John J. Yackel

Local and regional albedo observations of arctic first‐year sea ice during melt ponding

The shortwave surface albedo is a critical climatological parameter for sea ice, especially during the spring melt period when the ice contains a mixture of highly reflective (snow) and absorptive (melt ponds) surfaces. Broadband and spectral albedo measurements were made over numerous surface types on first-year sea ice during the melt period in Wellington Channel, Nunavut during the Collaborative-Interdisciplinary Cryospheric Experiment 1997. Albedo measurements ranged from 0.75 (moist snow) to 0.21 (dark melt ponds) with many unique intermediate surfaces. Aircraft videography collected throughout Wellington Channel and Lancaster Sound was processed to reveal four main surface cover types (wet snow, mixed type, light ponds, and dark ponds). Surface albedo data were applied to the aircraft observations to upscale surface albedo measurements to regional scales. Aircraft video-derived regional albedo (mean = 0.55±0.02) were comparable to helicopter and satellite-derived (Advanced Very High Resolution Radiometer) albedo estimates.

Sea ice type and open water discrimination using dual co-polarized C-band SAR

We investigate the utility of dual co-polarized C-band synthetic aperture radar (SAR) imagery for discriminating sea ice types and open water during winter. We base our analysis on ENVISAT ASAR alternating vertical and horizontal polarization (VV, HH) medium-resolution imagery of thin sea ice, first-year sea ice, multiyear sea ice, and open water in the Canadian Arctic. We introduce a methodology for generating colour composite imagery (based on VV, HH, and co-polarization ratio channels) that substantially enhances visual discrimination. We statistically compare sampled vertical and horizontal backscatter from sea ice types and open water to assess backscatter magnitudes and polarization differences. The latter are presented as co-polarized ratio (VV/HH) values in decibels (dB). We conclude by presenting a decision-tree classifier using estimated statistical thresholds. Open water is unambiguously discriminated (>99% accuracy) from all sea ice types, except thin sea ice, using a co-polarized ratio threshold of 2 dB. Thin sea ice exhibits high variability in backscatter magnitude and polarization difference, making its discrimination from multiyear sea ice, rough first-year sea ice, and open water ambiguous. Thin sea ice is effectively discriminated (93% accuracy) from smooth first-year sea ice using a co-polarized ratio threshold of 1.3 dB. Smooth snow-covered first-year sea ice exhibits polarization differences that are attributed to volume scattering mechanisms within the brine-wetted snow cover.

Surface-Based Polarimetric C-Band Scatterometer for Field Measurements of Sea Ice

A portable surface-based polarimetric C-band scatterometer for field deployment over sea ice is presented. The scatterometer system, its calibration, signal processing, and near-field correction are described. The near-field correction is shown to be effective for both linear polarized and polarimetric backscatter. Field methods for the scatterometer are described. Sample linear polarized and polarimetric backscatter results are presented for snow-covered first-year sea ice (FYI), multiyear hummock ice, and rough melt pond water on FYI. The magnitude of backscatter signature variability due to system effects is presented, providing the necessary basis for quantitative analysis of field data.

Melt ponds on sea ice in the Canadian Archipelago: 1. Variability in morphological and radiative properties

The morphological and radiative properties of melt ponds on first-year sea ice (FYI) were investigated during the summer of 1997 in the Canadian Archipelago as part of the Collaborative-Interdisciplinary Cryospheric Experiment (C-ICE) near Resolute Bay, Nunavut. In this paper we (1) describe a classification technique used to identify surface cover types from airborne videography during the melt pond season, (2) use the classification results to examine the fractional coverage of surface types and morphological characteristics of melt ponds, and (3) provide an estimate of the integrated shortwave albedo of this surface. Cluster analysis on the videography data identified four distinct surface cover types during the summer melt season: snow, saturated snow, light-colored melt ponds, and dark-colored melt ponds. Melt pond coverage was found to be highly variable over our study area. We found that pond sizes tended to be twice as large in areas of high melt pond density compared to ponds in areas of low pond density owing to their interconnective nature. Video data also identified an elongated melt pond morphology pattern over the smoothest FYI within our study region. A derived estimate of the integrated shortwave albedo was strongly related to the fractional cover of snow on the surface (R2 = 0.86). An analysis of combined fractional coverage of light and dark melt ponds from two aerial survey dates (Julian Days 181 and 184) revealed an aerial increase in melt ponds of ∼10.3%.

