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The improved dropsondes, relying on GPS navigation and improved
humidity sensors, could now be deployed in clouds and hurricane eyewalls,
which AOML/HRD did in Hurricane Guillermo (1997). Dropwindsondes
released from the P-3s in 1997 provided the first high-resolution wind and
thermodynamic profiles in a TC eyewall. Research and operational aircraft
since have obtained more than 1000 such profiles (e.g., Fig. 3). Franklin et al.
(2003) showed that the mean surface to 700 hPa (the standard flight level for
reconnaissance aircraft in hurricanes) wind speed ratio was about 90% in the
eyewall in several hundred over-ocean profiles. They determined an adjustment
factor for TC flight-level wind interpretation: without additional information,
NHC operationally assesses surface maximum 1-min wind speed estimates to
be about 90% of the peak 10-s wind speed observed at 700 hPa. This new
understanding of TC boundary layer wind and better methodology for adjusting
flight-level wind speed measurements to the surface are also applied to historical
TC reanalysis.
Fig. 3: Frequency distribution of the horizontal wind speed (m s -1 ) with altitude from
1200 eyewall dropwindsondes. The black line denoted the mean of the sample and the
two blue lines the 95 and 5 percentile of the distribution.
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