The determination of surface salinity with the European SMOS space mission

The European Space Agency Soil Moisture and Ocean Salinity (SMOS) mission aims at obtaining global maps of soil moisture and sea surface salinity from space for large-scale and climatic studies. It uses an L-band (1400-1427 MHz) Microwave Interferometric Radiometer by Aperture Synthesis to measure brightness temperature of the earth's surface at horizontal and vertical polarizations (T/sub h/ and T/sub v/). These two parameters will be used together to retrieve the geophysical parameters. The retrieval of salinity is a complex process that requires the knowledge of other environmental information and an accurate processing of the radiometer measurements. Here, we present recent results obtained from several studies and field experiments that were part of the SMOS mission, and highlight the issues still to be solved.

Related papers:

title: An improved model for the dielectric constant of sea water at microwave frequencies
abstract: The advent of precision microwave radiometry has placed a stringent requirement on the accuracy with which the dielectric constant of sea water must be known. To this end, measurements of the dielectric constant have been conducted at S -band and L -band with a quoted uncertainty of tenths of a percent. These and earlier results are critically examined, and expressions are developed which will yield computations of brightness temperature having an error of no more than 0.3 K for an undisturbed sea at frequencies lower than X -band. At the higher microwave and millimeter wave frequencies, the accuracy is in question because of uncertainties in the relaxation time and the dielectric constant at infinite frequency.

title: Sea Surface Salinity: The Next Remote Sensing Challenge
abstract: A brief history of salinity remote sensing is presented. The role of sea surface salinity (SSS) in the far north Atlantic and the influence of salinity variations on upper ocean dynamics in the tropics are described. An assessment of the present state of the technology of the SSS satellite remote sensing is given.

title: Error sources and feasibility for microwave remote sensing of ocean surface salinity
abstract: 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.

title: Sea surface salinity from space: Science goals and measurement approach
abstract: Aquarius is a NASA/Earth System Science Pathfinder (ESSP) mission that proposes to make the first-ever global measurements of sea surface salinity. These measurements will enable improved understanding of oceanic thermohaline circulation and of the changes in oceanic circulation that are related to seasonal to interannual climate variability. Aquarius science goals also address tropical ocean-climate feedbacks and freshwater budget components of the coupled ocean-atmosphere system. These oceanographic science requirements for Aquarius dictate measurements of global sea surface salinity that are accurate to 0.2–0.3 psu, as averaged monthly and over 100–200 km areas. Key aspects of the Aquarius salinity mission design include the instrument with its high-stability L-band radiometers, the precise calibration of the measurements, and the salinity retrieval algorithm. The Aquarius mission will meet the science needs by providing complete global coverage of ocean surface salinity, with an 8 day cycle of observations using a three beam, L-band radiometer/scatterometer flying in a 6 am/6 pm polar orbit. This conceptual design has been verified using observations from aircraft flight instruments. The radiometer design for the instrument and the needed precise calibration is based on proven, temperature-stabilized radiometer designs with internal references, plus vicarious calibration approaches developed in the course of previous space missions.

title: Sea Surface Salinity mapping with SMOS space mission
abstract: The European Space Agency SMOS (Soil Moisture and Ocean Salinity) mission is scheduled for launch in early 2007. SMOS will exploit an innovative instrument designed as a 2D interferometer acquiring globally brightness temperatures at L-band (1.4GHz) to retrieve soil moisture fields over the land surfaces and ocean salinity fields over the oceans. Considering the exploratory nature of the salinity measurement with SMOS, the GODAE open ocean requirement (0.1, practical salinity scale, over 200 km every 10 days) represents a technically challenging objective that has been set as a goal for the mission.

title: Passive active L- and S-band (PALS) microwave sensor for ocean salinity and soil moisture measurements
abstract: A passive/active WS-band (PALS) microwave aircraft instrument to measure ocean salinity and soil moisture has been built and tested. Because the L-band brightness temperatures associated with salinity changes are expected to be small, it was necessary to build a very sensitive and stable system. This new instrument has dual-frequency, dual polarization radiometer and radar sensors. The antenna is a high beam efficiency conical horn. The PALS instrument was installed on the NCAR C-130 aircraft and soil moisture measurements were made in support of the Southern Great Plains 1999 experiment in Oklahoma from July 8-14, 1999. Data taken before and after a rainstorm showed significant changes in the brightness temperatures, polarization ratios and radar backscatter, as a function of soil moisture. Salinity measurement missions were flown on July 17-19, 1999, southeast of Norfolk, VA, over the Gulf Stream. The measurements indicated a clear and repeatable salinity signal during these three days, which was in good agreement with the Cape Hatteras ship salinity data. Data were also taken in the open ocean and a small decrease of 0.2 K was measured in the brightness temperature, which corresponded to the salinity increase of 0.4 psu measured by the M/V Oleander vessel.

