The large number of radial flights legs flown into Elena over two and a half days make it possible to examine the changes in storm structure that occur as a storm intensifies into a strong category three hurricane and then weakens. The graph below is a time series of the best track maximum tangential winds (triangles, solid line, right scale) and minimum surface pressure (circles, dotted line, left scale) during Elena’s evolution. The gray horizontal lines denote the Saffir-Simpson hurricane strength categories. The red vertical lines and numbers show the five time periods that the data was divided into for compositing, and the red shaded rectangle represents a time of no flight data.

The composite of flights 1-28 represent Elena as a disorganized category two hurricane that was slowly intensifying or steady. Flights 29-48 capture the beginning of the rapid deepening of Elena and flights 49-62 are the middle of rapid deepening. Elena at its peak intensity as a very strong category three storm is captured by flights 63-76. Finally, Elena begins to weaken, according to the best track data set, during flights 77-86.

In order to composite the flight level variables on a single meaningful radius without the smearing of details that comes from straight radius composites, the inner edge of the eyewall was chosen to composite on. In each flight leg this radius was determined to be the distance from the eye where the tangential wind speed stopped increasing rapidly with increasing radius. This technique is shown in two examples below.

As can be seen above, the radius for composite (RC) and the radius of maximum wind (RMW) are not always the same. This is primarily the case for early flight legs and the composites of flights 1-28 and 29-48 due to outer wind maxima and little to no well-defined eyewall. Examining how the RC and the RMW change with time, we can see below that the RC has a much smoother variation, and thus will be our chosen radius for all composites.

The height of the 850-hPa

surface showed a linear

increase with increasingly

radius when Elena was

steady and at the beginning

of intensification. As Elena

continued to strengthen,

height falls occurred at all

radii, but rapid falls occurred

only within 20 km of the

RMU. In the last time period,

heights rose at all radii as

Elena began to weaken.

In the figures below, line graph composites of various reconnaissance flight variables will be shown. Each graph will have five lines, each representing one of the composite time periods detailed above. The composites are shown with respect to the inner edge of the eyewall as discussed above and expressed as distances away from that radius. Only the area –18 to + 60 km from the compositing radius will be shown as these distances approximately represent the radii where data coverage fell below 75%. The compositing radius will be referred to below as the radius of maximum updraft or RMU. The reason for this will become clear shortly.

Changes to the tangential wind profile of Elena include: winds decreasing in the core (-6 to –18 km from the RMU), winds increasing –6 to 24 km from the RMU, and little change to the winds outside of 24 km from the RMU. Two discrepancies between the flight level and best track data show up: winds weaken slightly between time periods 1 and 2 even as the best track data shows intensification and, the flight level winds are higher in the final time period when the best track shows Elena to be weakening.

While the values of relative vorticity increase steadily in magnitude during time periods 1-3, the core of Elena is in a state of solid body rotation with a small peak just inside the RMU. The transition between periods 3 and 4 is dramatic with a suppression of high vorticity values in the core, and a sharp rise just inside the RMU to 60 x 10^-4 s^-1. As Elena begins to weaken, values decrease near the RMU and rise to their highest levels in the core.

At 850 hPa Elena is dominated by outflow at all times and radii, except for at its peak intensity. Only at that time is inflow seen to reach the RMU. As Elena begins to weaken a sharp reversal in radial wind speed occurs and a peak in outflow is seen at the RMU. Considering the strong upward motion in the figure below, inflow must be confined to below the 850-hPa level.

During the steady and intensification time periods in Elena, temperatures rose all at radii with maximum values in the storm core. At peak intensity and the beginning of weakening, peak values moved radially outward to -8 km from the RMU. Temperatures also decreased during these two time periods at all radii greater than –4 km from the RMU.

Dew points in Elena rose at all radii during intensification (time periods 1-3), with maximum values inside –6 km from the RMU. At peak intensity, the maximum dew point moved radially outward to near the RMU. During the last time period, the maximum returns to the eye and values decrease everywhere outside –6 km from the RMU.

During the intensification of Elena, values of theta e increase at all radii, with peak values within or very near the storm core. At peak intensity, the largest values of theta e are found just inside the RMU, but the peak returns to the eye during the weakening stage. Values of theta e decrease in the last two time periods near and outside the RMU to increase the radial gradient of theta e.

Interestingly, the radial gradient of theta e is a maximum within 3 km of the RMU during all time periods, attaining maximum value during the “weakening” time period. This result lends evidence to the idea of the eyewall as a front and the intensification process as a frontal collapse (Emanuel 1993).