Abstract. An important consideration in understanding sea ice mechanics is the integration of observed sea ice behavior on a floe neighborhood scale (1-10 km) into ice dynamics on a regional scale 0(50 km). We investigate sea ice kinematics from October 1993 through April 1994 using relative motions from 13 drifting buoys with Global Positioning System navigation in a 20-km array centered on the Sea Ice Mechanics Initiative ice camp, and we compare these motions to synthetic aperture radar (SAR)-derived ice velocities over a 100-by 500-km region in the Beaufort Sea. There is excellent correspondence between the deformation of the buoy array and that from the SAR. Inferred ice dynamics from analysis of the two major northerly wind convergence events of the winter are consistent with a granular hardening plastic conceptual model for Beaufort sea ice. Under continued northerly winds the ice from the Alaskan shore to the camp failed in shear and convergence, in a progressive manner away from the coast. The continuum scale O(10 km) is an order of magnitude larger than the grain, i.e., floe, size O(1 km). The ice motion often forms aggregates of 20-200 km separated by narrow (<10 km) shear zones, similar to granular materials. At moderate forcing, i.e., wind stress multiplied by fetch, the ice appears to fail along slip lines that occur at an acute angle to each other and to the direction of the wind forcing, characteristic of a plastic material at critical state. With longer fetch the ice appears to fail in compression, perpendicular to the wind direction. Sea ice appeared to harden on a regional scale after the first event. During the second northerly wind event there was a sea ice breakout toward the west, apparently due to a lack of lateral confining stress. Our observations suggest that the ice floes advect through relatively stationary stress fields, created by the wind forcing and coastal boundaries. For example, while the SAR and advanced very high resolution radiometer images indicated the presence of the shear feature at the same geographic location for nearly a week, buoys would show shearing only for several days as they transited across the region of shear. There is a high correspondence between the major internal ice deformation events and persistent weather patterns on a 3-to 5-day temporal scale. This implies that SAR data collection and analysis for regional sea ice dynamics should be consistent with the wind forcing and have a sampling of less than 3 days.
AVHRR imagery has been used to document the sites of 22 polynyas in the Bering and Chukchi seas. Two principal classes of polynyas have been identified that tend to be negatively correlated: 1) persistent polynyas, which are present much of the time and form off southand west-facing coasts, and 2) north coast polynyas, which are only occasionally open and form off north-facing coasts. Median extent values characterizing 17 of these polynyas for six years have been compiled for the winter and spring months, and the results of attempts to correlate these values with synoptic meteorological measurements are reported. These attempts were not very successful, suggesting that more sophisticated approaches to the problem are required. Other factors, such as currents, may play a principal role in determining actual polynya extent.
Abstract. We estimate the influence of the mosaic distribution of surface temperatures of sea ice on regional surface sensible and radiative heat fluxes on the basis of advanced very high resolution radiometer (AVHRR) temperatures. The AVHRR data were used to derive -1 km 2 surface temperature values for 100 x 100 km 2 regions. Regional flux estimates are compared to direct flux measurements taken during the Surface Heat Budget of the Arctic Ocean (SHEBA) experiment in the Beaufort Sea. We describe 48 cases of clear-sky conditions between December 1997 and February 1998. The distribution of surface temperatures within each region is skewed: most temperatures fall near the value observed at the ice camp, but the distribution has a warm tail corresponding to subregions with thinner ice. Sensible heat fluxes at the ice camp were downward, from the air to the ice. Although the camp was representative of the majority of the region, the upward flux from thin ice generally balanced the downward flux to thicker ice, suggesting a regional equilibrium of near-zero sensible heat flux. This was true whether the net downward flux at the camp was large or small or whether the ice was less compact, as in January, or more compact, as in December and February. The only exception to this generalization occurred when the ice was under strong compression. The net regional radiative loss from the ice surface was -22% greater than that measured on the floe at the SHEBA camp. This increase is due to the fourth-power temperature dependence of the radiative flux acting on the skewed distribution of surface temperatures. The implications for modeling are that sea ice tends toward small net surface sensible heat flux on an aggregate scale, an emergent property of the system, and that parameterizing the regional mosaic of surface temperatures to resolve correctly the radiative balance in winter is important. IntroductionSea ice has a heterogeneous surface in both its thermal and roughness characteristics. Understanding and modeling the regional implications of this heterogeneity on heat and momentum fluxes are a major challenge for observations and global climate models (GCMs). For the atmosphere-ice momentum flux this heterogeneity leads to the notion of a regional effective drag coefficient, which is about twice as large as one measured locally near the center of floes because of form drag [Overland, 1985] A goal of the Surface Heat Budget of the Arctic Ocean (SHEBA) program is to develop and implement models that improve the simulation of the present-day Arctic climate. Strategic objectives include how the state of the ice changes in response to forcing from the atmosphere and how to extend these relationships from local scales to the aggregate scales suitable for climate models. The main thermodynamic role of sea ice in winter is to be an insulator between the relatively warm ocean (temperatures --1.7øC) and the cooler atmosphere. Thinning of the Arctic ice could have a major climate impact by raising ice surface temperatures and incre...
Africa is poised for a revolution in the quality and relevance of weather predictions, with potential for great benefits in terms of human and economic security. This revolution will be driven by recent international progress in nowcasting, numerical weather prediction, theoretical tropical dynamics and forecast communication, but will depend on suitable scientific investment being made. The commercial sector has recognized this opportunity and new forecast products are being made available to African stakeholders. At this time, it is vital that robust scientific methods are used to develop and evaluate the new generation of forecasts. The GCRF African SWIFT project represents an international effort to advance scientific solutions across the fields of nowcasting, synoptic and short-range severe weather prediction, subseasonal-to-seasonal (S2S) prediction, user engagement and forecast evaluation. This paper describes the opportunities facing African meteorology and the ways in which SWIFT is meeting those opportunities and identifying priority next steps.Delivery and maintenance of weather forecasting systems exploiting these new solutions requires a trained body of scientists with skills in research and training; modelling and operational prediction; communications and leadership. By supporting partnerships between academia and operational agencies in four African partner countries, the SWIFT project is helping to build capacity and capability in African forecasting science. A highlight of SWIFT is the coordination of three weather-forecasting “Testbeds” – the first of their kind in Africa – which have been used to bring new evaluation tools, research insights, user perspectives and communications pathways into a semi-operational forecasting environment.
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