Use of Reflected GPS to Map Wind Fields for Hurricane Genesis Applications

1. Use of Reflected GPS to Map Wind Fields for Hurricane Genesis Applications Stephen J. Katzberg†(Stephen.J.Katzberg@nasa.gov) And Jason Dunion‡(Jason.Dunion@noaa.gov) † Distinguished Research Associate, NASA-Langley Research Center ‡University of Miami-HRD/ NOAA

2. Bi-Static GPS Applications to IFEX-like Experiments “Very few aircraft measurements have been made in tropical disturbances over the past 25 years, largely because it is so difficult to collect data in these systems.” -HRD IFEX05 description. THIS PRESENTATION WILL ILLUSTRATE BI-STATIC GPS APPLICATIONS TO TROPICAL CYCLOGENESIS.

3. GPS Surface Reflection at Low Wind Speeds • The GPS Surface reflection technique is a configuration that uses GPS satellites as bi-static microwave illuminators • Disturbances on the ocean surface give rise to scattering away from the specular point. • Scattering away from the specular point represents additional signal delay and can be measured. • The source of the scattering is non-zero surface slopes.

4. Previous Work at High Wind Speed Calibration against COAMPS models† Comparison with dropsondes at high wind speeds‡ • †Katzberg, Stephen J., Omar Torres, and George Ganoe, “Calibration of reflected GPS for • tropical storm wind speed retrievals”; Geophys. Res. Lett., 33, L18602, doi:10.1029/2006GL026825, 2006 • ‡Katzberg, S. J., and J. Dunion , “Comparison of reflected GPS wind speed retrievals with dropsondes in tropical cyclones,” Geophys. Res. Lett., 36, L17602, doi:10.1029/2009GL039512., 2009

5. GPS Surface Reflection at Low Wind Speeds (cont.) • Even very low wind speeds generate non-zero ocean surface slopes, which can be detected in a bi-static configuration. • Extensive GPS and dropsonde data were acquired during hurricane genesis experiments, such as IFEX 2005. • These data sets have been compared for these flights to assess GPS retrieval accuracy at lower wind speeds.

6. IFEX July 14-16, 2005 Fit to ax+b With Outliers: slope 0.69, intercept 2.03 r.m.s: 1.3115 m/s Without Outliers: slope 0.88, intercept 1.21 r.m.s: 1.11 m/s

7. Gulfstream IV 2004 GPS derived winds compared to dropsondes from high altitude During circumnavigation of Hurricane Charley* *Altitude 13,000-14,000 m (39,000-42,000 ft.)

8. Detecting Wind Direction • Cox and Munk showed that ocean surface slopes are dependent on wind direction. • Scatterometers such as QuickSCAT make use of the anisotropy in backscattered power to determine wind direction. • At L-band where GPS transmits, the relationship between anisotropy and an assumed skewed bivariate Gaussian –like slope probability density has: • A twice per azimuth angle variation and • A weaker, once per azimuth angle component.

9. Detecting Wind Direction (cont.) • While there likely is a single-cycle-per-azimuth- rotation component, • With the current receiver, only the dominant twice per azimuth cycle is extracted. • This leaves, at least for now, a 180° ambiguity. • In the illustration which follows, the Flight Level and Dropsonde wind directions have been repeated twice per cycle to account for this ambiguity.

10. Detecting Wind Direction (concluded)

11. Conclusions • The GPS bi-static technique has been shown to yield accurate low wind speeds. • This GPS bi-static technique has been shown to be capable of determining wind direction, within a 180° ambiguity. • Existence of a single cycle per azimuth rotation, yet to be demonstrated, can eliminate the ambiguity. The GPS technique can be used to reconstruct surface wind fields in weaker systems, including those undergoing  tropical cyclogenesis.