Wu-Yang Tsai

Detection of snowmelt regions on the Greenland ice sheet using diurnal backscatter change

Snowmelt regions on Greenland ice are mapped daily with the SeaWinds wideswath Ku-band (13.4 GHz) scatterometer on the QuikSCAT satellite. The approach exploits the high temporal resolution of SeaWinds/QuikSCAT data for the melt mapping using diurnal backscatter change independent of the absolute calibration. The results reveal several pronounced melting and refreezing events, and effects of topography are evident in the melt patterns. The spatial resolution is sufficient to identify melt features on the Sukker-toppen Iskappe west of the main ice sheet. An anomalous warming event, caused by downward mixing of warm air, is detected in late September 1999 over the west flank of the southern Greenland ice sheet. Time-series images of melt regions are presented over the period from summer to the fall freeze-up. The satellite observations are verified with in situ measurements from the Greenland Climate Network stations.

Global snow cover monitoring with spaceborne K/sub u/-band scatterometer

This paper presents a study to demonstrate the potential of a spaceborne K/sub u/-band scatterometer to monitor global snow cover. Global Ku-band data were acquired by the NASA Scatterometer (NSCAT) operated on the Advanced Earth Observing Satellite (ADEOS) from September 1996 to June 1997. NSCAT backscatter patterns aver the northern hemisphere reveals boundaries between different snow classes, defined by the Cold Regions Research and Engineering Laboratory (CRREL), Hanover, NH, snow classification system at different times of the snow season. They show the evolution of the backscatter signature throughout the entire seasonal snow cycle. Within the snow extent determined by the National Oceanic and Atmospheric Administration (NOAA), Washington, DC, and Climate Prediction Center (CPC), operational snow product, Ku-band backscatter data expose detailed features and rapid changes as observed in in-situ snow depth data from surface weather stations in U.S., Canada, and Russia. Sensitivity of Ku-band backscatter to snow conditions is illustrated with the dramatic change over the U.S. northern plains and the Canadian prairie region corresponding to the snow event leading to the 1997 Flood of the Century. They discuss snow field experiments and data analysis plan to understand snow scattering mechanisms, to interpret snow backscatter, and to derive its relationship with snow physical parameters.

Polarimetric scatterometry: a promising technique for improving ocean surface wind measurements from space

Spaceborne wind scatterometers provide useful measurements of ocean surface winds and are important to climatological studies and operational weather forecasting. Past and currently planned scatterometers use measurements of the copolarized backscatter cross-section at different azimuth angles to infer ocean surface wind speed and direction. Although successful, current scatterometer designs have limitations such as degraded wind performance in the near-nadir and outer regions of the measurement swath and a reliance on external wind information for vector ambiguity removal. Theoretical studies of scattering from the wind-induced ocean surface indicate that polarimetric measurements provide orthogonal and complementary directional information to aid the wind retrieval process. In this paper, potential benefits of making polarimetric backscatter measurements to improve wind retrieval performance are addressed. To investigate the performance of a polarimetric scatterometer, a modified version of the SeaWinds end-to-end simulator at the Jet Propulsion Laboratory (JPL), Pasadena, CA, is employed. To model the effect of realistic measurement errors, expressions for polarimetric measurement variance and bias are derived. It is shown that a polarimetric scatterometer can be realized with straightforward and inexpensive modifications to a current scanning pencil-beam scatterometer system such as SeaWinds. Simulation results show that such a system ran improve wind performance in the nadir region and eliminate the reliance on external wind information.

