G. F. Cunningham

Ice, Cloud, and land Elevation Satellite (ICESat) over Arctic sea ice: Retrieval of freeboard

(1) Total freeboard (snow and ice) of the Arctic Ocean sea ice cover is derived using Ice, Cloud, and land Elevation Satellite (ICESat) data from two 35-day periods: one during the fall (OctoberNovember) of 2005 and the other during the winter (FebruaryMarch) of 2006. Three approaches are used to identify near-sea-surface tiepoints. Thin ice or open water samples in new openings, typically within 1� 2 cm of the sea surface, are used to assess the sea surface estimates. Results suggest that our retrieval procedures could provide consistent freeboard estimates along 25-km segments with uncertainties of better than 7 cm. Basin-scale composites of sea ice freeboard show a clear delineation of the seasonal ice zone in the fall. Overall, the mean freeboards of multiyear (MY) and first-year (FY) ice are 35 cm and 14 cm in the fall, and 43 cm and 27 cm in the winter. The increases of � 9 cm and � 12 cm on MY and FY sea ice are associated with the 4 months of ice growth and snow accumulation between data acquisitions. Since changes in snow depth account for >90% of the seasonal increase in freeboard on MY ice, it dominates the seasonal signal. Our freeboard estimates are within 10 cm of those derived from available snow/ice thickness measurements from ice mass balance buoys. Examination of the two residual elevations fields, after the removal of the sea ice freeboard contribution, shows coherent spatial patterns with a standard deviation (S.D.) of � 23 cm. Differencing them reduces the variance and gives a near random field with a mean of � 2 cm and a standard deviation of � 14 cm. While the residual fields seem to be dominated by the static component of unexplained sea surface height and mean dynamic topography (S.D. � 23 cm), the difference field reveals the magnitude of the time-varying components as well as noise in the ICESat elevations (S.D. � 10 cm).

Backscatter characteristics of the winter ice cover in the Beaufort Sea

The microwave backscatter statistics of the sea ice cover in the winter Beaufort Sea are characterized using C band synthetic aperture radar (SAR) data collected by the European Earth Resources Satellite 1. Sea ice backscatter signatures were sampled from SAR image data collected during the winter of 1991–1992 and winter of 1992–1993. The spatial and temporal variabilities of the backscatter signatures of different ice types are discussed. The results show considerable seasonal stability of the backscatter signature of multiyear ice as well as of first-year ice. Small amplitude regional variations of the multiyear ice backscatter can be observed. Consistent contrast between multiyear ice and first-year ice is maintained throughout the season. The highest radiometric variability is observed in sea ice in leads. On the basis of these observations the backscatter from the principal ice types (multiyear and first-year) are consistent with scatterometer observations and can be easily identified under winter conditions. Correlations of the regional variations of multiyear ice signatures to physical processes are suggested.

Area balance of the Arctic Ocean perennial ice zone: October 1996 to April 1997

We use National Aeronautics and Space Administration scatterometer (NSCAT), RADARSAT, and ice motion data to examine the perennial ice zone (PIZ) of the Arctic Ocean between October 1996 and April 1997. The PIZ is identified by a simple backscatter-based classification of the gridded NSCAT backscatter fields. The area of the PIZ at the beginning of October occupies an area of 5.32 × 106 km2, ∼76% of the Arctic Ocean. By the first of May, only 4.54 × 106 km2 of that area remains, a decrease of 0.78 × 106 km2 over the 7-month period. This area loss can be explained almost entirely by ice export. Over this period the total area flux of sea ice through Fram Strait, estimated using satellite passive microwave ice motion, is 0.80 × 106 km2 or 12% of the Arctic Ocean. Approximately 0.70 × 106 km2 or 88% of the exported area is from the PIZ. Nares Strait outflow is small at 34,000 km2 and is estimated by summing the high backscatter areas exported into northern Baffin Bay. After accounting for the outflow through the Fram and Nares Straits an unexplained residual of 46,000 km2 remains. We attribute this residual to errors in our estimation process, the unaccounted for ice flux through the Canadian Archipelago, and the net divergence and convergence of the PIZ over the period. Our interpretation of the radiometry seems to be sound on the basis of the area balance arguments provided here. This study shows that (1) the PIZ converage of the Arctic Ocean can be derived from NSCAT backscatter fields and (2) the decrease in PIZ area over the winter is a good estimate of the PIZ net ice area exported through Fram Strait.

