Introduction to special section: Small‐Scale Sea Ice Kinematics and Dynamics

[1] Away from the ice margins, the response of the ice cover to large-scale gradients in atmospheric and oceanic forcing is concentrated along narrow zones of failure (up to tens of kilometers in width) resulting in openings, closings or shears. In winter, openings dominate the local brine production and heat exchange between the underlying ocean and the atmosphere. Convergence of the pack ice forces the ice to raft or pile up into pressure ridges and to be forced down into keels, increasing the ice-ocean and ice atmosphere drag. A combination of openings and closings is typical when irregular boundaries are sheared relative to one another. These processes shape the unique character of the thickness distribution of the ice cover and have profound impacts on the strength of the ice and its deformation properties over a wide range of temporal and spatial scales. Understanding the basin-scale mechanical character of the sea ice cover is thus of importance in modeling its behavior in a changing climate and in facilitating operational applications. [2] In widely used models of sea ice, the representation of these processes is typically included in an aggregate and parameterized form based on simplifying assumptions. In the past, progress in model validation and improvements has been slowed by the lack of suitable observations. Except for focused field campaigns, observations of the above processes from buoy drift are limited by spatial sampling that is typically several hundred kilometers. Only with sea ice kinematics derived from high resolution Synthetic Aperture Radar (SAR) imagery have we been able to approach the spatial length scale required to observe these processes. In the late 1980s and most of the 1990s, the availability of small volumes of ice motion data from the European SAR satellites (ERS-1, 2) have allowed examination of sea ice strain rates at 5–10 km length scales and demonstrated the utility of these measurements for sea ice studies. However, the narrow swath of these early SAR missions obscures the spatial extent of the deformation patterns beyond 100 km. Launched in November of 1996, the wide-swath coverage of the RADARSAT imaging radar offers a tool capable of providing high resolution ( 100 m) observations of the Arctic ice cover. Since 1997, routine 3-day RADARSAT data of the Arctic Ocean have been acquired and processed into imagery at the Alaska Satellite Facility. The NASA-funded RADARSAT Geophysical Processing System (RGPS) [Kwok, 1998], a joint project of the Alaska Satellite Facility and the Jet Propulsion Laboratory is a program for producing fine-scale sea ice motion products. The program objective is to provide a dataset suitable for understanding the basin-scale behavior of sea ice kinematics on a seasonal and inter-annual time scale, and for improving ice dynamics. Thus far, four winters and three summers of the RADARSAT acquisitions have been processed and the data products are posted at the following website: working2/radarsat.html. [3] The availability of ice motion data from the RGPS program has allowed a more detailed and unprecedented look at the small-scale time-varying deformation of the ice cover [Kwok, 2001]. The RGPS observations point to the importance of understanding the consequence of ice pack as an anisotropic material with large-scale oriented fracture patterns. With the increasing resolution of coupled iceocean models that approaches the widths of leads, high resolution observations like that of the RGPS are needed for model development and validation. Simulation results can now be examined in detail. For climate studies, the impact of an anisotropic ice cover on surface heat and mass balance is not well understood. The RGPS dataset is a crucial component in the testing of new models that accounts for the spatial and temporal characteristics of these patterns. [4] At the 2004 RGPS Science Working Group meeting in Seattle, the participants showed that there were sufficient potential papers and new results to justify a Journal of Geophysical Research special section to showcase recent work on this topic. They highlight the importance of highresolution satellite observations for sea ice studies. There are also, of course, other papers that discuss these topics in other issues of JGR, as well as other journals. We encourage interested readers to examine these sources.