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), Christophe Baehr (METEO-FRANCE) Diana Baertels (NOAA) 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 (939 hPa) and satisfies all the FASTEX criteria.