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SHEBA: An Interdiscplinary Study of the Surface Heat Budget of the Arctic Ocean, Donald K. Perovich, 1998 Fall Meeting of the American Geophysical Union.

SHEBA is an large, interdisciplinary research program examining the surface heat budget of the Arctic Ocean. The primary goals of SHEBA are: 1) to determine the ice-ocean-atmosphere processes that control the ice-albedo and the cloud radiation feedback mechanisms and 2) to improve the treatment of the Arctic in General Circulation Models. SHEBA is addressing these goals through observations, data assimilation and modeling. A central component of SHEBA is the recently completed year-long field experiment. The icebreaker CCGC Des Groseilleirs served as the base of operations for Ice Station SHEBA from October 1997 through October 1998. Measurements were made at the ice station, as the icebreaker drifted with the Arctic pack ice. The focus was on obtaining a dataset of simultaneous and contiguous observations from the top of the atmosphere, through the sea ice and into the upper ocean over an entire annual cycle. A key element was making measurements for a complete annual cycle. The measurements include atmospheric profiles of temperature, humidty and wind speed; cloud properties; longwave and shortwave radiation fluxes; surface albedo; shortwave extinction in the ice; snow depth and snow properties; ice mass balance and ice morphology; the thermohaline structure of the upper ocean; and the turbulent energy exchange between the atmosphere, ice and ocean. This integrated and comprehensive dataset is being used to understand the processes governing the surface heat budget, to develop and test parameterizations suitable for climate models, and to evaluate single column models.

The Seasonal Evolution of Albedo during SHEBA, Thomas C. Grenfell, Donald K Perovich, Bonnie Light, Jacqueline Richter Menge, Walter B. Tucker III, Gary A Maykut, 1998 Fall Meeting of the American Geophysical Union.

As part of ice-albedo feedback studies during SHEBA, we measured spectral and wavelength-integrated albedos. Measurements were made every 2.5 m along a 200-m survey line from April through October. Initially this line was completely snowcovered, but as the melt season progressed it became a mixture of bare ice and melt ponds. Observed changes in albedo were a combination of a gradual evolution due to seasonal transitions and abrupt shifts resulting from synoptic weather events. The surface albedo was high (0.8-0.9) and spatially uniform in April. In May there was a gradual decrease in albedo as the snow cover warmed and the snow grain size increased. Rain in late May caused rapid coarsening of the snow and a sharp drop in albedo from 0.8 to 0.7. While this event marked the onset of melt there were periods of cooler temperatures and light snow during the next two weeks, with attendant increases in albedo. After this period of "flickering" there was a steady decrease in average albedo for the remainder of June and July. By the end of July the average albedo along the line was 0.4. The spatial variability in albedo was greatest at this time with values ranging from 0.1 for deep, dark ponds to 0.65 for bare white ice. Starting in late July and early August there were intermittent periods with air temperatures below freezing. During these periods ice skims formed on the surface of ponds and there were occasional snow flurries resulting in an increase of 0.1 in average albedo. Again there was a few weeks of "flickering" during freezeup, with albedos increasing and decreasing depending on the synoptic weather. By the end of August surface temperatures were consistently below freezing and the albedo increased as the snowcover got deeper. By October average albedos returned to their springtime maxima of 0.8 to 0.9 and were spatially uniform.

The Many Faces of Melt, Walter B. Tucker III, Donald K Perovich, Jacqueline Richter Menge, Thomas C. Grenfell, Bonnie Light, Hajo Eicken, Jinro Ukita, Gary A Maykut Bruce Elder, 1998 Fall Meeting of the American Geophysical Union.

During the summer melt season major changes occur in the morphology and the thickness of Arctic sea ice. As part of SHEBA we documented these changes qualitatively through photographs of the ice cover, and quantitatively through mass balance measurements. In late-May the ice was snow-covered and homogeneous in appearance. As the snow melted in June the surface took on a variegated appearance, with melt ponds forming. As melt progressed these ponds grew, both in areal extent and in depth. Throughout June and July the melt ponds deepened, in some cases completely melting through to the ocean. Once a pond had a salt water connection to the ocean, melting accelerated. Average pond depths at the end of July were were roughly 0.5 m. In some cases the ice beneath the ponds would weaken to a point where sections would break due bouyancy and float to the surface. The surface of the unponded ice also changed during melt. In some cases the surface ice would deteriorate into small (1-3 mm) ice grains, with an appearance similar to old snow. In other cases the ice melting would occur at crystal and platelet boundaries. This resulted in surfaces with a "fur-like" appearance or consisting of large (~10 cm) shards of ice. This surface would typically melt faster than the small ice grains. Ablation in ridge sails typically was greater than that in undeformed bare ice. There was little surface ablation after the beginning of August, though bottom ablation continued into September. There was about 3-4 m of lateral ablation around the edges of floes. Ice floes continually fragmented into smaller pieces as the cracks and small leads that had formed and froze in the winter melted.

