David L. Porter

Mesoscale and upper ocean variabilities during the 1989 JGOFS bloom study

Abstract Altimetric data from Geosat and some critical hydrographic measurements were used to estimate in real time the mesoscale physical oceanographic environment surrounding the Joint Global Ocean Flux Study (JGOFS) 1989 North Atlantic Bloom Experiment. Three cyclonic eddies, including an exceptionally large one, evolved and interacted over the 10 weeks of observations. Subsequent analysis of all available hydrographic data confirmed the real time estimates and provided further quantitative information concerning the mesoscale and submesoscale structure of the upper ocean. Remotely sensed indicators of near-surface chlorophyll content reveal significant biological variability on these wavelengths. The altimetric and hydrographic data have been assimilated into a dynamical model to produce optimal estimates of physical fields of interest as they evolve in time for use in physical and biological process studies.

A synthetic geoid validation of Geosat mesoscale dynamic topography in the Gulf Stream region

A validation study is carried out for the Geosat altimeter in the Gulf Stream meander and ring region. A synthetic geoid, containing the medium scales of interest for mesoscale research, is constructed by subtracting an independent estimate of the oceanographic mean dynamic topography from the altimetric sea surface. The oceanographic mean dynamic topography which is concurrent with the altimetric mean is derived from a year of real-time nowcasts and forecasts (Gulfcast) based on assimilating a composite data set into a dynamical model. Using the synthetic geoid, absolute dynamic topography from Geosat and Gulfcast for a year and 4 months in 1988 and 1989 compare very well in signal shape and feature locations. Sixteen cases of comparison of dynamic heights derived from AXBTs along Geosat tracks in 1987 are made with Geosat absolute topography. The rms difference is less than 10 cm. Thus the comparison of these two independent data sets validates the altimeter and the synthetic geoid within the noise and error levels of the system.

Continental Shelf Parameters Inferred from SAR Internal Wave Observations

Abstract This paper presents preliminary results on the use of synthetic aperture radar (SAR) imagery to extract oceanographic information about the continental shelf. From late spring through early fall the thermocline on the shelf is fully developed and the entire Mid-Atlantic Bight is replete with internal waves. The surface manifestation of these internal waves is observed using the SAR on board the European Research Satellite-1. The depth of the pycnocline and the density of the surface layer can be estimated from the dispersion relation obtained from a two-layer fluid model. The other parameters that are required to make this estimate are obtained by assuming climatological bottom density, estimating the speed of propagation from the tidal-generated wave packets, and calculating the depth of the pycnocline based on the isobath where the internal waves are dissipated. This robust method for obtaining quantitative information about the structure of the shelf’s water column shows good agreement with th...

Radar-derived interferometric surface currents and their relationship to subsurface current structure

Radar-derived ocean surface currents are analyzed in conjunction with in situ acoustic Doppler current profiler (ADCP) measurements. The interferometric measurements were collected by an X-band imaging Doppler radar in a manner analogous to those of along-track interferometric synthetic aperture radar (ATI-SAR). While the advent of ATI-SAR has provided a new, potentially powerful technique for current mapping, the relationship between surface currents and interferometric velocity measurements is not yet clearly understood. This paper presents comparisons between radar-derived and in situ current measurements. To develop a precise method for estimating the surface current from interferometric measurements, the influence of long wave orbital velocities and the influence of Bragg resonant waves are studied. We find that coupling between the orbital velocity and backscattered power (i.e., the modulation transfer function) can bias surface current estimates, potentially by up to 20 cm s -1 in an upwind viewing orientation. Furthermore, experimental observations verify a cos 2n (0/2) analytical model for the directional spreading of Bragg resonant waves. Extending our analysis to include subsurface currents, case studies are presented under varying environmental conditions for which the vertical current structure changes considerably. Analysis of radar imagery yields both radial surface currents and vector subsurface current estimates derived from long wave dispersion characteristics. Combining these with coincident ADCP measurements yields a vertical profile of current. Using these measurement techniques, we make several observations within the upper meter of the ocean. These profiles reveal the sensitivity of X-band interferometric measurements to wind-drift and the near-surface current structure.

WITTEX: an innovative three-satellite radar altimeter concept

The Water Inclination Topography and Technology Experiment (WITTEX) consists of three TOPEX-class radar altimeters on individual satellites in the same orbit plane. If built with delay-Doppler radar altimeter technology, the WITTEX satellites will be sufficiently small that all three can be launched simultaneously by one vehicle. Even though the WITTEX satellites will be in a common inertial orbit plane, Earth rotation separates their tracks at nadir. Cross-track separation will be proportional to intersatellite spacing. WITTEX will provide near-simultaneous and accurate measurement of sea surface heights along three parallel tracks, providing oceanographic data not previously available, including the two-dimensional (2-D) surface gradient. WITTEX offers a variety of beneficial solutions to the classic time/space sampling tradeoff that confronts oceanic radar altimetry.

