observing variations in global sea ice extent.

Sea ice concentration contour maps derived from the 0.81 cm wavelength channels on the Nimbus 7 scanning multichannel microwave radiometer (SMMR) are compared with surface and aircraft data and with sea ice charts derived from the TIROS-N satellite imagery. The regional ice and weather data and the derivation of the TIROS ice charts are discussed, and the Nimbus 7 SMMR data and the physical basis of the algorithm used to retrieve ice concentrations are described. A detailed comparison of the SMMR imagery with all available surface, satellite, and meteorological data for two days in March, 1979 is made. The results show that the ice concentration maps derived from the 0.81 cm wavelength SMMR radiances provide accurate information on both the ice edge position and the location of low and high ice concentration regions within the Bering Sea pack ice.

Aircraft remote sensing data collected during the 1984 summer Marginal Ice Zone Experiment in the Fram Strait are used to compare ice concentration estimates derived from synthetic aperture radar (SAR) imagery, passive microwave imagery at several frequencies, aerial photography, and spectral photometer data. The comparison is carried out not only to evaluate SAR performance against more established techniques but also to investigate how ice surface conditions, imaging geometry, and choice of algorithm parameters affect estimates made by each sensor.Active and passive microwave sensor estimates of ice concentration derived using similar algorithms show an rms difference of 13 percent. Agreement between each microwave sensor and near-simultaneous aerial photography is approximately the same (14 percent). The availability of high-resolution microwave imagery makes it possible to ascribe the discrepancies in the concentration estimates to variations in ice surface signatures in the scene.

The data acquired during the summer 1984 Marginal Ice Zone Experiment in the Fram Strait-Greenland Sea marginal ice zone, using airborne active and passive microwave sensors and the Nimbus 7 SMMR, were analyzed to compile a sequential description of the mesoscale and large-scale ice morphology variations during the period of June 6 - July 16, 1984. Throughout the experiment, the long ice edge between northwest Svalbard and central Greenland meandered; eddies were repeatedly formed, moved, and disappeared but the ice edge remained within a 100-km-wide zone. The ice pack behind this alternately diffuse and compact edge underwent rapid and pronounced variations in ice concentration over a 200-km-wide zone. The high-resolution ice concentration distributions obtained in the aircraft images agree well with the low-resolution distributions of SMMR images.

A technique is presented which improves existing methods of calculating sea ice concentrations from microwave radiances by reducing weather-related effects over open ocean areas and in the vicinity of marginal sea ice zones. Winds, atmospheric water vapor, cloud liquid water, and rain increase the microwave emission over these regions and thus result in erroneous values of computed sea ice concentration. The method described is based on the microwave spectral properties of sea ice and ice-free ocean and utilizes ratios of the polarized radiances at the 0.81-cm (37 GHz) and 1.7-cm (18 GHz) wavelengths. Following a discussion of the physical basis for this technique, examples are provided which demonstrate its utility. While the technique was developed for use with the Nimbus 7 scanning multichannel microwave radiometer data, it is applicable also to data from other microwave radiometers operating in a similar wavelength range.