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ABSTRACT: Successful naval operations require excellent knowledge of the ocean wind speed and direction. In addition, the global ocean wind vector is a key element for weather forecasting and for climate and oceanography studies. WindSat, a satellite-borne multifrequency polarimetric microwave radiometer developed by the Naval Research Laboratory, has demonstrated the ability to remotely sense the global ocean wind vector from space.1 The wind direction signal measured by WindSat is about two orders of magnitude smaller than the background scene, and only a little larger than the radiometer noise floor. Therefore, any small uncertainties in the geophysical model used to retrieve the wind direction will introduce errors in the retrieved wind vector. One such uncertainty is the contribution of sea foam on the wind direction signal. Down-looking radiometers, such as WindSat, receive energy emitted from the ocean surface and the atmosphere. The energy from the ocean surface, quantified as the brightness temperature, is related to surface physical temperature by T(B) = e(s) . T(W), where T(B) is the brightness temperature, T(W) is the physical temperature of ocean water surface, and e(s) is surface emissivity, which depends on the measurement frequency, polarization, incidence angle, and the azimuth angle between wind direction and the direction from which observations are made. The emissivity of the sea surface also depends on physical properties of the water surface such as temperature, salinity, and surface roughness, which is primarily wind-driven. The presence of foam and roughness created by wind and breaking waves greatly increases the surface emission at microwave frequencies. The surface emissivity is sometimes written as e(s) = f . e(F) + (1-f).e(r), where f is the fraction of the surface covered with foam, e(F) is the emissivity of foam, and e(R) is the emissivity of the foam-free, rough water surface.
11/2012;
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ABSTRACT: WindSat, the first satellite polarimetric microwave radiometer, and the NPOESS Conical Microwave Imager/Sounder both have as a key objective the retrieval of the ocean surface wind vector from radiometric brightness temperatures. Available observations and models to date show that the wind direction signal is only 1-3 K peak-to-peak at 19 and 37 GHz, much smaller than the wind speed signal. In order to obtain sufficient accuracy for reliable wind direction retrieval, uncertainties in geophysical modeling of the sea surface emission on the order of 0.2 K need to be removed. The surface roughness spectrum has been addressed by many studies, but the azimuthal signature of the microwave emission from breaking waves and foam has not been adequately addressed. Recently, a number of experiments have been conducted to quantify the increase in sea surface microwave emission due to foam. Measurements from the Floating Instrumentation Platform indicated that the increase in ocean surface emission due to breaking waves may depend on the incidence and azimuth angles of observation. The need to quantify this dependence motivated systematic measurement of the microwave emission from reproducible breaking waves as a function of incidence and azimuth angles. A number of empirical parameterizations of whitecap coverage with wind speed were used to estimate the increase in brightness temperatures measured by a satellite microwave radiometer due to wave breaking in the field of view. These results provide the first empirically based parameterization with wind speed of the effect of breaking waves and foam on satellite brightness temperatures at 10.8, 19, and 37 GHz.
