Snow-covered Sea Ice Research Articles

Comparison of <i>In Situ</i> and AVHRR-Derived Broadband Albedo over Arctic Sea Ice

Advanced Very High Resolution Radiometer (AVHRR) data are used to extract broadband sea ice surface albedos from narrowband channel 1 and 2 top of the atmosphere (TOA) radiances. Corrections for the intervening atmosphere, viewing geometry and sensor spectral response are applied to the satellite data. Atmospheric correction increases TOA albedos by 27 to 32%. After removing the effects of viewing geometry, surface albedo variability between orbits decreases. The satellite-derived surface albedo over snow-covered sea ice corrected for viewing geometry ranged from 0.68 to 0.82. The residual diurnal variability is attributed to uncertainties in the atmospheric and anisotropic corrections of the satellite data. After comparison with coincidental in situ measurements, AVHRR pixel. In order to develop a reliable methodology for using these satellite data to derive sea ice albedo, an improved understanding of both the atmosphere's behavior over the long path lengths common to the Arctic and the anisotropic nature of snow-covered sea ice reflectance is required. Furthermore, any seasonal characteristics of these factors must be addressed. Key words: sea ice, albedo, remote sensing, AVHRR, anisotropy

Theoretical approach describing the thermal regime of snow-covered sea ice

As the greater part of sea-ice area is covered with snow, the thermal regime of sea ice is characterized by the thermal behavior of snow-covered sea ice. In this paper the thermal regime of snow-covered sea ice is quantitatively investigated with a one-dimensional non-linear boundary model which contains: compaction of snow cover; internal absorption of solar radiation; evaporation–condensation within snow cover; equilibrium phase change of brine within sea ice; and vertical oceanic heat flux from seawater to ice. Penetration of air temperature oscillations into the snow-covered sea ice increases remarkably with increasing snow density. As internal melting within the snow-covered sea ice appears with increasing solar radiation, the rise in air temperature and increase of solar radiation in the springtime produce a corresponding change in the thermal state of sea ice, causing a drastic retreat of sea-ice cover. A case study for warm sea ice is presented describing the thermal state during the melting season.

Theoretical approach describing the thermal regime of snow-covered sea ice

As the greater part of sea-ice area is covered with snow, the thermal regime of sea ice is characterized by the thermal behavior of snow-covered sea ice. In this paper the thermal regime of snow-covered sea ice is quantitatively investigated with a one-dimensional non-linear boundary model which contains: compaction of snow cover; internal absorption of solar radiation; evaporation–condensation within snow cover; equilibrium phase change of brine within sea ice; and vertical oceanic heat flux from seawater to ice. Penetration of air temperature oscillations into the snow-covered sea ice increases remarkably with increasing snow density. As internal melting within the snow-covered sea ice appears with increasing solar radiation, the rise in air temperature and increase of solar radiation in the springtime produce a corresponding change in the thermal state of sea ice, causing a drastic retreat of sea-ice cover. A case study for warm sea ice is presented describing the thermal state during the melting season.

Scattering of electromagnetic waves by a two-layer random medium

The authors discuss the scattering of electromagnetic waves by a two-layer random medium. They derive the analytic and closed expressions of like and cross-polarized backscattering coefficients from the field equations. The theoretical results are in good agreement with the experimental data obtained in the case of snow-covered sea ice. The 'polarized backscattering ratio law' is demonstrated. Significantly, the theory applies to electromagnetic waves at all wavelengths and scatterers of various shapes if the T matrix can be calculated by average field theories.

A scattering model for snow-covered sea ice

The special properties of a robust radiative transfer model for scattering from layers of inhomogeneous rough-boundary slabs are presented. The model is applied to backscattering from saline and desalinated ice. Comparisons are made at single and multiple frequencies with some of the most complete sets of measurement data available, using measured physical and electrical characteristics of the ice as inputs to the model where possible. The results show close agreement. For example, for the saline ice backscatter data set, which consisted of measurements at two like and two cross polarizations at 5 and 13.9 GHz, the agreement with model predictions is within 2 dB except at 13.9-GHz cross polarization. Backscattering from >15-cm-thick saline ice is generally dominated by scattering from the top surface while backscattering from <8-cm-thick saline ice can be strongly influenced by returns from the ice/water interface, particularly at frequencies less than about 5 GHz.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Seasonal and diurnal variations in SAR signatures of landfast sea ice