Topological and statistical behavior classifiers for tracking applications

This paper introduces a method to integrate target behavior into the multiple hypothesis tracker (MHT) likelihood ratio. In particular, a periodic track appraisal based on behavior is introduced. The track appraisal uses elementary topological data analysis coupled with basic machine-learning techniques, and it adjusts the traditional kinematic data association likelihood (i.e., track score) using an established formulation for feature-aided data association. The proposed method is tested and demonstrated on synthetic vehicular data representing an urban traffic scene generated by the Simulation of Urban Mobility package. The vehicles in the scene exhibit different driving behaviors. The proposed method distinguishes those behaviors and shows improved data association decisions relative to a conventional, kinematic MHT.

Abyss-Lite: improved bathymetry from a dedicated small satellite delay-Doppler radar altimeter

We describe the rationale, scientific basis, and implementation of a mission to map the oceans bottom topography with a spatial resolution of 6 km based on a high-precision radar altimeter on a dedicated free-flying spacecraft.

Remotely sensed ocean observations of the Coastal Mixing and Optics site from synthetic aperture radars and advanced very high resolution radiometers

In this paper, we describe the remote sensing data set collected in support of the Coastal Mixing and Optics experiment (CMO). The data set consists of 33 synthetic aperture radar (SAR) images concentrated during the main CMO experimental period supplemented by nearly concurrent advanced very high resolution radiometer imagery. Discussion of how the two-dimensional spatial view of the ocean surface provided by the imagery can greatly reduce the difficulty in interpreting in situ data from point sensors or ship transects is presented. After a brief discussion of the imaging physics we examine interesting signatures ubiquitous in SAR imagery of the coastal ocean, such as internal wave packets, flow over variable bottom topography, and boundary layer turbulence. In particular, we demonstrate how the SAR imagery can be used in conjunction with the mooring data to estimate the speed and propagation direction of coastal internal waves within the study area. Furthermore, we use a recently developed technique to show how estimates of the thermocline depth and surface layer density can be obtained using the SAR imagery and a simple two-layer model of the water column stratification.

Measurements of dynamic topography during synop utilizing a Geosat synthetic geoid

Dynamic topography estimates were obtained for the Gulf Stream region employing a previously derived synthetic geoid and Geosat radar altimeter measurements. Ascending tracks passed through the central and eastern arrays of the Synoptic Ocean Prediction experiment. By utilizing the geoid estimate instead of the mean altimetric signal, the axis position and velocity can be computed. A time series of the dynamic height profiles crossing the Gulf Stream and a table of the positions of the axis of the Stream along two tracks are given. Time series of the maximum velocity perpendicular to the ground track were computed for both arrays. It was found that there was a seasonal cycle in this time series with a maximum in November.

Airborne expendable bathythermograph survey of the Kuroshio Extension and comparison with simultaneous altimeter measurements during the Geosat Exact Repeat Mission

Extensive airborne expendable bathythermograph (AXBT) sections across the Kuroshio Extension during the Geosat Exact Repeat Mission (ERM) were used to assess the ability of the Geosat altimeter to monitor the locations of the Kuroshio and its associated cold- and warm-core eddies. The AXBT sections in themselves provide detailed descriptions of the Kuroshio Extension to about 160°E. These observations demonstrated that surface or near-surface temperatures in this area may lead to incomplete and deceptive descriptions of the deeper thermal structure. Kuroshio-eddy interactions in particular were not accurately portrayed by the near-surface observations. For this reason, accurate mapping of the location of the Kuroshio Extension and of nearby eddies may not be practical from infrared (IR) imagery alone but requires augmentation from other measurement systems such as altimetry. In the present analysis, an estimate of the sea surface height was obtained from the altimeter ranges by time averaging sea surface heights in lieu of a geoid estimate. Sea surface height anomalies computed using the mean altimetric sea surface (i.e., the geoid plus the temporal mean oceanographic signal) can be significantly different from the actual oceanographic topography induced by mesoscale oceanographic features. Nevertheless, it is shown that the positions of the Kuroshio Extension and of the stronger cold- and warm-core eddies associated with the Kuroshio Extension are easily located from these altimetric sea surface anomalies.