A vessel transit assessment of sea ice variability in the Western Arctic, 1969–2002: implications for ship navigation

Recent investigations have shown reduced sea ice extents in Arctic regions and subsequently suggested that the Northwest Passage (NWP) might be able to sustain a prolonged shipping season. To date, no scientific evidence has been presented, within a ship navigation framework, to support increased marine traffic. The Arctic Ice Regime Shipping System (AIRSS) ice numeral (IN), which controls shipping activity in Canadian Arctic waters, was spatially assimilated with the Canadian Ice Service (CIS) historical digital database utilizing a geographic information system (GIS). INs provide a quantifiable framework for examining historical ice states in the context of ship navigation. Results provide a spatial and temporal assessment of ship navigation variability, within a ship transit framework from 1969 to 2002 for the western portion of the NWP. Feasible routes through the NWP experience extreme interannual variability in INs over the past 34 years. Yearly fluctuations of the IN can be attributed to the frequency of multiyear ice (MYI) encounters. The western coast of Banks Island experienced lower INs since 1991 and may be a potential barrier to completely navigating the NWP. Decreases in INs were also found to be associated with the positive signal of the arctic oscillation (AO) index. High-latitude MYI invasions into NWP shipping lanes appear be a major pitfall of future navigation routing in the face of climate warming.

Evaluation of C-band SAR polarimetric parameters for discrimination of first-year sea ice types

In this study, the classification potential of polarimetric parameters derived after Cloude–Pottier decomposition, Touzi decomposition, Freeman–Durden decomposition, normalized radar cross section measurements, phase differences, and statistical synthetic aperture radar correlation measures is evaluated by relating them to three pre-identified sea ice types and wind-roughened open water. A combined approach that constitutes a visual inspection of estimated probability densities of the polarimetric parameters and quantitative analysis using supervised classifications (k means and maximum likelihood) is adopted. Polarimetric parameters are iteratively combined in pairs and triplets to test for their ice type discrimination potential. Sensitivity of polarimetric parameters to radar incidence angle is also examined. Our results demonstrated strong but variable sensitivity of polarimetric parameters to different ice types, which was dependent on radar incidence angle. Results of parameter evaluation demonstrated that no single parameter discriminates significantly (>60%) between all the ice types considered in the study. Combining two low correlated parameters increased the classification accuracy by 10%–22%. Combining the third polarimetric parameter did not necessarily improve the classification results. However, the best classification results were achieved using a combination of three parameters.

On the estimation of spring melt in the North Water polynya using RADARSAT‐1

Abstract This paper evaluates the utility of time series RADARSAT‐1 Synthetic Aperture Radar (SAR) to detect melt onset (MO) and pond onset (PO) in the North Water (NOW) Polynya. SAR detected MO was associated with the approach of the daily average near‐surface air temperature (Tair) toward freezing, the beginning of a daily average surplus in the net all‐wave radiation flux (Q*), and a sharp decrease in the integrated shortwave albedo (α) at the surface. Relationships between microwave scattering and air, snow surface and ice surface temperature, up until MO, proved sensitive to the diurnal acquisition times of RADARSAT‐1. Time series results demonstrate that, despite its ‘uncalibrated’ status, RADARSAT‐1 ScanSAR data unambiguously identifies MO and PO over landfast First‐Year Ice (FYI) within the NOW Polynya. Spatially, results suggest a significantly earlier spring melt on the Greenland coast compared to the Ellesmere coast (10 to 15 days for regions between 77°N and 79°N and up to 45 days for regions between 76°N and 77°N). The regional near‐surface air temperature pattern was consistent with the spatial pattern of a SAR detected date of MO and PO.