Relating Measurements of Pack Ice Stress and Deformation in the Alaskan Beaufort Sea, J.A. Richter-Menge, J.E. Overland S.L. McNutt, 1998 Fall Meeting of the American Geophysical Union.

A governing factor in the exchange of energy between the atmosphere, sea ice cover, and ocean is the thickness distribution of the ice cover. A complex series of dynamic and thermodynamic processes define the thickness distribution. Key to the dynamic processes is the mechanical behavior of the ice cover. During the field programs supported by the Sea Ice Mechanics Initiative (SIMI) and the Surface HEat Budget of the Arctic Ocean (SHEBA), we took concurrent measurements of the internal ice stress and ice motion to improve the understanding of the mechanical behavior of the ice pack through direct measurements. The design and extent of our stress and deformation array, the use of satellite-derived ice motion vectors, and the length of the measurement period were critical components to our investigation. The results show that it is indeed possible to explore the relationship of ice stress and deformation through direct measurements. There was good agreement between stress and deformation events, but it was apparent that this relationship had a seasonal and scale dependence. For instance in the winter, internal ice stresses measured at the edge of an individual ice floe corresponded to deformational processes that occurred on a scale of 10 - 50 km. In the summer, the stresses in a floe were due to local interactions. The seasonal change in the relationship between stress and deformation reflects the changes in the continuity, extent, and thickness of the ice cover. This behavior is consistent with a granular plastic material, where the grains are defined by the individual ice floes and plasticity develops as the floes interact to form large aggregate plates of 20 - 150 km. We also observed that the movement of the position buoys validated the satellite-derived ice motion vectors. The stress and deformation events were Lagrangian in nature, as they occurred when the ice floes advected through relatively stationary stress fields. Using these techniques we can directly validate the internal ice stress parameters for sea ice dynamics models.

Integrated observations at SHEBA, D.K. Perovich, 1999 Meeting of the American Meteorological Society.

The Surface Heat Budget of the Arctic Ocean program (SHEBA) is directed towards understanding the ice-albedo and cloud radiation feedbacks, with an ultimate goal of improving climate models. These feedbacks are complex and depend on such factors as the incident radiation field, the cloud conditions, the atmosphere and ocean boundary layers and the ice morphology. An interdisciplinary approach using an integrated observation system is a central theme of the year-long SHEBA field experiment. For example, determining the distribution of incident solar radiation between reflection, absorption in the ice and transmission to the ocean entailed combining observations from more than a dozen radiometers. These radiometers were installed on the surface, mounted in aircraft, helicopters and balloons, frozen into the ice and lowered into the ocean. Several radiometers measured the temporal evolution of albedo, while others were used to conduct surface and aerial surveys investigating the spatial variability of albedo. To understand the observed variability in the distribution of sunlight it was necessary to combine the radiation observations with a description of the properties of the clouds, the ice and the upper ocean.

Thin and thinner: Mass balance measurements during SHEBA, D.K. Perovich, T.C. Grenfell, J.A. Richter-Menge, W.B. Tucker III, B. Light, H. Eicken, and G.A. Maykut, 1999 Fall Meeting of the American Geophysical Union.

As part of a large interdisciplinary study of the surface heat budget of the Arctic Ocean (SHEBA), we installed more than 100 ice thickness gauges in Fall 1997 to determine the sea ice mass balance. While installing these gauges we found that much of ice cover was only 1 m thick, considerably thinner than expected. Over the course of the year-long field experiment we monitored the mass balance for a wide variety of ice types including first year ice, ponded ice, unponded ice, multiyear ice, hummocks, new ridges, and old ridges. Initial ice thickness for these sites ranged from 0.3 m to 8 m and snow depths varied from a few centimeters to more than a meter. However, for all of their differences and variety, these thickness gauges sites shared a common trait; at every site there was a net thinning of the ice during the SHEBA year. The thin ice found October 1997 was even thinner in October 1998. The annual cycle of ice thickness was also similar at all locations. There was a steady increase in thickness through the winter that gradually tapered off in the spring. This was followed by a steep dropoff in thickness during summer melt and another tapering in late summer and early fall as freezeup began. Maximum surface melting was in June and July, while bottom ablation peaked in August. Combining results from the sites we found an average winter growth of 0.5 m and a summer melt of 1.05 m that consisted of 0.55 m of surface melt and 0.5 m of bottom melt. There was a weak trend for thicker ice to have less winter growth and greater net loss for the year, however ice growth was also impacted by the snow depth. Considerable variability was observed between sites in both accretion and ablation. Total accretion during the 9 month growth season ranged from zero for thick ridged ice to more than a meter for young ice. Ponds tended to have a large amount of surface melting, while ridges experienced considerable bottom ablation.

