Constituent Ice Research Articles

Scattering kernel of an array of floating ice floes: application to water wave transport in the marginal ice zone

A radiative transfer model of water wave scattering in the marginal ice zone is considered. In this context, wave energy redistribution across the directional components of the spectrum as a result of scattering by the constituent ice floes is typically modelled via a scattering kernel describing the far-field directionality of the scattered wave field produced by a single floe in isolation. Recognizing the potential importance of the floe size distribution (FSD) on wave scattering, we propose an enhanced scattering kernel constructed from the far-field scattering pattern of a circular array of floes. This is achieved by solving the self-consistent multiple scattering of a time-harmonic plane wave by a large array of floating circular floes with radii sampled from a prescribed FSD. A fast multipole method is implemented to accelerate the numerical estimation of the solution. Simulations are then conducted to characterize the properties of the scattering kernel for a range of configurations. It is found that the scattering kernel obtained for a wide array has a large, narrow transmission peak in the forward direction, while it uniformizes low-amplitude scattered waves in other directions. An idealized application to radiative transfer theory is also considered.

Using snowflake surface-area-to-volume ratio to model and interpret snowfall triple-frequency radar signatures

Abstract. The snowflake microstructure determines the microwave scattering properties of individual snowflakes and has a strong impact on snowfall radar signatures. In this study, individual snowflakes are represented by collections of randomly distributed ice spheres where the size and number of the constituent ice spheres are specified by the snowflake mass and surface-area-to-volume ratio (SAV) and the bounding volume of each ice sphere collection is given by the snowflake maximum dimension. Radar backscatter cross sections for the ice sphere collections are calculated at X-, Ku-, Ka-, and W-band frequencies and then used to model triple-frequency radar signatures for exponential snowflake size distributions (SSDs). Additionally, snowflake complexity values obtained from high-resolution multi-view snowflake images are used as an indicator of snowflake SAV to derive snowfall triple-frequency radar signatures. The modeled snowfall triple-frequency radar signatures cover a wide range of triple-frequency signatures that were previously determined from radar reflectivity measurements and illustrate characteristic differences related to snow type, quantified through snowflake SAV, and snowflake size. The results show high sensitivity to snowflake SAV and SSD maximum size but are generally less affected by uncertainties in the parameterization of snowflake mass, indicating the importance of snowflake SAV for the interpretation of snowfall triple-frequency radar signatures.

What do triple‐frequency radar signatures reveal about aggregate snowflakes?

AbstractA large data set of volume element models of aggregate snowflakes was created, building the snowflakes from various models of ice crystals found in the atmosphere: dendrites, needles, plates, and bullet rosettes, as well as spheroidal crystals for comparison. Several different sizes for the constituent crystals were also used. The radar backscattering cross sections of the snowflakes were computed from the models using the discrete dipole approximation (DDA) at 13.6 GHz (Ku band), 35.6 GHz (Ka band) and 94.0 GHz (W band), and the effects of the choice of crystal model and size on the Ku/Ka band and Ka/W band dual‐wavelength ratios (DWR) was investigated. It was found that the aggregate DWRs were very similar for all naturally occurring ice crystal types investigated in this study. This implies that the choice of crystal type is at most of secondary importance in the forward model of scattering used for snowfall retrievals but also, conversely, that the identification of the crystal type from triple‐frequency observations is likely to be difficult. In contrast, the size of the constituent ice crystals does have a nonnegligible impact on the triple‐frequency signatures. Additionally, it was found that the triple‐frequency signatures found in some experimental data, resembling those resulting from spheroidal model snowflakes, cannot be reproduced using the aggregates with any of the crystal types that were investigated. This suggests that besides aggregation, other mechanisms of snowflake formation from ice crystals must be considered in snowfall retrieval algorithms.

Experimental simulation of ice-wedge casting: processes, products and palaeoenvironmental significance

Abstract In six experiments, model ice wedges 150 mm high and ≤50 mm wide were thawed from the surface downward at 30 g in a geotechnical centrifuge, simulating ice-wedge casting during progressive active-layer deepening through permafrost. The frozen host soils ranged from sand to silt to clayey silt. Resulting ice-wedge pseudomorphs indicated that the degree of deformation during casting was determined by factors that control thaw consolidation. Hence, deformation increased as the host sediments became finer-grained and more ice-rich. Thaw of host sand at 15 and 20% gravimetric ice content and host silt at 20 and 40% ice content resulted in the formation of partially-developed ice-wedge pseudomorphs with an upper ‘plug’ of sediment derived largely from the cover soil, a central tunnel and a basal plug of sediment. Thaw of host clayey silt at 30 and 60% ice content resulted in the formation of fully developed pseudomorphs that were significantly narrower and shorter than the initial ice wedges. The experiments support the hypothesis that ice-wedge pseudomorphs tend to be better preserved in coarser-grained sediments that are not ice-rich and therefore deform little during thaw of constituent ice wedges. This selective preservation has probably led to underestimation of ice-wedge pseudomorphs in fine-grained soils that were originally ice-rich and therefore a bias towards reconstructing cold temperatures.

