The dielectric properties of brine in sea ice at microwave frequencies

The dc conductivity and complex dielectric constant at frequencies of 7.50, 9, 11, 30, and 40 GHz of 16 samples of sea water brine in equilibrium with sea ice with freezing temperatures ranging from -2.8degC to-25.0degC have been measured. The data is analyzed to yield parameters occurring in a Debye relaxation equation so that the dielectric constant of brine may be calculated throughout the microwave region of the electromagnetic spectrum.     

A physical model to determine snowfall over land by microwave radiometry

Falling snow is an important component of global precipitation in extratropical regions. This paper describes the methodology and results of physically based retrievals of snow falling over land surfaces. Because microwave brightness temperatures emitted by snow-covered surfaces are highly variable, precipitating snow above such surfaces is difficult to observe using window channels that occur at low frequencies (/spl nu/<100 GHz). Furthermore, at frequencies /spl nu//spl les/37 GHz, sensitivity to liquid hydrometeors is dominant. These problems are mitigated at high frequencies (/spl nu/>100 GHz) where water vapor screens the surface emission, and sensitivity to frozen hydrometeors is significant. However, the scattering effect of snowfall in the atmosphere at those higher frequencies is also impacted by water vapor in the upper atmosphere. The theory of scattering by randomly oriented dry snow particles at high microwave frequencies appears to be better described by regarding snow as a concatenation of "equivalent" ice spheres rather than as a sphere with the effective dielectric constant of an air-ice mixture. An equivalent sphere snow scattering model was validated against high-frequency attenuation measurements. Satellite-based high-frequency observations from an Advanced Microwave Sounding Unit (AMSU-B) instrument during the March 5-6, 2001 New England blizzard were used to retrieve snowfall over land. Vertical distributions of snow, temperature, and relative humidity profiles were derived from the Mesoscale Model (MM5) cloud model. Those data were applied and modified in a radiative transfer model that derived brightness temperatures consistent with the AMSU-B observations. The retrieved snowfall distribution was validated with radar reflectivity measurements obtained from a ground-based radar network.     

Airborne retrievals of snow and ice surface emissivity atmillimeter wavelengths

Passive microwave radiometers (24-157 GHz) have been flown over Baltic Sea ice and snow sites in April 1995 and on March 15, 1997. Data from these instruments are analyzed with reference to ground measurements of snow and ice conditions, and emissivity spectra are presented for 12 classifications of surface type. A simple model based on dielectric permittivity can accurately represent the microwave spectra of sea ice, but cannot be extended to the behavior of dry snow above 100 GHz without the addition of an extra term to represent volume scattering. The parameterization presented is intended to provide a background for temperature and humidity retrievals from satellite sounders, but the results will be of interest to the snow and ice remote-sensing communities     

Investigating of snow wetness parameter using a two-phase backscattering model

A two-phase backscattering model with nonsymmetrical inclusions is applied to calculate radar backscatter from a half-space of wet snow using strong fluctuation theory. Wet snow is assumed to consist of dry snow (host) and liquid water (inclusions). The shape and size of water inclusions are considered using an anisotropic and azimuth symmetric correlation function. The relationship between correlation lengths and snow wetness is presented by comparing strong fluctuation theory with the experimental data at 1.2, 8.6, 17, and 35.6 GHz. In the comparisons, correlation lengths are used as free fitting parameters. The effect of snow wetness on the backscattering coefficient is investigated. Numerical results of comparison between the two-phase backscattering model with nonsymmetrical inclusions and the experimental data are illustrated at 1.2, 8.6, 17, and 35.6 GHz. The effect of size and shape of water inclusions at different snow wetness values to backscatter level is shown. The comparison of angular response of backscattering coefficient (decibels) to wet snow between the model and the experimental data is presented at 2.6, 8.6, 17, and 35.6 GHz.     

Backscatter from snow and ice surfaces at near incident angles

The radar backscatter of natural snow surfaces was measured at 10 GHz and 35 GHz and at grazing angles from1degto0.3deg. For horizontal polarized radiation the terrain clutter per unit area (m²) at 10 GHz of a flat snow terrain decreases from -50 dB at1degto -70 dB at0.4deg. The return is approximately 10 dB lower for vertical polarized radiation. The terrain clutter was found to depend on the free water content of the snow. The radar cross sections of ice blocks placed on the snow surface is roughly proportional to the square of the area of the ice block facing the radar at 10 and 35 GHz and is approximately 20 dBsm below the return expected for a perfectly reflecting plane surface. At 95 GHz the ice blocks become diffuse reflectors.     