The annual temperature cycle of Arctic sea ice, B. Elder and D.K. Perovich, 1999 Fall Meeting of the American Geophysical Union.

Vertical profiles of ice temperature were measured at seven sites from October 1997 through September 1998 using thermistor strings and autonomous dataloggers. These sites included young ice, ponded ice, undeformed ice, a hummock, a consolidated ridge, and a new blocky ridge. All of the sites were within 5 km and had similar environmental forcing. In general, ice temperatures followed a pattern of 1) a cold front propagating down through the ice in the fall, 2) cold ice temperatures and ice growth in late fall, winter and early spring, and 3) warming to the freezing point in the summer. An exception to this pattern was an eight meter thick unconsolidated ridge where the cold front didn’t quite penetrate to the ice bottom during winter and where a portion of the interior remained below freezing during the summer. There was considerable horizontal variability in ice temperature during winter. For example, snow-ice interface temperatures varied by more than 10 C between sites. The coldest ice temperatures were observed in a consolidated ridge with a thin snowcover, while the warmest were in ponded ice. The warm pond temperatures were a result of two factors; the initial cooling in the fall was retarded by freezing of pond water and the depressed surface of pond was quickly covered by a deep layer of snow (0.6 m).

Reassessment of the Thermal Conductivity of the Seasonal Snow at SHEBA, M. Sturm, D.K. Perovich, J. Holmgren, 1999 Fall Meeting of the American Geophysical Union.

One hundred and one measurements of the thermal conductivity of the snow cover at SHEBA were made using a heated needle probe apparatus. These appear to be the first direct measurements of the thermal conductivity of snow measured on Arctic sea ice. Measurements were keyed to individual snow layers: 6 to 8 layers were present in most places within 20 km of the SHEBA ship. Measured values ranged from 0.285 ±0.118 W m-1 K-1 (n=32) for a dense, hard slab, to 0.080 ±0.039 W m-1 K-1 (n=24) for a low density porous depth hoar layer that was present near the base of the pack. The mean value for all measurements was 0.185 W m-1 K-1; several methods of computing a spatially-averaged bulk thermal conductivity value are possible and produce values ranging from 0.123 to 0.24 W m-1 K-1. Tests of the needle probe on standards and cross-comparison with a guarded hot-plate apparatus suggest the error in the measurements was ± 0.03 W m-1 K-1. Heat flow balance calculations using vertical temperature profiles from the ice and snow, measured at hourly intervals at four locations, suggest a bulk thermal conductivity in excess of 0.30 W m-1 K-1, and this value increased 30% or more during periods of stable, cold ambient temperature. This temperature-derived value is more consistent with the values traditionally used in modeling studies than the value we computed from direct measurements. Sets of snow-ice interface temperatures indicate lateral variations in excess of 5°C m-1 existed in many places around the SHEBA ship. These data suggest three possible explanations for the lack of consistency in the thermal conductivity values : 1) the needle probe or temperature measurements are in error, 2) natural and forced air convection in the snow pack increases the effective thermal conductivity of the snow, affecting the temperature measurements, but not the needle probe, or 3) natural irregularities in the snow cover, as evidenced by the large variation in interface temperature, account for a preponderance of the heat losses in the winter.

The Temporal and Spatial Variability of the Snow Cover at SHEBA, M. Sturm, J. Holmgren, D.K. Perovich, 1999 Fall Meeting of the American Geophysical Union.