Thermo-mechanically coupled ice-sheet response — cold, polythermal, temperate

AbstractClassical mixture concepts are the appropriate vehicle for describing the dynamics of ice masses containing some water. We review and derive, respectively, the theoretical formulations of cold, polythermal and temperate ice masses, emphasize the peculiarities of the model equations and point to difficulties that were encountered with the proposed models. The focus is both on the adequate physical motivation of the models and the consistency of their mathematical representation. The paper also has a tutorial character.As usual, cold ice is treated as a single-component incompressible heat-conducting viscous fluid, while two different models are presented for temperate ice. When it arises in a polythermal ice mass, the water content is small and a simple diffusive model for the moisture content suffices. This diffusive model is further simplified by taking its appropriate limit, when the moisture diffusivity tends to zero. Temperate ice in a wholly temperate — Alpine — glacier is treated as a two-phase flow problem, i.e. the momentum-balance laws of both constituents ice and water are properly accounted for. Such Darcy-type models are suggested because the water arises in a greater proportion; so its dynamic role can no longer be ignored.The constituent ice is treated as an incompressible non-linearly viscous isotropic body with constitutive properties similar to those of cold ice. The interstitial water is a density-preserving ideal or perfect fluid. The two interact with an interaction force that is proportional to the “porosity” and the seepage velocity. Internal melting that arises will lead to a generalization of the familiar Darcy law.When water is present, the boundary and transition conditions across internal singular surfaces take special, more complicated forms and involve statements on drainage to the base. These conditions are also discussed in detail.

Thermo-mechanically coupled ice-sheet response — cold, polythermal, temperate

AbstractClassical mixture concepts are the appropriate vehicle for describing the dynamics of ice masses containing some water. We review and derive, respectively, the theoretical formulations of cold, polythermal and temperate ice masses, emphasize the peculiarities of the model equations and point to difficulties that were encountered with the proposed models. The focus is both on the adequate physical motivation of the models and the consistency of their mathematical representation. The paper also has a tutorial character.As usual, cold ice is treated as a single-component incompressible heat-conducting viscous fluid, while two different models are presented for temperate ice. When it arises in a polythermal ice mass, the water content is small and a simple diffusive model for the moisture content suffices. This diffusive model is further simplified by taking its appropriate limit, when the moisture diffusivity tends to zero. Temperate ice in a wholly temperate — Alpine — glacier is treated as a two-phase flow problem, i.e. the momentum-balance laws of both constituents ice and water are properly accounted for. Such Darcy-type models are suggested because the water arises in a greater proportion; so its dynamic role can no longer be ignored.The constituent ice is treated as an incompressible non-linearly viscous isotropic body with constitutive properties similar to those of cold ice. The interstitial water is a density-preserving ideal or perfect fluid. The two interact with an interaction force that is proportional to the “porosity” and the seepage velocity. Internal melting that arises will lead to a generalization of the familiar Darcy law.When water is present, the boundary and transition conditions across internal singular surfaces take special, more complicated forms and involve statements on drainage to the base. These conditions are also discussed in detail.

High‐frequency anderice acoustic backscattering: A simulation model study

A simulation model that evaluates the backscatter of a high‐frequency acoustic pulse from the underice surface of pack ice regions, characteristic of the interior Arctic, has been used to investigate the extent to which various morphologic, geometric, and acoustic parameters, used to describe the underice surface, influence the backscatter. The simulation model utilizes a three‐dimensional underice canopy synthesized from two‐dimensional acoustic profile data. The large scale scattering features of the underice surface are modeled as first‐year ice keels and sloping flat ice regions. The small scale surface structure is modeled as an ensemble of randomly oriented ice blocks. The Kirchhoff approximation is used to evaluate the scattering strength of an individual ice block which, in turn, is used to calculate the scattering strength of the underice surface. The sonar equation is used to calculate reverberation. The parameters used in the model may be divided into four groups: canopy, ice keel, ice block, and acoustic parameters. Parameters used to identify and classify the constituent ice features of the canopy are varied, and it is shown that, for physically reasonable parameters, reverberation is essentially independent of the feature identification and classification criteria. Reverberation is also shown to be largely independent of the parameters that describe only the large scale structure of the ice keel, but is strongly dependent upon parameters that effect the area and spatial orientation of individual ice blocks.

Alignment of Cirrus Clouds along the Magnetic Meridian

CIRRUS clouds appear very often in a regular array of parallel bands. Alexander von Humboldt1 discovered in the course of his meteorological observations in South America and Siberia that these arrays follow frequently the line of the geomagnetic meridian of the locality; he invented the term “bandes polaires” for this phenomenon. Even very casual observations confirm the truth of his statement. He suspected some connexion between the appearance of these ‘polar bands’ and that of the aurora borealis. In the light of present-day knowledge of the composition of cirrus clouds, it is possible to put forward an explanation of this alignment. Since the constituent ice particles are known to form around dust particles of partly meteoric origin, they will align along the lines of force of the geomagnetic field in those cases in which the dust core consists of a magnetic material. It follows incidentally that the concentration of magnetic meteoric dust in the upper atmosphere should increase from the equator towards the poles, a hypothesis which is confirmed by Sobermann's2 recent work on the composition of noctilucent clouds above northern Sweden.

The changes occurring in whalemeat during storage in the frozen state

AbstractA study was made of the changes that occurred in frozen whalemeat during storage at — 10%. A rancid flavour developed in meat which was exposed to air. The taint was found to be restricted to a thin surface‐layer and to be caused by oxidation of the highly unsaturated fat. No rancidity was detectable in meat from the interior of blocks after storage for 22 months or in the surface of meat which had been encased in ice for 10 months. Of several wrapping materials tested, only polyvinylidene chloride film gave any marked protection to the meat. The results indicated that the rate and extent of oxidation were dependent on the degree to which the exposed surfaces of the frozen meat had become dehydrated through sublimation of the constituent ice.