James J. Simpson

Multivariate Alteration Detection (MAD) and MAF Postprocessing in Multispectral, Bitemporal Image Data: New Approaches to Change Detection Studies

Abstract This article introduces the multivariate alteration detection (MAD) transformation which is based on the established canonical correlations analysis. It also proposes using postprocessing of the change detected by the MAD variates using maximum autocorrelation factor (MAF) analysis. The MAD and the combined MAF/MAD transformations are invariant to linear scaling. Therefore, they are insensitive, for example, to differences in gain settings in a measuring device, or to linear radiometric and atmospheric correction schemes. Other multivariate change detection schemes described are principal component type analyses of simple difference images. Case studies with AHVRR and Landsat MSS data using simple linear stretching and masking of the change images show the usefulness of the new MAD and MAF/MAD change detection schemes. Ground truth observations confirm the detected changes. A simple simulation of a no-change situation shows the accuracy of the MAD and MAF/MAD transformations compared to principal components based methods.

Failures in Detecting Volcanic Ash from a Satellite-Based Technique

Abstract Immediate and accurate detection of airborne volcanic ash is an operational imperative of the aviation industry, especially jet aircraft. Ash encounters place passengers aboard these aircraft at severe risk and significantly impact, via forced rerouting, both the safety and profit margins of freight carriers due to their limited fuel supply. Moreover, the airlines can suffer high economic costs for repair and replacement of equipment. Operational detection and tracking of volcanic ash by most national weather services has relied heavily on a split window differencing technique of thermal longwave infrared channels on currently operational satellites. Unfortunately, prior work on volcanic ash detection has not emphasized the dynamical interaction between the erupting volcano and the effects of overlying atmospheric water vapor, phreatic and phreatomagmatic water sources. Six volcanic ash eruptions from around the globe were chosen for study because they have wide variation in ambient atmospheric water vapor, available ground and surface water and different magma types. Results show that the present differencing technique is not uniformly effective in properly classifying volcanic ash pixels in the satellite scene and often falsely interprets meteorological clouds as volcanic ash clouds and conversely. Moreover, it is not always a robust early detector, an operational aviation requirement. Seasonal variability in global integrated atmospheric water vapor, coupled with the geographical distribution of currently active volcanoes, suggests the concerns discussed herein with regard to six specific eruptions, have applicability to the global aviation industry. Operational implications are discussed and a strategic proposal is presented on necessary steps to improve detection.

The Relationship between Downward Irradiance and Upper Ocean Structure

Abstract The relationship between downward irradiance and upper ocean structure has been studied using a numerical model. Two general classes of irradiance parameterizations were utilized. The first (case I) employed a single attenuation length while the second (cast II) involved two attenuation lengths. The latter formulation provided for enhanced absorbance in the upper few meters. Wind speeds of 0, 1, 2, 3, 4, 5, 10 and 20 m.s−1 were used for the simulations in order to characterize heat versus wind dominated regimes. A one-dimensional second moment turbulent closure model was selected for the study so that heat could be treated differentially with depth. The case II results indicated warmer surface temperatures, shallower mixed layers, and more intense thermoclines than case I for wind speeds <10 m s−1. Results converged for higher wind speeds. There was considerably greater sensitivity to wind speed for case II when compared with case I. Mean horizontal velocity as well as thermal structure was sensi...

An offshore study in the California current system preface

Letude dun tourbillon de mesoechelle, quasi-permanent et quasi-stationnaire, dans le systeme du courant de Californie, comprend quatre parties: dynamique au sein du systeme, manifestations a la surface, structure chimique et distribution du plancton

An offshore eddy in the California current system Part I: Interior dynamics

Abstract From January 9 to 17, 1981, detailed observations of the horizontal and vertical structure beneath one of the quasi-permanent semi-stationary mesoscale offshore eddy signatures in the California Current System (CCS) discussed by Bernstein, Breaker and Whritner (1977), Burkov and Pavlova (1980), and Simpson (1982) were made. The vertical sections of temperature and density show the presence of three-layer system. A subsurface warm-core eddy, whose diameter is about 150 km at the 7°C isotherm, is the dominant feature. A warm surface layer, which extends to a depth of 75 m, lies over the eddy. Between the warm surface layer and the subsurface warm-core eddy, there is a cold-core region which extends to a depth of about 200 m. There is a high degree of symmetry about the vertical axis of rotation. Vertical sections of salinity and dissolved oxygen are entirely different from sections of temperature and density. Diagrams of water mass characteristics confirm that the core of the eddy, found between 250–600 m, consists of inshore water from the California Undercurrent (CU). Below about 700 m, local waters from the Deep Poleward Flow (DPF) have been incorporated into the eddy. The observed distributions of properties (T, S, δ θ , O 2 ) are inconsistent with a single, local generation process for the eddy system. Radial distributions of angular velocity, normalized gradient velocity and relative vorticity support the use of a Gaussian radial height field as an initial condition in eddy models. Possible reasons why CCS eddies may differ dynamically from Gulf Stream rings are given in the text. At the time the observations were made, the system as a whole was in near geostrophic balance. Local geostrophic balance, however, cannot explain the observed distribution of properties and structure. The observed symmetry in the structure of the eddy system, chemical evidence (Simpson, 1984), biological distributions (Haury, 1984) and satellite images of the CC (Koblinsky, Simpson and Dickey, 1984) suggest that lateral entrainment of warm (oceanic) and cold (coastal) water into the upper two layers of the three-layer system by the subsurface eddy is a likely generation mechanism for the cold-core region. The coastal origin of the frontal structure along the northeastern quadrant and the oceanic origin of the frontal structure along the southwestern quadrant of the eddy system further support lateral entrainment as a generation mechanism for the cold core. This entrainment makes the CCS eddy system different from cold-core rings in the Gulf Stream and rather similar to some warm-core eddies found in the East Australian Current. The presence of CU water in the core of this eddy raises the question of how CU water was transported from the continental slope. Eddy generation mechanisms, other than baroclinic instability of the CC, may be required to explain the distribution, persistence, and core composition of offshore mesoscale eddies in the CCS. There is evidence that barotropic, in addition to baroclinic, processes may be important.