The seasonal and diurnal changes in total distributions of three fast ice sites, validated during the Seasonal Sea Ice Monitoring Site (SIMS '90) experiment, are presented. Rationale for the observed changes are provided using previous scatterometer and SAR observations of the seasonally varying snow-covered sea ice surface. The implications of the stability and rapidity of the observed changes are considered important in the development of semiautomated feature extraction algorithms, required by the operational sea ice community, and in monitoring the seasonal evolution of a snow-covered sea ice surface. >

Spatial and Temporal Variation of Sea Ice Geophysical Properties and Microwave Remote Sensing Observations: The SIMS'90 Experiment

In this paper we present results from a sea ice field experiment conducted coincidentally with overflights of orbital and aerial remote sensing instrumentation in Resolute Passage and Barrow Strait, Northwest Territories, Canada. Our principal focus is to describe the spatial and temporal distribution of selected geophysical variables in the context of how microwave energy interacts with this seasonally varying snow-covered sea ice surface. Over the duration of the experiment, snow crystal size, structure, and snow volume salinities changed sufficiently to affect synthetic aperture radar (SAR) scattering; thermal profiles through the snow cover were diurnally driven; ice surface microscale roughness increased due to sublimation of water vapour from the snow pack onto the ice surface; and bulk ice surface; and bulk ice salinities did not change. Results from the SAR data analysis indicate that the geophysical structure of multiyear ice created a larger and more rapid change in the seasonal SAR scattering signature than did the structure for early consolidated smooth first-year ice. These results are considered fundamental to measurement and monitoring of the seasonal evolution of the snow-covered arctic sea ice surface using SAR remote sensing.Key words: snow, sea ice, synthetic aperture radar, seasonal evolution, remote sensing

Albedo Of Snow, Ice Sheets and Snow-Covered Sea Ice In General Circulation Models

We have developed a physically-based parameterization for snow albedo, for the visible and near-infrared spectral regions used in general circulation models (GCMs). Snow albedo depends primarily on snow grain size, and also on solar zenith angle, snow thickness, impurity content, and atmospheric transmittance. This parameterization is now available as a Fortran subroutine. Simpler, but less accurate, parameterizations have also been developed which depend only on grain size or thickness. Since GCMs do not compute snow grain size, we also developed a method to estimate grain size based on the air temperature and the snow age.Our parameterization for snow albedo is being incorporated in the NCAR Community Climate Model (CCM) in place of the existing empirical parameterization for snow albedo, to determine the effect of this improvement on the model's performance, and the results will be discussed. However, additional aspects of the treatment of the radiative properties of snow and ice were also capable of improvement and are being changed in the CCM. In particular, it is important to recognize that sea ice is often snow-covered and in that case has an albedo as high as that of snow, and that southern hemisphere sea ice is nearly always snow-covered, even through the melting season. The surface albedo for the Antarctic ice sheet should be about 0.83, but it had been set to 0.71 in the CCM, The CCM has been calculating temperatures too warm over Antarctica, and this low albedo contributed to that error.

Albedo Of Snow, Ice Sheets and Snow-Covered Sea Ice In General Circulation Models

We have developed a physically-based parameterization for snow albedo, for the visible and near-infrared spectral regions used in general circulation models (GCMs). Snow albedo depends primarily on snow grain size, and also on solar zenith angle, snow thickness, impurity content, and atmospheric transmittance. This parameterization is now available as a Fortran subroutine. Simpler, but less accurate, parameterizations have also been developed which depend only on grain size or thickness. Since GCMs do not compute snow grain size, we also developed a method to estimate grain size based on the air temperature and the snow age. Our parameterization for snow albedo is being incorporated in the NCAR Community Climate Model (CCM) in place of the existing empirical parameterization for snow albedo, to determine the effect of this improvement on the model's performance, and the results will be discussed. However, additional aspects of the treatment of the radiative properties of snow and ice were also capable of improvement and are being changed in the CCM. In particular, it is important to recognize that sea ice is often snow-covered and in that case has an albedo as high as that of snow, and that southern hemisphere sea ice is nearly always snow-covered, even through the melting season. The surface albedo for the Antarctic ice sheet should be about 0.83, but it had been set to 0.71 in the CCM, The CCM has been calculating temperatures too warm over Antarctica, and this low albedo contributed to that error.

