Benjamin Holt

Satellite observations of small coastal ocean eddies in the Southern California Bight

This study describes the characteristics of extensive small-scale coastal ocean eddies in the Southern California Bight. These surface features were primarily detected by using ERS-1 and ERS-2 synthetic aperture radar (SAR) satellite imagery from 1992 to 1998. The eddies, predominantly cyclonic in their rotation, appeared to result from several forcing mechanisms. They were mainly observed within the Santa Barbara Channel and the Santa Monica-San Pedro Basin regions and appeared to be seasonal in their distribution. Observed eddy diameters were all less than 50 km, with over 70% less than 10 km. The SAR data were complemented by sea surface temperature measurements derived from advanced very high resolution radiometer satellite imagery, as well as in situ data from moorings and drifters that provided substantial verification of the small-scale eddies. These findings are significant in that the eddies were, in general, smaller in size and more abundant than previously reported. Additionally, these results provide further evidence of the complex near-surface circulation within the Southern California Bight, with important implications for nutrient flux, productivity, plankton patchiness, larval transport and recruitment, and dispersal of pollutants.

Identification of Sea Ice Types in Spaceborne Synthetic Aperture Radar Data

An approach for identification of sea ice types in spaceborne synthetic aperture radar (SAR) image data is presented. The unsupervised classification approach involves cluster analysis for segmentation of the image data followed by cluster labeling based on previously defined look-up tables containing the expected backscatter signatures of different ice types measured by land-based scatterometer. The particular look-up table used for labeling a segmented image is selected based on the seasonal and meteorological conditions at the time of data acquisition. The extensive scatterometer observations and experience accumulated in field campaigns during the last 10 years were used to construct these look-up tables. These tables are expected to evolve as sea ice observations from the European ERS-1 SAR become available. This paper presents the classification approach, its expected performance, the dependence of this performance on radar system performance, and expected ice scattering characteristics. Results using both aircraft and simulated ERS-1 SAR data are presented. The results are compared to limited field ice property measurements and coincident passive microwave imagery. An algorithm based on this experimental approach has been implemented in the geophysical processor system at the Alaska SAR Facility for classification of sea ice data in ERS-1 C band SAR data. The importance of an integrated postlaunch program for validation and improvement of this approach is discussed.

Polarimetric Analysis of Backscatter From the Deepwater Horizon Oil Spill Using L-Band Synthetic Aperture Radar

We analyze the fully-polarimetric Uninhabited Aerial Vehicle Synthetic Aperture Radar (UAVSAR) data acquired on June 23, 2010, from two adjacent, overlapping flight tracks that imaged the main oil slick near the Deepwater Horizon (DWH) rig site in the Gulf of Mexico. Our results show that radar backscatter from both clean water and oil in the slick is predominantly from a single surface scatterer, consistent with the tilted Bragg scattering mechanism, across the range of incidence angles from 26° to 60°. We show that the change of backscatter over the main slick is due both to a damping of the ocean wave spectral components by the oil and an effective reduction of the dielectric constant resulting from a mixture of 65-90% oil with water in the surface layer. This shows that synthetic aperture radar can be used to measure the oil volumetric concentration in a thick slick. Using the H/A/α parameters, we show that surface scattering is dominant for oil and water whenever the data are above the noise floor and that the entropy (H) and α parameters for the DWH slick are comparable to those from the clean water. The anisotropy, A, parameter shows substantial variation across the oil slick and a significant range-dependent signal whenever the backscatter in all channels is above the instrument noise floor. For slick detection, we find the most reliable indicator to be the major eigenvalue of the coherency matrix, which is approximately equal to the total backscatter power for both oil in the slick and clean sea water.

Overview of results of Spaceborne Imaging Radar-C, X-Band Synthetic Aperture Radar (SIR-C/X-SAR)

The Spaceborne Imaging Radar-C, X-Band Synthetic Aperture Radar (SIR-C/X-SAR) was launched on the Space Shuttle Endeavour for two ten day missions in the spring and fall of 1994. Radar data from these missions are being used to better understand the dynamic global environment. During each mission, radar images of over 300 sites around the Earth were obtained, returning over a terabit of data. SIR-C/X-SAR science investigations were focused on quantifying radars ability to estimate surface properties of importance to understanding global change; and focused studies in geology, ecology, hydrology and oceanography, as well as radar calibration and electromagnetic theory studies. In addition, the second flight featured an interferometry experiment, where digital elevation maps were obtained by interfering data from the first and second shuttle flight, and from successive days on the second flight. SIR-C/X-SAR data have been used to validate algorithms which produce maps of vegetation type and biomass; snow, soil and vegetation moisture; and the distribution of wetlands, developed with earlier aircraft data. >

Estimation of the thin ice thickness and heat flux for the Chukchi Sea Alaskan coast polynya from Special Sensor Microwave/Imager data, 1990-2001

