Topic: Eyewall Wind Profiles in Hurricanes Determined By GPS Dropwindsondes

[Astronuc]

James L. Franklin¹, Michael L. Black², and Krystal Valde^1
1NOAA/NWS/TPC/National Hurricane Center, Miami, Florida
²NOAA/AOML/Hurricane Research Division, Miami, Florida

One of the more difficult problems for operational tropical cyclone forecasters is the assessment of the cyclone's maximum sustained surface wind. Even when aircraft reconnaissance data are available, these are typically obtained from the 700 mb (10,000 ft) level; from these flight-level observations, the forecaster is left to estimate the surface winds. Based on comparisons of flight-level and buoy data, Powell and Black (1990) recommended that a ratio of 63%-73% be used to reduce reconnaissance flight-level wind observations. While operational practices at the National Hurricane Center (NHC) have varied over time, in recent years surface winds have typically been taken to be 80%-90% of the flight-level wind. In view of studies such as Powell and Black, use of these relatively high ratios has periodically resulted in criticism of NHC intensity estimates.

In 1997, the National Oceanic and Atmospheric Administration (NOAA) and Air Force Reserve Command (AFRC) hurricane reconnaissance aircraft began to deploy Global Positioning System (GPS)-based dropwindsondes (Hock and Franklin 1999) in the hurricane eyewall. These instruments provide for the first time, detailed, accurate profiles (15 ft vertical resolution, with 1-4 mph accuracy) of the inner core of a hurricane from flight level (typically 700 mb) down to the surface. More than 350 such profiles have been obtained through the 1999 hurricane season.

http://www.nhc.noaa.gov/aboutwindprofile.shtml