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/ETL), 
            Gwenaelle Hello (METEO-FRANCE), 
            Tom Rosemond (NRL), 
            Diana Bartels (NOAA/NSSL)


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.  
 
 End