For decades, meteorologists used dropwindsondes, weather instrument packages deployed from aircraft, to make atmospheric soundings below flight level. Starting in the 1970s they used Omega dropwindsondes (ODWs) that employed the very low-frequency Omega navigation signals to estimate the ODW’s motion relative to the aircraft, and subsequently calculate the wind. Because of the limitations of this system, winds could not be calculated if the instrument fell into cloudy or precipitating areas. The land-based Omega system was terminated in 1997, and the ODW was replaced with a new instrument based on the satellite-based Global Positioning System (GPS).
On 30 July 1997, Hurricane Guillermo formed in the Eastern Pacific Ocean. Seeing an opportunity to conduct a Vortex Motion Experiment in Guillermo, HRD deployed the two NOAA P3 aircraft to Puerto Vallarta, Mexico on 1 August, and subsequently conducted two experiments before returning to base in Florida. During the second set of missions on 3 August, when Guillermo was a strong category-4 hurricane, HRD scientist Mike Black thought that it would be interesting to see if the new GPS dropwindsonde could work in clouds, so a sonde was released in the northern eyewall at 1942 UTC. It quickly became clear that the sonde was able to transmit accurate data to near the surface, so a total of 11 sondes were released in the eyewall during that mission, ten of which were good. This marks the first time that high-resolution profiles of temperature, humidity, and wind velocity in a hurricane eyewall were obtained. The highest wind speed measured by dropwindsonde during the mission was about 78 ms-1.
Max Mayfield wrote in the Guillermo report, “Profiles of wind speed versus altitude showed considerable variations among the individual ‘drops’. Figure 4 [reproduced below] shows a profile from one of the GPS sondes dropped within the southwest quadrant of the eyewall at 2342 UTC. In this example, it is noted that the winds are strongest in the low levels, and in fact, are a little stronger [italics in the original] at the surface than at flight level. These data will continue to be studied to better understand the relationship between flight-level winds and surface winds. In-depth analyses from HRD scientists are in progress, and publication of these analyses are eagerly awaited.”
Before this, the main technique to estimate tropical cyclone intensity from aircraft reconnaissance was to reduce flight-level winds by a certain percentage, depending on the altitude of the aircraft. A year later, dropwindsonde observations in the eyewall of Atlantic Hurricane Georges were used to operationally assess that the hurricane had a maximum sustained wind speed of 135 kt, just below category 5, and specialists noted that the surface wind speed was equal to or greater than those at the 3-km flight level. When Georges made landfall on the Mississippi coast a week later, NHC forecasters noted that the surface wind speed was 20-30% below those at the 3-km flight level.
The great variability eventually led to the Franklin et al. (2003) paper that quantified the mean wind speed profile in the hurricane eyewall and the expected variability about that mean. It also led to a reassessment of the best tracks of multiple tropical cyclones, including the famous upgrade of Hurricane Andrew (1992) to category 5 at landfall near Homestead, FL. This study garnered the authors a Department of Commerce Gold Medal.
Franklin, J. L., M. L. Black, and K. Valde, 2003: GPS dropwindsonde wind profiles in hurricanes and their operational implications. Wea. Forecasting, 18, 32–44.