8 Satellite Microwave Radar Observations of Antarctic Sea Ice
shown in parallel to thermistor profiles at a variety of sites with differing snow depth
(Fig. 18b). Thermistors indicate that protracted cold air temperatures after day 63
enable the snow layer to develop a strong thermal gradient. This cools- the basal snow
sufficiently quickly that the slush freezes around day 67.
Large numbers of individual multiyear ice floes were tracked in ERS-1 SAR images
to derive the microwave backscatter trend shown in the upper trace of Fig. 18c. For comparison, global mean SAR-image and EScat-image erG vv values measured over the identical 100 x 100km SAR-imaged areas, each report essentially the same decreasing trend.
The largest decrease in 0° vv is observed before and after the freeze- up of tlIe slush. Thus,
the change from rough surface scattering from snow-covered slush to rough surface
scattering from dry snow-covered perennial ice appears to be responsible for tlIe
observed shift in the multiyear ice peak in Fig. 17.
8.6.2.3
Meitponding
Additional effects of seasonal melting and transformation at the surface of perennial
ice floes are noted here as a consequence of summer processes and periodic winter
excursions in air temperature due to storm systems. Large warm excursions in air temperature have been observed to have considerable impact upon the microwave
backscattering characteristics of sea ice, in response to the changing surface snow and
ice characteristics (Drinkwater et al.1995a; Massom et al.1997). Though the appearance
of classical meltponds in tlIe Weddell Sea appears to be checked largely by cold dry
southerly winds and the latitude or extent of the southward wandering of the zero
degree seasonal isotherm, a recent summer experiment in 1995 recorded surface meltponding in the Weddell Sea (Low 1995). The appearance and expression of meltponding in SAR images is discussed in more detail by Low (1995) by use of coincident SAR
and aerial photography. Ongoing studies of the spatio-temporal expression of seasonal melt in the EScat image time series (1992 to the present day) will reveal the context
of these SAR-observed features in the long-term record. Furthermore, these studies are
expected to confirm whether extreme melt events noted in 1992 and 1995 are associated with climatic anomalies such as the 4-year period in sea-surface temperature and
reduced poleward wind stress observed by White and Petersen (1996).
8.7
Conclusions
ERS satellite data have for tlIe first time been extensively studied and applied in studies of sea-ice dynamics and characteristics in Antarctica. The main result of analysis of
these data is an understanding of how microwave signatures and their time-varying
properties can be used as a basis for understanding geophysical changes in the Antarctic sea-ice cover. Examples show that satellite radar is a valuable addition to the arsenal of tools now available for building up the long-term observational database necessary for addressing climate-related issues. Furthermore, algorithms which have been
applied to tlIese data will be refined and ultimately lead to data products including ice
kinematics fields and area flux information, with which to address mass- or freshwater balance questions in the Southern Ocean.
shown in parallel to thermistor profiles at a variety of sites with differing snow depth
(Fig. 18b). Thermistors indicate that protracted cold air temperatures after day 63
enable the snow layer to develop a strong thermal gradient. This cools- the basal snow
sufficiently quickly that the slush freezes around day 67.
Large numbers of individual multiyear ice floes were tracked in ERS-1 SAR images
to derive the microwave backscatter trend shown in the upper trace of Fig. 18c. For comparison, global mean SAR-image and EScat-image erG vv values measured over the identical 100 x 100km SAR-imaged areas, each report essentially the same decreasing trend.
The largest decrease in 0° vv is observed before and after the freeze- up of tlIe slush. Thus,
the change from rough surface scattering from snow-covered slush to rough surface
scattering from dry snow-covered perennial ice appears to be responsible for tlIe
observed shift in the multiyear ice peak in Fig. 17.
8.6.2.3
Meitponding
Additional effects of seasonal melting and transformation at the surface of perennial
ice floes are noted here as a consequence of summer processes and periodic winter
excursions in air temperature due to storm systems. Large warm excursions in air temperature have been observed to have considerable impact upon the microwave
backscattering characteristics of sea ice, in response to the changing surface snow and
ice characteristics (Drinkwater et al.1995a; Massom et al.1997). Though the appearance
of classical meltponds in tlIe Weddell Sea appears to be checked largely by cold dry
southerly winds and the latitude or extent of the southward wandering of the zero
degree seasonal isotherm, a recent summer experiment in 1995 recorded surface meltponding in the Weddell Sea (Low 1995). The appearance and expression of meltponding in SAR images is discussed in more detail by Low (1995) by use of coincident SAR
and aerial photography. Ongoing studies of the spatio-temporal expression of seasonal melt in the EScat image time series (1992 to the present day) will reveal the context
of these SAR-observed features in the long-term record. Furthermore, these studies are
expected to confirm whether extreme melt events noted in 1992 and 1995 are associated with climatic anomalies such as the 4-year period in sea-surface temperature and
reduced poleward wind stress observed by White and Petersen (1996).
8.7
Conclusions
ERS satellite data have for tlIe first time been extensively studied and applied in studies of sea-ice dynamics and characteristics in Antarctica. The main result of analysis of
these data is an understanding of how microwave signatures and their time-varying
properties can be used as a basis for understanding geophysical changes in the Antarctic sea-ice cover. Examples show that satellite radar is a valuable addition to the arsenal of tools now available for building up the long-term observational database necessary for addressing climate-related issues. Furthermore, algorithms which have been
applied to tlIese data will be refined and ultimately lead to data products including ice
kinematics fields and area flux information, with which to address mass- or freshwater balance questions in the Southern Ocean.