title: High-stability L-band radiometer measurements of saltwater
abstract: L-band radiometer brightness temperature measurements of a saltwater pond were made as a function of salinity and temperature. A precision L-band radiometer with stability better than 0.1 K per day was used for these measurements. The L-band measurements are in good agreement with three dielectric constant models over a temperature range from 8/spl deg/C to 32/spl deg/C and a salinity range from 25-40 psu. Based on this experiment, these dielectric models will provide an excellent basis for the algorithm development and design of the future National Aeronautics and Space Administration Aquarius satellite mission.

title: Determination of sea surface salinity and wind speed by L-band microwave radiometry from a fixed platform
abstract: In May 1999, the European Space Agency (ESA) selected SMOS (Soil Moisture and Ocean Salinity) as an Earth Explorer Opportunity mission. One of its goals is the generation of global sea surface salinity (SSS) maps. The satellite sensor is an L-band interferometric radiometer with full-polarimetric capability called MIRAS. The retrieval of SSS from microwave measurements is based on the fact that the brightness temperature (TB ) of seawater is a function of the dielectric constant, temperature and sea surface state (roughness, foam…). The sensitivity of TB to SSS is maximum at L-band, but it is necessary to quantify the other effects to have reliable SSS retrieval. In order to improve the present understanding of these effects on TB , ESA sponsored the Wind and Salinity Experiment (WISE) 2000 and 2001 field campaigns. These experimental results are of great importance for the development of sea surface emissivity models that will be used in the future SMOS SSS retrieval algorithms. This paper presents the influence of the emissivity models on the derived SSS from the data obtained in both campaigns. It also presents the impact on the retrieved SSS of using in situ measured or satellite derived wind information, or even simultaneously estimating the wind speed from the measured multi-angular TB .

title: Microwave Remote Sensing, Active and Passive
abstract: EN BIBLIOTECA: V.1: MICROWAVE REMOTE SENSING FUNDAMENTALS AND RADIOMETRY. V.2: RADAR REMOTE SENSING AND SURFACE SCATTERING AND EMISSION

title: Small-slope expansion for thermal and reflected radiation from a rough surface
abstract: Small-perturbation and small-slope expansions are applied and compared for calculation of thermal emission from a rough sea surface and reflected sky radiation. The comparison shows that the expansions are identical. This permits us to use the small-perturbation expansion to calculate microwave thermal radiation from the ocean instead of the two-scale model and the small-slope expansion. At grazing observation angles multiple scattering and shadowing become crucial and high-order terms of the expansion must be taken into account. This imposes a limitation on the applicability of the small-slope approximation.

title: Theoretical study of the small slope approximation for ocean polarimetric thermal emission
abstract: Analytical models for ocean surface polarimetric thermal emission based on the small perturbation method (SPM) have shown success in matching brightness temperature azimuthal variations from aircraft based measurements. It has also been shown that use of the small perturbation method for calculation of surface emissivity results in a series in surface slope, not surface height, so that the method remains accurate for large height surface emission even when it fails for the corresponding scattering calculations. This paper presents a detailed analysis of the SPM/small slope approximation (SPM/SSA) for ocean surface polarimetric thermal emission, and investigates the extent to which varying ocean surface length scales contribute to brightness temperature zeroth and second azimuthal harmonics. It is found that ocean waves of lengths both comparable to and much greater than the electromagnetic wavelength can contribute to these harmonics, depending on the extent to which the ocean surface spectral model places asymmetry in these length scales. In addition, the SPM/SSA is approximated for the contributions of both very long and very short ocean length scales compared to the electromagnetic wavelength, and it is found that both long and short wave contributions can be expressed in simple equations involving either standard or modified ocean surface slope variances.

