Richard D. West

Error sources and feasibility for microwave remote sensing of ocean surface salinity

A set of geophysical error sources for the microwave remote sensing of ocean surface salinity have been examined. The error sources include the sea surface temperature, sea surface roughness, atmospheric gases, ionospheric Faraday rotation, and solar and Galactic emission sources. It is shown that the brightness temperature effects of a few kelvin can be expected for most of these error sources. The key correction requirements for accurate salinity measurements are the knowledge accuracy of 0.5/spl deg/C for the sea surface temperature (SST), 10 mbar for the surface air pressure, 2/spl deg/C for the surface air temperature, 0.20 accuracy for the Faraday rotation, and surface roughness equivalent to 0.3 m s/sup -1/ for the surface wind speed. We suggest the use of several data products for corrections, including the AMSR-type instruments for SST and liquid cloud water, the AMSU-type product for air temperature, the scatterometer products or numerical weather analysis for the air pressure, coincidental radar observations with 0.2 dB precision for surface roughness, and on-board polarimetric radiometer channel for Faraday rotation. The most significant sky radiation is from the Sun. A careful design of the antenna is necessary to minimize the leakage of solar radiation or reflection into the antenna sidelobes. The narrow-band radiation from Galactic hydrogen clouds with a bandwidth of less than 1 MHz is also significant, but can be corrected with a radio sky survey or minimized with a notched (band-rejection) filter centered at 1.421 GHz. The other planetary and Galactic radio sources can also be flagged with a small data loss. We have performed a sampling analysis for a polar-orbiting satellite with 900 km swath width to determine the number of satellite observations over a given surface grid cell during an extended period. Under the assumption that the observations from different satellite passes are independent, it is suggested that an accuracy of 0.1 psu (practical salinity unit) is achievable for global monthly 10 latitude by 10 longitude gridded products.

Polarimetric scatterometry: a promising technique for improving ocean surface wind measurements from space

Spaceborne wind scatterometers provide useful measurements of ocean surface winds and are important to climatological studies and operational weather forecasting. Past and currently planned scatterometers use measurements of the copolarized backscatter cross-section at different azimuth angles to infer ocean surface wind speed and direction. Although successful, current scatterometer designs have limitations such as degraded wind performance in the near-nadir and outer regions of the measurement swath and a reliance on external wind information for vector ambiguity removal. Theoretical studies of scattering from the wind-induced ocean surface indicate that polarimetric measurements provide orthogonal and complementary directional information to aid the wind retrieval process. In this paper, potential benefits of making polarimetric backscatter measurements to improve wind retrieval performance are addressed. To investigate the performance of a polarimetric scatterometer, a modified version of the SeaWinds end-to-end simulator at the Jet Propulsion Laboratory (JPL), Pasadena, CA, is employed. To model the effect of realistic measurement errors, expressions for polarimetric measurement variance and bias are derived. It is shown that a polarimetric scatterometer can be realized with straightforward and inexpensive modifications to a current scanning pencil-beam scatterometer system such as SeaWinds. Simulation results show that such a system ran improve wind performance in the nadir region and eliminate the reliance on external wind information.

Sea-ice characterization measurements needed for testing of microwave remote sensing models

The nature and accuracy of ice-characterization measurements needed to test two microwave backscattering models are clarified by examining the sensitivities of these models to variations in the geophysical parameters they require as input. First, the Bragg, or small perturbation, model for rough surface scattering, which appears appropriate for backscattering from new ice types at L-band, is considered. The sensitivities of this model to variations in the dielectric constant of the ice and to the power spectrum of surface roughness are examined. The dense-medium radiation-transfer model at X-band is considered for backscattering from air bubbles embedded in multilayer ice. The sensitivities of this model to air-bubble size, air-volume fraction, and dielectric loss in the ice are examined. Based on these sensitivities, quantitative characterization guidelines for model testing are discussed. >

