Azimuthal Variation of the Microwave Emissivity of Foam (original) (raw)
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IEEE Transactions on Geoscience and Remote Sensing, 2000
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.
WindSat, the first polarimetric microwave radiometer on orbit, and the NPOESS Conical Microwave Imager/Sounder, scheduled for launch in 2010, are both designed to retrieve the ocean surface wind vector from radiometric brightness temperatures. Available observations and models 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. Therefore, quantitative knowledge of the dependence of the ocean surface emissivity on properties such as surface roughness and wave breaking is critical for wind vector retrieval. The dependence of surface emission on roughness has 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.
This paper presents a model of the Stokes vector emission from the ocean surface. The ocean surface is described as an ensemble of facets with Cox and Munk's Gram-Charlier slope distribution. The study discusses the impact of different up-wind and cross-wind rms slopes, skewness, peakedness, foam cover models and atmospheric effects on the azimuthal variation of the Stokes vector, as well as the limitations of the model. Simulation results compare favorably, both in mean value and azimuthal dependence, with SSWI data at 53" incidence angle and with JPL's WINDRAD measurements at incidence angles from 30" to 65", and at wind speeds from 2.5 to 11 d s . "
Journal of Geophysical Research, 1982
On the SEASAT-A satellite, a microwave scatterometer was used to determine the vector wind over the world's oceans. The technique is based on the sensitivity of microwave radar backscatter to the centimeter length ocean waves created by the action of the surface wind. This paper describes the algorithm used to convert the scatterometer' s measurements of ocean normalized radar cross section, •, to the neutral stability vector wind at 19.5 m height and the comparison of these winds with high quality surface observations. The wind vector algorithm used an empirical • model function to describe the dependence of the ocean • on the 19.5-m neutral stability wind vector. Two model functions, developed from a limited base of aircraft and satellite o • measurements, were evaluated by using an independent set of in situ surface wind observations from the Joint Air Sea Interaction Experiment (JASIN). Although these model functions were found to have some weaknesses, the results of these comparisons produced better results than the SEASAT specifications of wind speed accuracy of +-2 m/s and wind direction accuracy of +-20 ø over the 0-16 m/s range of winds observed during JASIN. An improved model function was later developed by 'tuning' to these JASIN data so that the remaining biases between the observed surface winds and the scatterometer-derived winds were minimized. Results are presented for this model function compared against other surface wind observations from the Gulf of Alaska SEASAT Experiment and the SEASAT Storms (Hurricane) Experiment. INTRODUCTION On June 28, 1978, •he National Aeronautics and Space Administration (NASA) launched SEASAT, the first satellite dedicated to establishing the utility of microwave sensors for remote sensing of the earth's oceans [Born et al., 1981]. This concept had its beginning in the mid-1960's when a conference called 'On the Feasibility of Conducting Oceanographic Explorations from Aircraft, Manned Orbital and Lunar Laboratories' was held at Woods Hole Oceanographic Institute, Woods Hole, Mass., in August 1964 [Ewing, 1965]. At this conference, the rudiments of many of the remote sensing systems for measuring oceanographic parameters were described that eventually were orbited on Skylab, Geos-3, and SEASAT. A few years later, a second conference sponsored by the National Academy of Sciences at Woods Hole made a broader study of potential areas of activity for NASA, including the study of the oceans. The concepts of high precision radar altimetry and of using radar backscatter to measure the winds both received considerable attention [National Research Council, 1970]. A third conference at Williamstown, Mass. [Kaula, 1970] also investigated the general subject; and ocean and atmospheric scientists postulated that satellite technology could provide the mecha
2006 IEEE MicroRad, 2006
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.
Effects of air-sea interaction parameters on ocean surface microwave emission at 10 and 37 GHz
IEEE Transactions on Geoscience and Remote Sensing, 2000
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 Manuscript 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 emissivity 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.
Azimuthal variations of the microwave radiation from a slightly non-Gaussian sea surface
Microwave radiation from the ocean rough surface is considered using a small slope expansion. The emphasis is on azimuthal variations of the brightness temperature, related to the anisotropy of wind-generated sea waves. The first and second harmonics, describing the upwind-downwind and upwind-crosswind differences, respectively, are considered using existing models of the sea spectrum. For modeling of the first harmonic the small deviation from Gaussian statistics of the sea surface is introduced to estimate the effect of longwave asymmetry and the distribution of ripples over the long waves. Numerical analysis shows that the longwave asymmetry cannot explain the observed values of the first harmonic in azimuthal variations of the brightness temperature. The ripple modulation by long waves is a possible mechanism explaining the first harmonic in moderate winds, although foam and wave breaks have to be included in electromagnetic modeling to achieve a quantitative agreement with experiments.