title: Modeling of wind direction signals in polarimetric sea surface brightness temperatures
abstract: There has been an increasing interest in the applications of polarimetric microwave radiometers for ocean wind remote sensing. Aircraft and spaceborne radiometers have found a few Kelvins wind direction signals in sea surface brightness temperatures, in addition to their sensitivities to wind speeds. However, it was not clear what physical scattering mechanisms produced the observed brightness dependence on wind direction. To this end, polarimetric microwave emissions from wind-generated sea surfaces are investigated with a polarimetric two-scale scattering model, which relates the directional wind-wave spectrum to passive microwave signatures of sea surfaces. Theoretical azimuthal modulations are found to agree well with experimental observations for all Stokes parameters from near nadir to 65/spl deg/ incidence angles. The upwind and downwind asymmetries of brightness temperatures were interpreted using the hydrodynamic modulation. The contributions of Bragg scattering by short waves, geometric optics scattering by long waves and sea foam are examined. The geometric optics scattering mechanism underestimates the directional signals in the first three Stokes parameters, and predicts no signals in the fourth Stokes parameter (V). In contrast, the Bragg scattering was found to dominate the wind direction signals from the two-scale model and correctly predicted the phase changes of the upwind and crosswind asymmetries in T/sub /spl upsi// and U from middle to high incidence angles. The phase changes predicted by the Bragg scattering theory for radiometric emission from water ripples is corroborated by the numerical Monte Carlo simulation of rough surface scattering. This theoretical interpretation indicates the potential use of polarimetric brightness temperatures for retrieving the directional wave spectrum of short gravity and capillary waves.

title: The apparent temperature of the sea at microwave frequencies
abstract: A discussion of the apparent temperature of the sea at microwave frequencies is presented using a theory based on the Kirchhoff approximation for scattering from rough, finitely conducting surfaces. Detailed numerical results on the variation of the temperature with wind speed are discussed for a frequency of 19.4 GHz. The dependence of the rms slope of the sea on wind speed, which is needed in the theory, is based on experimental measurements. It is found that significant temperature changes occur with increasing wind speed for horizontally polarized radiation. The temperature of vertically polarized radiation is much less affected by the wind and, at an angle of 50\deg (at 19.4 GHz), is almost independent of the state of the sea.

title: Extension of Kirchhoff method under stationary phase approximation to determination of polarimetric thermal emission of the sea
abstract: Wind direction over the sea can be determined from radiometric measurements of the third Stokes parameter (T/sub U/). The Kirchhoff method is extended under the stationary phase approximation to compute the dependence of T/sub U/ on frequency, wind speed and incidence angle. Numerical predictions, in agreement with experimental measurements, are presented.

title: Microwave Scattering and Emission Models and their Applications
abstract: First-order radiative transfer solution first-order radiative transfer solution passive sensing formulation of the surface scattering problem surface model and special cases ranges validity of the IEM model matrix doubling formulations for scattering and emission scattering and emissions models for snow and sea ice comparisons of model predictions with backscattering and emission measurements from snow and ice.

title: An extension of the IEM/IEMM surface scattering model
abstract: The integral equation model (IEM) has been developed over the last decade and, since its first presentation by Fung and Pan (1986  Pennsylvania State University Press) pp 701–14), it has become one of the theoretical models most widely used for rough surface scattering in microwave remote sensing. The aim of this model was the study of the scattering by random rough surfaces under more general conditions than the Kirchhoff or the small-perturbation approximations. Furthermore, the IEM was meant to include multiple-scattering effects at second order. The IEM has been gradually corrected in two later releases by its original authors (Hsieh C-Y et al 1997  ). However, the model still presents several theoretical hiatuses in its current formulation which call for a new revision. Most importantly, the IEM in its current form does not reduce in the general bistatic context to the small-perturbation method (SPM) when the scattering surface is slightly rough. A good description of multiple-scattering mechanisms implies that the single scattering is correctly described. This condition is not met by IEM as given hitherto. In the work presented here, a corrected version of IEM reproducing SPM for small roughness is proposed. Since it is also compliant with the physical and geometrical optics results, this new integral equation model is an appropriate candidate to bridge the gap between the Kirchhoff approximation and the SPM.

title: A physical radar cross-section model for a wind-driven sea with swell
abstract: A new spectrum model for the ocean surface is proposed. We determine the two unknown parameters in this spectrum by fitting it to radar observations. We find that this spectrum combined with two-scale scattering theory can predict much of the observed dependence of the radar cross section on radar frequency, polarization, angle of incidence, and wind velocity at incidence angles in the0\deg-70\degrange. The spectrum model is combined with a model for swell to examine the effect of swell on the radar cross section. We find that the effect of swell is significant for low radar frequencies (Lband) and near normal incidence but can be nearly eliminated by using higher frequencies (K_{u}band) and large angles of incidence (\approx 50\deg).

