Chief Scientist -- Willoughby
Doppler Scientist -- Landsea
Observers -- J. Lawrence (U. Houston), S. Jones (U. Munich)
This document is divided into 3 sections (Each section is written by the Chief Scientist):
Flight 960912H was a tasked reconnaissance mission into Hurricane Hortense, originating and terminating at McDill AFB. HRD participants were: Hugh Willoughby, Chris Landsea, Sarah Jones (Univ. of Munich) J. Lawrence (Univ. of Houston) and J. Donnelly (UMASS). The tasking required N42RF to obtain the 06Z and 12Z fixes and to fly ALFA patterns between fixes. Because the hurricane was more than 600 nmi from Tampa, the ability to remain on station for the required time between fixes was a concern. The ALFA patterns were oriented at 45 degrees to the cardinal directions (314-135, 045-225) with 110 nmi legs. Chosen mission altitude was 5000 ft.
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N42RF took off from McDill at 0331 UT on 12SEP96, arrived within radar range of the storm at 0546, and approached the eye at 5000 ft on a nominal track of 135. Initially the eye was 15 nmi in diameter, open to the S, defined by several wispy bands of convection with maximum reflectivities ~40 dB(Z). We reached the center 0609 UT, extrapolated a MSLP of 963 hPa, and encountered 110 kt winds on exit to the SE. We continued beyond the eye on the same track to a point SE of the center and turned north to a point NE of the center. The nominal track on the second penetration was 225, perpendicular to the first penetration's. We measured 115 kt winds on penetration and reached the center at 0734 UT when the MSLP was 962 hPa. We continued beyond the eye to a point 110 nmi the SW of the center and turned downwind to the west. After the second penetration, the character of the eyewall changed. It became a continuous arc of reflectivity occupying the northern semicircle. Maximum reflectivities increased to nearly 50 dB(Z). Before this time we had flown through only light turbulence in the eyewall and essentially no turbulence elsewhere. We entered the eye from the SE through a broad ~3 m/s updraft in the low reflectivities of the southern semicircle. By 0859 the MSLP had fallen to 958 hPa. On exit from the eye toward the NW we flew through alternating up and downdrafts with peak vertical velocities > 10 m/s. During one transition from down to updraft we encountered severe turbulence that imposed +3g and -1 g accelerations on the airplane. After the third penetration the eyewall expanded to 25 nmi diameter and extended through 270 degrees of arc, but remained open to the south. We flew downwind around the storm to a point east of the center and made the final penetration from east to west. When we reached the center at 1103 UT, the pressure remained unchanged from the third fix. A line of relatively low reflectivity extended diametrically across the eye from SW to NE, and the boundaries of an extensive anvil extending 200 nmi from eye showed clearly on the PPI radar. By this time, the sun had risen. The aircraft was in stratocumulus cloud at the 5000 ft flight level in the eye with blue sky visible above through breaks in the clouds. We exited on a 300 deg track, remaining at 5000 ft until we were 110 nmi from the center and then climbed to ferry altitude. N42RF recovered at McDill at 1335 UT.
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Airplane, and instrumentation worked well. The g-loading imposed by severe turbulence led to a required inspection for structural damage, but none was found. J. Donnelly made some progress on interference between the scatterometer and radiometer prototypes for ultimate installation on the AFRES aircraft.
Clearly, some influence weakened Hortense during the early part of the flight. Then the hurricane accumulated energy that erupted in the surpercell and produced a 3-4 hPa fall in MSLP as well as reformation of the eyewall at larger radius. This transformation was the start of a deepening trend that lasted throughout the day on 13SEP96.
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