Analysis of surface roughness and morphology of first-year sea ice melt ponds: implications for microwave scattering

Variations in wind forcing over summer first-year sea ice (FYI) melt ponds occur at hourly to weekly scales and are a significant contributor to microwave backscatter (/spl sigma//spl deg/) variability observed from spaceborne synthetic aperture radar (SAR) platforms (e.g., ENVISAT-ASAR and RADARSAT-1). This variability impairs our ability to use SAR to derive information on summer sea ice thermodynamic state and energy balance parameters such as albedo and melt pond fraction. The surface roughness contribution of FYI melt ponds in the Canadian Arctic Archipelago to like-polarized, C-band /spl sigma//spl deg/ estimates is analyzed through a spectral and statistical analysis of surface wave height profiles for varying wind speeds, upwind fetch lengths, and melt pond depths. A unique derivation of melt pond surface wave height spectra is presented based on digital video of melt pond surface wave trains. Significant scale surface roughness was observed even at wind speeds of 3 m/spl middot/s/sup -1/, resulting in small perturbation model estimates of /spl sigma//spl deg/ (HH) ranging from -5 dB at 20/spl deg/ incidence to -22 dB at 50/spl deg/ incidence. Results from a multivariate linear regression analysis show that 53.5% of observed variance in /spl sigma//spl deg/ (HH or VV) can be explained by wind speed, upwind fetch from melt pond edges, and melt pond depth, with no appreciable difference in the relative contribution of explanatory variables. Modeled omnidirectional /spl sigma//spl deg/ as a function of wind speed and incidence angle for 100-m transects collected throughout the melt pond season act to elaborate the role of fetch and depth, as well as the modulating effect of hummocks, on /spl sigma//spl deg/.

Observations of Snow Water Equivalent Change on Landfast First-Year Sea Ice in Winter Using Synthetic Aperture Radar Data

In this paper, we examine the utility of synthetic aperture radar (SAR) backscatter data to detect a change in snow water equivalent (SWE) over landfast first-year sea ice during winter at relatively cold temperatures. We begin by reviewing the theoretical framework for linking microwave scattering from SAR to the thermodynamic and electrical properties of first-year sea ice. Previous research has demonstrated that for a given ice thickness and air-temperature change, a thick snow cover will result in a smaller change in the snow-ice interface temperature than will a thin snow cover. This small change in the interface temperature will result in a relatively small change in the brine volume at the interface and the resulting complex permittivity, thereby producing a relatively small change in scattering. A thin snow cover produces the opposite effect-a greater change in interface temperature, brine volume, permittivity, and scattering. This work is extended here to illustrate a variation of this effect over landfast first-year sea ice using in situ measurements of physical snow properties and RADARSAT-1 SAR imagery acquired during the winter of 1999 in the central Canadian Archipelago at cold (~-26degC) and moderately cold (~-14degC) snow-sea-ice interface temperatures. We utilize in situ data from five validation sites to demonstrate how the change in microwave scattering covaries and is inversely proportional with the change in the magnitude of SWE. These changes are shown to be detectable over both short (2 days) and longer (45 days) time durations

On the utility of SeaWinds/QuikSCAT data for the estimation of the thermodynamic state of first-year sea ice

The thermodynamic state of sea ice is important to accurately and remotely monitor in order to provide improved geophysical variable parameterizations in sea ice thermodynamic models. Operationally, monitoring the thermodynamic state of sea ice can facilitate eased ship navigation routing. SeaWinds/QuikSCAT (QuikSCAT) dual-polarization [i.e., horizontal (HH) and vertical (VV)] active microwave data are available at a sufficiently large spatial scale and high temporal resolution to provide estimates of sea ice thermodynamics. This analysis evaluated the temporal evolution of the backscatter coefficient (/spl sigma//spl deg/) and VV/HH copolarization ratio from QuikSCAT for estimating sea ice thermodynamics. QuikSCAT estimates were compared against RADARSAT-1 synthetic aperture radar (SAR) imagery and the Canadian Ice Service (CIS) prototype operational ice strength algorithm. In situ data from the Collaborative Interdisciplinary Cryospheric Experiment (C-ICE) for 2000, 2001, and 2002 were used as validation. Results indicate that the temporal evolution of /spl sigma//spl deg/ from QuikSCAT is analogous to RADARSAT-1. The QuikSCAT /spl sigma//spl deg/ temporal evolution has the ability to identify winter, snow melt, and ponding thermodynamic states. Moreover, the copolarization VV/HH ratio of QuikSCAT could provide a second estimate of the ponding state in addition to identifying the drainage state that is difficult to detect by single-polarization SAR. QuikSCAT detected thermodynamic states that were found to be in reasonable agreement to that of in situ observations at the C-ICE camp for all years. Operational implications of this analysis suggest QuikSCAT is a more effective and efficient medium for monitoring ice decay compared to RADARSAT-1 and can be utilized to provide more robust modeled ice strength thresholds.