Microwave emission from density-stratified Antarctic firn at 6 cm wavelength

Previous observations have shown spatial covariances between microwave emission from Antarctic firn at 6cm wavelength, physical firn temperature and firn-density stratification. Such observations motivate us to understand the physics underlying such covariances and, based on that understanding, to develop estimation methods for firn temperature and layering parameters. We present here a model for 6cm emission from firn in which density, and therefore dielectric permittivity, varies randomly in discrete layers with mean thicknesses on the order of centimeters. The model accounts for depth profiles of the physical temperature, mean density and variance of random density fluctuations from layer to layer. We also present a procedure to estimate emission-model input parameters objectively from in situ density-profile observations, as well as uncertainties in the input parameters and corresponding uncertainties in theoretical brightness-temperature predictions. We compare emission-model predictions with ground-based observations at four diverse sites in Antarctica which span a range of accumulation rates and other parameters. We use coincident characterization data to estimate model inputs. At two sites, layered-medium emission-model predictions based on the most probable input parameters i.e. with no model tuning agree with observations to within 3.5% for incidence angles ≤50°. Corresponding figures for the other two sites are 7.5% and 10%. However, uncertainties in the input parameters are substantial due to the limited length and depth resolution of the characterization data. Uncertainties in brightness-temperature predictions are correspondingly substantial, Thus brightness-temperature predictions for the last-mentioned sites based on only slightly less probable input parameters are also in close agreement with observations, The significance of agreements and discrepancies could be clarified using characterization measurements with finer depth resolution.

Dense medium radiative transfer theory for two scattering layers with a Rayleigh distribution of particle sizes

Dense medium radiative transfer theory is applied to a three-layer model consisting of two scattering layers overlying a homogeneous half space with a size distribution of particles in each layer. A model with a distribution of sizes gives quite different results than those obtained from a model with a single size. The size distribution is especially important in the low frequency limit when scattering is strongly dependent on particle size. The size distribution and absorption characteristics also affect the extinction behavior as a function of fractional volume. Theoretical results are also compared with experimental data. The sizes, permittivities, and densities used in the numerical illustrations are typical values for snow. >

Dual-polarized Ku-band backscatter signatures of hurricane ocean winds

The Ku-band dual-polarized backscatter signatures of ocean surfaces are described in this paper with the airborne scatterometer measurements collected in the Hurricane Ocean Wind Experiment in September 1997. The data collected from flights over Hurricane Erika provide a direct evidence that there are wind direction signals in the vertically and horizontally polarized Ku-band backscatter of ocean surfaces under the influence of hurricane force winds. At 46/spl deg/ incidence angle, the vertically polarized backscatter acquired at the upwind direction increases by about 1 dB as the wind speed increases from 22 m/spl middot/s/sup -1/ to 35 m/spl middot/s/sup -1/, while the horizontally polarized backscatter appears to be twice as sensitive with a change of about 2 dB. At 35 m/spl middot/s/sup -1/ wind speeds, the difference between upwind and crosswind observations of vertically polarized backscatter is about 1.5 dB, smaller than the 2 dB difference for the horizontally polarized backscatter. This demonstrates that the horizontal polarization has a greater sensitivity to wind speed and direction than the vertical polarization in the high wind regime. The data also suggest that the upwind and downwind asymmetry of Ku-band backscatter decreases with increasing wind speed and can fall below 0 dB at small incidence angles (<35/spl deg/) for the vertical polarization. A combined interaction of the geometric optics scattering and the short wave modulation by long waves is proposed to interpret this phenomenon and appears to agree with the dependence of the signature on incidence, wind speed, and polarization. The aircraft flight data support the feasibility of dual-polarized Ku-band radar for hurricane ocean wind measurements, although the data do suggest a reduced wind speed and direction sensitivity in the high wind regime. Also, the differing polarization backscatter signatures-suggest the relative contributions of various surface scattering mechanisms. An improved Ku-band GMF is described.

