The rapid intensification of Hurricane Guillermo (1997) as viewed with GPS dropwindsondes

On August 2nd and 3rd of 1997 the two NOAA WP-3D aircraft simultaneously sampled Hurricane Guillermo in the eastern Pacific as part of the Vortex Motion and Evolution Experiment. This was the first experiment to deploy Global Positioning System dropwindsondes (GPS sondes) into a hurricane. Over 70 GPS sondes were jettisoned from 500 and 700 hPa within 300 km of the hurricane center over the two days. On each day, the lower aircraft visited the center 10 times as the hurricane progressed steadily westward near 5 m/s. This pattern allows for frequent monitoring of the reflectivity fields of the eyewall with the lower fuselage and tail radars.

Sampling of the storm occurred just as rapid intensification (RI) commenced on August 2nd with the hurricane deepening 25 hPa by the time the aircraft returned on August 3rd. Deepening may have been as much as 2 hPa/h on the 2nd. The experiment provides us with a unique opportunity to observe how the inner core of a hurricane evolves during an RI period. Better understanding of how the eyewall interacts with the immediate environment during RI may aid in forecasting RI.

The data from the GPS sondes were quality controlled and treated with a cubic spline to produce horizontal fields from 4 km altitude to near the sea surface. To do this we have combined the sondes for each day, assuming that the storm is quasi-stationary during the 6 hours of sampling. The flight paths and distribution of the GPS sondes, although denser on the 3rd, were similar enough to the 2nd so the composite fields from each day could be compared, save for the eyewall region. In-situ aircraft data with 1 Hz resolution were used to define the sharp radial gradients associated with the eyewall.

Several similarities and differences, some unexpected, are noted. The reflectivity field shows that the eye diameter was reduced by 10 km during the RI. However, echo tops, and rain rate remained similar for the two days. The shrinkage of the eyewall coupled with a nearly constant rain rate implies a reduction of net latent heat release during the RI. The shape of the eyewall was more spiral on the 2nd and becomes more circular on the 3rd. The inflow was asymmetric with maximum values observed along the east-southeast portion of the eyewall on the 2nd. After RI concluded on the 3rd the inflow was stronger and axisymmetric. Both days exhibit modest outflow at only 2000 m altitude less than one degree latitude north of the circulation center. Eye temperatures above 3 km increased as much as 5 °C and equivalent potential temperature near the sea surface increased 15 K during the RI.

In addition to the analysis of the inner structure of Guillermo, synoptic conditions will be examined through ECMWF 1.25° resolution data for the two days. Large-scale influences will then be compared with the inner structure changes to determine if there were large-scale clues to the RI.