Mark R. Drinkwater

Snow on Antarctic sea ice

Snow on Antarctic sea ice plays a complex and highly variable role in air-sea-ice interaction processes and the Earths climate system. Using data collected mostly during the past 10 years, this paper reviews the following topics: snow thickness and snow type and their geographical and seasonal variations; snow grain size, density, and salinity; frequency of occurrence of slush; thermal conductivity, snow surface temperature, and temperature gradients within snow; and the effect of snow thickness on albedo. Major findings include large regional and seasonal differences in snow properties and thicknesses; the consequences of thicker snow and thinner ice in the Antarctic relative to the Arctic (e.g., the importance of flooding and snow-ice formation); the potential impact of increasing snowfall resulting from global climate change; lower observed values of snow thermal conductivity than those typically used in models; periodic large-scale melt in winter; and the contrast in summer melt processes between the Arctic and the Antarctic. Both climate modeling and remote sensing would benefit by taking account of the differences between the two polar regions.

Azimuth variation in microwave scatterometer and radiometer data over Antarctica

While designed for ocean observation, scatterometer and radiometer data have proven very useful in a variety of cryosphere studies. Over large regions of Antarctica, ice sheet and bedrock topography and the snow deposition, drift, and erosional environment combine to produce roughness on various scales. Roughness ranges from broad, basin-scale ice-sheet topography at /spl sim/100 km wavelengths to large, spatially coherent dune fields at /spl sim/10 km wavelength to erosional features on the meter scale known as sastrugi. These roughness scales influence the microwave backscattering and emission properties of the surface, combining to introduce azimuth-angle dependencies in the satellite observation data. In this paper, the authors explore the use of NASA scatterometer (NSCAT) data, European remote sensing (ERS) advanced microwave instrument (AMI) scatterometer mode data, and special sensor microwave/imager (SSM/I) data to study surface roughness effects in Antarctica. All three sensors provide strong evidence of azimuth modulation, which is correlated with the surface slope environment and results in a katabatic wind flow regime. Due to its broad azimuth coverage, NSCAT data appears to be the best suited for azimuth-angle observations. A simple empirical model for the azimuth variation in the radar backscatter is developed, and an algorithm for computing the parameters of the model from NSCAT data at a fine scale is presented. Results indicate relationships exist between the azimuthal variation of the data and the orientation of the surface slope and small-scale roughness relative to the sensor-look direction.

LIMEX '87 ice surface characteristics: implications for C-band SAR backscatter signatures

Ice surface characterization data collected in 1987, during the Labrador Ice Margin Experiment, are analyzed to estimate the changes in snow and ice properties at the onset of melt. Surface measurements were made from an ice research vessel on several days (some of which had coincident remote sensing flights) at a number of locations in the marginal ice zone. These data are used as input parameters in a simple scattering model to simulate the effects of variations in material properties upon C-band scattering signatures. >

Winter snow cover on sea ice in the Weddell Sea

Measurements of snow thickness, temperature, salinity, density, and stratigraphy acquired during the 1992 Winter Weddell Gyre Study are presented. Results indicate that the winter snow cover on sea ice in the Weddell Sea is extremely variable. Extreme fluctuations in Antarctic synoptic conditions (air temperature, precipitation, humidity, and wind speed) occur during the austral winter. They result in unique modifications and additions to the snow layer during the aging process and act to stabilize an otherwise easily wind-redistributed shallow snow cover and develop well-packed drift features. The latter occur even over relatively undeformed areas of sea ice and have a significant localized effect on the snow thickness distribution. Significant variability in snow grain size (mean 2.73±3.12 mm) and density (0.32±0.09 g cm−3) is observed as a result of cyclical switches between high- and low-temperature gradient metamorphism. Multiple icy layers indicate multiple thaw-freeze events. One such event occurred during a 3-day station, during which the air temperature rose by 22°C in 12 hours (to approximately 0°C). This paper also examines mechanisms for flooding of the snow-ice interface, including snow loading. Even where the latter is not a factor, the layer of snow immediately above the snow-ice interface is commonly damp and saline (>10‰). Limitations in the data set are discussed, and comparisons are drawn with other experiments.

Global ice and land climate studies using scatterometer image data

Scatterometers have provided continuous synoptic microwave radar coverage of the Earth from space for nearly a decade. NASA launched three scatterometers: the current SeaWinds scatterometer onboard QuikSCAT (QSCAT, 13.4 GHz) launched in 1999; the NASA scatterometer (NSCAT, 14.0 GHz), which flew on the Japanese Space Agencys ADEOS-1 platform during 1996–1997; and the Seasat-A scatterometer system (SASS, 14.6 GHz), which flew in 1978. The European Space Agencys (ESA) 5.3-GHz scatterometer (ESCAT) has been carried onboard both the ERS-1 and ERS-2 satellites since 1991. properties, including the phase state, of a particular surface type. Varying response from the surface also results from different polarizations, viewing angles and orientations, and radar frequencies. The wide swath of scatterometers provides near daily global coverage at intrinsic sensor resolutions that are generally between 25–50 km.

