Simultaneous Passive and Active Microwave Observations of Near-Shore Beaufort Sea Ice

Abstract

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.