Mission Brief:

The plan called for the aircraft to fly a rotating figure-4 pattern (Fig. 1) in a developing system just northeast of the Windward Islands. The system has convective bursts associated with it, which have been persistent (though intermittent) for the past 24 h. The IP would be set up on the southeast side, with 105 nm legs. First pass would be flown at 5000 ft to hunt for the center at that altitude, and then the aircraft would climb to 12,000 ft for the rest of the mission. If possible, we would fly a convective burst module around a mesoscale convective system, sampling convective and stratiform areas. Sondes would be dropped at all turn points where there are no scatterers, plus one center drop from 12,000 ft. In addition, any turn points within the convective burst module would have sondes dropped with them. No AXBT's were planned for this flight.

Mission Synopsis:

AL92 continued to try to become better organized prior to and during this mission. The system was experiencing light-to-moderate southwesterly 850-200 hPa shear of 10-15 kt over the center and 20 kt shear to the north of the system (Fig. 2). GOES visible imagery during the mission (Fig. 3) showed some deep convection just northeast of the Windward islands. The orientation of the cloud patterns in this visible image suggest a midlevel vortex was present, if not a low-level vortex. An infrared image at this same time (Fig. 4) showed a broad shield of -60°C cloud tops with an isolated area of -70°C cloud tops near the center of the shield. Inspection of visible animations indicated that this cloud shield was displaced to the northeast of any possible lower tropospheric circulation. This was consistent with the shear pattern from Fig. 2.

The mission, and the resultant flight track, was very complicated as indicated by the flight track (Fig. 5). Air-traffic control issues were a significant impediment at the beginning of the mission. On the first leg at 5000 ft, we had to loop, and then deviate track, so we could not execute the flight plan as originally planned. Eventually we climbed to 12,000 ft because of lack of communications, setting up a new IP on the northeast side. We proceeded southwest, then deviated to the south to try to find the midlevel center (winds were northerly along this leg). We then proceeded south, then turned to the southwest, in clear air. We continued there for a distance, then ended the leg and headed toward a point southeast of the center. We flew northwest, then downwind to a point west of the center. Then we flew back east through the convective system to the east of the supposed low-level center. At the end of the eastbound leg, we flew a convective burst module where we flew along the back edge of stratiform rain, then turned to the southwest to sample the convective system from the rear to the front, then turned west-southwest to sample newly-developing convection on the west side of the MCS (possibly developing over the low-level center). Flight-level winds (Fig. 6a) showed a circulation center at this altitude associated with the precipitation shield. Then we turned back to sample the convection at the leading edge of the original MCS. We flew front-to-rear, through the stratiform region. We then turned back to the west, then south, sampling new convection again. We dropped sondes at the major turn points, the turns of the burst module, and at midpoints of the front-to-rear and rear-to-front legs (Fig. 6b). A total of 13 dropsondes were released. Toward the end of the pattern satellite microwave imagery showed that the convection on the southwest side of the burst module had become the dominant area of convection, while the precipitation area intensively sampled earlier had much lower microwave brightness temperatures (Fig. 7).

The system was very complex. There was clear evidence of a midlevel center, displaced to the east of a possible low-level center. We never sampled a low-level center, though, because one most likely did not exist at this time. Peak flight-level (SFMR) winds of about 40 (35) kt were observed in the precipitation shield. One radar leg (Fig. 8), flown from west to east, sampled well the midlevel vortex, which was coherent from 3 km up to 10 km. Below that level was southerly flow. Vorticity was maximized at the 4-7 km layers in this pass, with scattered weaker areas of positive vorticity at 1.5 km. We also possibly got good data from the burst module, which may have sampled a burst in its mature-to-decaying phase and in its initiation stage.

Problems :

There were some problems with the inertials at the beginning of the flight, which may have an impact on the radar data at this time. Two dropsondes during the burst module had problems with fast fall.

Robert Rogers
8/17/08

Mission Data