39 research outputs found
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Scatterometer-Based Assessment of 10-m Wind Analyses from the Operational ECMWF and NCEP Numerical Weather Prediction Models
Wind measurements by the National Aeronautics and Space Administration (NASA) scatterometer (NSCAT) and the SeaWinds scatterometer on the NASA QuikSCAT satellite are compared with buoy observations to establish that the accuracies of both scatterometers are essentially the same. The scatterometer measurement errors are best characterized in terms of random component errors, which are about 0.75 and 1.5 m sâ»Âč for the along-wind and crosswind components, respectively.
The NSCAT and QuikSCAT datasets provide a consistent baseline from which recent changes in the accuracies of 10-m wind analyses from the European Centre for Medium-Range Weather Forecasts (ECMWF) and the U.S. National Centers for Environmental Prediction (NCEP) operational numerical weather prediction (NWP) models are assessed from consideration of three time periods: September 1996âJune 1997, August 1999âJuly 2000, and February 2002âJanuary 2003. These correspond, respectively, to the 9.5-month duration of the NSCAT mission, the first 12 months of the QuikSCAT mission, and the first year after both ECMWF and NCEP began assimilating QuikSCAT observations. There were large improvements in the accuracies of both NWP models between the 1997 and 2000 time periods. Though modest in comparison, there were further improvements in 2002, at least partly attributable to the assimilation of QuikSCAT observations in both models.
There is no evidence of bias in the 10-m wind speeds in the NCEP model. The 10-m wind speeds in the ECMWF model, however, are shown to be biased low by about 0.4 m sâ»Âč. While it is difficult to eliminate systematic errors this small, a bias of 0.4 m sâ»Âč corresponds to a typical wind stress bias of more than 10%. This wind stress bias increases to nearly 20% if atmospheric stability effects are not taken into account. Biases of these magnitudes will result in significant systematic errors in ocean general circulation models that are forced by ECMWF winds
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The Relationship between Winds, Surface Roughness, and Radar Backscatter at Low Incidence Angles from TRMM Precipitation Radar Measurements
One year of collocated, rain-free normalized backscatter cross-section measurements from the Tropical Rainfall Mapping Mission (TRMM) precipitation radar (PR) and wind speed estimates from the TRMM Microwave Imager are used to construct fully empirical model functions relating cross section to wind speed for incidence angles from 0° (nadir) to 18°. With the exception of a ~1.9-dB offset, the TRMM PR model function at nadir compares well with TOPEX and Geosat results. For the first time using spaceborne data, least squares fits of the TRMM PR model functions to the geometric optics scattering formulation allow direct solution for the magnitudes and wind speed dependencies of Ku-band effective nadir reflectivity and effective mean square slope [s(u)]. Effective reflectivity is found to decrease slightly with increasing wind speed above 3.5 m sâ»Âč. A logarithmic dependence of s(u) is found for wind speeds between 1 and 10 m sâ»Âč. Both linear and two-branch logarithmic dependencies (with a slope break at 10 m sâ»Âč) are excellent and statistically indistinguishable parameterizations for s(u) for wind speeds between 5 and 19 m sâ»Âč. Calculations using the Elfouhaily et al. wave model suggest that the effective Ku-band s(u) corresponds to a cutoff wavelength of 3 to 6 times the radar wavelength for wind speeds from 5 to 25 m sâ»Âč; at lower wind speeds, the cutoff wavelength increases rapidly consistent with earlier observations
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Sampling Errors in Wind Fields Constructed from Single and Tandem Scatterometer Datasets
