G4 Flight in support of FASTEX IOP 17 
Low 41: the Fastex Cyclone
Part III: Post MSA flight: actual structures documentation
Date: February 19-20, 1997
(prepared by Jean-Pierre Cammas)

Take-off Time Shannon (approx):  1746Z February 19, 1997
Landing Time  Shannon (approx):  0050Z February 20, 1997
Flight duration: 7hr 04 mn

Scientists: Jean-Pierre Cammas (LA/CNRS), 
            Melvin Shapiro (NOAA/ETL), 
	    Christophe Baehr (METEO-FRANCE)
            Diana Bartels (NOAA/NSSL)


Purpose:

This is a post-MSA flight (North Sea, Norway Sea) over Low 41. Previous flights 
have given observations on both FUS/NUS/MSA regions. The purpose of this third 
flight with the GS-IV was to document actual structures involved in the mature 
phase of Low 41. Specific objectives of the flight were to sample the warm 
front and its apex part, the low level easterly jet ahead of the warm front, 
the outlow jet aloft, the structure of the low center itself. Another objective
was to assess the impact by orographic effects over the western coastal
mountains of Norway on the low level inflow of the low.  



Description of the flight:

All the flight plan has been flown at high levels (180 hPa). This was thanks 
to the late time in the day and a clever dealing with the 5 different ATC 
areas involved over North Sea, Norway Sea and Iceland (Jose Meitin). 
Though the meteorological situation was very interesting, only 39 dropsondes 
have been used during the flight (saving considerations). Three were bad, one 
was lost after reaching 825 hPa. The horizontal resolution of the dropsondes 
was inequal. The best resolution was over the warm front and above Norway where 
orographic effects were expected. The resolution was coarser over the north
part 
of the track in the outflow jet.   


The first leg going northeast after taking off intersected the warm front 
over the North Sea before the Norway coasts. A southeasterly low level jet 
(approx 30 ms-1) was observed after crossing the surface front.  

A tangential cut of the warm front was made going northwest on the second leg.
The apex part of the front was approached at the end of the leg. A short third 
leg going northeast crossed again the north part of the warm front. 

The fourth leg ended over Norway north of the warm front. As expected a dry 
low level layer was observed before to reach the Norway coasts. It is likely 
the orographic impact on the southeasterly flow forced to go down (subsidence) 
in the lee of the Norway coastal mountains. Such an inflow of a dry layer 
is expected to have a large effect on the behaviour of the mature low 41, as 
opposed with lows that deepen over open waters. 

The fifth and sixth legs over the north part of the flight plan were 
dedicated to the observation of the outflow jet aloft. A large 
diffluence of the upper level flow was indeed observed. Those data 
should help to quantify the outflow mass aloft and to further control 
the numerical simulation of this process. 

The end of the flight plan involved a "butterfly" pattern over the 
low center. The lowest surface pressure recorded with the dropsondes 
on this figure was 939 hPa (62.5N-7.1W), very close from the numerical 
guidance. A barotropic structure was described by the nearest dropsondes 
around the low center. On a larger scale, from the apex of the warm front 
to the low center, preliminary interpretation of the data show what appears 
to be a warm core seclusion structure. South of the low center and on the 
way to Shannon, strong northwesterly low level winds were observed 
(in excess of 50 knots). 


 Overall Assessment of the Flight:

 This successful flight ends a very complete IOP (FUS/NUS/MSA/Post-MSA) 
 over what has been called The Fastex Cyclone. Though the horizontal 
 resolution of the dropsondes was not as high as expected (period of 
 savings for the dropsondes), a very interesting dataset has 
 been sampled. It should be recall that all the flight plan has been 
 flown at high levels (approx 180 mb). Topics of interest are the  
 low level front and jet structures (warm front, seclusion over the low 
 center, southeasterlies north of the warm front, northwesterlies south 
 of the low center, ...), the impact of orographic effects over Norway on the 
 mature phase of the low and the outflow jet aloft. This low is the deepest 
 one studied during the field season (139 hPa) and satisfies all the FASTEX 
 criteria. 
 
 End