Pre-Tropical Storm Dolly

Tropical Cyclogenesis
(960817I Aircraft 43RF -- single aircraft)

Scientific Crew

Chief Scientist -- Marks

Doppler Scientist -- Dodge

Doppler Operator -- Landsea

Cloud Physics Scientist -- Bracken

This document is divided into 3 sections (Each section is written by the Chief Scientist):

Mission Briefing

The proposed mission was Tropical Cyclogenesis in the tropical depression (to become Tropical Storm Dolly on 19 August) just S of the Mona passage (between Hispaniola and Cuba) on our ferry from Barbados back to Miami. Additionally we would carry out another GPS sonde/ODW intercomparison in transit to the depression. Both aircraft would depart Barbados at 1500 UTC and climb to 16 kft in formation, perform the dropsonde intercomparison and aircraft intercomparison, then at the IP (16.5 N, 70 W) 43RF would descend to 10 kft (700 mb), while 42RF stayed near 18 kft (500 mb) and execute a stacked Figure 4 pattern through the depression, leaving the pattern heading N in the Mona passage, and return to Miami. At the briefing it was apparent that there were two main convective features in the depression, one just S of Hispaniola centered along 17 N latitude, and a second further W, SE of Jamaica along 16 N latitude. We chose to track W along 16.5 N latitude to 76 W longitude to locate the trough axis, hoping to sample both convective systems. Then we would drop SE to a point 15 N, 74 W, and track N into the Mona Passage completing the Figure 4. The only aspect of the plan in the book missing was the presence of the AFRES at low-levels, as they were not tasked to fly until the next day.

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Mission Synopsis

Both aircraft took off at 1500 UTC. We climbed in formation to 16 kft commencing the dropsonde comparison as soon as we got W of the Windward Islands (1530-1630 UTC). Four pairs of drops were made with 43RF calling the drops and videotaping the GPS sondes as the exited 42RF. All but the third ODW had good winds, and the first sonde had questionable humidity. At 1655 the aircraft separated with 43RF descending to 10 kft (700 mb) as we proceeded to the IP. ATC requested 43RF climb to 12 kft for the pattern. I checked the altitude change with Pete Black at 1727 UTC. No problem, so we climbed. At 1737 UTC we reached the IP (16.5 N, 70 W) and tracked W heading for a nice NNE-SSW line of convection along 72 W longitude (the E convective system in the satellite imagery). We entered the band at 1800 UTC. A second smaller band was just W of the first and we crossed it at 1812 UTC getting a sharp wind shift from SE to E winds at 1815 UTC. There should be excellent Doppler and microphysics data in this series of bands. At 1825 UTC (16.5 N, 73.8 W) the wind swung back to the SE at 30 kts (the jet) as we entered the NE edge of an immense stratiform rain area (the W convective system in the satellite imagery). Over the next 6-7 min the winds changed magnitude rapidly, gradually getting real light and variable at 1829 UTC (74 W) and then it flip flopped all over for the next few min. At 1837 UTC (74.8 W) we deviated around a strong reflectivity region in a NNW-SSE oriented convective line embedded in the large stratiform region. after that the wind became more E indicating we had crossed the trough axis around 74.5 W. The data system crashed from 184634 to 184701 UTC in the stratiform region and the work station was down most of the time since we entered the precipitation (Peter Dodge left it off until we started our Ferry back to Miami).

At 1853 UTC (76 W) we broke out of the stratiform precipitation area just before we turned SE. Spoke to Pete about moving S point to 15 N 74.5 W to put us up the center of the large stratiform region and the trough axis. Pete concurred said they had good wind shift in both convective features. He had strong NE flow after crossing E system where we had strong ESE flow. Apparently the E system was stronger at 500 mb. I mentioned that our estimated surface pressure was the lowest at our W turn point and that possibly a surface circulation might be W of the turn. The low clouds appeared to be spiraling into a region NW of our turn point. 42RF extended its W track to 76.5 W and dropped a GPS sonde at their W turn point. The splash pressure was 1010 mb with ENE winds of 10 kts. From 1900-1920 we tracked SE along the W edge of the large stratiform area (excellent radar presentation) as the winds shifted from E to SE to SSW and increased. At 1921 UTC (15.2 N, 74.68 W) 42RF was directly overhead, 3 min from our S turn point.

At 1924 UTC we tracked N in the middle of the monstrous stratiform region (as big as I ever saw in TOGA COARE - 14 December 1992 comes to mind). We were half way between two intense NNW-SSE bands of convection ( both lines were 20-30 km away). As we tracked N between the lines from 1900-1939 UTC the winds were from the SE at 20-30 kts, there was lightning visible, and extremely heavy rain at the aircraft (great 2D-P data). At 1938 UTC we penetrated an intense reflectivity region encountering a strong wind shift from 160° to 90°, a 5 m s-1 updraft, graupel, and a lot of electrical activity. Pete said 42RF had a similar experience in the same location at 500 mb. They attempted a GPS-sonde drop in the feature with no success. Apparently, there was a pretty good mid-level mesocyclone at that spot. At 2010 UTC we reached the end of our N leg and climbed to ferry altitude for the return to Miami.

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Mission Evaluation and Problems

This was a really good Genesis mission. Both aircraft found strong indications of a closed circulation with vertical continuity from 12 kft to 20 kft altitude forming in an immense region of stratiform precipitation in the western most convective feature in the wave axis. The stratiform rain contained intense convective rainbands in the generally strong SE flow at mid-levels just E of the wave axis. This strong SE flow was similar to that seen just W of Barbados and the Windward Islands during the mission on 15 August. The E convective feature also contained a mesocyclone at both altitudes, however the features were displaced horizontally between flight levels. The Doppler radar and cloud physics data sets should be outstanding. The radar should help us determine the vertical extent of the two mesocyclone features associated with both convective features. It seems clear that while the disturbances we sampled were associated with the wave axis, it was pretty clear that these were different convective features and mesocyclones than sampled on the 15 August mission. The cloud microphysics data in the extremely heavy rain within the western convective feature should provide some excellent 2D-P data.

The dropsonde intercomparison went very well. Four pairs of sondes were dropped, with 43RF videotaping the GPS-sonde drops from 42RF. Three of the four ODWs worked well. 42RF performed maneuvers on the last two drops to test how the GPS navigation performed in turns.

1. We had power problems with the workstation, similar, but much worse than on the 15 August mission. The problems resulted in Peter Dodge shutting down the workstation for much of the Figure 4 pattern. However, there were other problems noted with the 60-Hz power on the aircraft this time. The 60-Hz bus converter alarm kept going off during turbulence and monitors flickered. Apparently, the workstation is very sensitive to jitters in the 60-Hz power converters. We should get a rack mounted UPS for the work station to prevent this from occurring in the future (AOC talked to Joe Griffin about possible UPS models). After we left the turbulent air Peter Dodge ran the workstation all the way back to Miami with no problems. Hence, we decided not to pull the CPU off. He backed up the whole system and planned to take the DAT back to Joe Griffin to check things out. Subsequent to the flight AOC replaced a bad 60-Hz power converter.

2. The right camera was not functioning during the flight.

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