IEEE Transactions on Geoscience and Remote Sensing 04/2006; · 2.89 Impact Factor
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ABSTRACT: WindSat, the first polarimetric radiometer on orbit, launched in January 2003, provides the promise of passive ocean wind vector retrievals on a continuous basis, simultaneous with the retrieval of many other geophysical variables such as sea surface temperature, atmospheric water vapor, cloud liquid water, and sea ice extent and concentration. WindSat also serves as risk reduction for the upcoming National Polar-orbiting Operational Environmental Satellite System (NPOESS) Conical Scanning Microwave Imager/Sounder (CMIS). Since the dependence of microwave brightness temperatures on wind direction is small relative to that of other parameters such as wind speed, wind direction retrieval relies on increasingly accurate knowledge of the ocean surface microwave emission, which depends upon surface properties such as roughness and foam due to wave breaking. Coordinated near-surface measurements of ocean surface microwave emission and air-sea interaction parameters are needed to quantify the effects of the processes mentioned above in surface emission models to improve the accuracy of wind vector retrievals. Such coordinated observations were performed during the Fluxes, Air-Sea Interaction, and Remote Sensing (FAIRS) experiment conducted on the R/P Floating Instrument Platform (FLIP) in the northeastern Pacific Ocean during the Fall of 2000. X- and Ka-band partially polarimetric radiometers were mounted at the end of the port boom of R/P FLIP to measure ocean surface emission at incidence angles of 45°, 53°, and 65°. A bore-sighted video camera recorded the fractional area of foam in the field of view of the radiometers. Air-sea interaction parameters that were measured concurrently include wind speed, friction velocity, heat fluxes, and significant wave height. The measured dependence of ocean surface emissivity on wind speed and friction velocity is in good agreement with, and extends, earlier observations and empirical models based on satellite data. Concurrent radiometric measurements and fractional area foam coverage data strengthen the possibility of retrieval of sea surface foam coverage using airborne or spaceborne radiometry. The dependence of emissivity on atmospheric stability is shown to be much smaller than the dependence of em- issivity on wind speed. Analysis of emissivity dependence on atmospheric stability alone was inconclusive, due to the variation in atmospheric stability with wind speed. The effect of long-wave incidence angle modulation on sea surface emissivity for near-surface measurements was found to be negligible when emissivity measurements were averaged over tens to hundreds of long waves.
IEEE Transactions on Geoscience and Remote Sensing 09/2005; · 2.89 Impact Factor
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ABSTRACT: WindSat, the first polarimetric microwave radiometer on orbit, has as its primary objective the demonstration of robust retrieval of the sea surface wind vector from measured brightness temperatures. The ocean surface wind vector is one of the key environmental data records for the NPOESS Conical Microwave Imager/Sounder (CMIS) instruments, first planned for launch in 2009. To date, aircraft and satellite measurements, as well as modeling results, indicate that brightness temperature variations with wind direction are small, on the order of 1-3 K peak-to-peak. Therefore, quantitative understanding of the dependence of the ocean surface emissivity on properties such as surface roughness and wave breaking is critical for wind vector retrieval. Despite the importance of this, some basic physical properties such as the azimuthal angle dependence of the microwave emission from foam have not been well characterized to date. Recent measurements from the R/P FLIP indicated that the increase in ocean surface emission due to breaking waves may depend on both the incidence and azimuthal angles. The need to quantity this dependence motivated systematic measurement of the emissivity of reproducible breaking waves at varying incidence and azimuthal angles. Results from these recent field measurements provide the first parameterization with wind speed of the change in brightness temperatures due to breaking waves.
Geoscience and Remote Sensing Symposium, 2004. IGARSS '04. Proceedings. 2004 IEEE International; 10/2004
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ABSTRACT: The foam-covered ocean surface is treated as densely packed air bubbles coated with thin layers of seawater. We apply Monte Carlo simulations of solutions of Maxwell's equations to calculate the absorption, scattering, and extinction coefficients at 10.8 and 36.5 GHz. These quantities are then used in dense-media radiative transfer theory to calculate the microwave emissivity. Numerical results of the model are illustrated as a function of foam parameters. Results of emissivities for both horizontal polarization and vertical polarizations at 10.8 and 36.5 GHz are compared with experimental measurements.
IEEE Transactions on Geoscience and Remote Sensing 05/2003; · 2.89 Impact Factor
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ABSTRACT: The Airborne Polarimetric Microwave Imaging Radiometer (APMIR) has been developed at the Naval Research Laboratory. This instrument was designed primarily as a calibration tool for satellite sensors. As such, the system design began with a challenging error budget. The design and construction followed from the error budget. The system has flown several times. This paper focuses on the design of the instrument and preliminary results.
