The rapid intensification of Guillermo (1997)

From 0300 UTC on the 2nd of August to 0900 UTC on the 3rd Hurricane Guillermo deepened 54 hPa over 28 C waters in the Eastern Pacific. The two NOAA WP-3Ds flew dual aircraft missions with similar patterns on both days capturing a portion of this rapid intensification (RI). This is a rare opportunity to observe the inner core of a hurricane while it is undergoing RI over 24 h with nearly identical missions.

Seventy global positioning system (GPS) dropwindsondes (sondes) have been used to produce horizontal maps of the inner 250 km radius region from 10 m to 4000 m altitude. We use these GPS analyses, along with data from the lower fuselage and tail radars, and the aircraft in-situ 1 Hz sensors to answer the following questions about the RI of Guillermo.

(1) Does the eyewall reflectivity become more symmetric, with taller echo tops, higher rain rates, a smaller diameter and greater net latent heat release during RI?

(2) Are the expected increases in equivalent potential temperature (theta_e) occurring only under and near the eyewall or do these increases extend well into the strength region?

(3) Is the increase in the tangential wind component constrained only to the eyewall region or does it also extend well beyond the inner core?

(4) Does the inflow become more symmetric, stronger and deeper through the intensification period?

(5) How do the rainbands within 250 km of the circulation center evolve as the TC deepens? Does a more robust eyewall suppress nearby bands?

(6) Is there evidence that downdrafts are becoming less effective in limiting the boundary layer theta_e?

(7) Were there any synoptic scale clues leading one to believe RI was imminent?

Our preliminary findings show that the large scale factors, derived from NCEP-NCAR reanalyses, provide no clues that RI will soon occur. If the large scale fields matter, then a lagged response of more than a day is occurring. The radar reveals that the echo tops do not vary from one day to the next, but rain rates do increase by 20% within 60 km of the circulation center. Eye diameter reduces by about 12 km, but the process differs from a concentric ring undergoing uniform contraction. Instead we see the northern eyewall spiral into the eye repeatedly, and eventually producing a smaller radius eyewall with a more circular shape. Rainbands remain active on the east side of the TC throughout RI but these bands are not capable of producing downdrafts that dramatically reduce the theta_e of the inflow. Increases in the tangential wind and theta_e are observed only within 100 km radius of the circulation center. Subtraction of a particular field from one day to the next will be used to highlight the differences during the 25 hPa deepening that occurred between the two missions.

We feel that the spiraling in of the eyewall into the eye is a crucial step in the RI. However, this tightening of the eye by itself would not lead to a long-lasting higher category state. We will explore how surface fluxes adjust to maintain the more intense vortex.