A method for measuring snow depth using fine-resolution radars

A new method for determining snow depth over a horizontal surface uses the fading statistics for a radar that has excess range bandwidth. The number of independent samples per measurement with such a radar depends on the depth of the snow contributing to volume scatter. With deeper snow, and thus more volume scatter, the variance of the signals is reduced. Thus, the variance may be used to estimate the snow depth. Dry snow is required so that the volume scatter from within is not attenuated significantly. The radar used in such a measurement must have a bandwidth considerably greater than that required for the range resolution used. Such a radar could be deployed on a helicopter or low-flying aircraft, as well as on a sled. The method was tested using a wide-band beamlimited FMCW scatterometer mounted on a sled pulled over snow-covered sea ice. Although only a single test was possible, the results indicate the validity of the method.

Review of signature studies for microwave remote sensing of snowpacks

A long-term program of microwave-signature studies at the alpine test site, Weissfluhjoch (2550m altitude), was completed in 1987. Besides passive microwave data at frequencies between 5 and 100 GHz backscatter data at 10 GHz were collected together with dielectric and structural properties as well as classical snow data. The same instrumentation was used from icebreakers to measure snow-covered sea ice. Complementary snow signatures were recently obtained by a new multichannel (1–12 GHz) radiometer-scatterometer at lower-altitude test sites for taking into account possible spatial variability of snow properties, and for extending the spectral coverage to lower frequencies. The results include algorithms for classifying snow types, for mapping snow, for determining the liquid water content of the snow surface, for monitoring melt and refreeze cycles, for estimating energy loss and water equivalent, respectively. We conclude by an updated definition of an optimum snow sensor system.

Optical studies of Antarctic sea ice

During the austral winter 1986 the spectral albedo of both snow-covered and bare sea ice was measured over the wavelength range 400–1350 nm in the eastern Weddell Sea. The albedo shows high values and a week wave-length dependence in the visible part of the spectrum and a strong decrease with increasing wavelength in the near IR. Maxima in albedo correspond with minima in ice absorption. For relatively thin ice the albedo depends strongly on ice thickness. Additionally the bidirectional reflectance of sea ice was studied in the laboratory. For ice thicknesses of a few millimeters nearly specular reflectance is found. With growing ice thickness more scattering within the ice occurs and consequently the intensity of the sidewards scattered radiation increases. The reflectance distribution shows a peak in the forward direction which is less developed with thicker ice and smaller crystal size.

Modelling Changes in Scattering Properties of the Dielectric and Young Snow-Covered Sea Ice at GHz Requencies

Abstract Observations of the physical properties of the snow cover and underlying young fast ice in Resolute Passage, N.W.T., Canada, were made during the winter of 1982. Detailed measurements of snow density, and ice and snow temperatures, salinities, and brine volumes were made over a period of 46 d, beginning when the ice was 0.4 m thick and about 8 d old. The recorded values are used in a theoretical mixture model to predict the dielectric properties of the snow cover over the microwave frequency range. The results of this analysis are then used to investigate the effects of the snow properties on the radar back-scatter signatures of young sea ice. The results show that back-scatter is a function of the incidence angle and can change significantly over short periods of time during the early evolutionary phase of ice and snow-cover development. This has important consequences for the identification of young ice forms from Synthetic Aperture or Side Looking Airborne Radar images.

Modelling Changes in Scattering Properties of the Dielectric and Young Snow-Covered Sea Ice at GHz Requencies

AbstractObservations of the physical properties of the snow cover and underlying young fast ice in Resolute Passage, N.W.T., Canada, were made during the winter of 1982. Detailed measurements of snow density, and ice and snow temperatures, salinities, and brine volumes were made over a period of 46 d, beginning when the ice was 0.4 m thick and about 8 d old. The recorded values are used in a theoretical mixture model to predict the dielectric properties of the snow cover over the microwave frequency range. The results of this analysis are then used to investigate the effects of the snow properties on the radar back-scatter signatures of young sea ice. The results show that back-scatter is a function of the incidence angle and can change significantly over short periods of time during the early evolutionary phase of ice and snow-cover development. This has important consequences for the identification of young ice forms from Synthetic Aperture or Side Looking Airborne Radar images.