Lead Scientist | P. Black |
Radar | F. Marks |
Workstation | P. Leighton |
GPS sonde scientist | M. Black |
AXBT scientist | J. Cione |
Observer | W. Bracken |
Mission Briefing:
This flight was part of a 5-plane (two NOAA WP-3D Orion aircraft, NOAA G-IVSP Gulfstream jet aircraft, NASA DC-8 jet aircraft and high-altitude NASA ER-2 aircraft), synoptic-flow mission to drop GPS sondes in the environment around a developing tropical storm. This flight was the third in a series. This mission dropped 22 GPS sondes into the inner core (within 50 nm of the center. Of these, 11 were droped into the eyewall along 8 radial legs in 8 octants of the storm (two failed) and 3 were dropped in the eye (one failed). Seven AXBTs were dropped into the eyewall (1 failed) and one in the eye which failed. Three interesting drops were also obtained in the feeder band wrapping around the storm at a radius of 50 mi from E to N to W. The flight was flown at max altitude 500-450 mb, except for the two rotating Figure 4 patterns in the inner core which were flown at 550 mb (15 kft). The purpose of the flight was to provide improved initial conditions for track models and to discern the eyewall structure of a minimal stationary hurricane.
Mission Synopsis
The flight departed St. Croix International (TISX) at 1745 UTC,
23 August and landed at MacDill AFB, FL at 0145 UTC, 24 August.
A total of 33 GPS sondes and 8 AXBTs were dropped during this
mission, from 20 kft (15 kft in the inner core), 8 of which were
coincident. Two AXBTs and 3 GPS sondes failed. The Figure 4
legs were oriented SE-NW, SW-NE, N-S and E-W. Maximum
flight-level wind in the inner core was 85 kt in the western
feeder band and SE eyewall, 70 kt at the surface and 106 kt at
925 mb in the E eyewall. Minimum central pressure was 956 mb
and the inner eye diameter was 25 nm, while the outer
eye/rainband diameter wa 80-90 nm. The storm was stationary.
A major feeder band consisting of several thinner bands was
observed spiraling into the inner core to form an outer eyewall
from the SE, E, through NE, N and NW. Strongest eyewall
convective bands, with cloud turrets extending to 55-60 K ft
(as observed by the ER-2),were along the E sector of the inner
eyewall (45 dBZ reflectivities). The eyewall was open to the
west. A strong CB developed within the clear eye region on the
south side as we traversed from N-S, and containing the flight
level wind max of 85 kt. 85 kt flight level wind maxima were
also observed in the outer band to the N, NW and W of the
center. The inner E eyewall had only 65 kt winds at flight
level, but 105 kt at 925 mb.
GPS dropsonde wind profiles in the W eyewall, where convection
was weak, differed dramatically from those in the E eyewall,
where convection was very strong. Profiles in the W showed
nearly constant 65-70 kt winds with height down to 400m, near
the top of the well-defined boundary layer. Winds decreased
linearly from there to the 10-m level where winds were 45 kt.
In the E, winds increased downward from 60 kt to 106 kt at 850
mb, maintaining nearly constant wind with height to 60 m, then
decreasing rapidly to 70 kt at the 10-m level. In the 106 kt
high wind layer, strong inflow was observed with wind direction
veering inward by more than 60 deg between 850 mb and the
surface. The convection was therefore associated with an
enhanced inflow jet in the lowest levels on the E side of the
storm. The strong inflow, enhanced southeasterly horizontal
flow and strong convection associated with strong updrafts on
the east side coupled with enhanced horizontal flow in the
upper levels on the W side, some outflow in the low levels and
subsidence in the W suggest the vortex was interacting with
westerly or northwesterly shear (strong environmental SE low
level winds, weak upper level winds). This notion is supported
by hodograph plots of mean storm-domain winds computed from TA
Doppler radar data.
Doppler radar showed a strong low level wind max of over 100 kt
on the east side of the center. The 500 mb center seemed to be
displaced about 12 km NE of the surface center, which could be
clearly identified by a swirl in the low clouds within the 500
mb eye.
Excellent tail Doppler and lower fuselage radar data were also
obtained (3 radar composites were
transmitted to NHC in real-time, but no EVTD wind fields were
sent). We also collected some good F/AST data along a N-S line
of convection 10 nm W of the west eyewall. Cloud microphysics
data were also collected (good ice data in the rainbands W of
the center and rain data in the inner core).
The successful AXBT launches in the S, SE and E eyewalls
reported SSTs between 27.2-26.2C, about 2-3 deg cooler than the
29.0 C SSTs observed 2 days ago. Estimates of ocean mixed layer
depth in this region ranged between 60-90 m in this area, about
20-30 m deeper than 2 days ago, suggesting that strong mixing
had occurred beneath the storm in the 12-18 hrs that it had
remained stationary. This cooling under the eye and E eyewall
semicircle may have been priarily responsible for the cessation
of the storm deepening and decrease in convection.
Evaluation:
This flight is part of a landmark 5-plane synoptic flow
experiment for determining the environmental flow structure of
the atmosphere around a developing TC while also determining
the oceanic thermal structure beneath the inner core. It marks
the first time in history that 5 research aircraft have flown a
coordinated pattern simultaneously in a hurricane.
Problems:
Difficulties were again encountered in deploying GPS sonde
pairs on either side of the convective eyewall during the SW-NE
transit due to a GPS receiver channel failure in the aircraft.
This lead to missed drop points in the eyewall.
Peter G. Black
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