Frost Flowers Research Articles

Search for proxy indicators of young sea ice thickness

The determination of young sea ice thickness from space remains an elusive goal for those interested in the interaction of the oceans and the atmosphere, the thermal and chemical state of the ocean, and sea ice dynamics. Recent experiments and models have shown relationships between active and passive microwave signatures of new, growing ice and ice thickness. The two processes that dominate in determining the microwave signature are changes in dielectric properties and changes in surface roughness. In this paper we investigate the competition between these two processes in determining radar backscatter, the usefulness of surface roughness as an indicator of young ice thickness, and the optimum sensor parameters for observing changes in scattering linked to ice thickness. We present simulations that are based on radar observations made on laboratory‐grown saline ice. These observations confirm that surface scattering dominates over volume scattering for 13.9 GHz radar backscatter from young, rough ice at most angles and for young, smooth ice below 30°. Although rms roughness and backscatter (at 5.3 and 13.9 GHz, 23° incidence, and VV polarization) increase together after about 10 cm of ice growth under quiet conditions, it is unlikely that surface roughness and ice thickness are simply connected in real sea ice, where surface roughness can change rapidly due to the action of wind, waves, and snow. Simulations show, however, that formation of frost flowers is detectable by spaceborne radar and can serve to classify ice of roughly 5–20 cm thickness since it is a distinct, transient event that occurs under physical conditions that constrain the thickness of the ice. Our experimental data show that future sensors operating near 12° incidence may offer potential for probing the relationship between near‐surface dielectric properties and ice thickness, since the effects of variability in roughness and snowfall are minimized near this angle.

EtTect of frost flowers, rough saline snow and slush on the ERS-l SAR backscatter of thin Arctic sea-ice

Abstract We present ERS-l Synthetic Aperture Radar (SAR) backscatter measurements and scattering model calculations based on in situ data for thin Arctic sea ice covered with frost flowers, rough saline snow, or slush. The data were acquired in September–October 1991 during the ARCTIC–91 expedition when the air temperature dropped from O°C to –16°C. The ERS–I SAR signatures have a large variability and change rapidly due to environmental conditions. Rough and wet saline snow gave the highest backscattering coefficient of –6dB, whereas smooth slush gave the lowest of –16dB. Newly formed frost flowers gave an intermediate value of –14dB. Application of the backseattering model shows that surface scattering dominates in all cases, except possibly for new frost flowers. Two cases were found where the measured surfaces were too rough for the single scattering model to be used, which calls for multiple scattering effects to be included. Discrepancies between the model and measurements in the remaining cases wer...

Polarimetric scattering from layered media with multiple species of scatterers

Geophysical media are usually heterogeneous and contain multiple species of scatterers. In this paper a model is presented to calculate effective permittivities and polarimetric backscattering coefficients of multispecies‐layered media. The same physical description is consistently used in the derivation of both permittivities and scattering coefficients. The strong permittivity fluctuation theory is extended to account for the multiple species of scatterers with a general ellipsoidal shape whose orientations are randomly distributed. Under the distorted Born approximation, polarimetric scattering coefficients are obtained. These calculations are applicable to the special cases of spheroidal and spherical scatterers. The model is used to study effects of scatterer shapes and multispecies mixtures on polarimetric signatures of heterogeneous media. The multispecies model accounts for moisture content in scattering media such as snowpack in an ice sheet. The results indicate a high sensitivity of backscatter to moisture with a stronger dependence for drier snow and ice grain size is important to the backscatter. For frost‐covered saline ice, model results for bare ice are compared with measured data at C band and then the frost flower formation is simulated with a layer of fanlike ice crystals including brine infiltration over a rough interface. The results with the frost cover suggest a significant increase in scattering coefficients and a polarimetric signature closer to isotropic characteristics compared to the thin saline ice case.

A laboratory study of frost flower growth on the surface of young sea ice

Frost flowers are fragile ice crystals containing salt which grow to a height of 10–30 mm on the surface of young sea ice. Such flowers are observed all over the Arctic. The importance of the flowers and their accompanying slush layer is that they provide a rapid way to change the surface albedo and increase the surface roughness of young sea ice. This paper describes a laboratory technique for growing frost flowers and the physical processes which accompany the growth. The study was carried out in a saltwater tank located in a cold room. To grow frost flowers, we alternately cool the surface of the growing sea ice with a fan, then supply it with water vapor from a vaporizer. For these conditions and a room temperature of −22°C, the frost flowers begin to grow when the ice thickness reaches 5–8 mm. The flowers form at random locations on the ice and grow vertically to a height of 10–15 mm while spreading laterally from their original sites. Beneath the flowers, the surface is initially dry; then as the flowers spread laterally, a high‐salinity slush layer forms beneath them. This layer, which forms only under the flowers, grows to a thickness of 5 mm in 48 hours and has a characteristic lateral scale of 100–200 mm. The salinity of the slush layer is about 80 psu, compared with a frost flower salinity of 100 psu. Within 24 hours of their appearance, the flowers grow to cover 75–90% of the surface. A surface water budget for the flowers and slush layer shows that most of the water in the flowers and slush layer comes from the ice interior, not from the vaporizer. This implies that an external vapor source may be important in determining the initial growth of the flowers but not in their subsequent development.

