R.G. Onstott

Electromagnetic and physical properties of sea ice formed in the presence of wave action

Estimating the magnitude of brine flux to the upper ocean requires an ability to assess the dynamics of the formation of sea ice in a region. Brine storage and rate of expulsion is determined by the environmental conditions under which the sea ice forms. In this paper, the physical and electromagnetic properties of sea ice, formed under wave-agitated conditions, are studied and compared with results obtained from ice formed under quiescent conditions. Wave agitation is known to have a profound effect on the air-ice interface and internal ice structure. A variety of sensors, both active and passive, optical and microwave, were used to perform this characterization. Measured electromagnetic parameters included radar backscatter, microwave emission, and spectral albedo in the visible and infrared. Measured physical properties included ice structure, brine and temperature distribution, profiles of the vertical height of the air-ice interface, and ice formation processes. Results showed that emission, backscatter, and albedo all take different signature paths during the transformation from saline water to young sea ice and that the paths depend on sea surface state during ice formation.

Characterization Of Intrinsic Optical Properties For Sea Ice And Snow

A narrow He1ium:Neon (633nm) laser was used to measure transmission loss and spreading in thin samples of sea ice and snow collected from the Beaufort Sea during LEADEX ’91 and from sites in the high Arctic during March 1987. The sea ice types sampled include first year, multiyear, melt pond, and various stages of young sea ice in the thickness range of 2 to 20 centimeters. Snow samples include old snow from multiyear floes as well as fresher snow from first year ice types. These data were analyzed to characterize intrinsic beam transmission and small angle scattering optical properties. In sea ice the latter scattering properties are highly dependent on the intricate structure of air bubbles, brine channels, and internal platelet boundaries and considered important to the estimation of the volume scattering function and visible radiative transfer through the Arctic sea ice cover. In this technique beam spreading is first used to derive the modulation transfer function (MTF) using a Fourier-Bessel transform. The spatial frequency decay function derived from the MTF can be used to estimate the volume scattering function for small angles. For thin snow and many sea ice samples the beam spread functions could be characterized as having Gaussian shape and therefore easily parameterized for the numerical transforms. Beam transmission measurements derived from snow and sea ice samples compared favorably with previously derived extinction estimates.

Study of the relationship between the scale of sea ice deformation and radar backscatter intensity using ERS-1 SAR

Knowledge of the relationship between the backscatter response and physical properties of deformed sea ice is important in documenting the information which may be retrieved using synthetic aperture radar (SAR). Investigations have been conducted in the use of SAR to assess the state of deformation of Arctic sea ice and the robustness of the inversion of the SAR signatures to estimates of deformation state, ice thickness, and ridge height. This paper presents relevant issues and illustrates SAR signatures for a variety of deformed ice cases.

Study of the polarization behaviour of complex natural and man-made clutter at middle and grazing angles

A special mapping mission using an aircraft synthetic aperture radar (SAR) was conducted at an airport near a large metropolitan area to characterize the clutter found in association with airports and surrounding areas. These data were obtained to support the building of a simulator for design of future generation radars which will operate from commercial aircraft to detect meteorological features. In addition, these data document the polarization and angular response behavior for a radar parameter region and clutter environment rarely studied. Polarization and angle response behaviors are linked to land use patterns, a topic of increasing importance for rapidly developing communities and countries with large land area.

Results of satellite and in-situ remote sensing measurement and modeling studies of Arctic sea ice which support the monitoring of changes in the global climate

Coordinated satellite, in situ monitoring, and theoretical modeling studies of Arctic sea ice have been supported by both the Office of Naval Research and the National Aeronautic and Space Administration over the last 2 decades. Application of the results from these efforts is being applied to the monitoring of changes in the global climate. Two geophysical parameters are now retrieved on an operational basis: the extent of ice covered waters, and the fraction of multiyear ice, first year ice and open water. Efforts are on-going to increase the number of ice type categories and to invert signal statistics (e.g., emission and backscatter) to ice thickness. Knowledge of the distribution of ice thickness is critical to improving the ability to directly measure a geophysical response in the polar regions due to small changes in the global climate.