title: Coupled sea surface-atmosphere model: 2. Spectrum of short wind waves
abstract: A physical model of the short wind wave spectrum in the wavelength range from a few millimeters to few meters is proposed. The spectrum shape results from the solution of the energy spectral density balance equation. Special attention is paid to the description of the capillary range of the short wave spectrum. It is assumed that in this range the spectrum shape is determined mainly by the mechanism of generation of parasitic capillaries. This is described as the cascade energy transfer from the gravity to the capillary waves. Thus the capillary wave spectrum results through the balance between generation of capillaries and their viscous dissipation. The short gravity wave spectrum results through the balance between wind input and dissipation due to wave breaking. A parameterization of wind input is obtained in part 1 of the present paper. To describe the dissipation due to wave breaking, the approach developed by Phillips [1985] is used. The spectral rate of energy dissipation is presented in the form of a power dependence of the ratio of the saturation spectrum to some threshold level. It is further shown that the threshold level depends on the drift current shift in the water viscous sublayer, which affects the energy losses by wave breaking. To obtain a short wave spectrum which is valid in the whole wavenumber domain, the capillary and the short gravity wave spectra are patched in the vicinity of the wavenumber corresponding to the minimum phase velocity. This short wave spectrum is incorporated into the wind over waves coupled model developed in part 1 of the present paper. The measured statistical properties of the sea surface related to the short waves, such as the spectral shape of omnidirectional and up-wind spectra, their wind speed dependence and angular spreading, and the wind speed dependence of integral mean square slope and skewness parameters, are well reproduced by the model. Also the model well reproduces the measured wind speed dependence of the drag coefficient and of the coupling parameter.

title: A Model for the Microwave Emissivity of the Ocean's Surface as a Function of Wind Speed
abstract: A quantitative model is presented, which describes the ocean surface as an ensemble of flat facets with a normal distribution of slopes. The variance of the slope distribution is linearly related to frequency up to 35 GHz and constant at higher frequencies. These facets are partially covered with an absorbing nonpolarized foam layer. Experimental evidence is presented for this model.

title: The WISE 2000 and 2001 field experiments in support of the SMOS mission: sea surface L-band brightness temperature observations and their application to sea surface salinity retrieval
abstract: Soil Moisture and Ocean Salinity (SMOS) is an Earth Explorer Opportunity Mission from the European Space Agency with a launch date in 2007. Its goal is to produce global maps of soil moisture and ocean salinity variables for climatic studies using a new dual-polarization L-band (1400-1427 MHz) radiometer Microwave Imaging Radiometer by Aperture Synthesis (MIRAS). SMOS will have multiangular observation capability and can be optionally operated in full-polarimetric mode. At this frequency the sensitivity of the brightness temperature (T/sub B/) to the sea surface salinity (SSS) is low: 0.5 K/psu for a sea surface temperature (SST) of 20/spl deg/C, decreasing to 0.25 K/psu for a SST of 0/spl deg/C. Since other variables than SSS influence the T/sub B/ signal (sea surface temperature, surface roughness and foam), the accuracy of the SSS measurement will degrade unless these effects are properly accounted for. The main objective of the ESA-sponsored Wind and Salinity Experiment (WISE) field experiments has been the improvement of our understanding of the sea state effects on T/sub B/ at different incidence angles and polarizations. This understanding will help to develop and improve sea surface emissivity models to be used in the SMOS SSS retrieval algorithms. This paper summarizes the main results of the WISE field experiments on sea surface emissivity at L-band and its application to a performance study of multiangular sea surface salinity retrieval algorithms. The processing of the data reveals a sensitivity of T/sub B/ to wind speed extrapolated at nadir of /spl sim/0.23-0.25 K/(m/s), increasing at horizontal (H) polarization up to /spl sim/0.5 K/(m/s), and decreasing at vertical (V) polarization down to /spl sim/-0.2 K/(m/s) at 65/spl deg/ incidence angle. The sensitivity of T/sub B/ to significant wave height extrapolated to nadir is /spl sim/1 K/m, increasing at H-polarization up to /spl sim/1.5 K/m, and decreasing at V-polarization down to -0.5 K/m at 65/spl deg/. A modulation of the instantaneous brightness temperature T/sub B/(t) is found to be correlated with the measured sea surface slope spectra. Peaks in T/sub B/(t) are due to foam, which has allowed estimates of the foam brightness temperature and, taking into account the fractional foam coverage, the foam impact on the sea surface brightness temperature. It is suspected that a small azimuthal modulation /spl sim/0.2-0.3 K exists for low to moderate wind speeds. However, much larger values (4-5 K peak-to-peak) were registered during a strong storm, which could be due to increased foam. These sensitivities are satisfactorily compared to numerical models, and multiangular T/sub B/ data have been successfully used to retrieve sea surface salinity.

