G4 Flight in support of FASTEX IOP 17 Low 41: the Fastex Cyclone Part II: Near upstream flight: actual structures documentation Date: February 18, 1997 (prepared by Jean-Pierre Cammas) Take-off Time Saint Johns (approx): 1526Z Landing Time Shannon (approx): 2157Z Flight duration: 6hr 31 mn Scientists: Jean-Pierre Cammas (LA/CNRS), Melvin Shapiro (NOAA), Gwenaelle Hello (METEO-FRANCE), Tom ... (NRL), Diana Baertels (NOAA) Purpose: Low 41 was entering the middle part of its life cycle over the Central Atlantic. The purpose of this second flight over the frontal wave was to document actual structures involved in its development. The flight plan from Saint Johns to Shannon has been devoted to sample as well as possible the upper-level precursor, the cold and warm fronts, the strongest low level winds in the warm sector, the low itself. With the advantage of strong tail winds all the way over the Atlantic a total distance of about 3000 nm has been covered. Some of the legs of the flight plan also involved objective targets, with a verification time the day after either with european ground stations data or with data of the third flight of the NOAA GS-IV. This would allow to complete the predictability study of this frontal wave that began the day before (see part I). Description of the flight: The first leg going southeast after taking off entered into the NY ATC area. It was dedicated to the documentation of the upper-level trough over the rear part of the system and the southern part of the surface cold front. It has been flown at high levels (180 hPa) from 43.7N-47.8W to 40.0N-40.0W involving 10 dropsondes. The end of this first leg sampled the cyclonic shear side of a southwesterly upper-level jet stream (merging of the polar and artic jets, see below) and crossed the surface cold front. The second leg going north has been flown at lower levels (about 345 mb). It completed the observation of the cyclonic shear side of the upper-level jet stream and gave a second cut of the cold front. The end point of this leg was southwest of the surface low. Two upper-level jet axis (artic and polar jets) were observed along this leg, which was confirmed later on the third leg. The third leg, zonal at 46N and from 40W to 26W, again intersected the cold front. For the first time in the field season, an intercomparison with ships data has been possible: a dropsonde and a radiosonde have been launched right over the SUROIT (46N-37W) which was west of the surface cold front. Then a rainband was observed on the onboard precipitation radar when crossing the cold front (46N-34W). At about 33W we observed a clear separation between the two upper-level jets previously mentionned. Winds in excess of 40 ms-1 were recorded in the Low Level Jet (850-925 hPa) nearby 30W. The fourth leg (46N-26W to 52.5N-23W) was designed to document the low itself. It appears that this leg did not exactly cross the low center but has intersected its eastern part. At 1943Z southerly winds with a minimum surface pressure of 987 hPa were recorded at 50.6N-24.0W, the 1800Z surface analysis indicates a low center pressure closed to 985 hPa. The fifth leg going southeast put the aircraft in the right location to do a transverse cross section of the warm front on the way to Shannon. A large zonal band of precipitation was noticed on the radar before to begin this leg (52.5N-23.0W). A strong southeasterly low level jet was observed (51.3N-22.2W). A total of 54 dropsondes were launched, two were bad, seven had missing winds. Overall Assessment of the Flight: Taking-off from Saint Johns, tail winds allowed a far better coverage of the system than the one that could have been done with a Shannon to Shannon flight. This flight was very successful in documenting actual structures. There are very good data to cover the life cycle of this frontal wave, e.g. the deepening phase before the frontal wave reaches the MSA west of Ireland. Actual features sampled include a complex structure aloft (upper-level precursor of the system, two upper-level jet streams merging at the base of the trough and a strong diffluence downstream), sharp surface fronts and intense low-level jets. Based on the 18H-Arpege forecast the flight plan has been very efficient, except for the fourth leg which was designed to cross the surface low center. Instead of this, preliminary results show it is the eastern part of the low center that has been sampled. Real time modifications of the track to go right over the low center were not possible, unless to jeopardize the sampling of the following legs.