Polarimetric properties of simulated sea ice with special focus on property retrieval and important scattering processes

To improve the ability to retrieve geophysical properties using remote sensing sensors and model the electromagnetic properties of sea ice, an intensive, well-integrated multidisciplinary program is sponsored by the Office of Naval Research to study simulated and natural sea ice of a variety of forms. As part of this effort the author has made polarimetric scattering measurements over a wide range of micro- and millimeter-wave frequencies (e.g. 0.5 to 95 GHz) and incidence angles (0/spl deg/ to 70/spl deg/ with fine angle resolution). An additional aspect of these measurements is that they have been made with fine temporal resolution to allow correlation with small changes in ice sheet properties, in the air-ice interface, and meteorological properties. Results from this work are used to demonstrate how polarimetric radar data at a single frequency, or a suite of frequencies may be used to assess sea ice age, state, and thickness.

Characterization of ice in the Chukchi Sea at the start of the growing season using satellite SAR

The distribution of ice forms and thickness is believed to be important to the study and monitoring of changes in our global environment. Distribution information is known to vary with region. Satellite and in-situ observations were conducted in the Chukchi Sea with the intent to learn more about sea ice physical and microwave properties during the period of transition from summer to winter. Comparisons are made with observations from a well studied region, the Beaufort Sea. A regional knowledge of sea ice properties and microwave behaviour is important to the application of SAR remote sensing methods to retrieve geophysical properties.

Prediction of radar backscatter for gravity-capillary waves with precise spatial and temporal measurements

Water waves were generated mechanically in the laboratory to investigate the electromagnetic response for a roughened air-water interface. A FM-CW radar operating at 3 cm wavelength was used to monitor the water surface while precise concurrent nonintrusive surface elevation profiles were recorded using high-speed video imaging. A wavemaker produced wave frequencies of 6-12 Hz in increments of 0.05 Hz, a regime replete with internal resonances of the water waves. Precise surface and radar backscatter measurements facilitate the direct comparison between surfaces of known roughness properties and scattered fields. An analytical method was used to predict Bragg scattering. Measured versus predicted radar backscatter results show the ability to predict radar backscatter precisely.

Use of satellite remote sensing to monitor lead dynamics

How much radiation is reflected or absorbed at the Earths surface is important for use in global climate models. The Earths energy balance is determined by the spectral properties of the atmosphere and terrain surfaces. In the polar ocean, this balance is determined by the presence of open water, the thickness of sea ice, and the thickness of snow cover. Snow-covered ice causes much of the incident energy to be reflected; while, in the case of open water, a significant proportion is absorbed. Detection of and monitoring the openings in the polar pack during winter is important in assessing the coverage of open water and the production of new ice. This information is required in the study of the air-ice-ocean processes and the determination of an important contributor to ocean-atmosphere heat flux and ice-ocean brine flux. In this paper, the authors utilize multisensor observations to assess changes during the formation and closure of lead systems.

Microwave study of the formation of brine layers on homogeneous saline ice sheets

When sea water freezes into sea ice, brine and pure ice are produced. Due to the effects of the expulsion of brine from the sea ice interior and the wicking action of snow and frost flowers deposited on the air-ice interface, brine may accumulate on the upper ice sheet surface. Brine is a concentrated solution of sea salts and water, hence is highly lossy and has a large dielectric constant. Sea ice which is several hours old transforms to a layer which, as a bulk dielectric constant, is considerably less than that of the sea water from which it was born. Questions arise as to the impact of the surface brine layer. Potentially, the air-ice interface the reflectivity and transmission characteristics of this upper most ice layer may be perturbed. The purpose of this work is to quantify the importance of a brine layer on an ice surface, the type of changes in the microwave signature which may occur, and the expected signal dynamics. Predicted results are produced by the integration of an n-layer reflectivity model for use in determining the effective reflectivity of the air-brine-ice-water system, a sea-ice electrical-property model for predicting the ice sheet permittivity profile based on the temperature and salinity profile, an electrical-property model for brine, and EM models based on Kirchoff methods to predict backscatter for a rough surface. These results are compared with in-situ observations conducted during the recent Office of Naval Research Accelerated Research Initiative of the Electromagnetics of Sea Ice. Model predictions and measured data extend the microwave and millimeter-wave (0.5 to 95 GHz) portion of the electromagnetic spectrum.<<ETX>>