Feasibility of sea ice typing with synthetic aperture radar (SAR): Merging of Landsat thematic mapper and ERS 1 SAR satellite imagery

ERS 1 synthetic aperture radar (SAR) and Landsat thematic mapper (TM) images were acquired for the same area in the Beaufort Sea, April 16 and 18, 1992. The two image pairs were colocated to the same grid (25‐m resolution), and a supervised ice type classification was performed on the TM images in order to classify ice free, nilas, gray ice, gray‐white ice, thin first‐year ice, medium and thick first‐year ice, and old ice. Comparison of the collocated SAR pixels showed that ice‐free areas can only be classified under calm wind conditions (<3 m s−1) and for surface winds >10 m s−1 based on the backscattering coefficient alone. This is true for pack ice regions during the cold months of the year where ice‐free areas are spatially limited and where the capillary waves that cause SAR backscatter are dampened by entrained ice crystals. For nilas, two distinct backscatter classes were found at −17 dB and at −10 dB. The higher backscattering coefficient is attributed to the presence of frost flowers on light nilas. Gray and gray‐white ice have a backscatter signature similar to first‐year ice and therefore can not be distinguished by SAR alone. First‐year and old ice can be clearly separated based on their backscattering coefficient. The performance of the Geophysical Processor System ice classifier was tested against the Landsat derived ice products. It was found that smooth first‐year ice and rough first‐year ice were not significantly different in the backscatter domain. Ice concentration estimates based on ERS 1 C band SAR showed an error range of 5 to 8% for high ice concentration regions, mainly due to misclassified ice‐free and smooth first‐year ice areas. This error is expected to increase for areas of lower ice concentration. The combination of C band SAR and TM channels 2, 4, and 6 resulted in ice typing performance with an estimated accuracy of 90% for all seven ice classes.

Surface characteristics of lead ice

Under cold conditions, ice growth in a lead occurs rapidly, giving young sea ice a high bulk salinity. The surface characteristics of this ice type are dominated by a thin, highly saline surface skim and often by the formation of frost flowers. These surface characteristics are of particular interest because of their significant temporal variability and large impact on the electromagnetic properties of sea ice. As part of the Lead Experiment held during March and April of 1991 and 1992 in the Alaskan Beaufort Sea, ice properties and surface conditions typical of springtime leads were monitored at three sites during the initial few days of growth. Observations indicate that once the ice thickness reached approximately 2 cm, a thin (∼1 mm), highly saline (∼100 parts per thousand (ppt)) skim of brine formed on the surface. After only a few hours of growth the initially smooth surface of the sea ice developed some small‐scale roughness. Frost flowers, the result of ice grown from the vapor phase, quickly formed on the surface of the sea ice and continued to develop during the observations. Depending on the temperature, the frost flowers were composed of various ice crystal types, including clumps, stellar dendrites, and needles. The initially fresh frost flowers quickly became salty, reaching salinities as high as 100 ppt. The salinity of both the frost flowers and the surface skim decreased at night as temperatures dropped and additional hoarfrost accumulated on the surface. These decreases were also due in part to the accumulation of snow on the surface of the leads. Combining these observations with simple calculations, possible mechanisms for the development and evolution of the surface skim and frost flowers are discussed. They include the hypothesis that the source of the surface skim is brine expelled upward from the sea ice as it cools, that the surface protrusions serve as nucleation sites for the frost flowers, and that the source of the excess water vapor necessary for frost flower growth is the wet surface skim.

Notes concerning certain Linear Marks in a Sedimentary Rock

Abstract The marks described in the paper occur in a fine–grained argillaceous sandstone referred by the U.S. Geological Survey to the Triassic or Jura–Trias period, which is found on a low tableland within 2 miles of the bluffs overlooking Glen Canyon. The marks commonly appear as straight lines intersecting at right angles, but some have a pinnate distribution, suggesting engravings of frost–flowers. A description of the markings is given, and various experiments made in the laboratory to illustrate the effects of formation of crystals formed over sediment are described. (See a paper by the same Author in the Utah University Quarterly for December 1895.)