Surface winds in hurricanes from GPS-sondes: Comparisons with observations

The Hurricane Research Division (HRD) of NOAA began dropping NCAR Global Positioning System (GPS) sondes in the vicinity of tropical cyclones (TC) on an experimental basis in 1996. High-resolution vertical profiles of wind speed and direction, temperature, dew point, and pressure were collected from flight-level to the surface. The boundary layer and near surface winds measured by sondes are considered particularly important, since few surface wind instruments operate in sustained wind speeds greater than 50 m/s. There have been some uncertainties about whether the profiles of boundary layer (especially near the surface) winds and thermodynamic measurements made by sondes in TC convective eyewalls were representative of conditions observed at the surface. In addition, the averaging times of sonde winds relative to fixed earth measurements from standard anemometers have been uncertain.

Based on the preliminary results from 17 cases in relatively high winds from three hurricanes in 1998, the sondes appeared to be recording very accurate and representative wind speeds when compared to in situ measurements from marine platforms. There appeared to be a slight negative bias in the sondes' reported surface wind speeds, while there was hardly any bias in the wind speeds adjusted to 10 m using the mean boundary layer winds averaged over the lowest 500 m layer. The pressure and temperature observations also showed good agreement between the sondes and surface platforms. Although these results are very preliminary, it appeared that the sondes provided accurate measurements in the boundary layer and near surface observations which are extremely beneficial for operational and research purposes. The sample size has been increased during the 1999 hurricane season. Therefore, more definitive statistics for sondes and surface platform comparisons that were available during the 1996-1999 hurricane seasons will be presented at the conference.