title: Local and non-local curvature approximation: a new asymptotic theory for wave scattering
abstract: We present a new asymptotic theory for scalar and vector wave scattering from rough surfaces which federates an extended Kirchhoff approximation (EKA), such as the integral equation method (IEM), with the first and second order small slope approximations (SSA). The new development stems from the fact that any improvement of the ‘high frequency’ Kirchhoff or tangent plane approximation (KA) must come through surface curvature and higher order derivatives. Hence, this condition requires that the second order kernel be quadratic in its lowest order with respect to its Fourier variable or formally the gradient operator. A second important constraint which must be met is that both the Kirchhoff approximation (KA) and the first order small perturbation method (SPM-1 or Bragg) be dynamically reached, depending on the surface conditions. We derive herein this new kernel from a formal inclusion of the derivative operator in the difference between the polarization coefficients of KA and SPM-1. This new kernel is as simple as the expressions for both Kirchhoff and SPM-1 coefficients. This formal difference has the same curvature order as SSA-1 + SSA-2. It is acknowledged that even though the second order small perturbation method (SPM-2) is not enforced, as opposed to the SSA, our model should reproduce a reasonable approximation of the SPM-2 function at least up to the curvature or quadratic order. We provide three different versions of this new asymptotic theory under the local, non-local, and weighted curvature approximations. Each of these three models is demonstrated to be tilt invariant through first order in the tilting vector.

title: Sea surface emissivity observations at L-band: first results of the Wind and Salinity Experiment WISE 2000
abstract: Sea surface salinity can be measured by passive microwave remote sensing at L-band. In May 1999, the European Space Agency (ESA) selected the Soil Moisture and Ocean Salinity (SMOS) Earth Explorer Opportunity Mission to provide global coverage of soil moisture and ocean salinity. To determine the effect of wind on the sea surface emissivity, ESA sponsored the Wind and Salinity Experiment (WISE 2000). This paper describes the field campaign, the measurements acquired with emphasis in the radiometric measurements at L-band, their comparison with numerical models, and the implications for the remote sensing of sea salinity.

title: Sea state effect on the sea surface emissivity at L-band
abstract: In May 1999, the European Space Agency (ESA) selected the Earth Explorer Opportunity Soil Moisture and Ocean Salinity (SMOS) mission to obtain global and frequent soil moisture and ocean salinity maps. SMOS' single payload is the Microwave Imaging Radiometer by Aperture Synthesis (MIRAS), an L-band two-dimensional aperture synthesis radiometer with multiangular observation capabilities. At L-band, the brightness temperature sensitivity to the sea surface salinity (SSS) is low, approximately 0.5 K/psu at 20/spl deg/C, decreasing to 0.25 K/psu at 0/spl deg/C, comparable to that to the wind speed /spl sim/0.2 K/(m/s) at nadir. However, at a given time, the sea state does not depend only on local winds, but on the local wind history and the presence of waves traveling from far distances. The Wind and Salinity Experiment (WISE) 2000 and 2001 campaigns were sponsored by ESA to determine the impact of oceanographic and atmospheric variables on the L-band brightness temperature at vertical and horizontal polarizations. This paper presents the results of the analysis of three nonstationary sea state conditions: growing and decreasing sea, and the presence of swell. Measured sea surface spectra are compared with the theoretical ones, computed using the instantaneous wind speed. Differences can be minimized using an "effective wind speed" that makes the theoretical spectrum best match the measured one. The impact on the predicted brightness temperatures is then assessed using the small slope approximation/small perturbation method (SSA/SPM).

title: Passive Microwave Measurements of Sea Surface Roughness
abstract: Passive microwave measurements of the sea surface were made from Argus Island tower at 1.41, 8.36, and 19.34 GHz over a range in wind speeds from calm to 15 m/s. These measurements show a definite frequency-dependent correlation between the microwave brightness temperature and wind speed. This dependence results from roughness effects of the compact sea surface associated with wind-driven waves.

title: Spectral characteristics of the microwave emission from a wind-driven foam-covered sea
abstract: Aircraft observations of the microwave emission from the Bering Sea show that the spectral character of brightness temperature variation with wind speed in a fully developed sea is due to a combination of surface roughness and a surface layer with varying dielectric constant over a physical depth of a few millimeters. The varying dielectric layer is identified with cusps formed by the bubbles associated with thin foam streaks. Computations of the radiative transfer through such a layer show that the change in microwave emissivity due to the presence of that layer decreases as microwave wavelength increases. Since foam streak coverage on the ocean surface increases with surface wind speed, the increase in microwave brightness temperature with increasing wind speed will be less at longer wavelengths, a characteristic that is observed in the experimental results presented. In addition, it has been observed that in fetch-limited situations the microwave brightness temperature depends in the same way on surface wind speed but is essentially independent of varying foam coverage, the implication being that the surface wave structure becomes more important in determining the microwave brightness temperatures as fetch approaches zero. It appears that microwave brightness temperatures are directly related to ocean surface wind speeds, regardless of fetch.