Propagation constant and the velocity of the coherent wave in a dense strongly scattering medium

Frequency- and time-domain experiments are conducted to study the effective propagation constant of the coherent wave in a dense strongly scattering medium. A wide-band microwave signal (10-40 GHz) is propagated through randomly distributed glass spheres with a 5.73 mm average diameter and separated into incoherent and coherent fields. The real and imaginary parts of the propagation constant are obtained from the coherent field. The narrow size distribution of the particles enables the authors to study scattering from the Rayleigh region through the Mie resonance scattering region. The results of the experiments are compared to independent scattering, effective-field approximation (Foldys), and the higher order quasi-crystalline approximation (QCA) using Mie scattering coefficients and the Percus-Yevick approximation for the pair-distribution function. The phase and group velocities of the coherent wave are obtained from the effective propagation constant and compared with theory. In addition, the velocity of the coherent wave in random media is measured using the time-domain technique. It is shown that the velocity of the coherent wave in random media is neither phase nor group velocity.

The influence of layering and grain size on microwave emission from polar firn

This paper studies the relative influence of scattering by ice grains and by density stratification on microwave emission at 37 GHz and 6.6 GHz. Polar firn exhibits density stratification on centimeter depth scales, and variable particle sizes on sub-millimeter length scales. At 37 GHz, the dominant scattering mechanism is particle scattering from the ice grains. Good agreement between theory and satellite observations can be obtained with particle scattering effects alone. At 6.6 GHz, the dominant scattering mechanism is levered scattering from the abrupt interfaces between layers with different densities. Including lever scattering gives improved agreement between theory and satellite observations compared to results obtained with particle scattering alone.<<ETX>>

On Microwave Sea Ice Signature Modeling: Connecting Models to the Real World

Signature modeling for sea ice is based partly on an idealized representation of the actual sea ice. Scattering is computed for the idealized medium in the expectation that the result mimics the actual scattering. In idealizing the scattering medium, choices must be made to emphasize some features at the expense of others; the choices are not straightforward, but rather depend on judgement. The choices can strongly affect computed signatures and their agreement with observations, regardless of the underlying merit of approximations in the scattering model itself. We present a two quantitative examples. The first comes from the 1988 CRRELEX experiment and involves an artificial grey ice sheet. We use the many-layer strong fluctuation theory to compute effective emissivities based on two plausible physical models for the ice sheet. The differing physical models for depth variation of brine volume, as well as particulars of brine pocket geometry cause qualitatively differing signature behavior. The second example comes from the 1988 Coordinated Eastern Arctic Experiment (CEAREX) and involves old ice. We use dense medium radiative transfer @MRT) to compute backscattering cross sections at 10 GHz. The ice in this case is so bubbly that it is unclear how to derive a reasonable discrete scatterer model. We investigate the effects of differing assumptions, finding that differing size distribution assumptions can easily lead to cross sections differing by more than 7 dB. We argue that signature modeling for sea ice requires some hard thinking about how we represent the actual sea ice, in addition to the consideration of various scattering approximations that has so far received the greater share of modelers’ attention.

Extinction Behavior Of Snow Between 18 GHz And 90 GHz: Comparison Between Theory And Experiments

This paper studies the frequency dependence of the extinction rate in snow for the frequency range of 18 to 90 GHz. The results of three approaches are compared: independent scattering, dense medium theory using the quasicrystalline approximation (QCA), and dense medium theory using the quasicrystalline approximation with coherent potential (QCA-CP). The latter two theories predict an extinction rate that is lower than independent scattering for most cases. &CA-CP in particular compares very well with Monte Carlo simulations, while QCA shows promising results when compared with measured data. INTRODUCTION The extinction rate of snow is an intrinsic property used in different versions of radiative transfer theory. Thus, it is important to study this basic intrinsic property which is unaffected by other parameters such as snow layer thickness, the permittivity of the lower half space, etc. In this paper, we study the frequency dependence of the extinction rate between 18 and 90 GHz. In this frequency range, volume scattering is an important part of the extinction rate. Snow is modelled as a random distribution of ice particles with a size distribution. The three theories used are: independent scattering, dense medium theory using &CA, and dense medium theory using QCA-CP. The latter two theories predict an extinction rate that is lower than independent scattering for most cases. Such behavior has been verified by controlled Iaboratory experiments, and by Monte Carlo simulations of direct solutions of Maxwell’s equations of a medium containing up to 4000 particles. Theoretical results are compared with results from controlled laboratory experiments, Monte Carlo simulations, and extinction measurements of dry snow. The emphasis is on matching the data for a sample over the entire frequency range with one set of physical parameters.