NASA Scatterometer Experiment

Abstract Satellite scatterometers are microwave radars capable of measuring near-surface vector winds (both speed and direction) over the oceans under all weather conditions. The data generated from these instruments are used in scientific studies of upper ocean circulation, tropospheric dynamics, air–sea interaction and climate change; in operational meteorology as a means to increase numerical weather forecast skill and the accuracy of storm warning predictions; and in commercial applications such as ship routing. The scatterometer wind measurement technique was demonstrated with the flight of the Seasat Scatterometer in 1978. This paper summarizes the scatterometry measurement technique, describes the design of the NASA Scatterometer (NSCAT) instrument recently launched aboard the National Space Development Agency of Japans (NASDA) Advanced Earth Observing Satellite (ADEOS), presents first results from the NSCAT instrument, and describes the future U.S. program for measuring surface marine wind vectors.

Sea ice identification using dual-polarized Ku-band scatterometer data

Describes a classification algorithm using dual-polarized scatterometer measurements to identify the edge of the sea ice cover. The distinct polarization scattering signatures of sea ice and open water are discussed and illustrated with the dual-polarized radar measurements from the Seasat-A scatterometer (SASS). The analysis of SASS data suggests that the ratio of vertical and horizontal polarization backscatter, denoted as the copol ratio, is a useful discriminator of sea ice and open ocean. A simple classification algorithm using the thresholds of the copol ratio and backscatter levels is proposed. The feasibility of this algorithm is demonstrated using the SASS data from the single-sided, dual-polarization mode. The results indicate that the dual-polarized measurements from the NASA scatterometer (NSCAT) can be used to produce routine maps of sea ice extents.

Postlaunch sensor verification and calibration of the NASA Scatterometer

Scatterometer instruments are active microwave sensors that transmit a series of microwave pulses and measure the returned echo power to determine the normalized radar backscattering cross section (sigma-0) of the ocean surface from which the speed and direction of near-surface ocean winds are derived. The NASA Scatterometer (NSCAT) was launched on board the ADEOS spacecraft in August 1996 and returned ten months of high-quality data before the failure of the ADEOS spacecraft terminated the data stream in June 1997. The purpose of this paper is to provide an overview of the NSCAT instrument and sigma-0 computation and to describe the process and the results of an intensive postlaunch verification, calibration, and validation effort. This process encompassed the functional and performance verification of the flight instrument, the sigma-0 computation algorithms, the science data processing system, and the analysis of the sigma-0 and wind products. The calibration process included the radiometric calibration of NSCAT using both engineering telemetry and science data and the radiometric beam balance of all eight antenna beams using both open ocean and uniform land targets. Finally, brief summaries of the construction of the NSCAT geophysical model function and the verification and validation of the wind products will be presented. The key results of this paper are as follows: The NSCAT instrument was shown to function properly and all functional parameters were within their predicted ranges. The instrument electronics subsystems were very stable and all of the key parameters, such as transmit power, receiver gain, and bandpass filter responses, were shown to be stable to within 0.1 dB. The science data processing system was thoroughly verified and the sigma-0 computation error was shown to be less than 0.1 dB. All eight antenna beams were radiometrically balanced, using natural targets, to an estimated accuracy of about 0.3 dB.

High-resolution measurements with a spaceborne pencil-beam scatterometer using combined range/Doppler discrimination techniques

Conically scanning pencil-beam scatterometer systems, such as the SeaWinds radar, constitute an important class of instruments for spaceborne climate observation. In addition to ocean winds, scatterometer data are being applied to a wide range of land and cryospheric applications. A key issue for future scatterometer missions is improved spatial resolution. Pencil-beam scatterometers to date have been real-aperture systems where only range discrimination is used, resulting in a relatively coarse resolution of approximately 25 km. In this paper, the addition of Doppler discrimination techniques is proposed to meet the need for higher resolution. The unique issues associated with the simultaneous application of range and Doppler processing to a conically scanning radar are addressed, and expressions for the theoretical measurement performance of such a system are derived. Important differences with side-looking imaging radars, which also may employ Doppler techniques, are highlighted. Conceptual design examples based on scatterometer missions of current interest are provided to illustrate this new high-resolution scatterometer approach. It is shown that spatial resolution of pencil-beam scatterometer systems can be improved by an order of magnitude by utilizing combined range/Doppler discrimination techniques, while maintaining the wide-swath and constant incidence angle needed for many geophysical measurements.