title: A Unified Directional Spectrum for Long and Short Wind-Driven Waves
abstract: Review of several recent ocean surface wave models finds that while comprehensive in many regards, these spectral models do not satisfy certain additional, but fundamental, criteria. We propose that these criteria include the ability to properly describe diverse fetch conditions and to provide agreement with in situ observations of Cox and Munk [1954] and Jahne and Riemer [1990] and Hara et al. [1994] data in the high-wavenumber regime. Moreover, we find numerous analytically undesirable aspects such as discontinuities across wavenumber limits, nonphysical tuning or adjustment parameters, and noncentrosymmetric directional spreading functions. This paper describes a two-dimensional wavenumber spectrum valid over all wavenumbers and analytically amenable to usage in electromagnetic models. The two regime model is formulated based on the Joint North Sea Wave Project (JONSWAP) in the long-wave regime and on the work of Phillips [1985] and Kitaigorodskii [1973] at the high wavenumbers. The omnidirectional and wind-dependent spectrum is constructed to agree with past and recent observations including the criteria mentioned above. The key feature of this model is the similarity of description for the high- and low-wavenumber regimes; both forms are posed to stress that the air-sea interaction process of friction between wind and waves (i.e., generalized wave age, u/c) is occurring at all wavelengths simultaneously. This wave age parameterization is the unifying feature of the spectrum. The spectrum's directional spreading function is symmetric about the wind direction and has both wavenumber and wind speed dependence. A ratio method is described that enables comparison of this spreading function with previous noncentrosymmetric forms. Radar data are purposefully excluded from this spectral development. Finally, a test of the spectrum is made by deriving roughness length using the boundary layer model of Kitaigorodskii. Our inference of drag coefficient versus wind speed and wave age shows encouraging agreement with Humidity Exchange Over the Sea (HEXOS) campaign results.

title: A new empirical model of sea surface microwave emissivity for salinity remote sensing
abstract: SMOS (Soil Moisture and Ocean Salinity) is a European Space Agency mission that aims at generating global ocean salinity maps with an accuracy of 0.1 psu, at spatial and temporal resolution suitable for climatic studies. The satellite sensor is an L-band (1400–1427 MHz) aperture synthesis interferometric radiometer. Sea surface salinity (SSS) can be retrieved since the brightness temperature of sea water is dependent on the frequency, angle of observation, dielectric constant of sea water, sea surface temperature and sea surface state. This paper presents a new empirical sea water emissivity model at L-band in which surface roughness effects are parameterized in terms of wind speed and significant wave height. For the SMOS mission these parameters can be obtained from external measurements and model diagnostics. An analysis has been done on the effect on SSS retrieval of different sources for this auxiliary information.

title: Measuring Ocean Salinity with ESA’s SMOS Mission – Advancing the Science
abstract: 1 ESA Directorate of Earth Observation, ESTEC, Noordwijk, The Netherlands 2 Polytechnic University of Catalonia, Barcelona, Spain 3 Institut de Ciències del Mar CMIMA-CSIC, Barcelona, Spain 4 CESBIO, Toulouse, France 5 Naval Research Laboratory, Stennis Space Center, USA 6 Nansen Environmental & Remote Sensing Center (NERSC), Bergen, Norway 7 Université Paris Pierre et Marie Curie, Paris, France 8 Technical University of Denmark, Lyngby, Denmark

title: Acoustically relevant bubble assemblages and their dependence on meteorological parameters
abstract: A detailed physical model of the life history of a typical bubble plume, from its formation by a breaking wave to its dissipation into the background bubble population, is given, and the relationship between the early, acoustically relevant stages in bubble-plume development and the associated, remotely detectable whitecap is described. The manner in which the fraction of the sea surface covered by stage A spilling crests and by stage B mature whitecaps depends upon wind speed and upon wind stress or friction velocity is investigated. Formal expressions are given whereby near-surface bubble concentrations can be estimated from observations of fractional whitecap coverage or from measurements of the 10-m elevation wind speed.

title: The emissivity of sea foam at microwave frequencies
abstract: A consistent picture of the emission characteristics of sea foam at microwave frequencies has emerged from a survey of published radiometric measurements. The results are summarized, as functions of frequency and angle, by means of simple equations. Available data on the reflection properties of foam are also examined and shown to be qualitatively, but not quantitatively, in agreement with the radiometric results.

