G4 Flight in support of FASTEX IOP 06 Low 20 MSA flight: actual structures documentation Date: January 23, 1997 (prepared by Jean-Pierre Cammas) Take-off Time Shannon (approx): 1757Z January 23, 1997 Landing Time Shannon (approx): 0120Z January 24, 1997 Flight duration: 7hr 23 mn Scientists: Jean-Pierre Cammas (LA/CNRS), Alain Joly (METEO-FRANCE), Francois Lalaurette (METEO-FRANCE), Nick Bond (NOAA), Diana Baertels (NOAA) Purpose: Low 20 was expecting to deepen and move to the southern tip of the Greenland by 0000Z January 23. It is associated with a very trailing active front and an upper-level potential vorticity maximum west of the cold front. The formation of a small low over the triple point was only supported by the Arpege forecast, which may be in relation with a two-finger shape of the upper-level PV maximum in the same forecast. Low 20 did not exhibit the characteristic patterns of a Fastex-like cyclone, the MSA flights were cancelled (lack of interest and case similar to previously documented cases earlier in the field season). However, for the second flight of the GS-IV, it was proposed a technical test and a scientific objective on Low 20: (i) to test the procedures with the Air Traffic Control and to qualify the dropsoundes at high levels (e.g., FL450) ; (ii) to sample the triple point and the upper-level PV structure. Description of the flight: Two hours before to take-off authorization from ATC was got to launch from FL450 in a zonal box (58N-60N, 15W-29W, located inside the initially reserved larger box). In consequence, the first half of the flight plan draws a track within the high altitude zonal box, which was conveniently located in the north part of the upper-level trough. The second part of the flight plan (FL260) was designed to sample the south part of the upper-level potential vorticity structure, the triple point and possibly the warm front. A total of 28 dropsondes were launched, among which 14 in the high altitude zonal box. Nineteen of them were good, 16 have been transmitted on the GTS, the two last one entered in the ARPEGE 0000Z assimilation cycle. Good high altitude data have been taken in the upper-level PV maximum in the north part of the system. This part also involved two cuts of the cold front. As forecasters in Shannon let us inform that the surface front was moving faster than expected (which was in agreement with our interpretation of the first dropsondes), the second part of the flight plan has been shortened to keep closer from the surface front. The triple point was sampled on the last leg. The survey of the upper-level PV structure has been nicely done, interesting observations of the upper-level frontal zone have been sampled behind the cold front. As it was not permitted to drop on the way back to Shannon over the warm front, we decided to fly at FL50 to take in situ measurements over the warm front (from 0018Z to0100Z). Overall assessment of the flight: This second flight of the GS-IV was mostly motivated by technical needs: test of ATC procedures and qualification of high altitude dropsounding. Though successful on this occasion, the dealing with ATC showed that the allowance of a high altitude box has some strong constraints (time of the day, zonal shape of the box, authorization got 2 hours before to take-off) that render difficult the design of a flight plan. On the meteorological point of view, the Low 20 was not supporting great enthousiasm from the FASTEX investigators. However, a nice survey of the upper-level potential vorticity structure behind of the cold front of Low 20 has been done. There are interesting observations of the upper-level jet-front system.