Microwave Dielectric Behavior of Wet Soil-Part 1: Empirical Models and Experimental Observations

This is the first paper in a two-part sequence that evaluates the microwave dielectric behavior of soil-water mixtures as a function of water content, temperature, and soil textural composition. Part I presents the results of dielectric constant measurements conducted for five different soil types at frequencies between 1.4 and 18 GHz. Soil texture is shown to have an effect on dielectric behavior over the entire frequency range and is most pronounced at frequencies below 5 GHz. In addition, the dielectric properties of frozen soils suggest that a fraction of the soil water component remains liquid even at temperatures of -24° C. The dielectric data as measured at room temperature are summarized at each frequency by polynomial expressions dependent upon both the volumetric moisture content m and the percentage of sand and clay contained in the soil; separate polynomial expressions are given for the real and imaginary parts of the dielectric constant. In Part II, two dielectric mixing models will be presented to account for the observed behavior: 1) a semiempirical refractive mixing model that accurately describes the data and requires only volumetric moisture and soil texture as inputs, and 2) a theoretical four-component mixing model that explicitly accounts for the presence of bound water.     

Bistatic scatter from rain

An experimental investigation of bistatic scatter from rain was conducted using a 143 km scatter path at frequencies of 4.5 and 7.7 GHz. The ratio of transmitted to received power (transmission loss) was measured for scattering angles ranging from6degto130deg. Simultaneous weather radar observations were made at a frequency of 1.3 GHz. Transmission loss estimates for the bistatic scatter path were computed using the weather radar data, the bistatic radar equation, and a model for the scattering cross section per unit volume of rain based upon Rayleigh scattering by an ensemble of water spheres. The measured and estimated transmission loss values were compared to test the use of the scattering model for the estimation of interference. The averaged ratio of measured-to-calculated transmission loss for the 4.5 GHz data is 1.2pm 0.4dB. The averaged ratio for the 7.7 GHz data is -1.6pm 0.5dB. Both these values are within the combined calibration uncertainties of each measurement system. The results show that the use of the simplified Rayleigh scattering cross section model for an ensemble of water spheres adequately describes bistatic scatter for a wide range of scattering angles and frequencies below 7.7 GHz for the hydrometeor types (rain, snow, and mixed rain and snow) encountered in New England.     

Snowcover Influence on Backscattering from Terrain

The effects of snowcover on the microwave backscattering from terrain in the 8-35 GHz region are examined through the analysis of experimental data and by application of a semiempirical model. The model accounts for surface backscattering contributions by the snow-air and snow-soil interfaces, and for volume backscattering contributions by the snow layer. Through comparisons of backscattering data for different terrain surfaces measured both with and without snowcover, the masking effects of snow are evaluated as a function of snow water equivalent and liquid water content. The results indicate that with dry snowcover it is not possible to discriminate between different types of ground surface (concrete, asphalt, grass, and bare ground) if the snow water equivalent is greater than about 20 cm (or a depth greater than 60 cm for a snow density of 0.3 g · cm-3). For the same density, however, if the snow is wet, a depth of 10 cm is sufficient to mask the underlying surface.     

Monitoring of melting refreezing cycles of snow with microwave radiometers: the Microwave Alpine Snow Melting Experiment (MASMEx 2002-2003)

A study of the melting cycle of snow was carried out by using ground-based microwave radiometers, which operated continuously 24 h/day from late March to mid-May in 2002 and from mid-February to early May in 2003. The experiment took place on the eastern Italian Alps and included micrometeorological and conventional snow measurements as well. The measurements confirmed the high sensitivity of microwave emission at 19 and 37 GHz to the melting-refreezing cycles of snow. Moreover, micrometeorological data made it possible to simulate snow density, temperature, and liquid water content through a hydrological snowpack model and provided additional insight into these processes. Simulations obtained with a two-layer electromagnetic model based on the strong fluctuation theory and driven by the output of the hydrological snowpack model were consistent with experimental data and allowed interpretation of both variation in microwave emission during the melting and refreezing phases and in discerning the contributions of the upper and lower layers of snow as well as of the underlying ground surface.     

On the estimation of snow depth from microwave radiometricmeasurements

Multiple-channel microwave radiometric measurements made over Alaska at aircraft (near 90 and 183 GHz) and satellite (at 37 and 85 GHz) altitudes are used to study the effect of atmospheric absorption on the estimation of snow depth. The estimation is based on the radiative transfer calculations using an early theoretical model of Mie scattering of single-size particles. It is shown that the radiometric correction for the effect of atmospheric absorption is important even at 37 GHz for a reliable estimation of snow depth. Under a dry atmosphere and based on single-frequency radiometric measurements, the underestimation of snow depth could amount to 50% at 85 GHz and 20-30% at 37 GHz if the effect of atmospheric absorption is not taken into account. The snow depths estimated from the 90-GHz aircraft and 85-GHz satellite measurements are found to be in reasonable agreement. However, there is a discrepancy in the snow depth estimated from the 37-GHz (at both vertical and horizontal polarizations) and 85-GHz satellite measurements     