title: Computed transmission through rain at microwave and visible frequencies
abstract: In this paper we present tables which contain the Mie scattering coefficient, absorption coefficient, extinction coefficient, equivalent medium index of refraction and phase delay for rains conforming to the Laws and Parsons drop-size distribution. These transmission characteristics have been calculated for microwave frequencies of interest in common carrier radio relay systems, 300 to 1.43 GHz, that is, 0.1 to 21.0 cm, at rain rates from 0.25 to 150.0 mm/hr. We also include the extinction coefficients for the visible wavelength 0.6328 μ. The microwave tables were generated by using a Mie scattering computer program similar to that designed and previously reported by Deirmendjian. The calculations at 0.6328 μ were made separately by employing the usual assumptions for droplets with very large circumference to wavelength ratios.

title: Detection of calibration drifts in spaceborne microwave radiometers using a vicarious cold reference
abstract: The coldest possible brightness temperatures observed by a downward-looking microwave radiometer from space are often produced by calm oceans under cloud-free skies and very low humidity. This set of conditions tends to occur with sufficient regularity that an orbiting radiometer will accumulate a useful number of observations within a period of a few days to weeks. Histograms of the radiometer's coldest measurements provide an anchor point against which very small drifts in absolute calibration can be detected. This technique is applied to the TOPEX microwave radiometer (TMR), and a statistically significant drift of several tenths of a Kelvin per year is clearly detected in one of the channels. TMR housekeeping calibration data indicates a likely cause for the drift, as small changes in the isolation of latching ferrite circulators that are used in the onboard calibration-switch assembly. This method can easily be adapted to other microwave radiometers, especially imagers operating at frequencies in the atmospheric windows. In addition to detecting long-term instrument drifts with high precision, the method also provides a means for cross-calibrating different instruments. The cold reference provides a common tie point, even between sensors operating at different polarizations and/or incidence angles.

title: Surface Salinity Retrieved from SMOS Measurements over the Global Ocean: Imprecisions Due to Sea Surface Roughness and Temperature Uncertainties
abstract: The Soil Moisture and Ocean Salinity (SMOS) mission recently led by the European Space Agency (ESA) intends to monitor soil moisture and sea surface salinity (SSS). Since the sensitivity of radiometric L-band signal to SSS is weak, measuring SSS with an acceptable accuracy is challenging: it requires both a very stable instrument and very precise corrections of other geophysical signals than the SSS affecting the L-band signal. Concentration is on the sea surface roughness and temperature (SST) effects and the extent to which they need to be corrected to optimize both SSS precision and retrieval complexity. In addition to uncertainties regarding SST and wind speed (W), realistic noise on the SMOS brightness temperatures (Tb’s) are considered and possible consequences of Tb biases are examined. In most oceanic regions, random noise in W, SST, and Tb should not hamper the SMOS SSS retrieval within the Global Ocean Data Assimilation Experiment (GODAE) requirements (a precision better than 0.1 pss over 200 km 3 200 km and 10 days). However, minimizing systematic bias errors over the time scale at which the SSS products will be averaged is critical: the GODAE requirement will not be met if Tb’s or W is biased in warm waters (258C) by 0.07 K and 0.3 m s21, respectively, and in cold waters (58C) by 0.03 K and 0.15 m s21, respectively, or if no a priori information on W is available. In order to minimize errors coming from the W natural variability, it is essential to use high-temporal-resolution wind data. The use of the first Stokes parameter instead of bipolarized Tb degrades the SSS precision by less than 10% in most regions, showing that Faraday rotation should not hamper SMOS SSS retrieval.

title: Galactic noise and passive microwave remote sensing from space at L-band
abstract: An important consideration for remote sensing measurements at 1.4 GHz (L-band) is radiation from extraterrestrial sources such as hydrogen. Unlike the cosmic background, this radiation is spatially and temporally variable in the local sky. Radio astronomy surveys are employed to produce maps of equivalent brightness temperature with sufficient spatial and radiometric accuracy to improve approximations which have been used in the past.

title: Ionospheric effects for L-band 2-D interferometric radiometry
abstract: Ionospheric effects are a potential error source for the estimation of surface quantities such as sea surface salinity, using L-band radiometry. This study is carried out in the context of the SMOS future space mission, which uses an interferometric radiometer. We first describe the way the Faraday rotation angle due to electron content along the observing path varies across the two-dimensional field of view. Over open ocean surfaces, we show that it is possible to retrieve the total electron content (TEC) at nadir from radiometric data considered over the bulk of the field of view, with an accuracy better than 0.5 TEC units, compatible with requirements for surface salinity observations. Using a full-polarimetric design improves the accuracy on the estimated TEC value. The random uncertainty on retrieved salinity is decreased by about 15% with respect to results obtained when using only data for the first Stokes parameter, which is immune to Faraday rotation. Similarly, TEC values over land surfaces may be retrieved with the accuracy required in the context of soil moisture measurements. Finally, direct TEC estimation provides information which should allow to correct for ionospheric attenuation as well.