A study of the onset of melt over the Arctic Ocean in RADARSAT synthetic aperture radar data

[1] The regional melt onset signal of Lagrangian elements of Arctic sea ice in RADARSAT synthetic aperture radar (SAR) data during the spring of 1998 is examined. The melt signal is clearly detectable not only in backscatter changes over multiyear ice but also first-year ice and mixtures of these ice types. This allows a more complete mapping of the progression of melt over the sea ice cover. For the spring of 1998 the onset dates range between 10 May and 29 June. The spatial pattern of melt onset is characterized by sharp boundaries delineating distinct regions with relatively uniform onset dates. This pattern appears to be associated with moisture and warm air brought in by a lowpressure trough and the timing of a rain event reported at the Surface Heat Budget of the Arctic Ocean (SHEBA) camp. Measurements from the SHEBA camp show good correspondence between the timing of melt in SAR imagery and the onset of albedo drop off associated with the beginning of summer. Onset dates are compared with the timing of the zero crossing of the temperature records from drifting buoys and the onset dates derived from satellite passive microwave brightness temperature fields. The timing of our estimates is within 1–2 days of the zero crossing of the buoy temperature records. Onset dates derived from passive microwave observations appear to be biased toward a later stage of melt. Comparison with results derived from a SAR data set from the spring of 1992 show that the onset of melt in 1998, derived from RADARSAT, in the Beaufort Sea occurred 2 weeks earlier. INDEX TERMS: 4540 Oceanography: Physical: Ice mechanics and air/sea/ ice exchange processes; 4207 Oceanography: General: Arctic and Antarctic oceanography; 4227 Oceanography: General: Diurnal, seasonal, and annual cycles; 4275 Oceanography: General: Remote sensing and electromagnetic processes (0689); KEYWORDS: melt onset, Arctic ice cover, radar remote sensing, Lagrangian ice motion

Ice lead orientation characteristics in the winter Beaufort Sea

The directional orientations of leads in the winter ice pack of the Beaufort Sea are studied both spatially and temporally. Data from the European Earth Resources Satellite-1 (ERS-1) synthetic aperture radar (SAR) was used. The SAR data was produced in image form at the Alaska SAR Facility (ASF) with 100m/spl times/100m pixel resolution. The lead ice pixels, which included all non-multiyear ice; were defined using a simple thresholding of the radar backscatter values. The orientations of the leads covering the Beaufort Sea during the period of January through March of 1992 were derived using a lead skeletonization technique. Results show a strong temporal persistence in the distribution and orientation of the leads during this period. The orientation of leads newly-formed within a 3-day period were then compared to both the local and large-scale ice deformation fields. Results show a consistent relationship between the orientations of the newly-formed leads and the principal direction of shear within the ice motion fields.<<ETX>>

Contributions of growth and deformation to monthly variability in sea ice thickness north of the coasts of Greenland and the Canadian Arctic Archipelago

Regional variability in monthly CryoSat-2 sea ice thickness is partitioned into contributions from dynamics and thermodynamics using ice deformation calculated from large-scale ice drift. For five winters (December to April, 2011–2015), over a region of persistent convergence north of the coasts of Greenland and the Canadian Arctic Archipelago, deformation explains ~34% of the overall variance (up to 69% in 2014/2015) in monthly thickness changes. Approximately 42–56% (or ~ 0.6 m) of the seasonal changes in mean regional ice thickness can be attributed to divergence and shear. The estimated area-averaged growth of 0.12 ± 0.03 m/month compares favorably with measurements from ice mass balance buoys. Examination of the time-variable thickness distributions shows areas covered by ice 3 m) increased. Albeit at fairly coarse resolution, this coupled analysis of thickness changes and deformation offered a first look at the character of the regional thickness redistribution process.

Preliminary Results From The Asf/gps Ice Classification Algorithm

The European Space Agency Remote Sensing Satellite (ERS-1) satellite carried a C-band synthetic aperture radar (SAR) to study the earths polar regions. The radar returns from sea ice can be used to infer properties of ice, including ice type. An algorithm has been developed for the Alaska SAR facility (ASF)/Geophysical Processor System (GPS) to infer ice type from the SAR observations over sea ice and open water. The algorithm utilizes look-up tables containing expected backscatter values from various ice types. An analysis has been made of two overlapping strips with 14 SAR images. The backscatter values of specific ice regions were sampled to study the backscatter characteristics of the ice in time and space. Results show both stability of the backscatter values in time and a good separation of multiyear and first-year ice signals, verifying the approach used in the classification algorithm.

Use of time series SAR data to resolve ice type ambiguities in newly-opened leads

The backscatter signature of sea ice in newly-opened leads frequently overlaps with that of older and thicker ice types. This phenomenon limits the accuracy of backscatter based ice type classification in single date SAR images. The authors use ice motion data derived from successive SAR observations to identify areas of recent openings in the winter sea ice cover. With the assumption that the backscatter of a new lead adds to a nominally invariant backscatter histogram, one can calculate the area of new ice which has been created and record the temporal evolution of backscatter of the new ice. This temporal signature is related to atmospheric conditions during the rapid growth phase of the new ice. They illustrate the use of time series information with ERS-1 SAR data from the Alaska SAR Facility.<<ETX>>

RADARSAT Geophysical Processor System: 2 years of production

The RADARSAT Geophysical Processing System has been processing geophysical data products at ASF and JPL since 1999. The RGPS data products provide an opportunity to improve our understanding of a myriad of processes that affect climate by generating a range of products over the Arctic at 3- and 6 day intervals. These processes include mass balance, heat transfer and momentum transfer between the Arctic Ocean and atmosphere. The RGPS Team is presently processing data acquired by RADARSAT during the winter of 1998-1999 for the seasonal ice zone and the Barents Sea. Data products from other seasons for the Arctic Basin are currently available on the web. We will summarize the some results from the products generated by the ASF RGPS and describe availability of this data set to the general community.