[1] High-resolution wind products based on space-borne scatterometer measurements by ASCAT and SeaWinds are used widely for various purposes. In this paper the quality of such products is assessed in terms of accuracy and resolution, using spectral analysis and triple collocation with buoy measurements and NWP model forecasts. An experimental ASCAT coastal product is shown to have a spectral behavior close to k−5/3 for scales around 100 km, as expected from theory and airborne measurements. The NWP spectra fall off more rapidly than the scatterometer wind spectra starting at scales of about 1000 km. Triple collocation is performed for four collocated data sets, each with a different scatterometer wind product: ASCAT at 12.5 km and 25 km, and SeaWinds at 25 km processed in two different ways. The spectral difference between scatterometer wind and model forecast is integrated to obtain the representation error which originates from the fact that global weather models miss small-scale details observed by the scatterometers and the buoys. The estimated errors in buoy winds and model winds are consistent over the data sets for the meriodional wind component; for the zonal wind component consistency is less, but still acceptable. Generally, enhanced detail in the scatterometer winds, as indicated at high spatial frequencies by a spectral tail close to k−5/3, results in better agreement with buoys and worse agreement with NWP predictions. The accuracy of the scatterometer winds is about 1 ms−1 or better. The calibration coefficients from triple collocation indicate that on average the ASCAT winds are slightly underestimated.

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On the quality of high‐resolution scatterometer winds

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.

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

Activity during the 2001 hurricane season was similar to that of the 2000 season. Fifteen tropical storms developed, with nine becoming hurricanes and four major hurricanes. Two tropical depressions failed to become tropical storms. Similarities to the 2000 season include overall activity much above climatological levels and most of the cyclones occurring over the open Atlantic north of 25°N. The overall “lateness” of the season was notable, with 11 named storms, including all the hurricanes, forming after 1 September. There were no hurricane landfalls in the United States for the second year in a row. However, the season's tropical cyclones were responsible for 93 deaths, including 41 from Tropical Storm Allison in the United States, and 48 from Hurricanes Iris and Michelle in the Caribbean.

ANNUAL SUMMARY: Atlantic Hurricane Season of 2001

Orbital remote sensing of the Earth is now poised to make three fundamental contributions towards reducing the detrimental effects of extreme floods. Effective Flood warning requires frequent radar observation of the Earth's surface through cloud cover. In contrast, both optical and radar wavelengths will increasingly be used for disaster assessment and hazard reduction.

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Flood warnings, flood disaster assessments, and flood hazard reduction: the roles of orbital remote sensing

The normalized radar cross section (NRCS) measured by satellite ocean radar systems is representative of the sea surface roughness at the scale of gravity-capillary waves, which are not only dominated by winds, but also modulated by some secondary factors such as sea surface temperature (SST) and sea surface salinity (SSS). In this paper, the variations of NRCS due to SST changes, depending on scatterometer radar frequency, polarization, and incidence angle, are investigated on the basis of a physics-based radar backscatter model and a dataset of collocated ASCAT C-band and RapidScat Ku-band scatterometer measurements. The study shows that the SST effects are substantial at Ku-band, but rather negligible for C-band NRCS measurements. Furthermore, the SST effects are wind speed dependent and more pronounced in VV polarization and at higher incidence angles. SSS effects, due to dielectric constant, surface tension, and dynamic viscosity variations, on scatterometer winds are limited (within 1%). This study concludes that it is necessary to take SST into account in scatterometer wind retrieval for radar wavelengths smaller than C-band.

SST Dependence of Ku- and C-Band Backscatter Measurements

The SeaWinds instrument is the first "pencilbeam" scatterometer to routinely measure ocean surface winds from space. Developed to fly on-board the Japanese ADEOS-II mission, the first flight of SeaWinds was moved up several years when the ADEOS-I satellite carrying the NASA Scatterometer (NSCAT) failed. Utilizing only a one year development time, a dedicated small spacecraft was purchased and the SeaWinds spare hardware was adapted for flight. The SeaWinds on QuikSCAT mission was successfully launched on June 19, 1999. The QuikSCAT mission, the SeaWinds instrument, and the SeaWinds ground data processing are briefly described.

SeaWinds on QuikSCAT: sensor description and mission overview

Satellite wind scstterometers are midrovave radar instruments designcd to measure both global radar backscattering cross section (sigm*O) and near-surface wind speed and direction over the gIobal ocean. NASA has a long term commitment to ocean wind remote sensing, starting with the Seasat-A SatelZite Scatterometer (SASS), through NASA Scatterometer (NSCAT), Sea Winds 071 QuikSCAT (QSCAT), Sea Winds on ADBOS-2 (Sea Winds) and Alpha-Scattcrornotsr on GCOM1B. QSCAT was launched on June 19, 1999 from Vandenberg Air Force Base, using a Titan I1 launch vehicle and Ball Aerospace’s Quikbird spacecraft. It was turned-on on July 7, 1999 and hm operated flawlessly since. The purpose of this paper is to present the QSCAT mission objectives, the spacecraft and instrument design concept, system parameters and performance, and the process and results of an intensive post-launch calibration and ver3ication (Cal/Val effort of the cnd-to-end QSCAT sensor system.

W.-T. Tsai

Papers

SeaWinds on QuikSCAT: sensor description and mission overview

SeaWinds on QuickSCAT: Overview of Sensor System and Post-Launch Calibration/Verification

Effect of leaf temperature on photosynthetic yield and relative injury of Phalaenopsis