title: L‐band sea surface emissivity: Preliminary results of the WISE‐2000 campaign and its application to salinity retrieval in the SMOS mission
abstract: Soil moisture and ocean salinity at surface level can be measured by passive microwave remote sensing at L-band. To provide global coverage data of soil moisture and ocean salinity with three-day revisit time, the Earth Explorer Opportunity Mission SMOS (Soil Moisture and Ocean Salinity) was selected by ESA (European Space Agency) in May 1999. SMOS' single payload is a Y-shaped 2-D aperture synthesis interferometric radiometer called MIRAS (Microwave Imaging Radiometer by Aperture Synthesis). SMOS presents some particular imaging peculiarities: variation of incidence and azimuth angles, different radiometric sensitivity and accuracy at each direction (pixels), and geometric polarization mixing. Therefore, the accuracy of the geophysical parameter retrieval depends on the knowledge of the angular dependence of the emissivity over a wide range of incidence and azimuth angles. The accuracy of the sea surface salinity retrievals depends on our capability to correct the wind-induced variation of the brightness temperatures. To better understand wind effects, ESA sponsored the WInd and Salinity Experiment 2000 (WISE-2000) from November 15, 2000, to January 16, 2001, in the Casablanca oil rig, at 40 km off the coast of Tarragona (Spain). This paper is divided into two parts. First, it presents the derived sensitivities of the brightness temperatures at vertical and horizontal polarizations with wind speed, and compares to Hollinger's measurements and numerical simulations. Second, these results are applied to the SMOS sea surface salinity (SSS) retrieval problem for different tracks within the swath. It is shown that, except for low SSS and sea surface temperature (SST), the retrieved SSS has a RMS error of approximately 1 psu in one satellite pass.

title: Faraday rotation and L band oceanographic measurements
abstract: Spaceborne radiometric measurements of the L band brightness temperature over the oceans make it possible to estimate sea surface salinity. However, Faraday rotation in the ionosphere disturbs the signals and must be corrected. Two different ways of assessing the disturbance directly from the radiometric measurements, and hence enabling a correction for the effect, are being discussed. Also, a method, which aims at circumventing the problem by using the first Stokes parameter in the salinity retrieval, is being discussed.

title: Sea surface salinity retrieval using multi-angular L-band radiometry: numerical study using the SMOS End-to-end Performance Simulator
abstract: In May 1999 the European Space Agency (ESA) selected the SMOS Earth Explorer Opportunity mission to provide global soil moisture and sea surface salinity (SSS) maps using the MIRAS L-band aperture synthesis interferometric radiometer. The sea salinity signature is very small and sea surface temperature (SST) and mainly wind speed (WS) effects must be accurately modelled. In order to achieve the required accuracy (0.1 psu), measurements must be averaged in time (10-30 days) and space (100 km). However, the geophysical parameters SSS, SST and WS change in time and space and the improvement factor by averaging remains open. We use the SMOS End-to-end Performance Simulator (SEPS) developed by EADS-CASA, GMV and UPC to simulate the SSS retrieval performance.

title: Retrieving sea surface salinity with multiangular L-band brightness temperatures: Improvement by spatiotemporal averaging
abstract: The Soil Moisture and Ocean Salinity (SMOS) mission was selected in May 1999 by the European Space Agency to provide global and frequent soil moisture and sea surface salinity maps. SMOS' single payload is Microwave Imaging Radiometer by Aperture Synthesis (MIRAS), an L band two-dimensional aperture synthesis interferometric radiometer with multiangular observation capabilities. Most geophysical parameter retrieval errors studies have assumed the independence of measurements both in time and space so that the standard deviation of the retrieval errors decreases with the inverse of square root of the number of measurements being averaged. This assumption is especially critical in the case of sea surface salinity (SSS), where spatiotemporal averaging is required to achieve the ultimate goal of 0.1 psu error. This work presents a detailed study of the SSS error reduction by spatiotemporal averaging, using the SMOS end-to-end performance simulator (SEPS), including thermal noise, all instrumental error sources, current error correction and image reconstruction algorithms, and correction of atmospheric and sky noises. The most important error sources are the biases that appear in the brightness temperature images. Three different sources of biases have been identified: errors in the noise injection radiometers, Sun contributions to the antenna temperature, and imaging under aliasing conditions. A calibration technique has been devised to correct these biases prior to the SSS retrieval at each satellite overpass. Simulation results show a retrieved salinity error of 0.2 psu in warm open ocean, and up to 0.7 psu at high latitudes and near the coast, where the external calibration method presents more difficulties.