OCEANS 2003. Proceedings; 02/2003
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ABSTRACT: Radiometric measurements of the microwave emissivity of foam were conducted during May 2000 at the Naval Research Laboratory's Chesapeake Bay Detachment using radiometers operating at 10.8 and 36.5 GHz. Horizontal and vertical polarization measurements were performed at 36.5 GHz; horizontal, vertical, +45°, -45°, left-circular, and right-circular polarization measurements were obtained at 10.8 GHz. These measurements were carried out over a range of incidence angles from 30° to 60°. Surface foam was generated by blowing compressed air through a matrix of gas-permeable tubing supported by an aluminum frame and floats. Video micrographs of the foam were used to measure bubble size distribution and foam layer thickness. A video camera was boresighted with the radiometers to determine the beam-fill fraction of the foam generator. Results show emissivities that were greater than 0.9 and approximately constant in value over the range of incidence angles for vertically polarized radiation at both 10.8 and 36.5 GHz, while emissivities of horizontally polarized radiation showed a gradual decrease in value as incidence angle increased. Emissivities at +45°, -45°, left-circular, and right-circular polarizations were all very nearly equal to each other and were in turn approximately equal to the average values of the horizontal and vertical emissivities in each case.
IEEE Transactions on Geoscience and Remote Sensing 01/2003; · 2.89 Impact Factor
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ABSTRACT: Radiometric measurements of the microwave emissivity of foam were
conducted in May 2000 at the Naval Research Laboratory's Chesapeake Bay
Detachment using radiometers operating at 10.8 and 36.5 GHz. Horizontal
and vertical polarization measurements were made at 36.5 GHz;
horizontal, vertical, +45, -45, left circular, and right circular
polarization measurements were obtained at 10.8 GHz. Surface foam was
generated by blowing compressed air through gas permeable tubes
supported by an aluminum frame and floats. Video micrographs of the foam
were used to measure bubble size distribution and foam layer thickness.
A video camera was boresighted with the radiometers to determine
beam-fill fraction of the foam generator
Geoscience and Remote Sensing Symposium, 2001. IGARSS '01. IEEE 2001 International; 02/2001
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ABSTRACT: Sea foam increases surface emission and brightness temperature at microwave frequencies. Together with the surface roughness, it is a key component of the signal used to obtain surface wind vector with satellite-borne radiometric and polarimetric instruments. Current knowledge of foam emissivity, however, is incomplete, particularly in regard to azimuthal effects. Since breaking waves on the open ocean are intermittent and highly variable, radiometric measurements of reproducible breaking waves were performed in an outdoor salt water wave tank in 2002 and 2004. The wave tank was configured to create foam-producing breaking waves using an underwater shoaling beach. Four radiometers, operating at frequencies of 6.8, 10.8, 18.7, and 37 GHz, were positioned at several incidence angles and azimuth look angles to record microwave emission from the breaking waves. A bore-sighted video camera recorded images of foam fraction within radiometers footprints. Auxiliary information on the wave field and breaking events (such as void fraction, bubble size spectrum, foam layer thickness, wave height, and subsurface turbulent dissipation) was provided by a suite of additional instruments. The results of wave tank measurements of the azimuthal dependence of foam emissivity at 6.8 and 10.8 GHz are presented, and possible contributors to the observed azimuthal changes of foam emissivity are discussed
IEEE MicroRad, 2006;
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ABSTRACT: The need to improve retrieval of the surface wind vector and sea surface temperature (SST) from WindSat and the upcoming NPOESS Conical Microwave Imager Sounder (CMIS) motivated measurements of the microwave emission from breaking waves, both on the open ocean and in a wave basin. Aircraft and satellite measurements have demonstrated that the wind direction dependence of ocean surface brightness temperatures is small, on the order of 1-3 K peak-to-peak. Therefore, the accuracy of wind vector retrieval depends strongly upon quantitative knowledge of the relationship of the ocean surface emissivity to surface properties, such as sea surface wave spectrum and wave breaking. The effects of the surface wave spectrum have been addressed by many studies, but the azimuthal dependence of the microwave emission from breaking waves and foam has not been adequately addressed. Recently, a number of experiments have been conducted to quantify the increase in sea surface microwave emission due to foam. The Polarimetric Observations of the Emissivity of Whitecaps EXperiment (POEWEX'04) was conducted during November 2004 to measure the azimuthal dependence of reproducible breaking waves in order to improve wind vector retrieval from spaceborne radiometric measurements, especially at wind speeds of 7 m/s and higher. The emissivity of breaking waves was shown to vary as a function of azimuth angle at four different WindSat frequencies
IEEE MicroRad, 2006;