The SeaWinds scatterometer instrument

The SeaWinds scatterometer instrument is currently being developed by NASA/JPL, as a part of the NASA EOS Program, for flight on the Japanese ADEOS II mission in 1999. This Ku-band radar scatterometer will infer surface wind speed and direction by measuring the radar normalized backscatter cross-section over several different azimuth angles. This paper presents the design characteristics of and operational approach to the instrument itself. The SeaWinds pencil-beam-antenna conical-scan design is a change from the fixed fan-beam antennas of SASS and NSCAT. The purpose of this change is to develop a more compact design consistent with the accommodation constraints of the ADEOS II spacecraft. The SeaWinds conical-scan arrangement has a 1-m reflector dish antenna that provides a time shared dual-antenna beam at 40 and 46 degree look angles. The dual-beam operation provides up to four azimuth look directions for each wind measurement cell. At an orbit height of 803 km, the conical scan provides a broad and contiguous wind measurement swath of about 1800 km for each orbit pass. Radiometric measurement performance from a conical scan is inherently stable because of a common antenna apparatus, a measurement cell well defined by the narrow antenna beamwidth, and only two fixed-beam incidence angles for the multiple azimuth looks. A tracking filter is required to accommodate variations in the Doppler shift of the echo during the scan period. Key specifications of the SeaWinds instrument and associated tradeoffs and performance are described.<<ETX>>

Dual-polarized Ku-band backscatter signatures of hurricane ocean winds

The Ku-band dual-polarized backscatter signatures of ocean surfaces are described in this paper with the airborne scatterometer measurements collected in the Hurricane Ocean Wind Experiment in September 1997. The data collected from flights over Hurricane Erika provide a direct evidence that there are wind direction signals in the vertically and horizontally polarized Ku-band backscatter of ocean surfaces under the influence of hurricane force winds. At 46/spl deg/ incidence angle, the vertically polarized backscatter acquired at the upwind direction increases by about 1 dB as the wind speed increases from 22 m/spl middot/s/sup -1/ to 35 m/spl middot/s/sup -1/, while the horizontally polarized backscatter appears to be twice as sensitive with a change of about 2 dB. At 35 m/spl middot/s/sup -1/ wind speeds, the difference between upwind and crosswind observations of vertically polarized backscatter is about 1.5 dB, smaller than the 2 dB difference for the horizontally polarized backscatter. This demonstrates that the horizontal polarization has a greater sensitivity to wind speed and direction than the vertical polarization in the high wind regime. The data also suggest that the upwind and downwind asymmetry of Ku-band backscatter decreases with increasing wind speed and can fall below 0 dB at small incidence angles (<35/spl deg/) for the vertical polarization. A combined interaction of the geometric optics scattering and the short wave modulation by long waves is proposed to interpret this phenomenon and appears to agree with the dependence of the signature on incidence, wind speed, and polarization. The aircraft flight data support the feasibility of dual-polarized Ku-band radar for hurricane ocean wind measurements, although the data do suggest a reduced wind speed and direction sensitivity in the high wind regime. Also, the differing polarization backscatter signatures-suggest the relative contributions of various surface scattering mechanisms. An improved Ku-band GMF is described.

Flood mapping over the Asian continent during the 1999 summer monsoon season

Backscatter data from the SeaWinds scatterometer on the QuikSCAT satellite are used to delineate floods over the Asian, continent. SeaWinds acquires Ku-band (13.4 GHz) data at the vertical polarization over a very large swath of 1800 km, and at the horizontal polarization over a 1400-km swath. The authors present the flood areas together with topography on land and wind field on ocean. Results show extensive floods in China, India, Bangladesh, and other Asian countries. Timely flood mapping can provide crucial information for flood relief efforts.