Microwave Dielectric Behavior of Wet Soil-Part II: Dielectric Mixing Models

This paper is the second in a series evaluating the microwave dielectric behavior of soil-water mixtures as a function of water content and soil textural composition. Part II draws upon the data presented in Part 1 [13] to develop appropriate empirical and theoretical dielectric mixing models for the 1.4-to 18-GHz region. A semiempirical mixing model based upon the index of refraction is presented, requiring only easily ascertained soil physical parameters such as volumetric moisture and soil textural composition as inputs. In addition, a theoretical model accounting explicitly for the presence of a hydration layer of bound water adjacent to hydrophilic soil particle surfaces is presented. A four-component dielectric mixing model treats the soil-water system as a host medium of dry soil solids containing randomly distributed and randomly oriented disc-shaped inclusions of bound water, bulk water, and air. The bulk water component is considered to be dependent upon frequency, temperature, and salinity. The soil solution is differentiated by means of a soil physical model into 1) a bound component and 2) a bulk soil solution. The performance of each model is evaluated as a function of soil moisture, soil texture, and frequency, using the dielectric measurements of five soils ranging from sandy loam to silty clay (as presented in Part I [13]) at frequencies between 1.4 and 18 GHz. The semiempirical mixing model yields an excellent fit to the measured data at frequencies above 4 GHz. At 1.     

Brightness Temperatures of Snow Melting/Refreezing Cycles: Observations and Modeling Using a Multilayer Dense Medium Theory-Based Model

The ability of electromagnetic models to accurately predict microwave emission of a snowpack is complicated by the need to account for, among other things, nonindependent scattering by closely packed snow grains, stratigraphic variations, and the occurrence of wet snow. A multilayer dense medium model can account for the first two effects. While microwave remote sensing is well known to be capable of binary wet/dry discrimination, the ability to model brightness as a function of wetness opens up the possibility of ultimately retrieving a percentage wetness value during such hydrologically significant melting conditions. In this paper, the first application of a multilayer dense medium radiative transfer theory (DMRT) model is proposed to simulate emission from both wet and dry snow during melting and refreezing cycles. Wet snow is modeled as a mixture of ice particles surrounded by a thin film of water embedded in an air background. Melting/refreezing cycles are studied by means of brightness temperatures at 6.7, 19, and 37 GHz recorded by the University of Michigan Truck-Mounted Radiometer System at the Local Scale Observation Site during the Cold Land Processes Experiment-1 in March 2003. Input parameters to the DMRT model are obtained from snow pit measurements carried out in conjunction with the microwave observations. The comparisons between simulated and measured brightness temperatures show that the electromagnetic model is able to reproduce the brightness temperatures with an average percentage error of 3% (~8 K) and a maximum relative percentage error of around 8% (~20 K)     

Influence of land-cover category on brightness temperature of snow

A helicopter-borne multifrequency radiometer (24, 34, 48 and 94 GHz vertical polarization) was used to investigate the behavior of the brightness temperature of snow in Sodankyla (latitude: 67.41 N, longitude: 26.58 E), Northern Finland. The measurements were carried out during dry snow, wet snow, and snow-free conditions. The angle of incidence was 45° in all measurements. The measurements and the main results are presented. The analysis is focused on the effect of vegetation and land type on the brightness temperature of snow. The main topics of this paper are: (a) the general behavior of the brightness temperature of snow for different land types, (b) the effect of forest vegetation on the brightness temperature of snow, and (c) the capability of the radiometer system to monitor snow extent in forests during the melting period     

Measurement and modeling of the millimeter-wave backscatterresponse of soil surfaces

The millimeter-wave (MMW) backscatter response of bare-soil was examined by conducting experimental measurements at 35 and 94 GHz using a truck-mounted polarimetric scatterometer and by developing appropriate models to relate the backscattering coefficient to the soil's surface and volume properties. The experimental measurements were conducted for three soil surfaces with different roughnesses under both dry and wet conditions. The experimental measurements indicate that in general the backscattering coefficient is comprised of a surface scattering component σ^s and a volume scattering component σ ^v. For wet soil conditions, the backscatter is dominated by surface scattering, while for dry conditions both surface and volume scattering are significant, particularly at 94 GHz. Because theoretical surface scattering models were found incapable of predicting the measured backscatter, a semiempirical surface scattering model was developed that relates the surface scattering component of the total backscatter to the roughness parameter ks, where k=2π/λ and s is the rms height, and the dielectric constant of the soil surface. Volume scattering was modeled using radiative transfer theory with the packed soil particles acting as the host material and the air voids as the scattering particles. The combined contribution of surface and volume scattering was found to provide good agreement between the model calculations and the experimental observations     