The development of the snow cover at SHEBA was monitored using automatic depth sounders at two locations, and by measuring the depth directly along a 500-m traverse line weekly. These measurements indicate that the snow cover developed rapidly in the fall, and then slowly increased through the rest of the winter, reaching maximum depth in early-May. The snow pack consisted of six layers of snow; at the base there was a thin layer of snow-ice, above this were two layers of well-developed depth hoar. They were capped by a thick wind slab overlain by soft slabs and fine-grained snow. Depth hoar comprised 52% of the snow pack, based on 194 snowpits. The wind slab formed over a period of several days in early December, 1997, and structurally dominated the snow pack, suggesting the importance of a single weather event in the snow cover development. The spatial variability of the snow cover was investigated at maximum depth using a variety of means. At 40 locations, one hundred meter long profiles of the snow and ice surface were measured by laser leveling and depth probing. The snow-ice interface temperature was measured along many of these lines. Stratigraphy, density, and snow water equivalent were measured in snow pits dug at six places. An FM-CW radar was used to extend the lines as much as 600 m, and some cases, to produce strip maps of snow depth over areas covering several thousand square meters. In all more than 21,169 measurements of depth were made by hand; the mean depth was 33.7 cm (= 19.3 cm). The mean density was 0.32 g cm-3, and the mean water equivalent was 10.3 cm (n=362). Snow cover depth and other properties were highly variable, with local variability nearly as high as over the entire sampling area. Ice structures (ridges, melt ponds, blocks, etc.) produced variations in the depth and affected the snow stratigraphy in complex ways. Snow-ice interface temperature measurements suggest that this complexity may have important ramifications for heat transfer through the snow pack.

The Impact of Summer Ice Dynamics on theSurface Heat Budget of the Arctic Ocean, J.A. Richter-Menge, D.K. Perovich,T.C. Grenfell, B. Light,H. Eicken, 1999 Fall Meeting of the American Geophysical Union.

During the winter season sea ice dynamics dramatically change the ice cover, through the formation of pressure ridges and leads. These changes play an important role in the energy budget of the air-ice-ocean system. More subtle to recognize is the influence of ice dynamics on the energy budget during the summer months. In the summer, thermodynamic processes weaken the ice pack, allowing even modest dynamic events to break it apart. As the pack loosens, the assemble of ice floes move in a state of free drift with little floe-floe interaction. Results from the recent SHEBA experiment have demonstrated that even under these conditions, summer ice dynamics do cause dramatic changes in the characteristics of the ice-ocean matrix that affect the energy budget. To illustrate this, we present observations taken before and after a period of sustained, moderate winds in late July, which was preceded by an extended period of low winds. These conditions resulted in significant differential motion of ice floes in the vicinity of SHEBA. The measurements include the salinity and temperature profile of the upper ocean in open water areas; the mass balance of the ice cover; and the distribution of ice and open water. The data show that after the storm there was a significant change in the amount and distribution of open water areas; that the layer of warm, fresh water that had formed at the top of the water column during melt had become mixed; and that there was an increase in the rate of melting at the bottom side of the ice cover.

A Comparison Between Measured Stress and Calculated Stress in Arctic Pack Ice, M. Hopkins and J.A. Richter-Menge, 1999 Fall Meeting of the American Geophysical Union.

The Arctic ice pack is composed of an aggregate of thick multi-year ice, thinner undeformed first-year ice, ridged ice, and open water. Deformation of the ice pack is driven by wind and water drag, Coriolis acceleration, and inertia. When deformation occurs ice parcels interact to form pressure ridges, rafted ice, and leads. Measurement of the stress that propagates through the pack as a result of these deformation processes was an important goal during the recently completed SHEBA field experiment. Stress sensors were placed at many locations in the ice floes surrounding the SHEBA camp. Stresses measured at these sites have been correlated with strain measured concurrently by an array of drifting buoys. An important question that remains is how the point stresses measured by individual sensors relate to the global average stresses in the ice pack. The latter is related to the internal stress term, a important component in sea ice dynamics models. To help answer this question, we have performed simulations of the ice pack using a granular model. Each simulation was initiated with 2000 floes frozen together in an undeformed square. The deformation was driven by kinematic boundary motions from RGPS ice motion data derived from SAR imagery and wind stress measured at the SHEBA site. Forces are created in the model using a parameterization of the ridging process. The model is described and demonstrated at http://www.crrel.usace.army.mil/ierd/hop1/seaice.html. The simulation results indicate that stress propagation through the pack may be highly non-uniform. This is consistent with the insight gained by comparing the time series of in situ stress results between various measurements sites. We present a detailed comparison of measured and modeled point stress during several periods the SHEBA experiment and show how these point stresses relate to the average stresses determined by the simulation.