Sampling patterns and sampling errors from various scatterometer datasets are examined. Four single and two tandem scatterometer mission scenarios are considered. The single scatterometer missions are ERS (with a single, narrow swath), NSCAT and ASCAT (dual swaths), and QuikSCAT (a single, broad swath obtained from the SeaWinds instrument). The two tandem scenarios are combinations of the broad-swath SeaWinds scatterometer with ASCAT and QuikSCAT. The dense, nearly uniform distribution of measurements within swaths, combined with the relatively sparse, nonuniform placement of the swaths themselves create complicated spaceâtime sampling patterns. The temporal sampling of all of the missions is characterized by bursts of closely spaced samples separated by longer gaps and is highly variable in both latitude and longitude. Sampling errors are quantified by the expected squared bias of particular linear estimates of component winds. Modifications to a previous method that allow more efficient expected squared bias calculations are presented and applied. Sampling errors depend strongly on both the details of the temporal sampling of each mission and the assumed temporal scales of variability in the wind field but are relatively insensitive to different spatial scales of variability. With the exception of ERS, all of the scatterometer scenarios can be used to make low-resolution (3° and 12 days) wind component maps with errors at or below the 1 m sâ»Âč level. Only datasets from the broad-swath and tandem mission scenarios can be used for higher-resolution maps with similar levels of error, emphasizing the importance of the improved spatial and temporal coverage of those missions. A brief discussion of measurement errors concludes that sampling error is generally the dominant term in the overall error budget for maps constructed from scatterometer dataset
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Scatterometer Beam Balancing Using Open-Ocean Backscatter Measurements
Calculation of vector winds from spaceborne fan-beam scatterometers requires that backscatter measurements from different antennas be relatively calibrated to high accuracy. A method is developed to perform postlaunch antenna calibration using global mean ocean backscatter measurements in conjunction with estimates of the statistical distribution of near-surface wind velocity and the model function relating backscatter to winds. Substantial analytic simplifications result from assuming that the wind speed and azimuth distributions are separable and that the upwindâdownwind asymmetry term in the model function is small. The analytic model allows quantitative examination of the sensitivity of the technique to errors in the approximate wind distributions and empirical model function. The approach and its assumptions are tested using 13 months of ERS-1 backscatter data, surface wind estimates from two operational weather analyses, and three empirical C-band model functions. It is shown that the ERS-1 antennas are relatively calibrated to within 0.2 dB, which is consistent with other published results obtained using ground receiving stations and Amazon forest data. The results are nearly insensitive to realistic errors in the estimated wind velocity distributions and model function. Analysis suggests that the ocean antenna calibration technique should be accurate to about 0.2 dB using as little as 3 weeks of scatterometer data
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ECMWF and ERS-1 Surface Winds over the Arabian Sea during July 1995
The European Centre for Medium-Range Weather Forecasts (ECMWF) and Institut Francais Pour la Recherche et lâExploitation de la Mer European Remote-Sensing Satellite, named IFR2, surface wind velocity data products are compared during July 1995 over the Arabian Sea. Substantial differences were found. The central positions of the maximum isotach were separated by 450 km, and the ECMWF maximum isotach was 2 m sâ»Âč higher than that of IFR2. IFR2 wind components contained about 10 times more variance than ECMWF winds for horizontal distances from 50 to 250 km. Along the 8.5°N southern boundary of the Arabian Sea, ECMWF southward Ekman transport was higher than that of IFR2 by an amount that could be observed with current measurements. The ECMWF and IFR2 difference in upward transport of water into the Ekman layer, computed from wind stress curl, was large enough to measure
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Satellite Observations of the Wind Jets off the Pacific Coast of Central America. Part I: Case Studies and Statistical Characteristics
Measurements of near-surface winds by the NASA scatterometer (NSCAT) from October 1996 through June 1997 are analyzed to investigate the three major wind jets along the Pacific coast of Central America that blow over the Gulfs of Tehuantepec, Papagayo, and Panama. Each jet is easily identifiable as locally intense offshore winds in the lee of low-elevation gaps through the Sierra Madre mountain range. The jets have relatively narrow cross-stream width but often extend several hundred kilometers or more into the Pacific. The Tehuantepec and Papagayo jets sometimes merge with the northeast trade winds of the Pacific.
