27th Conference on Hurricanes and Tropical Meteorology


Overview of RAINEX Flight Program

Robert A. Houze Jr., University of Washington, Seattle, WA

The 2005 Hurricane Rainband and Intensity Experiment (RAINEX) employed three P3 aircraft (2 from NOAA, 1 from NRL) to make intensive Doppler radar and dropsonde measurements in Hurricanes Katrina, Ophelia, and Rita. The aircraft were based in Tampa, Florida. The NRL P3 was equipped with the NCAR/ELDORA Doppler radar system. Satellite communications were used to transmit aircraft radar and flight-level data to a ground Operations Center. The real-time data flow made it possible for the RAINEX PIs in the Operations Center to coordinate the placement of the aircraft in real time via internet chat. The flights targeted the key mesoscale subfeatures of developing and mature storm vortices. The original idea of RAINEX was to sample the eyewalls and rainbands of mature storms to determine where vorticity was being concentrated on small scales within the storms. However, because the three hurricanes investigated all formed close to Florida, the RAINEX was able to sample the structure of tropical depression and tropical storm stages as well as mature hurricane stages. The NOAA aircraft flew landfall missions for Katrina and Rita and missions to document the extratropical transition of Ophelia. Thus, the RAINEX dataset covers all stages of storm lifecycle.

Ophelia: In the depression stage of Ophelia, the RAINEX aircraft circumnavigated the central "convective burst", which later became the center of the tropical depression. This feature was remarkably persistent in contrast to mesoscale convective features located radially outward from the storm center. In the tropical-storm stage of Ophelia, the RAINEX aircraft observed rainbands spiraling into the center of the storm. Ophelia never formed a small, intense eye and never reached the status of a Category 3 hurricane. In the hurricane phase of Ophelia, the coordinated RAINEX flights showed mesoscale inflows of apparently subsiding dry air entering the stratiform regions of rainbands. These observations may give insight into the mechanism by which environmental dry air inhibits storm development.

Katrina and Rita in the tropical-storm stage: Katrina and Rita were similar storms. Both formed in the western Atlantic, not far from Florida. RAINEX flights sampled them while they were still in the tropical-storm stage, just before they moved into the Gulf and rapidly intensified. In the tropical storm stage, Katrina and Rita already had large principle rainbands, and the RAINEX flights sampled these bands with Doppler radar and dropsondes. Preliminary analysis of the Katrina principle rainband shows that its cells were embedded in stratiform precipitation, and that the cells exhibited outward slopes.

Katrina and Rita as mature hurricanes: The RAINEX flights in the mature stages of Katrina and Rita included observations of outer rainbands, inner rainbands, rainbands trying to evolve into eyewall structures, mature eyewalls, shrinking and weakening eyewalls, and in Rita a fully concentric secondary eyewall. The highlights of the mature storm flights were:

Katrina, 27 August 2005: A large spiral principal rainband was well defined on the eastern side of the storm and was sampled and found to be primarily stratiform on its downwind end. An apparent eyewall replacement process was sampled. A secondary eyewall had apparently formed on the southeast side of the storm and had an outward slope, while the inner eyewall was a remnant that had lost its outward slope.

Katrina, 28 August 2005: Katrina was Category 5 with a well-defined eye. An inner rainband observed by aircraft on the northeast side of the storm had eyewall-like structure but did not appear to form a complete secondary eyewall. The principal rainband on the east side of the storm was sampled by coordinated flights of NRL and NOAA P3 aircraft. It consisted of two parallel lines of outward-sloping convective cells. Extraordinarily detailed ELDORA radar data were obtained on this portion of the principal rainband as the NRL P3 aircraft flew between two lines of convection. The ELDORA radar also observed a semicircular rainband on the northeast side of the eye that had the appearance of a secondary eyewall that was trying to form. However, satellite data show that a secondary eyewall did not form after this time.

Katrina, 29 August 2005: One of the NOAA P3 aircraft obtained Doppler radar and dropsonde data as the downwind portion of Katrina's principal rainband, and Katrina's eyewall (by then asymmetric) passed over the coastline between New Orleans and Biloxi. This portion of the principal rainband was a mix of active convection, decaying cells, and stratiform precipitation. The still active portion of the eyewall was well formed and sloping outward at landfall.

Rita, 21 September 2005: Rita had rapidly intensified to Category 5 just before this RAINEX mission. The central pressure during the mission was 897 mb. The NRL P3 and one of the NOAA P3 aircraft flew coordinated quadruple-Doppler radar tracks with intensive dropsonde coverage to document rainbands in each quadrant of the storm. The NOAA P3 aircraft sampled the eyewall before and after the quadruple-Doppler flights.

Rita, 22 September 2005: Rita had de-intensified in conjunction with an eyewall replacement cycle. When the RAINEX aircraft arrived, a concentric secondary eyewall was present. One NOAA P3 aircraft intensively sampled the two eyewalls with repeated radial penetrations. The NRL P3 circumnavigated the two eyewalls and observed them with the ELDORA radar. A second NOAA P3 aircraft flew broad "figure-four" patterns to map the entire vortex structure. This nested set of aircraft data was an unprecedented documentation of a strong hurricane undergoing eyewall replacement.

Rita, 23 September 2005: Rita had become an asymmetric. Again, all three RAINEX P3 aircraft flew a coordinated mission, with one NOAA P3 making repeated radial penetrations of the eyewall and inner rainbands immediately surrounding the eyewall. A second NOAA P3 aircraft again flew broad "figure-four" patterns to map the entire vortex structure. The NRL P3 with ELDORA circumnavigated the eye and observed both the eye and the inner rainbands near the eye. The NRL P3 entered the eye region for dropsonde documentation of the dry air affecting the asymmetric eye.


Session 12A, Special Session:- RAINEX I
Thursday, 27 April 2006, 10:30 AM-12:00 PM, Regency Grand BR 4-6

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