Numerical scattering analysis for two-dimensional dense randommedia: characterization of effective permittivity

A new numerical method for determining effective permittivity of dense random media in two dimensions is presented. The core of the method is to compare the average scattered field of a random collection of scatterers confined within an imaginary boundary with the scattered field from a homogeneous dielectric of the same shape as the imaginary boundary. The two-dimensional (2-D) problem is aggressively studied to provide insight into the dependence of the method's convergence on particle size, boundary shape, and boundary dimension. A novel inverse scattering method is introduced based on the method of moments (MoM), which greatly reduces the computation time and increases the flexibility of the procedure to analyze a variety of geometries. Results from this 2-D method may be used directly to compare with theoretical methods for determining effective permittivity such as the Polder-Van Santen (1946) mixing formula or field techniques such as the quasi-crystalline approximation     

Externally heated copper vapor laser for parametric studies

The construction and operation of a reliable externally heated copper vapor laser for parametric studies is presented. A long-life high-temperature heating element is described. A temperature measurement method has been developed that is compatible with laser operation and has an accuracy ofpm20degC over the range0-1700degC. The laser is operated in the burst mode to eliminate effects of discharge hearing and enable measurement of the time evolution of laser behavior. An electrical circuit was developed which eliminates transient electrical behavior at the beginning of the burst. An example of parameter measurement, the dependence of output power on tube temperature under specific operating conditions, is also given.     

Estimation of snow water equivalence using SIR-C/X-SAR. I. Inferring snow density and subsurface properties

Algorithms for estimating dry snow density and the dielectric constant and roughness of the underlying soil or rock use backscattering measurements with VV and HH polarization at L-band frequency (1.25 GHz). Comparison with field measurements of snow density during the first SIR-C/X-SAR overpass shows absolute accuracy of 42 kg m/sup -3/ (13% relative error). For the underlying soil, comparisons with the ground scatterometer measurements showed errors of 4% by volume for soil moisture estimation and 4 mm for the surface root mean square (RMS) height. Values of snow density and the properties of the underlying soil are necessary for the estimation of snow water equivalence.     

Efficient luminescence band created in (Al, Ga)As multilayers by athermal laser processing

A new luminescence band is observed in (Al, Ga)As three-layer structures which were subjected to well-controlled CW Krion laser irradiation at power densities of 0.55 MW/cm². The new band is shifted by 90 meV to lower energies with respect to the band-to-band recombination. The total photoluminescence efficiency at room temperature can be improved by as much as 80 percent. Measurements of the luminescence at 0.8 μm and of the infrared (IR) power emitted at 3.1 μm have been performed during processing. Maximum temperature rises of320-350degC for the laser heated zone have been evaluated from these measurements. The well-defined laser threshold power, necessary to create the new luminescence centers, does not depend on the substrate temperature in the range of20-250degC. It does increase linearly with decreasing laser photon energy.     

The liquid-phase epitaxial growth of low net donor concentration (5 × 1014-5 × 1015/cm3) GaSb for detector applications in the 1.3 - 1.6 µm region

We report on the liquid-phase epitaxial (LPE) growth of low doped n-GaSb for long wavelength photodiodes. Very low net donor concentration epilayers (5 times 10^{14}-5 times 10^{15}/cm³) were grown both from undoped Ga rich solutions in the300-375degC range and by compensation using Ga-rich solutions atsim500degC with intentionally added Te. The preparation of Au-GaSb Schottky diodes forC-Vprofiling is also discussed.     

Backscattering measurements of alpine snowcovers at 5.3 and 35 GHz

This paper describes two network-analyzer (NA)-based scatterometers at 5.3 (C-band) and 35 GHz (Ka-band) as well as snowcover measurements made in the Swiss and Austrian Alps between December 1993 and January 1996. First, the setup and the mode of operation of the scatterometers are discussed. Both instruments measure the backscattering coefficients γ at hh, νν, νh, and νh polarizations and for incidence angles ranging from 0 to 70°. The accuracy of γ is generally better than ±1.8 dB, and the scatterometers are well suited for signature studies of natural surfaces. During the two years, the authors performed many backscattering measurements of natural, strongly layered snowcovers and the authors investigated relationships between γ and physical parameters of the snowcover. All measurements were collected in a signature catalogue. They report on results at 40° incidence angle. They found that the combined use of active sensors at 5.3 and 35 GHz allows the discrimination of various snowcover situations, if multitemporal information is available. In addition, they observed a relationship of γ at 5.3 GHz with the integrated column height of liquid water and dependencies of γ at 35 GHz on the height of the dry snow, on the volumetric liquid water content at the snow surface, and on the thickness of the refrozen crust at the snow surface