The Tehuantepec jet was highly energetic with characteristic timescales of about 2 days. Events were triggered by high pressures associated with cold surges into the Gulf of Mexico that originated over the Great Plains of North America. The Papagayo and Panama jets were much more persistent than the Tehuantepec jets. The winds at both of these lower-latitude locations exhibited a strong seasonal variation with almost exclusively offshore flow from late November 1996 through late May 1997 and periods of onshore flow in October and November during the late stages of the 1996 Central American monsoon season. Superimposed on this low-frequency seasonal variation were events with characteristic timescales of a few days.
Based on NSCAT data, the spatial and temporal evolution of major wind events is described in detail for three representative case studies. In December 1996, the jets developed sequentially from north to south, consistent with the notion that wind events in the two lower-latitude jets are associated with cold-air outbreaks that trigger the Tehuantepec jet a day or so earlier. In November 1996 and March 1997, the Papagayo and Panama jets were strongly influenced by tropical phenomena that had little apparent association with the Tehuantepec jet. These latter two case studies, together with the distinction between the statistical characteristics of the three jets, suggest that the Papagayo and Panama jets are predominantly controlled by a mechanism that is very different from the across-gap pressure gradients associated with high pressure systems of midlatitude origin that control the Tehuantepec jet
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An optical technique for the measurement of longshore currents
We present an optical method (optical current meter) to measure the longshore
component of nearshore surface currents by measuring the alongshore drift of persistent
sea foam in the surf zone. The method uses short time series of video data collected from
an alongshore array of pixels. These space-time data are first Fourier transformed to a
frequency-wave number spectrum and, finally, to a velocity spectrum. A model of the
velocity spectrum is fit to the observed spectrum to estimate the foam drift velocity.
Confidence intervals and other measures of the input and output data quality are
calculated. Field test comparisons were made against an in situ bidirectional
electromagnetic current meter on the basis of 1 month of video data from the 1997 Sandy
Duck field experiment. The root mean square error between the two approaches was
0.10 m/s. Linear regression analysis showed the gain between the two instruments to not be
statistically different from one. Differences between the surface and interior measurements
were compared to forcing mechanisms that may cause surface velocity shear. Velocity
offsets and alongshore wind stress were well correlated for cases when waves and wind
were not aligned to within ±45°, when wind- and wave-forced currents are reasonably
separable. Calculated wind-dependent surface current shear, modeled as a surface boundary
layer, correlated well with the observed velocity offsets for observations of nonalignment
between wind and waves. This technique can be applied to study large-scale coastal
behavior
Observations of Coupling between Surface Wind Stress and Sea Surface Temperature in the Eastern Tropical Pacific
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On the Use of QuikSCAT Scatterometer Measurements of Surface Winds for Marine Weather Prediction
The value of Quick Scatterometer (QuikSCAT) measurements of 10-m ocean vector winds for marine weather prediction is investigated from two Northern Hemisphere case studies. The first of these focuses on an intense cyclone with hurricane-force winds that occurred over the extratropical western North Pacific on 10 January 2005. The second is a 17 February 2005 example that is typical of sea surface temperature influence on low-level winds in moderate wind conditions in the vicinity of the Gulf Stream in the western North Atlantic. In both cases, the analyses of 10-m winds from the NCEP and ECMWF global numerical weather prediction models considerably underestimated the spatial variability of the wind field on scales smaller than 1000 km compared with the structure determined from QuikSCAT observations. The NCEP and ECMWF models both assimilate QuikSCAT observations. While the accuracies of the 10-m wind analyses from these models measurably improved after implementation of the QuikSCAT data assimilation, the information content in the QuikSCAT data is underutilized by the numerical models. QuikSCAT data are available in nearâreal time in the NOAA/NCEP Advanced Weather Interactive Processing System (N-AWIPS) and are used extensively in manual analyses of surface winds. The high resolution of the QuikSCAT data is routinely utilized by forecasters at the NOAA/NCEP Ocean Prediction Center, Tropical Prediction Center, and other NOAA weather forecast offices to improve the accuracies of wind warnings in marine forecasts