Showing papers by "Robert Evans published in 1993"

Asymptotic decay of liquid structure: oscillatory liquid-vapour density profiles and the Fisher-Widom line

Abstract: Recent work has highlighted the existence of a unified theory for the asymptotic decay of the density profile ρ(r) of an inhomogeneous fluid and of the bulk radial distribution function g(r). For a given short-ranged interatomic potential ρ(r) decays into bulk in the same fashion as g(r), i.e. with the same exponential decay length (α0/-1) and, for sufficiently high bulk density (ρb) and/or temperature (T), oscillatory wavelength (2π/α1). The quantities α0 and α1 are determined by a linear stability analysis of the bulk fluid; they depend on only the bulk direct correlation function. In this paper we reintroduce the concept of the Fisher-Widom (FW) line. This line was originally introduced, in say the (ρb, T plane, as that which separates pure exponential from exponentially damped oscillatory decay of g(r). We explore the relevance of the FW line for the form of the density profile at a liquid-vapour interface. Using a weighted density approximation (WDA) density functional theory we locate the FW line fo...

Calibration of advanced very high resolution radiometer infrared channels: A new approach to nonlinear correction

Abstract: A detailed reanalysis of the calibration procedures for the National Oceanic and Atmospheric Administration (NOAA) advanced very high resolution radiometer (AVHRR) based on thermal-vacuum test data was performed as part of the National Air and Space Administration/NOAA AVHRR Pathfinder Project. This effort, a followup to work by Brown et al. (1985), was motivated by the finding that the AVHRR instruments on several NOAA platforms have been routinely operated outside the range of thermal-vacuum test results, and thus one could not interpolate nonlinear corrections directly from earlier methods. These new calibration procedures permit calculation of nonlinear temperature corrections for any AVHRR operating temperature based on a second-order polynomial regression with a total calibration accuracy relative to an external calibration standard of less than two digital counts (±0.2°C). Such an improvement is quite important to the absolute accuracy of surface thermal fields, which are derived from these data utilizing various multichannel atmospheric water vapor correction schemes. We find systematic differences in the newly derived nonlinear correction results and those reported previously by Weinreb et al. (1990) and the original reference material in the various addenda to NOAA NESS Technical Memorandum 107 (Lauritson et al., 1979). Calibration results for various AVHRR radiometers show instrument-similar corrections for each band. Radiometers on NOAA platforms 8-12 demonstrate similar nonlinearities.

Fluctuations, correlation functions and interfacial Hamiltonians for the complete wetting phase transition

Abstract: Exact statistical mechanical sum rules are combined with thermodynamic scaling arguments to determine the leading-order singular contributions to the transverse moments of the density-density correlation function G of a simple fluid undergoing a complete wetting phase transition, from off-bulk coexistence, at a structureless substrate (wall). Contrary to our earlier suggestion, capillary-wave-like fluctuations do manifest themselves throughout the wetting film and lead to a transverse correlation length ξ‖(a), for both particles near the wall, which diverges with the same critical exponent as ξ‖, the correlation length appropriate for both particles in the depinning liquid-gas interface. Explicit results from a mean-field (density functional) calculation are consistent with our general predictions. In the weak fluctuation regime (d < 3), scaling methods are used to derive the short distance expansions of the density profile and moments of G for short ranged forces. These expansions, which describe singula...

Comment on 'Aerosol and Rayleigh radiance contributions to Coastal Zone Colour Scanner images' by Eckstein and Simpson

Abstract: In a recent paper Eckstein and Simpson describe what they believe to be serious difficulties and/or errors with the CZCS (Coastal Zone Color Scanner) processing algorithms based on their analysis of seven images. Here we point out that portions of their analysis, particularly those dealing with multiple scattered Rayleigh radiance, are incorrect. We also argue that other problems they discuss have already been addressed in the literature. Finally, we suggest that many apparent artifacts in CZCS-derived pigment fields are likely to be due to inadequacies in the sensor band set or to poor radiometric stability, both of which will be remedied with the next generation of ocean color sensors.

A comparison of GEOSAT altimeter inferred currents and measured flow at 5400 m depth in the Argentine Basin

Abstract: A comparison is made between an 11 month duration current meter record obtained near the bottom at 40°27′S, 49°25′W in the mid-Argentine Basin in water depth 5400 m, and near-surface flow inferred from GEOSAT altimeter data. The GEOSAT has a repeat path time of 17 days, and inferred near-surface flow features with periods of 34 days or less should not be detectable. Thus the current meter record was filtered with a 34 day low pass filter. Since only fluctuating flows can be inferred with the GEOSAT altimeter data, the deep recorded mean current was added to the satellite-inferred flow for comparison purposes. The deep flow and the altimeter-inferred near-surface flow agree remarkably well, suggesting that for time scales ⩾34 days the flow was nearly barotropic in this region of the South Atlantic.