Thomas V. Martin

Analysis and retracking of continental ice sheet radar altimeter waveforms

The SEASAT-I radar altimeter data set acquired over both the Antarctic and Greenland continental ice sheets is analyzed to obtain corrected ranges to the ice surface. The radar altimeter functional response over the continental ice sheets is considerably more complex than over the oceans. Causal factors identified in this complicated response include sloping surfaces, undulating ice surfaces with characteristic wavelengths on the same spatial scale as the altimeter beam-limited footprint, off-track reflections, and dynamic lag of the altimeter tracking circuit. Retracking methods using the altimeter return pulse waveforms give range corrections that are typically several meters. The entire set of SEASAT-I altimetry over the continental ice sheets is being retracked by fitting a multi-parameter function to each waveform. Many waveforms have double ramps indicating near-normal reflections from two distinct portions of the ice surface within the altimeter beam. Two independent range measurements differing by less than 25 m are obtained from retracking the double-ramp waveforms.

Large-scale drift of Arctic Sea ice retrieved from passive microwave satellite data

A method of determining the large-scale sea ice drift using 85.5 GHz Special Sensor Microwave Imager data are presented. A cross-correlation method is applied to sequential images of gridded data covering the entire Arctic. Individual correlation results are validated with ice velocities derived from buoy data. The satellite-derived mean drift values and the variabilities of the ice drift correspond closely with the buoy data. Similarly, time series of buoy data and associated satellite data are in good agreement even over large time periods. An example of a satellite-retrieved 3 day mean drift field demonstrates the potential of the method for providing large-scale ice circulation patterns. Mean drift fields of the winter periods 1987-1988 and 1992-1993 indicate a considerable interannual variability of the sea ice drift pattern in the Arctic Ocean. The Arctic region is divided into seven larger areas, and the area flux between these regions has been derived. The Kara Sea and the Laptev Sea show the largest area ice export with 0.02 and 0.015 km 2 s -1 , respectively. The central Arctic export through Fram Strait amounts to 0.12 Sv during the winter of 1992-1993 with a maximum of 0.15 Sv in January.

Regional mean sea surfaces based on GEOS‐3 and SEASAT altimeter data

Abstract Altimetric sea surfaces provide a basis for detailed analyses of the earths gravity, crustal structure, and the oceanic surface circulation. We have computed long‐term mean surfaces for the Bering Sea, Northwest Atlantic Ocean, and Gulf of Mexico based on a combination of the entire SEASAT (three‐month) and GEOS‐3 (3.5‐year) altimeter data sets. The number of available passes ranged from 558 in the gulf to 1396 in the Atlantic. The large amount of data in these areas, coupled with the increased constraint provided by the combination of data from two orbital inclinations, has permitted the accurate removal of the effects of radial ephemeris error through crossing arc adjustments. The precision of these regional mean sea surfaces is approximately 15 cm, with horizontal resolutions approaching 25 km.

Mean sea surface computation using GEOS‐3 altimeter data

The mean surfaces of several regions of the worlds oceans have been estimated using GEOS‐3 altimeter data. Included in these regions are the northwest Atlantic, the northeast Pacific off the coast of California, the Indian Ocean, the southwest Pacific, and the Philippine Sea. These surfaces have been oriented with respect to a common earth center‐of‐mass system by constraining the separate solutions to conform to precisely determined laser reference control orbits. The same reference orbits were used for all regions assuring continuity of the separate solutions. Radial accuracies of the control orbits have been demonstrated to be on the order of 1 m. In the computation of these surfaces, the altimeter‐measured sea surface height crossover differences were minimized by the adjustment of tilt and bias parameters for each pass with the exception of laser reference control passes. The tilt and bias adjustments removed long wavelength errors, which were primarily due to orbit error. Ocean tides were modeled w...

Applications of satellite altimetry to oceanography and geophysics

Satellite-borne altimeters have had a profound impact on geodesy, geophysics, and physical oceanography. To first order approximation, profiles of sea surface height are equivalent to the geoid and are highly correlated with seafloor topography for wavelengths less than 1000 km. Using all available Geos-3 and Seasat altimeter data, mean sea surfaces and geoid gradient maps have been computed for the Bering Sea and the South Pacific. When enhanced using hill-shading techniques, these images reveal in graphic detail the surface expression of seamounts, ridges, trenches, and fracture zones. Such maps are invaluable in oceanic regions where bathymetric data are sparse. Superimposed on the static geoid topography is dynamic topography due to ocean circulation. Temporal variability of dynamic height due to oceanic eddies can be determined from time series of repeated altimeter profiles. Maps of sea height variability and eddy kinetic energy derived from Geos-3 and Seasat altimetry in some cases represent improvements over those derived from standard oceanographic observations. Measurement of absolute dynamic height imposes stringent requirements on geoid and orbit accuracies, although existing models and data have been used to derive surprisingly realistic global circulation solutions. Further improvement will only be made when advances are made in geoid modeling and precision orbit determination. In contrast, it appears that use of altimeter data to correct satellite orbits will enable observation of basin-scale sea level variations of the type associated with climatic phenomena.

Anomalies of sea-ice transports in the Arctic

Abstract In the Arctic, Sea-ice motion and ice export are prominent processes and good indicators of Arctic climate System variability. Sea-ice drift is Simulated using a dynamic–thermodynamic Sea-ice model, validated with retrievals from SsM/I Satellite observations. Both datasets agree well in reproducing the main Arctic drift patterns. In order to Study inner Arctic transports and ice volume anomalies, the Arctic Ocean is Split by ten boundaries, Separating the central Arctic Ocean from the Nordic and marginal Seas. It is found that the already dominant Sea-ice export through Fram Strait has increased at the expense of export through the Barents Sea in the most recent years investigated. Furthermore, ice export from the Eurasian marginal Seas increased Slightly, followed by greater ice production during the winter. In contrast to this, the Sea-ice volume moved within the Beaufort Gyre distinctly decreased. In total, the ice volume in the central Arctic decreased during the 40 year period covered by this Study. The changes in the ice volume correspond to two wind-driven circulation regimes of the Arctic Sea-ice motion, which recur approximately every 11 years. For the volume anomalies we derived a correlation of –0.59 to the North Atlantic Oscillation (NAO) index, lagging the NAO by 2 years.