G4 Flight in support of FASTEX IOP 17 Low 41: the Fastex Cyclone Part I: Far upstream targetting flight Date: February 17, 1997 (prepared by Jean-Pierre Cammas and Gwenaelle Hello) Ferry flight to Saint-Johns (New Foundland): Take-off Time Shannon (approx): 0800Z Landing Time Saint Johns (approx): 1245Z Flight duration: 4 hr 45 mn Targetting flight nearby Saint-Johns: Take-off Time (approx): 1500Z Landing Time (approx): 2030Z Flight duration: 5hr 30 mn Scientists: Jean-Pierre Cammas (LA/CNRS), Gwenaelle Hello (METEO-FRANCE), Melvin Shapiro (NOAA), Tom ... (NRL), Diana Baertels (NOAA) Purpose: The first flight was a ferry from Shannon to Saint-Johns. After landing and refueling, a first scientific flight was designed to target the far upstream sensible areas of Low 41 which was expected to explosively deepen in the MSA region the day after. The targets were designed as follow, verifying time 0219 00Z and target time 0217 18Z. Duration of this flight (5hr 30mn) was fixed upon the constraints of the maximum daily crew duty time, taking-off the day after as soon as 1530Z, a flight centered around 1800Z for data assimilation purposes. Some far upstream targets were shown to be east of St Johns according to adjoint and singular vectors computations both from Arpege (Meteo-France) and NOGAPS (NRL) models. Scheduled way-points were: St-Johns, WP1 50.0N-52.0W, WP2 48.0N-40.0W, WP3 44.0N-40.0W, WP4 40.0N-55.0W, St-Johns. The first leg cuts some upper level structures (other parts of its could be assessed by the US land soundings). The second leg sampled is devoted to low level features following maximum axis and the third leg cuts some maximum of the low level targets. The last waypoint is in fact in the north part of the deepening area. The area sampled is roughly north-east of the incipient surface low (approx 37N-62W) and its upper-level precursor (approx 41N-68W). Aloft the general situation involves a very strong zonal upper-level jet stream whom the axis is a few degrees south of St Johns. At the surface, the pressure field involves a saddle area between the parent low south of Iceland, the incipient low 41 south of St Johns, and two anticyclones over the eastern United States and over the Azores. Description of the flight: A total of 22 dropsondes were launched, all successfully except for a few missing and bad winds. A first part of the flight plan was flown at FL270 (approx 365 hpa) involving the first ten dropsondes. Entering the NY ATC area (west of 40.0 W), authorization was got to fly at FL 410 (approx 165 hPa), then at FL 450 (approx 150 mb) until the last dropsonde (46.0N-53.3W). Though the flight plan was entirely builded to follow targetting objectives, some interesting mesoscale structures have been sampled. Relatively to high levels, legs from WP1 to WP3 and from WP4 to St Johns involve two 45-degrees oriented cuts across the cyclonic shear side of the upper-level jet stream and the associated gradient of tropopause height (geopotentiel on the 2 PVU surface). The leg between WP3 and WP4 was along the jet-core with very strong winds ahead. The highest winds recorded with the dropsondes were about to 170 knots during the last leg to St Johns (approx 44.3N-53.7W). Relatively to the surface, the flight plan encompasses first the north-westerly winds in the cold air outbreak east of St Johns (approx 1018 hPa), then a region of higher surface pressure (approx 1028 hPa) before to enter the easterly winds downstream of the incipient low (approx 1012 hPa). Very clear transitions were observed between two different boundary layers in the cold air outbreak east of St Johns and over the Gulf Stream south of St Johns. Flying above the upper-level jet core at FL 410 and FL 450, wind measurements by the aircraft have been found questionable because they were greater than the maxima observed with the dropsondes. Overall Assessment of the Flight: Very successful flight. The target areas have been sampled as scheduled. Three dropsondes were analysed by the Arpege model in real time at 1800Z. The complete structure of the objective targets could be studied as the Lear Jet did a flight nearly in the same area but 6 hours later. Interesting actual structures have been documented, among which are found the cyclonic shear side of a very strong zonal upper-level jet, clear transitions between different boundary layers.