Passive Microwave Spectral Emission from Saline Ice at C-Band During the Growth Phase

The microwave emission properties of first-year sea ice were investigated from the R/V Polarstern during the Antarctic Winter Weddell Gyre Project in 1989. Radiometer measurements were made at 611 MHz and 10 GHz and were accompanied by video and visual observations. Using the theory of radiometric emission from a layered medium, a method for deriving sea ice thickness from radiometer data is developed and tested. The model is based on an incoherent reflection process and predicts that the emissivity of saline ice increases monotonically with increasing ice thickness until saturation occurs. 22,569 22,570 MENASHI ET AL.' LoW-FREQUENCY PASSIVE-MICROWAVE OBSERVATIONS OF SEA ICE 0i Greenland, Nature, 345, 795-797, 1990. Wadhams, P., M. A. Lange, and S. F. Ackey, The thickness distribution across the Atlantic sector of the Antarctic Ocean in midwinter, J. Geophys. Res., A. W. Lohanick, Naval Oceanographic and Atmospheric Research Laboratory, Polar Oceanography Branch Office, Hanover, NH 03755.

Section: ) Is That It Establishes a Monotonically Increasing Relationsupporting

confidence: 80%

Section: Optical Pathlength;mentioning

confidence: 85%

Section: Introductionmentioning

confidence: 99%

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Low‐frequency passive‐microwave observations of sea ice in the Weddell Sea

Menashi¹,

Germain²,

Swift³

et al. 1993

“…The first was a rapid initial rise in brightness temperature (Tb) from the relatively low values of open water to the higher values of thick ice, rising more quickly at higher frequencies. Similar behavior has been reported by Grenfell and Comiso [1986] and Swift et al [1986]. The second common feature was a sudden increase in surface temperature when the ice was between 1 and 2 cm thick.…”

supporting

confidence: 85%

Passive microwave remote sensing of thin sea ice using principal component analysis

Wensnahan¹,

Maykut²,

Grenfell³

et al. 1993

Time sequences of surface based measurements of passive microwave emission from growing saline ice reported by Wensnahan et al. (1993) are used to explore the possibility of developing a satellite based sea ice concentration algorithm which solves for the presence of thinner ice. It is shown that two classes of thinner ice can be distinguished from mixtures of open water (OW), first‐year (FY) ice, and multiyear (MY) ice. The two classes do not necessarily correspond to specific World Meteorological Organization ice types; rather, newly formed ice represents a brief transition spectrum between OW and thin ice. Newly formed ice appears to be optically thick at 37 and 90 GHz and has a relatively dry surface. The thin ice spectrum occurs when the ice is greater than 4 cm thick and appears to result from the accumulation of brine at the surface of the ice. Thin ice has a relatively stable spectrum characterized by high brightness temperatures, a near‐zero spectral gradient at vertical polarization, and a large difference between vertical and horizontal polarizations. Supervised principal component analysis (PCA) was done of laboratory data using 10 channels of passive data: vertical and horizontal polarization at 6.7, 10, 19, 37, and 90 GHz. Analyses were also done on subsets of the laboratory data at 6.7 to 37 GHz as well as 19 to 90 GHz, representing the scanning multichannel microwave radiometer (SMMR) and special sensor microwave imager (SSM/I) satellite frequencies, respectively. Using all of the channels or the SMMR subset makes it possible to solve for mixtures of OW and FY, MY, newly formed and thin ice but with large errors. However, any four of these scene types can be distinguished with reasonable accuracy. The SSM/I frequencies allow determination of at most four of these scene types but with moderate errors. PCA was used in a case study of SSM/I data from the Bering Sea for April 2, 1988. Winds from the north formed thin ice areas which the NASA Team algorithm interprets as large amounts of OW and MY ice. With PCA, these same areas are interpreted as 20–30% OW near the lee shores but otherwise as consisting almost entirely of thin ice. We conclude that thin ice can be detected using satellite data. However, questions remain as to how the thin ice spectrum varies with environmental conditions, how it evolves to that of FY, and how this evolution affects the predicted concentrations of thin ice.

“…A space heater was also installed to suppress excessive ice growth during periods of very cold weather. Other investigations [Grenfell and Comiso, 1986;Swift et al 1986] utilized remote sensing antennas and other instruments mounted on a steel gantry that spanned the pool and could be moved across the pavers to permit measurements on any part of the ice sheet. The pool was filled with water that was raised to the desired salinity (23-25%) by adding finely granulated sea salt and mixing thoroughly with circulating pumps.…”

Section: Introductionsupporting

confidence: 88%

Structure and dielectric properties at 4.8 and 9.5 GHz of saline ice

Arcone¹,

Gow²,

McGrew³

1986

The structure and salinity characteristics of saline ice slabs removed from ice sheets grown in an outdoor pool have been studied and related to the complex relative dielectric permittivity measured, utilizing free space transmission techniques, at 4.8 and 9.5 GHz. The saline ice closely simulated Arctic sea ice in its structural and salinity characteristics which were regularly monitored in a number of ice sheets grown during the winters of 1983–1984 and 1984–1985. In‐situ transmission measurements at similar frequencies were also made on the ice sheet itself using antennas located above and beneath the ice. The slab measurements were made during warming from −28°C to −2°C on slabs grown during the winter of 1983–1984 (4.75 GHz) and during a warming and cooling cycle over a slightly larger temperature range on slabs grown during the winter, 1984–1985 (4.80 and 9.50 GHz). The 1983–1984 results show both the real εr′ and imaginary εr″ parts to vary almost in direct proportion to the brine volume νB at values less than about 50‰. The 1984–1985 slabs showed only εr′ to depend strongly on νB while values of εr″ were considerably less than in the previous year. This difference may have been due to greater electrical conductivity in the 1983–1984 slabs, which could be related to distinct differences seen in brine pocket structure between the two winters. Values of εr′ as a function of νB were slightly lower on the average at 9.50 GHz than at 4.80, and most values of εr′ agreed with the results of others when their temperature and salinity values were converted to values of νB. Changes in εr′ due to thermal modification (laboratory warming) seem more strongly related to a gain in air volume due to brine drainage than to the redistribution of brine inclusions. The in‐situ measurements showed extremely high attenuation for the young (<12 cm) brine‐rich ice. Good agreement was found between data for the more desalinated samples and theoretical values predicted by a previously proposed dielectric mixing model that was modified to account for the brine pocket geometry observed in thin sections, and also by including a bulk conductivity term to account for the observed loss.