The authors compare the performance of seven wind-estimation algorithms, including the weighted least squares in the log domain, maximum-likelihood (ML), least squares, weighted least squares, adjustable weighted least squares, L1 norm, and least wind speed squares algorithms, for wind retrieval. For each algorithm, they present performance simulation results for the NASA scatterometer system planned to be launched in the 1990s. A relative performance merit based on the root-mean-square value of wind vector error is devised. It is found that performances of all algorithms are quite comparable. However, the results do indicate that the ML algorithm performs best for the 50-km wind resolution cell case and the L1 norm algorithm performs best for the 25-km wind resolution cell case.< >
An airborne K/sub u/-band scatterometer (NUSCAT) was used to measure ocean backscatter during the Surface Wave Dynamics Experiment (SWADE). The objective is to improve understanding of the relationship between ocean radar backscatter and near surface winds. Backscatter measurements are presented for the Gulf-Stream boundary, low wind, and large wave effects. Over the boundary crossings, large air-sea temperature differences are encountered and substantial changes in the radar cross section are observed. At low wind speeds, the behavior of the measured radar cross sections is different from model predictions in several cases and backscatters are occasionally very low in the cross-wind direction. Large waves due to swells have little impact at moderate winds but significantly increase backscatter at light winds.< >
The NASA/JPL Airborne Rain MApping Radar (ARMAR) was operated on the NASA DC-8 aircraft during TOGA COARE in early 1993. On 12 flights ARMAR observed stratiform precipitation associated with mesoscale convective systems. The statistics of 16 melting layer parameters, including maximum reflectivity, cooling rate, Doppler velocity, LDR, and the HH-VV correlation coefficient are presented and discussed.
The Jet Propulsion Laboratory NUSCAT K/sub u/-band scatterometer successfully acquired ocean backscatter data over a wide variety of oceanic and atmospheric conditions during the Surface Wave Dynamics Experiment (SWADE). Ten flights resulted in 30 h of data collection were conducted on the NASA Ames C130 aircraft. The SWADE experimental area was deployed with several buoys providing appropriate in situ measurements to correlate with the radar backscatter for incidence angles from 10 to 60/spl deg/ at both horizontal and vertical polarizations. The NUSCAT-SWADE backscatter data base was utilized in conjunction with buoy measurements to investigate ocean backscatter signatures over the range of conditions encountered during SWADE. To account for modulations and fluctuations in incidence angles, a backscatter analysis independent of a priori geophysical model functions was developed, tested, and implemented. Results for backscatter azimuth modulations in terms of upwind, downwind, and crosswind radar returns are compared to airborne RADSCAT results and to SASS-I and II geophysical model functions versus neutral wind speed (U/sub N/). NUSCAT-SWADE results are closest overall to SASS-II values, fit best with SASS-I at 10/spl deg/ incidence angle, and are significantly higher than RADSCAT. Effects of friction velocity (u*), wave age (c/sub o//u*), and significant wave height (H/sub 1/3/) on ocean backscatter are studied. The backscatter shows a good positive sensitivity to u*, an inverse dependence on c/sub o//u*, and no systematic trend with H/sub 1/3/ excluding cases of large swells. Coefficients of empirical relations between backscatter and neutral wind speed, friction velocity, and wave age are derived for 10-40/spl deg/ incidence angles at both horizontal and vertical polarizations. Covariance studies of backscatter with the derived relations to winds show an overall deviation factor in the order of 1 dB for ocean signatures including uncertainties in surface conditions.
Investigates the theory of polarimetric passive remote sensing of wind-generated sea surfaces. A two-scale polarimetric scattering and emission model of sea surfaces is developed to interpret existing active and passive remote sensing microwave signatures of sea surfaces, and to investigate the potential application of polarimetric radiometry to ocean surface winds. Theoretical backscattering coefficients are compared with SASS geophysical model function, the accuracy of which has been confirmed by NUSCAT data, to verify the accuracy of the two-scale model. Furthermore, it is found that model-predicted azimuthal modulations of Stokes parameters of thermal radiation agree reasonably well with existing Ku-, K-, and Ka-band radiometer data. The results indicate that the azimuthal modulations observed in the microwave backscatter as well as emission data could be responsible by the same anisotropic directional surface features caused by wind forcing. Finally, the authors discuss issues related to passive remote sensing of ocean surface winds.< >
During the Surface Wave Dynamics Experiment (SWADE), the Jet Propulsion Laboratory Scatterometer NUSCAT operating at Ku band was successfully used to acquire ocean backscatter data over a wide variety of atmospheric and oceanic conditions. Ten flights of data collection were conducted on the NASA Ames C130 aircraft, which resulted in 30 hours of data. The SWADE experimental area was deployed with several buoys providing appropriate in-situ data to correlate with the radar backscatter for incidence angles from 10/spl deg/ to 60/spl deg/ and both horizontal and vertical polarizations. The authors use the NUSCAT/SWADE backscatter data base in conjunction with buoy measurements to investigate backscatter azimuth modulations and compare the results under various physical conditions to airborne RADSCAT measurements and to predictions from SASS geophysical model functions.
Presents the first experimental evidence that the polarimetric brightness temperatures of sea surfaces are sensitive to ocean wind direction in the incidence angle range of 30 to 50/spl deg/. The experimental data were collected by a K-band (19.35 GHz) polarimetric wind radiometer (WINDRAD) mounted on the NASA DC-8 aircraft. A set of aircraft radiometer flights was successfully completed in November 1993. The authors performed circle flights over National Data Buoy Center (NDBC) moored buoys deployed off the northern California coast, which provided ocean wind measurements. The results indicate that passive polarimetric radiometry has a strong potential for global ocean wind speed and direction measurements from space.< >
Ocean radar backscatter in the presence of large waves is investigated using data acquired with the Jet Propulsion Laboratory NUSCAT radar at Ku band for horizontal and vertical polarizations and the University of Massachusetts C-SCAT radar at C band for vertical polarization during the Surface Wave Dynamics Experiment. Off-nadir backscatter data of ocean surfaces were obtained in the presence of large waves with significant wave height up to 5.6 m. In moderate-wind cases, effects of large waves are not detectable within the measurement uncertainty and no noticeable correlation between backscatter coefficients and wave height is found. Under high-wave light-wind conditions, backscatter is enhanced significantly at large incidence angles,with a weaker effect at small incidence angles. Backscatter coefficients in the wind speed range under consideration are compared with SASS-II (Ku band), CMOD3-H1 (C band), and Plant's model results which confirm the experimental observations. Variations of the friction velocity, which can give rise to the observed backscatter behaviors in the presence of large waves, are presented.< >
Attempting to understand and predict weather on a local and global basis has challenged both the scientific and engineering communities. One key parameter in understanding the weather is the ocean surface wind vector because of its role in the energy exchange at the air-sea surface. scatterometers, radars that measure the reflectivity of a target offer a tool with which to remotely monitor these winds from tower-, aircraft-, and satellite-based platforms. This paper introduces three current airborne scatterometer systems, and presents data collected by these instruments under low-, moderate-, and high-wind conditions. The paper focuses on airborne scatterometers because of their ability to resolve submesoscale variations in wind fields. Discrepancies between existing theory and the observations are noted and the concerns in measuring low-wind speeds discussed. Finally, the application of using this technology for estimating the surface-wind vector during a hurricane is demonstrated.< >
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.
