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/PMEL),
            Diana Bartels (NOAA/NSSL)


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 received 
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 informed us
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 FL050 to take in-situ 
measurements over the warm front (from 0018Z to 0100Z). 

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 received 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.