1. INTRODUCTION In this paper we present a comparison of horizontal winds from the NCEP/NCAR reanalysis (National Center for Environmental Prediction and National Center for Atmospheric Research) and those measured using VHF (50 MHz) wind profilers over the tropical Pacific (Figure 1).

2. DATA AND TECHNIQUES Wind profiler radial velocities were averaged using multiple averaging periods (1, 3, 6, 12, 24, and 48 hour), and a 3-hour sampling interval using a consensus averaging technique (Strauch et al. 1984). We limited the maximum comparison height to 12 km, as low signal to noise ratios above this level often limit the quality of the measurements.

The NCEP/NCAR reanalysis takes an analysis/forecast model to perform data assimilation (Kalnay et al. 1996). The assimilation system uses a three-dimensional variational scheme to optimally combine observations into a forecast model.

Using geopotential height at the 17 levels of the reanalysis, the reanalysis zonal and meridional winds were interpolated to the wind profiler observation heights. The reanalysis data were also interpolated in longitude and latitude to match the locations of the wind profilers.

3. COMPARISON OF WIND PROFILER AND NCEP/NCAR RE-ANALYSIS WINDS

The closest agreement between the wind profiler and reanalysis mean winds occurs at Christmas Island, a site at which profiler winds are assimilated into the forecast model. A good agreement is also shown at Darwin, where nearby rawinsonde observations are available. The poorest agreement occurs at Piura, where profiler winds are not assimilated, the closest rawinsonde is almost 2000 km from the profiler site, and topography is not adequately resolved in the reanalysis model.

4. CONCLUSIONS Differences outlined in this study suggest that some significant atmospheric processes across the equatorial Pacific may be better represented in the reanalysis, if a finer spatial resolution is used, and if profiler winds where available are assimilated

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Schafer, R., S. K. Avery, and K. S. Gage, 2002: A Comparison of VHF Wind Profiler observations and the NCEP/NCAR Re-analysis over the Tropical Pacific. In Press. J. Appl. Met.

Strauch, R.G., D.A. Merritt, K.P. Moran, K.B. Earnshaw, and J.D. Van de Kamp, 1984: The Colorado wind-profiling network. J. Atmos. Oceanic Technol., 1, 37-49.

Figure Caption Figure 1 The Trans-Pacific Profiler Network. Antenna symbols mark the locations of wind profiling radars. Crosses mark the closest reanalysis grid points