Alternative Parameterizations of Downward Irradiance and Their Dynamical Significance

Abstract The effect of solar flux divergence on upper ocean dynamics and energetics under both low and high wind speeds was determined using four different parameterizations of downward irradiance. The first (case I) involved only one attenuation length, the second (case II) involved two attenuation lengths, the third (case III) used a spectral decomposition of the incident solar flux over nine wavelength bands, and the fourth (case IV) used an arctangent model of downward irradiance. The Mellor-Yamada turbulence closure scheme (level 2½) was used for the simulations. Cases II–IV predict the existence of an intensified shallow shear zone which is consonant with recent observations. At low wind speeds, the turbulent energy budget is dominated by shear production, dissipation and the diffusion of turbulent kinetic energy, regardless of parameterization. At high wind speeds, shear production is balanced by dissipation. Specific recommendations are made for parameterizing the downward irradiance in the contex...

An offshore eddy in the California current system part II: Surface manifestation

Abstract Ship and satellite observations taken over the last thirty years show that mesoscale patterns of sea surface temperature (SST) in the California Current System are consistently found throughout the year and usually occur in approximately the same geographical locations. Typically, these patterns are more pronounced in fall/winter than in spring/summer. The temporal and spatial characteristics of these persistent feature were examined with satellite infrared (IR) measurements during winter 1980–1981. In January 1981, a ship surveyed the vertical structure of several physical, chemical, and biological parameters beneath one of these SST features centered near 32°N, 124°W. The surface IR pattern had a length scale of 200 km and a time scale of about 100 days. It disintegrated following the first two storms of the winter season. Motion studies of the pattern in late October indicated an anticyclonic rotation with maximum velocities of 50 cm s −1 at 50 km from the axis of rotation. As a unit, the pattern advected southward with an average speed of 1 cm s −1 . Thermal fronts, determined from the satellite imagery, were strongest (0.4°C km −1 ) along the rim of the pattern and were advected anticyclonically with the pattern; their length scales were 20–30 km in the along-front direction and less than 10 km wide. The hydrographic data revealed a three-layer structure beneath the surface pattern; a 75 m deep surface layer, a cold-core region from 75 to 200 m depth, and a warm-core eddy extending from 250 to 1450 m. The anticyclonic motion of the surface layer was caused by a geostrophic adjustment to the surface dynamic height anomaly produced by the subsurface warm-core eddy. The IR pattern observed from space reflects the horizontal structure of the surface layer and is consistent with a theoretical model of a mean horizontal SST gradient perturbed by a subsurface density anomaly. Ship of opportunity SST observations collected by the National Marine Fisheries are shown to resolve mesoscale patterns. For December 1980, the SST pattern near 32°N, 124°W represented a 2°C warm anomaly compared with the 20-year mean monthly SST pattern.

On the accurate detection and enhancement of oceanic features observed in satellite data

Abstract Various methods to accurately and efficiently compute gradients, detect edges, and enhance features in satellite data are presented. Error rate criteria and spatial localization criteria are used to statistically evaluate the performance of these methods for both noise-free and noise-contaminated images. Results from the tests using noise-contaminated images can be used to infer the performance of these operators on AVHRR images with partially undetected clouds. Operators most suited to remotely-sensed applications (e.g., support of real-time experiments at sea, image classification studies, and updating numerical models of ocean - atmosphere circulation) are identified, and a set of practical considerations for accurate gradient and/or edge detection in remotely-sensed data is given.

Automated cloud screening of AVHRR imagery using split-and-merge clustering☆

Previous methods to segment clouds from ocean in AVHRR imagery have shown varying degrees of success, with nighttime approaches being the most limited. An improved method of automatic image segmentation, the principal component transformation split-and-merge clustering (PCTSMC) algorithm, is presented and applied to cloud screening of both nighttime and daytime AVHRR data. The method combines spectral differencing, the principal component transformation, and split-and-merge clustering to sample objectively the natural classes in the data. This segmentation method is then augmented by supervised classification techniques to screen clouds from the imagery. Comparisons with other nighttime methods demonstrate its improved capability in this application. The sensitivity of the method to clustering parameters is presented; the results show that the method is insensitive to the split-and-merge thresholds.

The February 2001 Eruption of Mount Cleveland, Alaska: Case Study of an Aviation Hazard

Mount Cleveland, Alaska (528499N, 1698579W), located on Chuginadak Island, erupted on 19 February 2001. The atmosphere‐volcanic plume interactions that occurred as part of this event led to several serious encounters of commercial aircraft with the ash. A number of continental and oceanic air traffic control areas were affected. Here, a detailed case study of the eruption, subsequent movement of the airborne plume, and operational response is presented. The likelihood of such encounters in the future may be reduced as a result of lessons learned from this event. Some potential new assets for improving the detection of and response to the airborne volcanic ash hazard to aviation also are discussed.