Greenland snow accumulation estimates from satellite radar scatterometer data

Data collected by the C band ERS-2 wind scatterometer (EScat), the Ku band ADEOS-1 NASA scatterometer (NSCAT), and the Ku band SeaWinds on QuikScat (QSCAT) satellite instruments are used to illustrate spatiotemporal variability in snow accumulation on the Greenland ice sheet. Microwave radar backscatter images of Greenland are derived using the scatterometer image reconstruction (SIR) method at 3-day intervals over the periods 1991–1998 and 1996–1997 for EScat and NSCAT, respectively. The backscatter coefficient σ° normalized to 40° incidence, A, and gradient in backscatter, B, in the range 20°–60° are compared with historical snow accumulation data and recent measurements made in the Program for Arctic Regional Climate Assessment (PARCA) shallow snow pits. Empirical relationships derived from these comparisons reveal different exponential relationships between C and Ku band A values and dry snow zone mean annual accumulation, Q. Frequency difference images between overlapping scatterometer images suggest that C band data are more sensitive to snow layering and buried inhomogeneities, whereas Ku band data are more sensitive to volume scattering from recently accumulated snow. Direct comparisons between NSCAT B values and in situ Q measurements show a linear relationship between ln (Q) and B, with a negative rank correlation of R = −0.8. The root-mean-square residual in fitting regression line equation ln (Q) = 3.08 − 17.83B to the data is 0.05-m snow water equivalent. This simple Ku band empirical relationship is exploited to investigate decadal changes in dry snow zone accumulation between Seasat (1978) and NSCAT (1996). Additional comparisons between NSCAT and recent QSCAT (1999) data reveal significant upslope shifts in the dry snow line along the southwestern flank of the ice sheet. Recent acceleration in the increase in intensity of scattering is observed in the percolation zone, suggesting increased melting between 2000- and 3000-m elevation in the southern half of the ice sheet.

Recent changes in the microwave scattering properties of the Antarctic ice sheet

Time series, satellite microwave data are used to monitor and quantify changes in the scattering properties of the Antarctic ice sheet. Daily ERS scatterometer (EScat) and Special Sensor Microwave/Imager (SSM/I) image data, acquired since 1992, are analyzed to understand the seasonal and interannual changes over the ice sheet. For regions of the ice sheet where azimuthal modulation is negligible, seasonal cycles are observed in both the EScat (amplitude /spl sim/0.5 dB) and SSM/I data (amplitude /spl sim/10 K). These cycles are attributed to seasonal changes in surface temperature. Interannual variability in the time series signatures appears to be associated with accumulation. There is also evidence to suggest that shifts in the wind direction can alter the backscatter through azimuthal modulation. Over the period 1992-97, large trends are observed in the EScat( 1 K yr/sup -1/) signatures over several regions in Antarctica. These changes typically occur over ice shelves and at the margins of the ice sheet where previous melt events have occurred, and where accumulation is relatively high (>300 mm yr/sup -1/). It is likely the large changes result from the successive burial of an efficient scattering layer formed by refreezing after a melt event prior to 1992. There is also evidence to suggest that similarly large changes can be observed in the interior of the ice sheet due to the burial of depth hear layers. In order to monitor long term change in the properties of the Antarctic ice sheet, it is necessary to remove the seasonal cycle from time series microwave data. Such anomaly data can then be used to understand the link between EScat and SSM/I with accumulation and wind shifts.

Multifrequency Polarimetric Synthetic Aperture Radar Observations of Sea Ice

The first known fully polarimetric airborne synthetic aperture radar (SAR) data set of sea ice is introduced. Images were acquired in the Beaufort, Bering and Chukchi seas in March 1988, during a campaign for validation of Defense Meteorological Satellite Program Special Sensor Microwave Imager radiometer ice products. Statistics of the magnitude, phase and polarization of complex backscattered signals recorded by the Jet Propulsion Laboratory three-frequency SAR are examined in detail for various scenes with different ice characteristics. The full Stokes matrix information generated from C, L, and P band data characterize the scattering behavior of different ice types. Polarization ratios and phase differences between linear copolarized returns are used for discrimination between particular image features and mechanisms are suggested for the observed polarimetric characteristics. Results indicate that combinations of frequency and polarization enhance current capability to distinguish ice of different properties using single frequency, fixed polarization micro-wave radar. A specific example is the polarimetric identification of new ice formation which may not be easily distinguished in ERS-1 5.3GHz, VV polarization SAR data. Such findings are consistent with theoretical model simulations of scattering characteristics of sea ice. Overall, these preliminary results demonstrate that radar polarimetry will likely add a new dimension to our current capability for extracting geophysically important ice variables using radar remote sensing methods.

Polarimetric signatures of sea ice: 2. Experimental observations

Experimental observations of polarimetric signatures are presented for sea ice in the Beaufort Sea under cold winter conditions and interpreted with the composite model developed in Part 1. Polarimetric data were acquired in March 1988 with the Jet Propulsion Laboratory multifrequency airborne synthetic aperture radar (SAR) during the Beaufort Sea Flight Campaign. The experimental area was located near 75°N latitude and spanned 140°–145°W longitude. Selected sea ice scenes contain various ice types, including multiyear, thick first-year, and thin lead ice. Additionally, the C band SAR on the first European Remote Sensing Satellite provides supplementary backscattering data of winter Beaufort Sea ice for small incident angles (20°–26°) at vertical polarization. Sea ice characterization and environmental data used in the model were collected at the Applied Physics Laboratory drifting ice station to the northeast of Prudhoe Bay; additional data from field and laboratory experiments are also utilized in this analysis. The model relates sea ice polarimetric backscattering signatures to physical, structural, and electromagnetic properties of sea ice. Scattering mechanisms contributing to sea ice signatures are explained, and sensitivities of polarimetric signatures to sea ice characterization parameters are studied.

SMOS Validation and the COSMOS Campaigns

The Soil Moisture and Ocean Salinity (SMOS) mission is a joint ESA-CNES (F)-CDTI (E) mission within the ESA Living Planet Program, and it was the second ESA Earth Explorer Opportunity Mission to be selected. The mission objectives of SMOS are to provide soil moisture and ocean salinity observations for weather forecasting, climate monitoring, and the global freshwater cycle. This paper will describe the scientific campaigns performed to date, as well as the plans for the on-orbit calibration and validation activities.