[1] One of the largest Arctic polynyas occurs along the Alaskan coast of the Chukchi Sea between Cape Lisburne and Point Barrow. For this polynya, a new thin ice thickness algorithm is described that uses the ratio of the vertically and horizontally polarized Special Sensor Microwave/Imager (SSM/I) 37-GHz channels to retrieve the distribution of thicknesses and heat fluxes at a 25-km resolution. Comparison with clear-sky advanced very high resolution radiometer data shows that the SSM/I thicknesses and heat fluxes are valid for ice thicknesses less than 10–20 cm, and comparison with several synthetic aperture radar (SAR) images shows that the 10-cm ice SSM/I ice thickness contour approximately follows the SAR polynya edge. For the twelve winters of 1990–2001, the ice thicknesses and heat fluxes within the polynya are estimated from daily SSM/I data, then compared with field data and with estimates from other investigations. The results show the following: First, our calculated heat losses are consistent with 2 years of over-winter salinity and temperature field data. Second, comparison with other numerical and satellite estimates of the ice production shows that although our ice production per unit area is smaller, our polynya areas are larger, so that our ice production estimates are of the same order. Because our salinity forcing occurs over a larger area than in the other models, the oceanic response associated with our forcing will be modified. INDEX TERMS: 4540 Oceanography: Physical: Ice mechanics and air/sea/ice exchange processes; 3360 Meteorology and Atmospheric Dynamics: Remote sensing; 4504 Oceanography: Physical: Air/sea interactions (0312); 4572 Oceanography: Physical: Upper ocean processes; 4207 Oceanography: General: Arctic and Antarctic oceanography; KEYWORDS: Chukchi Sea, coastal polynya, remote sensing

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.

Waves in frazil and pancake ice and their detection in Seasat synthetic aperture radar imagery

A theoretical model of waves propagating into an ice cover composed of frazil and pancake ice is developed and compared with measurements of wavelength and direction derived from synthetic aperture radar (SAR) imagery obtained from Seasat in October 1978. The theoretical model is based on the concept that ice of these types, which consists of small crystals or cakes, has only a mass-loading effect on the water surface. We derive the dispersion relation for phase and group velocities, finding that there is an upper frequency limit for propagation into the ice. From the reflection coefficient at the ice edge we derive the wave radiation pressure exerted on the ice, showing that it will cause a slick of frazil ice backed by thicker floes to become more dense or thick with increasing penetration. The implications for radar scattering enabling detection on SAR are that the Bragg resonant wavelength corresponds to waves above the frequency limit for propagation, so that a frazil slick appears dark on an SAR image. When the frazil ice becomes transformed into pancake ice, through slick compression or other means, the raised edges of the pancakes cause the ice to appear bright despite the fact that there are no waves present at the Bragg wavelength. These results are applied to a Seasat SAR image obtained from the Chukchi Sea. The appearance of the ice in the image corresponds to what we expect for frazil ice gradually transforming itself into pancake ice, backed by thicker floes. We derive directional wave number spectra outside and inside the ice cover by digital Fourier analysis of image subscenes, and we find that the change of wavelength and angle of refraction of the dominant wave entering the ice field are both characteristic of the dispersion relation derived theoretically. Mean ice thicknesses extracted from the theory correspond to thicknesses expected for such slicks. The technique offers a possible means of extracting the thickness of fields of frazil and pancake ice from SAR imagery; this may be of considerable utility when ERS 1 SAR is used to study the advancing winter ice edge in the Antarctic, which consists of vast areas of these ice types.

Improvements in the estimates of ice thickness and production in the Chukchi Sea polynyas derived from AMSR‐E

For January-March 2003, we use 12.5-km resolution Advanced Microwave Scanning Radiometer (AMSR) data for the first time in a comparison with Synthetic Aperture Radar (SAR) and Special Sensor Microwave/Imager (SSM/I) data to study two Chukchi coast polynyas, one consisting of many, the other of only a few 25-km SSM/I pixels. Within these polynyas, the ice thicknesses are derived separately from the SMM/I 37-GHz and AMSR 36-GHz channels; the heat fluxes are derived by combining thicknesses with meteorological data. Comparison with ScanSAR data shows that for the large polynya, because AMSR provides better resolution of the surrounding coastline and first-year ice, the AMSR heat losses are greater than the SSM/I; for the small polynya, AMSR measures its variability even when its area is order of a single SSM/I pixel. This means that AMSR permits more accurate calculation of polynya heat losses, yielding the potential of improved estimates of Arctic polynya productivity.

SIR-C/X-SAR observations of rain storms☆

Abstract The spaceborne imaging radar-C, X-band synthetic aperture radar observations of rain storms are the first multipolarization and multifrequency observations of precipitation from space. In addition to numerous, often dramatic images of severe weather systems obtained by forming a synthetic aperture in the usual side-looking attitude, several data takes were performed while the radar antennas were parallel to the ground and the radar beams were pointing at nadir. These opportunities coincided with the passage of the Shuttle over Tropical Cyclone Odille in the southern Indian Ocean during the first flight and over Typhoon Seth in the western Pacific during the second flight. The resulting observations, or, more appropriately, the resulting measurements, demonstrate for the first time the capability of a spaceborne multifrequency multipolarization microwave radar system to quantify precipitation rates, to detect hydrometeor phase, and to classify rain type.

The effect of a storm on the 1992 summer sea ice cover of the Beaufort, Chukchi, and East Siberian Seas

For the summer Beaufort, Chukchi, and East Siberian Seas a variety of active and passive microwave satellite data are used to determine the response of the ice edge and interior to a storm. Specifically, the study concentrates on a low-pressure system that passed over the region between August 14 and 20, 1992, with peak geostrophic winds of about 18 m s−1. Through the use of the ERS-1 imaging radar and the Special Sensor Microwave/Imager the ice response is examined at two locations: at the ice edge for a nearly stationary location in the Chukchi Sea and in the ice interior along three swaths in the Beaufort, Chukchi, and East Siberian Seas. The ice edge observations show that the storm fractures the large floes into small floes, some of which are advected into the adjacent warm water. The ice interior observations show that the storm caused an increase in the open water amount and a shift in the floe size distribution toward smaller floes. In the ice interior, application of the cumulative number distribution N(d), where N is the number of floes per unit area that are no smaller than some floe diameter d, shows that for d>1 km, N behaves like d−α, where α lies in the range 1.8–2.9. Our analysis also shows that this slope is not affected by the storm and is slightly larger near the ice edge.